in the context of the present invention , “ encapsulation ” refers to the coating of particles with another material . it is well known to those knowledgeable in the art that the efficiency of the encapsulation process is variable and that the coating material in some particles may not cover the total outer surface of the material in the interior of the particle . hence particles with this reduced covering by the coating material are also considered to form part of this invention . suitable mixing techniques can be applied to obtain an intimate mixture of the components . these mixing techniques are well known in the pharmaceutical industry . at the same time , if so desired , the particle size of the components can be adjusted using well known particle size reduction techniques , examples being milling and wet milling , especially high pressure homogeneous mixing . in a preferred aspect of this invention the micro particles are made by encapsulation of the components in a suitable polymer or other material . processes for the encapsulation of tetracyclines forming particles are known e . g us 2004 / 0121003 . encapsulation agents can be chosen from any of those known from the literature , such as : polysaccharides ( e . g . starches , maltodextrines and gum arabic ), lipids ( e . g . stearic acid , mono and diglycerides , among others ) proteins ( e . g . gelatins , casein , soy ), polymers ( e . g . hydroxypropylmethyl cellulose and its derivatives ( e . g . hydroxypropylmethyl cellulose acetate succinate ( hpmcas ), polymethacrylate and its derivatives , polyvinylpirrolidone and its derivatives , poliethyleneglycol and its derivatives ). melt extrusion procedures may also be applied to obtain mixing and encapsulation . the particles , wherein the tetracycline and the antioxidant are encapsulated , not only offers improved stability of the tetracycline by protection from atmospheric moisture and oxygen but delivers both components simultaneously in precise and fixed proportions . the tetracycline can be chosen from any of the compounds belonging to this class that show antibacterial activity or one of its pharmaceutically acceptable salts . preferably the tetracycline is minocycline , doxycycline , tigecycline , tetracycline or one of their pharmaceutically acceptable salts . particularly preferred compounds for use in the invention include doxycycline hyclate and minocycline hydrochloride . the antioxidant can be chosen from any of the compounds having antioxidant activity such as ascorbic acid ( vitamin c ); tocopherols and tocotrienols ( e . g . vitamin e ); carotenes ; flavonoids ( e . g . quercetin ) or mixtures thereof . in a preferred embodiment of this invention the particles comprise either doxycycline or minocycline as the hydrochloride or base and ascorbic acid ( vitamin c ) or quercetin . in addition a preferred polymer is hydropropylmethyl cellulose acetate succinate ( hpmcas ). the particles may contain the tetracycline and the antioxidant in any variable molar proportions depending on the desired ratio . the particles can be produced by known physical methods ( e . g . pan - coating , air - suspension coating , centrifugal or melt extrusion , vibrational nozzle or spray - drying ) or chemical methods ( e . g . interfacial polymerization , in - situ polymerization , matrix polymerization ) or any other known method as referred to in the literature , as will be clear to those skilled in the art . in a preferred embodiment of this invention the particles are made by spray drying . the particles can be formulated with any adequate pharmaceutically acceptable excipient for oral , parenteral , topical , periodontal , pulmonary , vaginal or ophthalmic delivery . the formulated particles can be used in the treatment of any infectious disease responsive to treatment with a tetracycline , namely mucosae or skin infections such as acne or rosacea , but other infections are also treatable using this new and novel formulation , namely infections of the mouth , the ear and respiratory or urinary tracts . in a preferred embodiment of the invention the tetracycline and the antioxidant are suspended in a non - solvent , to produce a homogeneous mixture and the suspension spray dried . dichloromethane and acetone or mixtures are preferred but other volatile solvents can be used . the resulting encapsulated material can be post dried if necessary to reduce the levels of the solvent . suitable spray drying equipment is available commercially to carry out the spray drying . a preferred nozzle is a two fluid nozzle with a cap and diameter of 1 . 4 and 0 7 mm respectively , although different types of nozzles and differing dimensions can be used to obtain good results . the following examples are illustrative and in no way restrict the scope of the invention . charge the required amount of dcm / acetone ( 75 / 25 % w / w ) in a flask ; add the required amount of polymer hpmcas under stirring ; stir until complete dissolution ; suspend in the above flask the required quantity of antioxidant ( sodium ascorbate or quercetin ) under stirring ; and suspend in the above flask the required amount of api ( minocycline hcl or doxycycline hyclate ) a laboratory scale spray dryer was used in these trials . the unit was equipped with a two fluid nozzle , where nozzle cap and diameter were 1 . 4 and 0 . 7 mm , respectively . a high - performance cyclone was used to collect the dried product . the spray drying unit was operated with nitrogen and in single pass mode , i . e . without recirculation of the drying nitrogen . the aspirator , blowing nitrogen , was set at 100 % of its capacity ( flow rate at maximum capacity is approximately 40 kg / h ). the flow rate of the atomization nitrogen was about 0 . 32 kg / h ( corresponding to 20 mm in the rotameter of the equipment ). before feeding the stock solution / suspension , the spray dryer was stabilized with solvent ( water ). during the stabilization period , the solvent flow rate was adjusted in order to give the target outlet temperature ( the inlet temperature of the drying gas was imposed through the heater set - point ). after stabilization of the outlet temperature , the feed of the spray dryer was commuted from the solvent to the product solution ( flow rate was then readjusted to maintain the outlet temperature in the target value ). at the end of the stock solution / suspension , the feed was once more commuted to solvent , in order to rinse the feed line and to carry out a controlled shut down of the unit . yield was calculated as the mass percentage of the dry product collected under the first cyclone in relation to the total solids in the solution / suspension fed to the spray dryer . the microencapsulated material powder was analyzed for impurity profile of the tetracycline by hplc ( see tables 4 and 5 ) and morphology by scanning electron microscopy ( see fig1 ). the figure shows that the particles are collapsed spheres and that the crystals of active materials are contained therein .
particles containing a tetracycline or one of its pharmaceutically acceptable salts and an antioxidant , formulations containing the same and their use in the treatment of infectious diseases are described . methods of encapsulation of a tetracycline or one of its pharmaceutically acceptable salts and an antioxidant are also disclosed .