it is well known that , during inhalation therapy or systemic absorption via the respiratory tract , the human lung separates particles based on the aerodynamic diameter , which is a function of the actual average particle diameter ( i . e ., the geometric diameter ), as well as the shape and the density of the particle . more specifically , a lower particle density will produce a smaller aerodynamic diameter for particles of equivalent geometric since size , as illustrated by equation 1 as follows : where d ae and d geo are the aerodynamic and geometric diameters , respectively , and p is the particle density . because of the spherical particulates being hollow , they have an actual density lower than that of the solid particulates currently employed for respirable medicaments . thus , applicants have unexpectedly discovered that the spherical hollow particulates should be perceived by the lung as being of a smaller aerodynamic size than the aerodynamic size of the solid particulates of substantially the same actual average particle diameter , and thus the spherical hollow particulates should be deposited deeper in the lungs . moreover , the spherical hollow particulates can be made with an actual average particle diameter ( i . e ., the geometric diameter ) greater than that of the solid particulates currently employed for respirable medicaments . in that event , the spherical hollow particulate form would likely improve the deaggregation properties of the medicament for entrainment in the inhalation stream , as the medicament is moving from an inhalator device into the patient &# 39 ; s lungs , due to the large spherical hollow particulates having less surface - to - surface contact with each other as compared to the relatively smaller solid particulates . as a result , an increase in the respirable fraction of a medicament formulation should be achieved with large spherical hollow particulates as compared to small solid particles , where the mass median aerodynamic diameter of the two is approximately the same . as noted above , methods for spray drying of particles are well known , and it is also well known that controlling selected conditions for spray drying , such as the temperature , the type of solvent , the concentration of the active ingredient and / or the optional excipient , can result in the spray dried particles being hollow structures instead of solid . any of the various well known spray drying methods may be employed for spray drying the medicament particles in accordance with the present invention to form spherical hollow structures useful for inhalation therapy or systemic absorption via the respiratory tract , including those spray drying methods disclosed in the above - mentioned u . s . pat . no . 4 , 590 , 206 to forrester et al ., u . s . pat . no . 4 , 127 , 622 to watanabe et al . and pct international publication no . wo 91 / 12823 to illum et al ., the disclosures of which are incorporated herein by reference . various solvents may be employed during spray drying , including , but not limited to , hydrocarbons , halogenated hydrocarbons ( i . e ., fluorinated hydrocarbons or chlorinated hydrocarbons ), alcohols , ketones , and the like . examples of suitable solvents include , but are not limited to , water , hexane , perfluoromethylcyclohexane , perfluorohexane , perfluoropentane , dichloromethane , ethanol , acetone , and combinations thereof . medicament particles which may be spray dried in accordance with the present invention to form spherical hollow particulates are suitable for use as respirable medicaments for inhalation therapy or systemic absorption via the respiratory tract to treat respiratory disorders such as asthma , bronchitis , chronic obstructive pulmonary diseases and chest infection . additional medicaments may be selected from any other suitable drug useful in inhalation therapy and which may be presented as a suspension or in a dry powder inhalator . appropriate medicaments may thus be selected from , for example , analgesics , e . g ., codeine , dihydromorphine , ergotamine , fentanyl or morphine ; anginal preparations , e . g . diltiazem ; antiallergics , e . g . cromoglycate , ketotifen or neodocromil ; antiinfectives e . g . cephalosporins , penicillins , stretomycin , sulphonamides , tetracyclines and pentamidine ; antihistamines , e . g . methapyrilene anti - inflammatories , e . g . fluticasone , flunisolide , budesonide , tipredane or triamcinolone acetonide ; antitussives , e . g . noscapine ; bronchodilators , e . g . salmeterol , salmbutamol , ephedrine , adrenaline , fenoterol , formoterol , isoprenaline , metaproterenol , phenylephrine , phenylpropanolamine , pirbuterol , reproterol , rimiterol , terbutaline , isoetharine , tulobuterol orciprenaline , pirbuterol , reproterol , rimiterol , terbutaline , isoetharine , tulobuterol orciprenaline , or (−)- 4 - amino - 3 , 5 - dichloro - α -[[[ 6 -[ 2 -( 2 - pyridinyl ) ethoxy ]- hexyl ] amino ] methyl ] benzenemethanol ; diuretics , e . g . amiloride ; anticholinergics , e . g . ipratropium , atropine , oxitropium ; hormones , e . g ., cortisone , hydrocortisone or prednisolone ; xanthines e . g . aminophylline , choline theophyllinate , lysine theophyllinate or theophylline and therapeutic proteins and peptides , e . g . insulin or glucagon . it will be clear to a person skilled in the art that , where appropriate , the medicaments may be used in the form of salts ( e . g . as alkali metal or amine salts or as acids addition salts ) or as esters ( e . g . lower alkyl esters ) or as solvates ( e . g . hydrates ) to optimise the activity and / or stability of the medicament . preferred medicaments are salbutamol , salmeterol , fluticasone propionate , beclomethasone dipropionate , terbutaline , cromoglycate , budesonide , and triamcinolone acetonide and / or salts thereof . moreover , the medicaments optionally may be together with excipients acceptable for inhalation into the human body , which may be organic excipients , such as polysaccharides ( i . e ., starch , cellulose , and the like ), lactose , glucose , mannitol , amino acids , and maltodextrins , or may be inorganic excipients , such as calcium carbonate and sodium chloride . the excipient may be included with the medicament via well known methods , such as by admixing , co - precipitating , and the like . when entrained in an inhalation stream for inhalation by the patient , the spherical hollow particulates typically should acquire a mass median aerodynamic diameter , particularly from about 0 . 5 μm to about 7 . 0 μm , more particularly from about 1 μm to about 4 . 5 μm , as perceived by the patient &# 39 ; s lungs as the spherical hollow particulates pass into the lungs . also , the spherical hollow particulates typically should have & gt ; 50 % of the mass of hollow particulates , more particularly & gt ; 70 % of the mass of hollow particulates , particularly having a mass median aerodynamic diameter & lt ; 6 μm , more particularly & lt ; 5 μm , as perceived by the patient &# 39 ; s lungs as the spherical hollow particulates pass into lungs . as noted in the above discussion of prior art inhalators , it is particularly useful that particles of respirable particle size range have more than 50 % thereof with a mass median aerodynamic diameter & lt ; 6 μm , more particularly & lt ; 5 μm , for appropriate deposition into the lungs , which should be achieved with the present invention . the ability to control the geometric density of the substantially spherical hollow particulates offers an additional advantage over current inhalator systems which use a suspension of medicament particulates in a propellant . in existing systems containing drug and propellant suspensions , the suspension may separate or stratify because of the differences in the densities of the medicament and propellant . separation may be either classified as “ creaming ” wherein the medicament rises to the top of the more dense propellant , or “ sedimentation ” wherein the medicament settles to the bottom of the less dense propellant . regardless of the classification , separation of the medicament and component may cause a lack of dosage uniformity per activation , i . e ., each dose may not provide an equal amount of drug over the life a multi - dose inhalator . the uniformity of dosages delivered by multi - dose inhalators is of critical importance to the efficacy of the device and must be within narrow parameters to meet regulatory criteria . the problem of separation of the suspension is generally addressed by vigorously shaking the inhalator immediately before it is used . however , patient compliance with this simple task is difficult to control and even slight delays between shaking and use effect dosage uniformity . the present invention , however , overcomes the problem separation and , in so doing , conceivably eliminates the need to shake the inhalator before use . by allowing the drug to be density matched to the selected propellant , the tendency of the medicament and propellant to stratify is removed . the drug and propellant are uniformly distributed in suspension and it can be assumed that each dose would then also be similarly uniform . medicament density may be pre - selected and controlled by adjusting the spray drying conditions under which the particulates are created , as previously mentioned . in particular , though , density may be controlled by adjusting the thickness of the walls of the spheres as compared to sphere diameter , and by adjusting the ratio of drug to excipient when creating composite medicament particulates . in some embodiments , however , it may be preferred to use pure medicaments without excipients . in short , the ability to pre - select and control the geometric density of the medicament particulates offers a significant advantage over existing medicament / propellant suspension systems . with respect to dry powder inhalators , the spherical hollow particulates of the present invention are suitable for use in any carrier in any dry powder inhalator , including , but not limited to , any of the dry powder inhalators disclosed in the above - mentioned patents and published patent applications . the spherical hollow particulates of the present invention are also suitable for use in any metered dose inhalator , including the pressurized aerosolized type ( where the particulates are together with a propellant and an optional suspending agent ). with respect to pressurized aerosol metered dose inhalators , as such pressurized aerosol containers are well known in the art , the spherical hollow particulates may be placed in a pressurized container with a suitable propellant , and optionally with a suitable suspending agent ( also known as a dispersing agent or a surfactant ) by any of the well known methods therefor , such as that shown in the above - noted respiratory drug delivery , p . 185 et seq . in general , adding a medicament to a pressurized aerosol container is accomplished as follows . medicament is added to a high shear blender ( i . e ., mixer ) which contains propellant and may also contain a suspending agent . it may also be preferred to add a polar substance to increase solubility of surfactant in a propellant , e . g . ethanol . propellants may be of the chlorofluorocarbon variety ( i . e ., trichloromonofluoromethane , sold by dupont under the registered trademark freon 11 and colloquially known as cfc - 11 , or dichlorodifluoromethane , sold by dupont under the registered trademark freon 12 and colloquially known as cfc - 12 ), which , as mentioned above , are being phased out by the environmental protection agency of the u . s . government as each of freon 11 and freon 12 has an odp = 0 . alternatively , propellants may be of the more recently developed environmentally safe varieties . suitable environmentally safe propellants include , but are not limited to , any of the above - mentioned perfluoroethane , monochlorodifluoromethane , 1 , 1 , 1 , 2 - tetrafluoroethane , 1 , 1 ,- difluoroethane , 1 , 1 , 1 , 2 , 3 , 3 , 3 - heptafluoro - n - propane , and combinations thereof . suitable optional suspending agents include , but are not limited to , oleic acid , span ® 85 ( registered trademark for the partial esters of the common fatty acids ( lauric , palmitic , stearic , and oleic ) and hexitol anhydrides ( hexitans and hexides ), that are derived from sorbitol and that tend to be oil - soluble and dispersible or insoluble in water }, lecithin , and combinations thereof . if the propellant has a low boiling point so that it would volatilize during procedures at or near room temperature , then the mixer needs to be maintained well below room temperature to prevent evaporation or alternatively a sealed mixer ( one in a closed system with the container ) may be employed . once a homogenous suspension is obtained , the suspension is filled into aerosol containers . during the filling , the mixer can be used to maintain adequate suspension throughout the entire filling circuit by continuously circulating the suspension through the concentrated filling unit . at this point , there exist two main options . with the first option , especially for products not using the environmentally unsafe propellant , cfc - 11 , the entire formulation is prepared in a low temperature pressure vessel and then filled through the valve into evacuated , previously crimped containers . care must be taken with propellants such as hfc - 134a , that have a high vapor pressure , as filling through the valve of the container is difficult with such propellants . the second option involves the manufacture of a lower volatility concentrate . with this alternative technique , filling is in a controlled environment into containers , after which the valves are crimped in place . subsequently , the high pressure propellant is added through the valve . the tensile strength of the spherical hollow particulates will vary depending on the particular medicament ( and optional excipient ) being spray dried . in the event that the spherical hollow particulates have a weak enough tensile strength so that a large storage container of them , such as a kilogram quantity , would result in upper hollow particulates crushing lower hollow particulates in the container prior to deposition of the hollow particulates in an inhalator device , then formation of hollow particulates should be accomplished in - line so that the formed hollow particulates can be deposited directly after formation into an inhalator device . medicament powder of each of the two medicaments , albuterol sulfate and amiloride hcl ( abbreviated herein as alb s and amil hcl , respectively ), is employed in this example . also , the excipient , lactose , is employed in this example . aliquots of each of the medicaments , and also of lactose , are spray dried as follows . 15 g of alb s ( lot no . w 1946 fb ) are dissolved in 300 ml of water to create a 5 % solution . similarly , 3 . 479 g of amil hcl ( lot no . 9007h 902 ) are diluted in water to 1000 ml to create a solution . likewise , 15 g of lactose ( lot no . 1nc25 , 605 from sheffield products of norwich , n . y .) are diluted in water to 150 ml to create a solution . each solution is respectively spray dried using a virtis ™ ( a spray dryer commercially available from the virtis company of gardiner , n . y .) with each of the air from the nozzle and from the blower set at its respective maximum value under the following conditions of temperature and rate , as summarized in table a below : table a flow rate spray dried inlet temp outlet temp setting particles (° c .) (° c .) ( ml / minute ) alb s 150 101 7 ( medicament ) amil hcl 150 92 12 ( medicament ) lactose 180 127 5 ( excipient ) spray drying produced the following average particle diameters ( i . e ., the geometric diameters ) as summarized in table b below : table b spray dried geometric hollow particles diameter or solid alb s 1 to 5 μm dimpled solids amil hcl 1 to 5 μm hollow spheres lactose 2 μm hollow spheres as noted in table b and as can be seen in the photomicrographs in fig1 and 2 , spray drying the alb s produced dimpled solid structures and did not produce spherical hollow particulates . on the other hand , spray drying the amil hcl produced spherical hollow particulates , as can be seen in the photomicrographs in fig3 - 5 . spray drying the lactose produced spherical hollow particulates , with the largest lactose particulate having an average particle diameter of about 17 μm , as can be seen in the photomicrographs in fig6 and 7 . while it is not intended to be bound to any theory , it is believed that the concentration of alb s in water , namely a 5 % solution of 15 g in 300 ml , was not low enough for the spray drying to result in spherical hollow particulates of alb s , and thus , lowering the concentration of alb s should result in spherical hollow particulates . also , it is believed that admixing the alb s with an excipient , such as lactose , during the spray drying should result in spherical hollow particulates . the following is a discussion of how a diskhaler ™ ( a medicament dispersing device , i . e ., an inhalator , commercially available from glaxowellcome , inc .) and an aerobreather ™ ( available from api of hadley , mass .) may be employed with the spherical hollow medicament particulates of the present invention , such as the amil hcl made in example i , to determine how the powdered medicament is dispersed and thus illustrate that the spherical hollow medicament particulates are useful in a dry powder inhalator . more particularly , the extent to which a medicament is dispersed may be measured by its mass median aerodynamic diameter ( mmad ) in micrometers , and the percentage that is less than 6 micrometers , particularly less than 5 micrometers , is indicative of desirable particle size for inhalation into the lungs . several diskhaler ™ devices should be employed . the diskhaler ™ has a screen which serves to direct an air jet , thus helping to entrain the particles in the air jet . the 4 - blister compartment would be removed from the holder portion of each diskhaler ™. a dosage of each of the spray dried spherical hollow particulates would be respectively loaded onto the bottom of the holder portion of a diskhaler ™, the bottom serving as a carrier surface . next , each diskhaler ™ with its respective medicament would be attached to an aerobreather ™ for dispersion of the medicament from the carrier . the aerobreather ™ is a device that simulates inspiration by a human through the mouth at 60 liters / minute , with an acceleration of 19 liters / second 2 and a total volume of 1 liter . the inspired powder ( which would be approximately 1 milligram ) then would be drawn into the aerosizer ™ unit for aerodynamic particle size analysis . the photomultiplier tubes of the aerosizer ™ would be operated at 1100 volts , and the data would be analyzed in an auto - combine mode with software version 5 . 02 . 37 available from api of hadley , mass . as noted above , the extent to which the powder is dispersed is measured by the mmad in micrometers , and the percentage that is less than 6 micrometers , particularly less than 5 micrometers , is indicative of desirable particle size for inhalation into the lungs . the results for the dispersed spray dried spherical hollow medicament particulates should be a mmad from about 0 . 5 to about 7 μm , particularly about 1 to about 4 . 5 μm , and a % mass & lt ; 6μm of about 30 % or more , particularly about 50 % or more , and most particularly about 70 % or more . also , the spherical hollow particulates of the present invention should be deposited deeper in the lungs than are conventional micronized solid particulates ( with substantially the same geometric diameter ) from a dry powder inhalator . the following is a discussion of how an inhalator that is a pressurized aerosol container with a valve may be employed with the spherical hollow medicament particulates of the present invention , such as the amil hcl made in example i . example formulations suitable for a metered dose inhalator according to this invention include ( i ) a suspension consisting essentially of spherical hollow medicament particulates of respirable size and 1 , 1 , 1 , 2 - tetrafluoroethane ; and ( ii ) a suspension of spherical hollow medicament particulates of respirable size , 1 , 1 , 1 , 2 - tetrafluoroethane , oleic acid and sufficient ethanol to solubulize the oleic acid . the spherical hollow medicament particulates should be added to a high shear blender ( i . e ., mixer ) which contains , for instance , 1 , 1 , 1 , 2 - tetrafluoroethane propellant ( colloquially known under the trade name , hfc - 134a ) and lecithin suspending agent . however , the vapor pressure of 1 , 1 , 1 , 2 - tetrafluoro - ethane propellant at 68 ° f . is 68 . 4 psig , and hence , the vapor pressure is too great to meet the u . s . government department of transportation requirements for use in aerosol containers when the containers are transported and temperatures can go up to 130 ° f . thus , a vapor pressure depressant , such as a glycol ether ( i . e ., 2 - butoxyethanol ) or an alkyl acetate ( i . e ., butyl acetate ) should be used together with 1 , 1 , 1 , 2 - tetrafluoroethane propellant so that the resultant suspension in the aerosol container meets the department of transportation requirements and has a vapor pressure of less than 180 psig at 130 ° f . also , since 1 , 1 , 1 , 2 - tetrafluoroethane propellant has a low boiling point of − 15 . 5 ° f . (− 26 . 5 ° c .) so that it would volatilize during procedures at or near room temperature , then the mixer should be maintained well below room temperature to prevent evaporation . alternatively , a sealed mixer ( one in a closed system with the container ) may be employed . once a homogenous suspension is obtained , it is filled into aerosol containers . during the filling , the mixer can be used to maintain adequate suspension throughout the entire filling circuit by continuously circulating the suspension through the concentrated filling unit . because , as noted , 1 , 1 , 1 , 2 - tetrafluoroethane propellant has a high vapor pressure , care must be taken during filling as filling through the valve of the container is difficult with such high pressure propellants . with one technique , the entire formulation is prepared in a low temperature pressure vessel and then filled through the valve into evacuated , previously crimped containers . with an alternative technique , the propellant is not placed in suspension with the medicament and suspending agent prior to filling . rather , filling of the suspension of medicament and suspending agent into each container is accomplished in a controlled environment , after which the valve is crimped in place onto the containers . subsequently , the high pressure 1 , 1 , 1 , 2 - tetrafluoroethane propellant is added through the valve . as noted above , the extent to which a medicament is dispersed may be measured by its mass median aerodynamic diameter ( mmad ) in micrometers , and the percentage that is less than 6 micrometers , particularly less than 5 micrometers , is indicative of desirable particle size for inhalation into the lungs . accordingly , like the results noted above in example ii for the spray dried spherical hollow medicament particulates dispersed from a dry powder inhalator , the results for the spray dried spherical hollow medicament particulates dispersed from pressurized aerosol containers should be a mmad from about 0 . 5 to about 7 μm , particularly about 1 to about 4 . 5 μm , and a % mass & lt ; 6μm of about 30 % or more , particularly about 50 % or more , and most particularly about 70 % or more . also , the spherical hollow particulates of the present invention should be deposited deeper in the lungs than are conventional micronized solid particulates ( with substantially the same geometric diameter ) from an aerosol inhalator . it will be understood that various details of the invention may be changed without departing from the scope of the invention . furthermore , the foregoing description is for the purpose of illustration only , and not for the purpose of limitation — the invention being defined by the claims .
a pressurized metered dose inhaler comprising a canister , said canister containing a liquid formulation comprising a pressurized liquid propellant and spherical hollow medicament particles of respirable size suspended therein . in preferred embodiments , the density of the particles is equivalent to that of the liquid propellant , such that uniform suspension formulation is formed .