Growth promoters for animals
This application is a division, of allowed U.S. application Ser. No. 07/523,249, filed May 15, 1990, now U.S. Pat. No. 5,288,496. 
    
    
     FIELD OF THE INVENTION 
     This invention is directed to a delivery system which may be used to provide growth promoters for animals. 
     BACKGROUND OF THE INVENTION 
     Recent advances in pharmaceutical bioengineering have stimulated the interest of the food industry in new delivery systems for hormones and other growth promoters. 
     Steroids, and especially anabolic steroids, are widely used to promote the growth of cattle and other food animals. Such growth promotion is desirable among food producers because it maximizes the weight gain per animal per amount of food consumed. 
     Currently, steroid products are usually supplied to the animal in the form of a non-biodegradable, implantable-steroid time-release pellet which is placed in the ear skin, part of the animal which is discarded upon slaughter. Such delivery systems have been recently reviewed (Sawyer, G. J., et al., Austral. Vet. J. 65:101-108 (1988). The pellet systems cannot provide a pulsed dose of steroid and are often reimplanted two or three times throughout the life of the animal with each implant being administered by a specially designed injector gun. 
     The use of pellet delivery systems has not met with complete success and they have many disadvantages. The main health concern is that misuse of the non-degradable pellet system may result in harmful levels of such steroids in the meat provided to consumers. Such misuse may arise if the pellet is misplaced. For example, if the pellet is misplaced in the neck or behind the ear instead of in the ear skin, the possibility is raised that the remains of the pellet could be ingested by the consumer. Also, a farmer may mistakenly inject more than one pellet at a time. Also, the pellets are often not successfully implanted and fall to the ground in the treatment pen. 
     Disadvantages of the currently available pellet implants further include their inability to allow for combination of different drugs with highly controlled release rates in the same unit, and the fact that only 40-70% of the active agent is released. Lastly, commercial implants do not offer a capability in adjusting the duration of action and because of the nondegradable polymeric excipient and the relatively low total surface area of the pellets, it is very difficult to adjust the dosage of the active agent. 
     Compudose R  is a polymeric controlled release implant device for use with beef cattle which is designed to provide for the continuous delivery of estradiol-17β) for 200 or 400 days (Ferguson, T. H. et al., J. Cont. Rel 8:45-54 (1988)). The compudose R  implant is made by coating a nonmedicated silicone rubber core with a thin layer of silicone runner which contains micronized crystalline estradiol-17β). However, the device is not biodegradable. 
     Biodegradable particles for use in the delivery of steroid hormones have been described (U.S. Pat. Nos. 4,329,331, 4,489,055, 4,683,288, 4,677,191, 4,675,189, 4,530,840, 4,542,025 and 4,389,330. 
     However, none of the above references have suggested or disclosed a method of increasing animal growth by providing a biodegradable microparticle delivery of steroid growth promoters in food animals and a need exists for better, safer delivery systems for steroids in food animals. 
     SUMMARY OF THE INVENTION 
     With the above-described needs in mind, the present inventor set out to develop a composition which could be adapted to accomplish the sustained release of steroid growth promoters in food animals in a manner which would not only more efficiently promote the growth of such animals but which would also provide a safer delivery system for the growth promoting compounds. 
     The invention further provides a method of delivering growth promoters to animals in a multiphasic manner. 
     The invention further provides a method of delivering two or more active agents at predetermined release rates with the release of each drug being at a different rate and duration by design. 
     In detail, the invention provides a method of stimulating growth in an animal by treating such animals with injectable, biodegradable microparticles, wherein such microparticles are loaded with at least one growth-promoting steroid. Populations of microparticles with different active steroids or other types of bioactive agents, such as antibiotics or vaccines can be blended to form a composite formulation. 
     In one embodiment, the growth-promoting steroid is selected from the group consisting of estradiol benzoate, 17β-estradoil, trenbolone acetate, zeranol, testosterone and testosterone derivatives or combinations thereof. 
     The invention further concerns the embodiment of the above-described method wherein the biodegradable microparticles comprise a homopolymer or copolymer of lactic/glycolic acid, polycaprolactone, polydioxonone, polyorthoesters, polyanhydides, and natural polymers including albumin, casein, and waxes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 graphically presents the data from Table III. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to provide a clearer and consistent understanding of the specification and claims, including the scope to be given such terms, the following definitions are provided. 
     By the term &#34;administered&#34; is intended any method of delivering the growth promoting microparticles of the invention to an animal, such as, for example, parenteral (intravenous, intramuscular, or subcutaneous) administration. 
     By the term &#34;food animal&#34; is intended any animal that is consumed as a source of protein in the diet of humans or other animals. Typical food animals include bovine animals, for example cattle; ovine animals, for example sheep; porcine, for example pigs; fowl, for example chickens and turkeys; rabbit, and the like. 
     By &#34;promoting growth&#34; or &#34;increasing growth&#34; is meant increasing the weight or weight gain of an animal by an amount not found in an animal whose growth was not being &#34;promoted.&#34; 
     By &#34;microparticles&#34; is meant sol id particles which contain an active agent, herein a growth promoting metabolic steroid, either in solution or in crystalline form. The active agent is dispersed or dissolved within the polymer which serves as the matrix of the particle. 
     The present invention concerns a method of increasing growth in food animals which comprises providing to such animals biodegradable microparticles loaded with a metabolic steroid. The method of the present invention provides advantages over methods known in the art such as, inter alia, an increased weight gain, a biodegradable system, an injectable system which prevents the loss of dose during treatment which often occurs with solid pellet implants, the ability to mix microparticles containing different drugs, the ability to program release (multiphasic release patterns) to give faster rates of drug release as the animal grows larger, and the ability to stimulate growth in poultry. 
     Further, the method of the invention provides the ability to design a unique blood hormone profile for the animal and a multiphasic hormone delivery system which provides pulsed doses of desired growth promoter, thus eliminating a need for consecutive implant treatments. 
     The product of the present invention offers the advantage of durations of action ranging from only 30 to 60 days to more than 200 days depending upon the type of microsphere selected. In the preferred embodiment the microspheres are designed to afford growth promotion in animals over a period of 150 to 250 days. The duration of action can be easily controlled by manipulation of the polymer composition, polymer:drug ratio and microsphere size. 
     Another important advantage of the present invention is that practically all of the active drug is delivered to the animal whereas with existing commercial implants only 40 to 75% of the active agent is released. Because the polymer used in the method of the invention is biodegradable, all of the entrapped drug can be released into the animal. 
     The preferred construction of microparticles of the invention are described in U.S. Pat. Nos. 4,389,330, and 4,530,840, fully incorporated herein by reference. The microparticles of the invention are composed of a polymer which is, preferably, either a homopolymer or copolymer of lactic or glycolic acids. Other bio-degradable polymers may be used, such as, for example, polycaprolactone, polydioxonene, polyorthoesters, polyanhydides, and natural polymers including albumin, casein, and waxes. 
     The present invention is directed to a method of stimulating growth in food animals by administering an injectable, long-acting biodegradable microparticle which contains a growth promoter for food animals. Suitable growth promoters include 17β)-estradiol, estradiol benzoate, trenbolone acetate, zeranol, testosterone, testosterone derivatives (and especially testosterone propionate) and any combination thereof. 
     In a preferred embodiment, administration of the growth promoter to food animals by the method of the invention is achieved by a single administration of the growth promoter loaded microparticles and the microparticles release the active growth promoter in a constant or pulsed manner into the animal and eliminates the need for repetitive injections. 
     The formulation of the present invention contains a growth promoter dispersed in a microparticle matrix material. The amount of growth promoter agent incorporated in the microparticles usually ranges from less than 1 wt. % to as high as 95 wt. %, preferably 10 to 50 wt. %. By weight % is meant parts of drug per parts of polymer by weight. For example, 10 wt. % would mean 10 parts drug per 90 parts polymer by weight. 
     The polymeric matrix material of the microparticles present invention must be a biocompatible and biodegradable polymeric material. The term biocompatible is defined as a polymeric material which is not toxic to an animal and not carcinogenic. The matrix material should be biodegradable in the sense that the polymeric material should degrade by bodily processes to products readily disposable by the body and should not accumulate in the body. The biodegraded products also should be biocompatible with the body in the sense that the polymeric matrix is compatible with the body. Suitable examples of polymeric matrix materials include poly (glycolic acid), poly(-d,l-lactic) acid, poly(1-1actic acid) and copolymers thereof, copolyoxalates, polycaprolactone, poly {lactic acid-caprolactone), and the like. Suitable polymeric materials also include waxes such as glycerol mono- and distearate and casein. 
     The molecular weight of the polymeric matrix material is of some importance. The molecular weight should be high enough so that it forms satisfactory polymer coatings, i.e., the polymer should be a good film former. Usually, a satisfactory molecular weight is greater than 5,000 daltons. However, since the properties of the film are also partially dependent on the particular polymeric material being used, it is very difficult to specify an appropriate molecular weight range for all polymers. The molecular weight of a polymer is also important from the point of view that molecular weight influences the biodegradation rate of the polymer. For a diffusional mechanism of drug release, the polymer should intact until all of the drug is released form the microparticles and then degrade. The drug can also be released from the microparticles as the polymeric excipient bioerodes. By an appropriate selection of polymeric materials a microparticle formulation can be made such that the resulting microparticles exhibit both diffusional release and biodegradation release properties; This is useful in affording multiphasic release patterns. 
     The microparticle product of the present invention can be prepared by any method which is capable of producing microparticles in a size range acceptable for use in an injectable composition. A preferred method of preparation is the method described in U.S. Pat. No. 4,389,330. In this method the desired growth promoting compound is dissolved or dispersed in an appropriate solvent. To the growth promoting agent-containing medium is added the polymeric matrix material in an amount relative to the active ingredient which gives a product of the desired loading of active agent. Optionally, all of the ingredients of the microparticle product can be blended in the solvent medium together. Suitable solvents for the growth promoter compound and the polymeric matrix material include organic solvents such as acetone, halogenated hydrocarbons such as chloroform, methylene chloride and the like, aromatic hydrocarbon compounds, halogenated aromatic hydrocarbon compounds, cyclic ethers, alcohols, ethyl acetate and the like. Preferred solvents for the growth promoter agent are methylene chloride or ethyl acetate. 
     The mixture of ingredients in the solvent is emulsified in a continuous-phase processing medium; the continuous-phase medium being such that a dispersion of microdroplets containing the indicated ingredients is formed in the continuous-phase medium. Naturally, the continuous-phase processing medium and the organic solvent must be immiscible, and most commonly is water although nonaqueous media such as xylene and toluene and synthetic oils and natural oils can be used. Usually, a surfactant is added to the continuous-phase processing medium to prevent the microparticles from agglomerating and to control the size of the solvent microdroplets in the emulsion. A preferred surfactant-dispersing medium combination is a 1 to 10 wt. % poly (vinyl alcohol) in water mixture. The dispersion is formed by mechanical agitation of the mixed materials. An emulsion can also be formed by adding small drops of the active agent-wall forming material solution to the continuous phase processing medium. The temperature during the formation of the emulsion is not especially critical but can influence the size and quality of the microparticles and the solubility of the drug in the continuous phase. Of course, it is desirable to have as little of the drug in the continuous phase as possible. Moreover, depending on the solvent and continuous-phase processing medium employed, the temperature must not be too low or the solvent and processing medium .will solidify or the processing medium will become too viscous for practical purposes, or too high that the processing medium will evaporate, or that the liquid processing medium will not be maintained. Moreover, the temperature of the medium cannot be so high that the stability of the particular active agent being incorporated in the microparticles is adversely affected. . Accordingly, the dispersion process can be conducted at any temperature which maintains stable operating conditions, which preferred temperature being about 30° C. to 60° C., depending upon the drug and excipient selected. 
     The dispersion which is formed is a stable emulsion and from this dispersion the organic solvent immiscible fluid can be partially removed in the first step of the solvent removal process. The solvent can easily be removed by common techniques such as heating, the application of a reduced pressure or a combination of both. The temperature employed to evaporate solvent from the .microdroplets is not critical, but should not be that high that it degrades the growth promoting agent employed in the preparation of a given microparticle, nor should it be so high as to evaporate solvent at such a rapid rate to cause defects in the wall forming material. Generally, from 5 to 75 %, preferably 1 to 25% of the solvent is removed in the first solvent removal step. 
     After the first stage, the dispersed microparticles in the solvent immiscible fluid medium are isolated from the fluid medium by any convenient means of separation. Thus, for example, the fluid can be decanted from the microparticle or the microparticle suspension can be filtered. Still other, various combinations of separation techniques can be used if desired. 
     Following the isolation of the microparticles from the continuous-phase processing medium, the remainder of the solvent in the microparticles is removed by extraction. In this step, the microparticles can be suspended in the same continuous-phase processing medium used in step one, with or without surfactant, or in another liquid. The extraction medium removes the solvent from the microparticles and yet does not dissolve the microparticles. During the extraction, the extraction medium with dissolved solvent must be removed and replaced with fresh extraction medium. This is best done on a continual basis, where the rate of extraction medium replenishment is critical. If the rate is too slow, agent crystals will protrude from the microparticles or grow in the extraction medium. Obviously, the rate of extraction medium replenishment or a given process is a variable which can easily be determined at the time the process is performed and, therefore, no precise limits for the rate must be predetermined. After the majority of the solvent has been removed from the microparticles, the microparticles are dried by exposure to air or by other conventional drying techniques such as vacuum drying, drying over a desiccant, or the like. This process is very efficient in encapsulating the growth promoting agent since core loadings of up to 80 wt. %, preferably up to 50 wt. % are obtained. 
     The microparticle product of the present invention is usually made up of particles of a spherical shape although sometimes the microparticles may be irregularly shaped. The microparticles can vary in size, ranging from submicron to millimeter diameters. Preferably, submicron to 250 μm, are desirable for growth promoter formulations allowing administration of the microparticles with a standard gauge needle. 
     The growth promoting agent bearing microparticles are obtained and stored as a dry material. Prior to administration to an animal or group of animals, the microparticles are suspended in an acceptable pharmaceutical liquid vehicle, and then the suspension is injected into the desired portion of the body of the animal. 
     The amount of growth promoting agent administered to the animal depends on the particular animal species, growth promoting agent, length of time of treatment, age of the animal and amount of growth promotion which is desired. Since the present invention is concerned with a unique method of promoting growth in food animals by administering growth promoters in a biodegradable microparticle formulation involving encapsulated growth promoting agents, one skilled in the art is well aware of the dosages required to treat a particular animal with a particular growth promoter. Commonly, growth promoters are administered in microgram to milligram amounts, depending upon the size of the animal and the length of treatment time. 
     The microparticles can be mixed by size or by type so as to provide for a delivery of growth promoters to animals in a multiphasic manner and/or in a manner which provides different growth promoters to the animal at different times, or a. mixture of growth promoters to the animal at the same time. Other biologically active agents commonly administered to animals may be blended with the growth promoter formulation. For example, antibiotics, anthelmintics, vaccines, or any desired active agent, either in microparticle form or in conventional, unencapsulated form may be blended with the growth promoter and provided to an animal by the method of the invention. 
     For example, the method of the invention may also be used to deliver a vaccine containing antigenic material such as an antigen or mixture of antigens of bacterial, viral, or cellular origin. Especially, for example, the method of the invention may be used to deliver Pasteurella multocide, Pasteurella haemolytica, Haemophilus influenzae, Hemophilus ducreyi, Escherichia coli, Salmonella abortus ovis, and IBR-BVD-PI3 virus antigen(s) and vaccines. 
     The following examples further describe the materials and methods used in carrying out the invention. The examples are not intended to limit the invention in any manner. 
     EXAMPLES 
     Example 1 
     Preparation of 20%-Loaded Estradiol Benzoate Microparticles 
     This example describes the preparation of 20% loaded estradiol benzoate microparticles with an 85:15 poly(d,l-lactide-co-glycolide) (PLGA) excipient. 
     To prepare a 150 g batch of 20% loaded estradiol benzoate microparticles, 30 g estradiol benzoate is dissolved in 706.06 g of 17% sterile-filtered 85:15 dl-PLGA (MW=135,964)/ethyl acetate solution. During dissolution, the contents of the Erlenmeyer flask are maintained at 60° C. Once the drug has totally dissolved, the drug/polymer solution is added to a 3000-ml jacketed reactor containing 144 g of 3% PVA having been saturated with 183 g of ethyl acetate stirring at 245 rpm at 55° C. 
     Once the embryonic microparticles have stabilized, the contents of the reaction flask are transferred to 110 liters of water stirring in two 60-liter stainless-steel beakers at 8° C. After having been stirred in the initial wash for 17 minutes, the hardened microparticles are wet sieved onto a 25-micron sieve. The retained product is a transferred to a second water wash of 50 liters stirring in a 60-liter stainless steel beaker at 8° C. After having been in the second wash for 15 minutes, the microparticles are again sieved onto a 25-micron sieve. The retained product is transferred to a third and final wash of 50 liters stirring at 8° C. After stirring in the third wash for about 4 hours, product is isolated by sieving through a sieve stack composed of 25, 45, 90 and 150 micron openings. 
     Each sieve is washed with liberal portions of fresh water and the product is allowed to air dry. 
     Example 2 
     Preparation of 15%-Loaded Estradiol Benzoate Microparticles 
     This example describes the preparation of 15% loaded estradiol benzoate microparticles with poly (d,l-lactide) excipient. 
     To prepare a 10 g batch of 15%-loaded estradiol benzoate microparticles, 1.5 g estradiol benzoate and 8.6 g dl-PLA, is dissolved in 35.7 g of ethyl acetate in an Erlenmeyer flask at 52° C. The drug/polymer solution is added to a 300-ml jacketed reaction flask containing 196 g of 3% aqueous polyvinyl alcohol having been saturated with 15.6 g of ethyl acetate stirring at 570 rpm at 52° C. 
     Once the embryonic microparticles have stabilized, the contents of the reaction flask are transferred to 8 liters of water stirring in 10-liter stainless steel beaker at 11° C. After stirring in the initial wash for approximately 4 minutes, the hardened microparticles are wet sieved onto a 25-micron sieve. Retained product is transferred to a second wash of 4 liters stirring at 11° C. After the product has stirred in the second wash for 6 hours, it is isolated by sieving through a sieve stack composed of 25, 45, 90, 150, and 212 micron openings. Each sieve is washed with liberal portions of fresh water to more effectively separate the different sizes of microparticles, and the product is allowed to air dry on sieves overnight. 
     Example 3 
     Preparation of 17β-Estradiol Microspheres 
     A 10-gram batch of 20%-loaded 17β-estradiol microspheres was prepared by weighing 8g dl-PLA and 2g 17β-estradiol into a 150 ml Erlenmeyer flask. Then 72 g of chloroform was added to 150 ml Erlenmeyer flask. Then 72 g of chloroform was added to the flask to dissolve the polymer. After the polymer was completely dissolved, 24 g of acetone was added to dissolve the drug. The flask was maintained at 56° C. during the drug/polymer dissolution stage. The solution was added to a 350 ml jacketed resin kettle containing 215 g of 5% polyvinyl alcohol stirring at 390 rpm and maintained at 56° C. After 4 minutes, the emulsion had stabilized and the soft microspheres were in the desired size range, the reactor contents were transferred into 8 liters of stirring water at 16° C. The microspheres were then sieved from the wash by a 25-micron sieve and the retained product was transferred into a second 8 liter stirring wash at 16° C. After stirring the product in the second wash for 3 hours, the particles were isolated by sieving through an 8 inch sieve stack composed of 25, 90,  and 212 micron mesh. Each sieve was washed with liberal portions of fresh water to effectively separate the different sizes of microspheres. After air drying overnight, the product was collected, weighed and placed under ambient temperature vacuum to dry. 
     Example 4 
     Preparation of 50%-Loaded Trenbolone Acetate Microparticles 
     This example describes the preparation of 50%-loaded trenbolone acetate microparticles with an 85:15 poly (d,l-lactide-co-glycolide) excipient. 
     To prepare a 50 g batch of 50%-loaded trenbolone acetate microparticles, 25 g of trenbolone acetate and 25 g of 85:15 dl-PLGA (MW=135,964), are dissolved in 110 g of ethyl acetate in an Erlenmeyer flask at 52° C. The drug/polymer solution is added to a 1000-ml jacketed reaction flask containing about 400 g of 3% aqueous polyvinyl alcohol having been saturated with 23 g of ethyl acetate stirring at 340 rpm at 52° C. 
     Once the embryonic microparticles have stabilized, the contents of the reaction flask are transferred to 45 liters of water stirring in a 60-liter stainless steel beaker at 11° C. After having stirred in the initial wash for approximately 10 minutes, the hardened microparticles are wet sieved onto a 25-micron sieve. The product retained by the sieve is transferred to a second wash volume of 40 liters of water stirring at 11° C. After 5 hours stirring in second wash, the microparticles are isolated by sieving through a sieve stack composed of 25, 90, 150, 212 and 500 micron openings. Each sieve is washed with liberal portions of fresh water to more effectively separate the different sizes of microparticles and the product is allowed to air dry on sieves overnight. 
     Example 5 
     Preparation of 30%-Loaded Trenbolone Acetate Microparticles 
     This example describes the preparation of 30%-loaded trenbolone acetate microparticles with an 85:15 poly (d,l-lactide-co-glycolide) excipient. 
     To prepare a 60 g batch of 30%-loaded trenbolone acetate microparticles, 18 g of trenbolone acetate and 42 g of 85:15 dl-PLGA (MW=135,964) are dissolved in 161 g of ethyl acetate in an Erlenmeyer flask at 82° C. The drug/polymer solution is added to a 1000-ml jacketed reaction flask containing about 585 g 3% aqueous polyvinyl alcohol having been saturated with 30 g of ethyl acetate stirring at 300 rpm. 
     Once the embryonic microparticles have stabilized, the contents of the reaction flask are transferred to 50 liters of water stirring in a 60-liter stainless-steel beaker at 12° C. After having stirred in initial wash for approximately 15  minutes, the hardened microparticles are wet-sieved onto a 25-micron sieve. The product retained by the sieve is transferred to a second wash volume of 40 liters of water stirring at 12° C. After three hours stirring in second wash, the microparticles are isolated by sieving through a sieve stack composed of 25, 90, 150, 212 and 300 micron openings. Each sieve is washed with liberal portions of fresh water to more effectively separate the different sizes of microparticles, and the product is allowed to air-dry on sieves. 
     Example 6 
     Preparation of 25%-Loaded Zeranol Microparticles 
     The example describes the preparation of 25%-loaded zeranol microparticles with an 85:15 poly (d,l-lactide-co-glycolide) excipient. 
     To prepare a 5 g batch of 25%-loaded zeranol microparticles, 1.25 g of zeranol and 3.75 g of 85:15 dl-PLGA are dissolved in 33.0 g of ethyl acetate in an Erlenmeyer flask at 59° C. The drug/polymer solution is added to a 100-ml jacketed reaction flask containing about 93 g of 5% aqueous polyvinyl alcohol having been saturated with 5 g of ethyl acetate at 59° C. and stirring at 300 rpm. 
     Once the embryonic microparticles had stabilized, the contents of the reaction flask are transferred to 3.5 liters of water stirring in a 4-liter beaker at 12° C. After having stirred in the initial wash for approximately 1/2 hour, the microparticles are wet sieved onto a 25-micron sieve. The retained product is transferred to a second wash of 3..5 liters of water at 12° C. After 3 hours in the second wash the microparticles are isolated by sieving through a sieve stack composed of 25, 45, 90, 150 and 212 micron pore openings. Each sieve is washed with liberal portions of water to more effectively separate the different sizes of microparticles, and the product is allowed to air dry on sieves. 
     Example 7 
     Preparation of 50%-Loaded Zeranol Microparticles 
     This example describes the preparation of 50%-loaded zeranol microparticles with a poly (d,l-lactide) excipient. 
     To prepare a 5 g batch of 50%-loaded zeranol microparticles 2.5 g of zeranol and 2.7 g of dl-PLA, dissolved in 63 g of ethyl acetate in an Erlenmeyer flask at 59° C. The drug/polymer solution is added to a 100 ml jacketed reaction flask containing about 90 g of 3% aqueous polyvinyl alcohol, having been saturated with 6 g of ethyl acetate at 58° C. and stirring at 200 rpm. 
     Once the embryonic microparticles have stabilized, the contents of the reaction flask are transferred to 4 liters of water stirring in a 4-liter beaker at 12° C. After having stirred in the initial wash for 23 minutes, the microparticles are wet sieved onto a 25-micron sieve. The product retained by the sieve is transferred to a second wash of 4 liters stirring at 90° C. After stirring for three hours in a second wash, the microparticles are isolated by sieving through a sieve stack composed of 25, 45, 90, 150 and 212 micron pore openings. Each sieve is washed with liberal portions of water, and the product is allowed to air dry on sieves. 
     Example 8 
     Effect of Injectable Microparticle Formulation on Promoting Weight Gain in Beef Cattle 
     Estradiol benzoate microparticles prepared by the methods described above were tested in beef calves to determine the effect on weight gain. Three groups of 22 calves each were used. An untreated control groups, T1, a test group T5, and a positive control group, T6. The test group (T5), received injections of 15 mg of estradiol benzoate microencapsulated in poly(lactide-co-glycolide excipient. The positive controls (T6) received the commercial implant Ralgro (zeranol active) 36 mg. The Hereford calves weighed less than 250 lbs at the time of treatment and the three groups were comprised of 10 male and 12 female animals per group. All calves remained with their dams during the 168-day study and then they were weaned. Calves were weighted, treatment site inspected, and other general health observations made at 28-day intervals. Animals were weighted on a portable Paul cattle scale equipped with a digital readout. The results of the study are shown in Tables I and II. The biodegradable microparticles were more effective than the commercial implants in increasing the weight of the animals during the treatment period. 
     
                       TABLE 1                                                     
______________________________________                                    
EFFECT OF INJECTABLE MICROPARTICLES                                       
ON GROWTH OF HEREFORD BEEF CALVES                                         
       Daily             Daily                                            
       Gain at           Gain at                                          
       Day 140                                                            
              Gain       Day 168  Gain                                    
       (lbs)  Over Control                                                
                         (lbs)    Over Control                            
______________________________________                                    
T1       1.66     --         1.62   --                                    
Untreated                                                                 
Control                                                                   
T5       1.86     (12%)      1.82   (12.3%)                               
Estrogen                                                                  
Microspheres                                                              
T6       1.73     (4.2%)     1.70    (4.9%)                               
Ralgro                                                                    
Estrogen          7.5% increase     7.1% increase                         
Micro-                                                                    
particles                                                                 
vs. Ralgro                                                                
______________________________________                                    
 
    
     
                                           TABLE II                                
__________________________________________________________________________
Weight Gain of Hereford Calves Treated with biodegradable microspheres    
Calf     Day 0 Wts.                                                       
                  D28                                                     
                     D56                                                  
                        D84                                               
                           D112                                           
                              Day 140 Wts                                 
                                       Day 168 Wts                        
                                                Total                     
Trt.                                                                      
   I.D.                                                                   
      Sex                                                                 
         1st                                                              
            2nd                                                           
               Avg.                                                       
                  WT WT WT WT 1st                                         
                                 2nd                                      
                                    Avg.                                  
                                       1st                                
                                          2nd                             
                                             Avg.                         
                                                Gain                      
__________________________________________________________________________
T5 11 H  155                                                              
            149                                                           
               152                                                        
                  196                                                     
                     238                                                  
                        312                                               
                           360                                            
                              417                                         
                                 418                                      
                                    418                                   
                                       464                                
                                          468                             
                                             466                          
                                                314                       
   24 H  159                                                              
            150                                                           
               155                                                        
                  189                                                     
                     220                                                  
                        295                                               
                           330                                            
                              412                                         
                                 403                                      
                                    408                                   
                                       447                                
                                          452                             
                                             450                          
                                                295                       
   25 H  131                                                              
            124                                                           
               128                                                        
                  169                                                     
                     216                                                  
                        288                                               
                           312                                            
                              399                                         
                                 404                                      
                                    402                                   
                                       459                                
                                          461                             
                                             460                          
                                                333                       
   31 H  185                                                              
            178                                                           
               182                                                        
                  208                                                     
                     258                                                  
                        335                                               
                           405                                            
                              492                                         
                                 484                                      
                                    488                                   
                                       536                                
                                          540                             
                                             538                          
                                                357                       
   32 H  223                                                              
            216                                                           
               220                                                        
                  263                                                     
                     312                                                  
                        388                                               
                           431                                            
                              508                                         
                                 494                                      
                                    501                                   
                                       545                                
                                          550                             
                                             548                          
                                                328                       
   50 H  151                                                              
            144                                                           
               148                                                        
                  176                                                     
                     214                                                  
                        286                                               
                           317                                            
                              399                                         
                                 386                                      
                                    393                                   
                                       420                                
                                          431                             
                                             426                          
                                                287                       
   54 H  169                                                              
            172                                                           
               171                                                        
                  197                                                     
                     235                                                  
                        325                                               
                           375                                            
                              452                                         
                                 454                                      
                                    453                                   
                                       499                                
                                          503                             
                                             501                          
                                                331                       
   103                                                                    
      H  148                                                              
            140                                                           
               144                                                        
                  189                                                     
                     236                                                  
                        309                                               
                           363                                            
                              467                                         
                                 435                                      
                                    451                                   
                                       475                                
                                          486                             
                                             481                          
                                                337                       
   109                                                                    
      H  176                                                              
            169                                                           
               173                                                        
                  212                                                     
                     250                                                  
                        325                                               
                           375                                            
                              454                                         
                                 448                                      
                                    451                                   
                                       493                                
                                          498                             
                                             496                          
                                                323                       
   110                                                                    
      H  115                                                              
            109                                                           
               112                                                        
                  144                                                     
                     179                                                  
                        230                                               
                           270                                            
                              325                                         
                                 307                                      
                                    316                                   
                                       366                                
                                          372                             
                                             369                          
                                                257                       
   113                                                                    
      H  163                                                              
            161                                                           
               162                                                        
                  189                                                     
                     230                                                  
                        294                                               
                           329                                            
                              390                                         
                                 373                                      
                                    382                                   
                                       418                                
                                          427                             
                                             423                          
                                                261                       
   127                                                                    
      H  146                                                              
            141                                                           
               144                                                        
                  174                                                     
                     204                                                  
                        274                                               
                           317                                            
                              384                                         
                                 375                                      
                                    380                                   
                                       428                                
                                          434                             
                                             431                          
                                                288                       
   29 S  154                                                              
            150                                                           
               152                                                        
                  185                                                     
                     214                                                  
                        283                                               
                           324                                            
                              421                                         
                                 377                                      
                                    399                                   
                                       435                                
                                          444                             
                                             440                          
                                                288                       
   33 S  180                                                              
            173                                                           
               177                                                        
                  205                                                     
                     256                                                  
                        343                                               
                           404                                            
                              491                                         
                                 480                                      
                                    486                                   
                                       526                                
                                          532                             
                                             529                          
                                                353                       
   34 S  150                                                              
            144                                                           
               147                                                        
                  181                                                     
                     226                                                  
                        299                                               
                           347                                            
                              433                                         
                                 411                                      
                                    422                                   
                                       459                                
                                          459                             
                                             459                          
                                                312                       
   57 S  186                                                              
            180                                                           
               183                                                        
                  217                                                     
                     255                                                  
                        333                                               
                           375                                            
                              462                                         
                                 443                                      
                                    453                                   
                                       504                                
                                          495                             
                                             500                          
                                                317                       
   63 S  119                                                              
            115                                                           
               117                                                        
                  148                                                     
                     180                                                  
                        248                                               
                           281                                            
                              375                                         
                                 335                                      
                                    355                                   
                                       389                                
                                          390                             
                                             390                          
                                                273                       
   80 S  168                                                              
            165                                                           
               167                                                        
                  197                                                     
                     237                                                  
                        314                                               
                           359                                            
                              434                                         
                                 420                                      
                                    427                                   
                                       486                                
                                          498                             
                                             492                          
                                                326                       
   84 S  135                                                              
            131                                                           
               133                                                        
                  158                                                     
                     180                                                  
                        218                                               
                           260                                            
                              322                                         
                                 309                                      
                                    316                                   
                                       360                                
                                          365                             
                                             363                          
                                                230                       
   85 S  192                                                              
            195                                                           
               194                                                        
                  226                                                     
                     268                                                  
                        346                                               
                           387                                            
                              479                                         
                                 432                                      
                                    456                                   
                                       504                                
                                          508                             
                                             506                          
                                                313                       
   116                                                                    
      S  160                                                              
            147                                                           
               154                                                        
                  190                                                     
                     224                                                  
                        296                                               
                           347                                            
                              433                                         
                                 418                                      
                                    426                                   
                                       467                                
                                          478                             
                                             473                          
                                                319                       
   121                                                                    
      S  206                                                              
            196                                                           
               201                                                        
                  241                                                     
                     279                                                  
                        348                                               
                           394                                            
                              462                                         
                                 459                                      
                                    461                                   
                                       508                                
                                          496                             
                                             502                          
                                                301                       
T6  1 H  160                                                              
            149                                                           
               155                                                        
                  182                                                     
                     205                                                  
                        273                                               
                           327                                            
                              401                                         
                                 396                                      
                                    399                                   
                                       449                                
                                          442                             
                                             446                          
                                                291                       
    2 H  179                                                              
            172                                                           
               176                                                        
                  208                                                     
                     257                                                  
                        329                                               
                           362                                            
                              467                                         
                                 436                                      
                                    452                                   
                                       477                                
                                          486                             
                                             482                          
                                                306                       
   46 H  174                                                              
            176                                                           
               175                                                        
                  202                                                     
                     234                                                  
                        305                                               
                           355                                            
                              411                                         
                                 414                                      
                                    413                                   
                                       452                                
                                          455                             
                                             454                          
                                                279                       
   64 H  147                                                              
            140                                                           
               144                                                        
                  181                                                     
                     220                                                  
                        286                                               
                           327                                            
                              383                                         
                                 373                                      
                                    378                                   
                                       436                                
                                          426                             
                                             431                          
                                                288                       
   66 H  127                                                              
            122                                                           
               125                                                        
                  164                                                     
                     202                                                  
                        274                                               
                           329                                            
                              413                                         
                                 391                                      
                                    402                                   
                                       454                                
                                          460                             
                                             457                          
                                                333                       
   73 H  150                                                              
            144                                                           
               147                                                        
                  186                                                     
                     197                                                  
                        262                                               
                           330                                            
                              368                                         
                                 344                                      
                                    356                                   
                                       398                                
                                          385                             
                                             392                          
                                                245                       
   92 H  193                                                              
            190                                                           
               192                                                        
                  223                                                     
                     260                                                  
                        333                                               
                           347                                            
                              447                                         
                                 433                                      
                                    440                                   
                                       498                                
                                          493                             
                                             496                          
                                                304                       
   106                                                                    
      H  166                                                              
            158                                                           
               162                                                        
                  192                                                     
                     224                                                  
                        252                                               
                           308                                            
                              399                                         
                                 357                                      
                                    378                                   
                                       408                                
                                          416                             
                                             412                          
                                                250                       
   131                                                                    
      H  139                                                              
            134                                                           
               137                                                        
                  169                                                     
                     199                                                  
                        264                                               
                           311                                            
                              396                                         
                                 361                                      
                                    379                                   
                                       414                                
                                          396                             
                                             405                          
                                                269                       
   132                                                                    
      H  152                                                              
            147                                                           
               150                                                        
                  184                                                     
                     219                                                  
                        299                                               
                           324                                            
                              392                                         
                                 394                                      
                                    393                                   
                                       439                                
                                          419                             
                                             429                          
                                                280                       
   139                                                                    
      H  103                                                              
             98                                                           
               101                                                        
                  127                                                     
                     155                                                  
                        215                                               
                           258                                            
                              278                                         
                                 276                                      
                                    277                                   
                                       336                                
                                          326                             
                                             331                          
                                                231                       
   140                                                                    
      H  155                                                              
            149                                                           
               152                                                        
                  172                                                     
                     209                                                  
                        272                                               
                           309                                            
                              404                                         
                                 378                                      
                                    391                                   
                                       414                                
                                          403                             
                                             409                          
                                                257                       
   12 S  131                                                              
            131                                                           
               131                                                        
                  160                                                     
                     197                                                  
                        257                                               
                           300                                            
                              367                                         
                                 355                                      
                                    361                                   
                                       403                                
                                          415                             
                                             409                          
                                                278                       
   13 S  182                                                              
            180                                                           
               181                                                        
                  207                                                     
                     241                                                  
                        305                                               
                           350                                            
                              426                                         
                                 412                                      
                                    419                                   
                                       470                                
                                          477                             
                                             474                          
                                                293                       
   16 S  159                                                              
            154                                                           
               157                                                        
                  186                                                     
                     233                                                  
                        299                                               
                           344                                            
                              429                                         
                                 422                                      
                                    426                                   
                                       475                                
                                          478                             
                                             477                          
                                                320                       
   28 S  143                                                              
            137                                                           
               140                                                        
                  N.W.                                                    
                     197                                                  
                        254                                               
                           306                                            
                              406                                         
                                 364                                      
                                    385                                   
                                       429                                
                                          428                             
                                             429                          
                                                289                       
   40 S  111                                                              
            107                                                           
               109                                                        
                  138                                                     
                     167                                                  
                        224                                               
                           270                                            
                              378                                         
                                 337                                      
                                    358                                   
                                       400                                
                                          405                             
                                             403                          
                                                294                       
   56 S  172                                                              
            163                                                           
               168                                                        
                  194                                                     
                     220                                                  
                        272                                               
                           308                                            
                              382                                         
                                 379                                      
                                    381                                   
                                       422                                
                                          413                             
                                             418                          
                                                250                       
   61 S  167                                                              
            164                                                           
               166                                                        
                  196                                                     
                     230                                                  
                        298                                               
                           324                                            
                              389                                         
                                 384                                      
                                    387                                   
                                       434                                
                                          442                             
                                             438                          
                                                273                       
   70 S  188                                                              
            188                                                           
               188                                                        
                  207                                                     
                     247                                                  
                        311                                               
                           374                                            
                              468                                         
                                 449                                      
                                    459                                   
                                       516                                
                                          522                             
                                             519                          
                                                331                       
   112                                                                    
      S  198                                                              
            188                                                           
               193                                                        
                  223                                                     
                     265                                                  
                        349                                               
                           408                                            
                              492                                         
                                 474                                      
                                    483                                   
                                       542                                
                                          536                             
                                             539                          
                                                346                       
   119                                                                    
      S  151                                                              
            144                                                           
               148                                                        
                  178                                                     
                     212                                                  
                        279                                               
                           306                                            
                              418                                         
                                 382                                      
                                    400                                   
                                       424                                
                                          424                             
                                             424                          
                                                277                       
T1 18 H  151                                                              
            149                                                           
               150                                                        
                  180                                                     
                     209                                                  
                        270                                               
                           328                                            
                              422                                         
                                 394                                      
                                    408                                   
                                       439                                
                                          443                             
                                             441                          
                                                291                       
   19 H  167                                                              
            161                                                           
               164                                                        
                  193                                                     
                     228                                                  
                        289                                               
                           334                                            
                              391                                         
                                 385                                      
                                    388                                   
                                       434                                
                                          411                             
                                             423                          
                                                259                       
   37 H  158                                                              
            159                                                           
               159                                                        
                  184                                                     
                     219                                                  
                        292                                               
                           330                                            
                              390                                         
                                 388                                      
                                    389                                   
                                       426                                
                                          413                             
                                             420                          
                                                261                       
   38 H  143                                                              
            135                                                           
               139                                                        
                  167                                                     
                     202                                                  
                        278                                               
                           326                                            
                              404                                         
                                 417                                      
                                    411                                   
                                       456                                
                                          442                             
                                             449                          
                                                310                       
   69 H  113                                                              
            106                                                           
               110                                                        
                  143                                                     
                     172                                                  
                        232                                               
                           269                                            
                              335                                         
                                 327                                      
                                    331                                   
                                       388                                
                                          388                             
                                             388                          
                                                279                       
   78 H  147                                                              
            149                                                           
               148                                                        
                  178                                                     
                     216                                                  
                        288                                               
                           332                                            
                              397                                         
                                 398                                      
                                    398                                   
                                       442                                
                                          445                             
                                             444                          
                                                296                       
   98 H  163                                                              
            160                                                           
               162                                                        
                  197                                                     
                     235                                                  
                        297                                               
                           324                                            
                              423                                         
                                 391                                      
                                    407                                   
                                       431                                
                                          423                             
                                             427                          
                                                266                       
   99 H  154                                                              
            147                                                           
               151                                                        
                  177                                                     
                     221                                                  
                        279                                               
                           312                                            
                              373                                         
                                 377                                      
                                    375                                   
                                       413                                
                                          418                             
                                             416                          
                                                265                       
   102                                                                    
      H  222                                                              
            215                                                           
               219                                                        
                  250                                                     
                     301                                                  
                        388                                               
                           414                                            
                              503                                         
                                 500                                      
                                    502                                   
                                       544                                
                                          544                             
                                             544                          
                                                326                       
   123                                                                    
      H  182                                                              
            172                                                           
               177                                                        
                  205                                                     
                     238                                                  
                        300                                               
                           339                                            
                              432                                         
                                 390                                      
                                    411                                   
                                       437                                
                                          446                             
                                             442                          
                                                265                       
   126                                                                    
      H  175                                                              
            168                                                           
               172                                                        
                  200                                                     
                     232                                                  
                        307                                               
                           357                                            
                              418                                         
                                 413                                      
                                    416                                   
                                       454                                
                                          442                             
                                             448                          
                                                277                       
   135                                                                    
      H  130                                                              
            130                                                           
               130                                                        
                  154                                                     
                     182                                                  
                        241                                               
                           280                                            
                              346                                         
                                 346                                      
                                    346                                   
                                       388                                
                                          384                             
                                             386                          
                                                256                       
    9 S  202                                                              
            201                                                           
               202                                                        
                  230                                                     
                     269                                                  
                        335                                               
                           381                                            
                              477                                         
                                 443                                      
                                    460                                   
                                       488                                
                                          508                             
                                             498                          
                                                297                       
   17 S  135                                                              
            132                                                           
               134                                                        
                  158                                                     
                     192                                                  
                        251                                               
                           291                                            
                              354                                         
                                 360                                      
                                    357                                   
                                       409                                
                                          407                             
                                             408                          
                                                275                       
   21 S  168                                                              
            158                                                           
               163                                                        
                  192                                                     
                     223                                                  
                        294                                               
                           335                                            
                              414                                         
                                 388                                      
                                    401                                   
                                       426                                
                                          434                             
                                             430                          
                                                267                       
   26 S  152                                                              
            147                                                           
               150                                                        
                  176                                                     
                     196                                                  
                        252                                               
                           290                                            
                              357                                         
                                 352                                      
                                    255                                   
                                       393                                
                                          394                             
                                             394                          
                                                244                       
   47 S  160                                                              
            158                                                           
               159                                                        
                  184                                                     
                     205                                                  
                        277                                               
                           307                                            
                              359                                         
                                 365                                      
                                    362                                   
                                       405                                
                                          408                             
                                             407                          
                                                248                       
   52 S  146                                                              
            139                                                           
               143                                                        
                  N.W.                                                    
                     189                                                  
                        244                                               
                           296                                            
                              350                                         
                                 345                                      
                                    348                                   
                                       397                                
                                          401                             
                                             399                          
                                                257                       
   68 S  117                                                              
            114                                                           
               116                                                        
                  141                                                     
                     160                                                  
                        217                                               
                           263                                            
                              341                                         
                                 321                                      
                                    331                                   
                                       389                                
                                          393                             
                                             391                          
                                                276                       
   101                                                                    
      S  183                                                              
            180                                                           
               182                                                        
                  198                                                     
                     235                                                  
                        306                                               
                           359                                            
                              413                                         
                                 405                                      
                                    409                                   
                                       455                                
                                          460                             
                                             458                          
                                                276                       
   111                                                                    
      S  189                                                              
            182                                                           
               186                                                        
                  212                                                     
                     241                                                  
                        313                                               
                           356                                            
                              438                                         
                                 423                                      
                                    431                                   
                                       472                                
                                          462                             
                                             467                          
                                                282                       
   137                                                                    
      S  177                                                              
            169                                                           
               173                                                        
                  198                                                     
                     219                                                  
                        286                                               
                           315                                            
                              412                                         
                                 374                                      
                                    393                                   
                                       403                                
                                          390                             
                                             397                          
                                                224                       
__________________________________________________________________________
 
    
     Example 9 
     Duration of Delivery of the Active Agent--Use of a Mixture of 20%- and 3Loaded Microparticles to Deliver Estradiol Benzoate 
     Two batches, 20% loaded and 30% loaded, of estradiol benzoate microparticles were formulated in an 80/20 blend (by weight and tested) and tested in 6 Holstein bull calves to determine serum estradiol levels. Calves weighing between 92 and 116 pounds at the time of treatment were injected in the ear (3 calves) and in the leg muscle (3 calves) with the test material. A dose of 40 mg of estradiol benzoate (plus slight excess to offset syringe retention) microencapsulated in an 85:15 lactide:glycolide copolymer was given as a 1 cc suspension. The suspending fluid was a 2% aqueous solution of carboxymethyl cellulose with 1% tween 20 surfactant. Serum samples were collected for approximately 130 days. The results are shown in Table III, and FIG. 1. These serum estradiol levels correlated well with actual animal weight gain demonstrated when the efficacy of this product was tested in a feed lot trial. This particular combination of steroid and polymeric excipient provided efficacious levels of estradiol in the animals for 168 days. 
     
                       TABLE III                                                   
______________________________________                                    
Estradiol Levels                                                          
             SR052-26B SR052-26B                                          
             Muscle    Ear                                                
Days         (pg/ml)   (pg/ml)                                            
______________________________________                                    
-7           10         4                                                 
-5            6         2                                                 
-3           19         10                                                
 0            6         4                                                 
 1           42        113                                                
 2           19         51                                                
 7           10         35                                                
10           17        178                                                
14           84         67                                                
17           56        422                                                
21           96        172                                                
24           95        326                                                
29           114       514                                                
31           105       762                                                
36           72        532                                                
38           75        667                                                
42           35        373                                                
45           22        318                                                
49           19        112                                                
52           26         81                                                
56           35        134                                                
59           44        104                                                
63           62         93                                                
66           87        335                                                
70           79        332                                                
73           56        445                                                
77           61        497                                                
80           39        225                                                
84           37        163                                                
87           52        151                                                
91           38        321                                                
94           28        244                                                
98           15        311                                                
101          16        105                                                
105          12        122                                                
108          14        114                                                
112          20         46                                                
115          13         58                                                
119          20         29                                                
122          21         25                                                
129          24         24                                                
______________________________________                                    
 
    
     Example 10 
     Duration of Delivery of the Active Agent--Use of a Mixture of 15%- and 30%-Loaded Microparticles to Deliver Estradiol Benzoate 
     Poly(d,l-lactide) was used to microencapsulate estradiol benzoate and the resulting product was administered to 6 Holstein calves to evaluate serum estradiol levels. Two batches, 15% and 30% loaded, were blended to afford the test material. The calves weighing between 101 and 145 pounds, were treated with 40 mg of mi croencapsulated estradiol benzoate intramuscularly at the base of the ear. The microparticles were suspended in an aqueous carboxymethyl-cellulose/tween 20 solution. Serum samples were collected at approximately 4-day intervals over a period of several months. The resulting data (Table IV) show that the biodegradable microparticles released the active agent for 200 days in the animals. 
     
                       TABLE IV                                                    
______________________________________                                    
Estradiol Levels                                                          
            SR046-120                                                     
            Ear                                                           
       Day  (pg/ml)                                                       
______________________________________                                    
       -4   14.0                                                          
        0   14.5                                                          
        1   40.9                                                          
        2   25.0                                                          
        3   22.7                                                          
        4   58.0                                                          
        5   27.7                                                          
        7   24.8                                                          
       10   14.7                                                          
       14   37.9                                                          
       17   28.4                                                          
       21   82.0                                                          
       24   63.6                                                          
       28   198.0                                                         
       31   189.8                                                         
       35   85.6                                                          
       38   334.1                                                         
       42   195.2                                                         
       45   186.2                                                         
       49   160.9                                                         
       52   100.3                                                         
       56   117.5                                                         
       59   70.5                                                          
       63   49.5                                                          
       67   40.2                                                          
       70   21.5                                                          
       74   22.6                                                          
       77   28.8                                                          
       80   100.4                                                         
       84   10.3                                                          
       87   28.0                                                          
       91   18.6                                                          
       94   47.8                                                          
       98   33.2                                                          
       101  32.6                                                          
       105  57.7                                                          
       108  71.3                                                          
       112  88.9                                                          
       115  91.5                                                          
       119  109.0                                                         
       122  189.7                                                         
       126  132.6                                                         
       129  352.5                                                         
       133  366.4                                                         
       138  106.1                                                         
       140  105.7                                                         
       143  107.4                                                         
       147  126.7                                                         
       150  92.7                                                          
       154  118.9                                                         
       157  100.3                                                         
       161  69.5                                                          
       168  58.9                                                          
       175  34.8                                                          
       182  51.0                                                          
       189  22.9                                                          
       196  19.1                                                          
       203  16.5                                                          
______________________________________                                    
 
    
     Example 11 
     Duration of Delivery of the Active Agent--Use of a Mixture of 30%- and 50%-Loaded Microparticles to Deliver Trenbolone Acetate 
     Trenbolone acetate was microencapsulated in an 85:15 copolymer of lactic and glycolic I  acids, and the resulting product evaluated in 6 Holstein bull calves, The formulation tested was a 50/50 blend of a 30% loaded batch and a 50% loaded batch, Calves weighing between 91 and 117 pounds were injected either i.m. in the leg (3 calves) or at the base of the ear (3 calves) with 200 mg of encapsulated trenbolone acetate, The microparticles were suspended in an aqueous carboxymethylcellulose/tween 20 injection vehicle. Serum samples were collected and analyzed for trenbolone acetate at selected intervals, Table V shows that elevated serum drug levels were achieved over the 150-day test period, This formulation was shown to be efficacious in increasing the weight gain of beef calves, 
     
                       TABLE V                                                     
______________________________________                                    
Trenbolone acetate levels                                                 
             SR046-100 SR046-100                                          
             Ear       Muscle                                             
Days         (pg/ml)   (pg/ml)                                            
______________________________________                                    
-4            39       13                                                 
-1            13       24                                                 
 0            26       47                                                 
 1           300       183                                                
 2           202       104                                                
 3           269       91                                                 
 4           233       135                                                
 5           290       124                                                
 7           295       147                                                
10           156       128                                                
14           146       92                                                 
17           300       91                                                 
21           272       88                                                 
24           145       62                                                 
28           130       30                                                 
31           141       94                                                 
35           318       43                                                 
38           285       150                                                
42           219       222                                                
45           466       295                                                
49           517       231                                                
52           349       128                                                
56           273       129                                                
59            77       120                                                
63            83       86                                                 
66           268       77                                                 
70           106       81                                                 
73            66       49                                                 
77           206       113                                                
80           130       111                                                
84           151       125                                                
87           113       107                                                
91           105       84                                                 
94            68       101                                                
98            61       65                                                 
101           89       69                                                 
105          108       57                                                 
108           46       90                                                 
112          103       79                                                 
115          140       83                                                 
119          115       68                                                 
122           97       122                                                
129           18       122                                                
136           53       78                                                 
143           35       73                                                 
150           35       50                                                 
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     Having now fully described the invention, it will be understood by those with skill in the art that the scope may be performed within a wide and equivalent range of conditions, parameters and the like, without affecting the spirit or scope of the invention or any embodiment thereof.