FIELD OF INVENTION

The present invention relates to a method of icroencapsulating a core material to form a microsphere with reduced residual solvent and water content.

BACKGROUND OF THE INVENTION

Microencapsulation is the process of coating a core material with a thin layer of a separate, encapsulating material to form microcapsules. The microencapsulation process has many applications, particularly in the pharmaceutical industry. In a pharmaceutical applica¬ tion, drugs are coated to obtain controlled-release of the drug, to improve chemical stability, and to permit the mixing and storage of reactive or incompatible drugs.

One of the methods of microencapsulation is a phase separation technique. For water-soluble or miscible core material, the phase separation process generally involves the technique of dispersing the solid core material of the desired particle size or an aqueous solution or suspension in a polymeric coating material dissolved in an organic solvent. The polymeric material is then deposited on the core material by gradual

precipitation of the polymer.. This is achieved by either the use of precipitants, by changes in tempera¬ ture, or by removal of the solvent by dilution or distillation. An example of this process is described in United States Patent No. 4,166,800 to Fong. In this patent, the polymer is precipitated by a phase separa¬ tion agent, a non-solvent for the polymer.

U.S. Patent No. 4,518,547 to Cuff et al.. describes a process for the microencapsulation of a hydrophilic core material by interfacial polycondensation. The process comprises dissolving the core material in a hydrophilic solvent, water, preferably together with an inert carrier material and adding a water immiscible organic solvent to form droplets containing the core material. Two complementary polycondensation monomer reactants are then added either sequentially or simulta¬ neously which causes interfacial polymerization of a membrane encapsulating the core material.

U.S. Patent No. 4,384,975 to Fong describes an oil- in-water emulsion process for producing icrospheres. This process comprises dissolving a polymer in a volatile, water-immiscible organic solvent in which the core material is not soluble; adding the core material; mixing the organic dispersion with an aqueous solution containing a carboxylic acid salt as the emulsifier to form a stable oil-in-water emulsion; and removing the organic solvent by evaporation to form the micro¬ capsules.

U.S. Patent No. 4,389,330 to Tice et al.. describes a process for preparing microcapsules containing a water insoluble core material. In this process, a polymer is dissolved in an organic solvent. The core material is dispersed or dissolved in the polymer-organic solvent.

This loaded mixture is then dispersed in a continuous- phase processing medium to form the microcapsules. The medium can be water or a non-aqueous media such as xylene or toluene or oils. The solvent is removed in a two-step removal process.

European Patent Application No. 81/305426.9 describes a microencapsulation process of water soluble polypeptides. In this procedure, a polymer, the wall- forming material, is dissolved in an organic solvent, methylene chloride. The core material is then added to the polymer-solvent solution. A non-solvent such as an oil compound which is soluble in the organic solvent, but is a non-solvent for the polymer is then added. In Example I, the ratio of non-solvent to organic solvent is about 1:3. The microcapsules that are formed are then quenched by mixing them with heptane.

With conventional phase separation encapsulation techniques, especially when used to microencapsulate a pharmaceutical water-soluble drug, there is a high solvent residue. A need, therefore, has continued to exist for a technique of preparing microcapsules and microspheres of high quality, with reduced residual solvent. Further, with microcapsules and microspheres encapsulated with biodegradable polymers, it is desir¬ able to avoid water in the finished product in order to enhance product stability. Water present in the biodegradable polymers will typically cause the polymers to hydrolize; for example, the mechanism of biodegrada- tion in polymers such as polylactides is by hydrolysis of ester linkages.

SUMMARY OF THE INVENTION

This invention comprises a process for microencap- sulating a core material such that the resulting microcapsule has reduced residual solvent and reduced water content. In one embodiment of this invention, the microsphere has reduced residual solvent with no residual water present. The process of this invention involves the steps of:

(a) dissolving a polymeric coating material in an organic solvent in which the core material is not soluble;

(b) adding a core material;

(c) adding a first non-solvent of the polymer selected from synthetic oil or vegetable oil compounds in a ratio of first non-solvent to the organic solvent of from about 1:5 wt/wt to about 3:1 wt/wt to form embryonic microcap¬ sules; and

(d) adding a second non-solvent of the polymer to harden the embryonic microspheres and to extract the first non-solvent from the microspheres.

The invention also comprises microspheres compris¬ ing a core material encapsulated by a polymer coating, the microspheres having a residual solvent content of, about one percent.

The inventors have discovered that the use of synthetic oils or vegetable oil compounds as the first non-solvent in combination with the high ratio of the first non-solvent to the organic solvent for the polymeric coating material reduces the residual solvent in the final microsphere product. By use of this

invention it is possible to reduce the level of residual solvent in the microspheres.

In one embodiment of this invention, when solid particles to be encapsulated are used, the final microspheres will have a reduced residual solvent with no residual water content. Thus, the invention also comprises microspheres comprising a core material encapsulated by a polymer coating, the microspheres having a reduced residual solvent content of about one percent and with no residual water present.

pT-TATf-Tϋr. nT-RCRIPTION OF THE PREFERRED EMBODIMENTS The process of this invention is based on a phase separation technique to microencapsulate a core materi¬ al. The general process of this invention involves the following steps:

(a) dissolving a polymeric coating material in an organic solvent;

(b) adding the core material as solid particles to form a suspension or as an aqueous solution to form a water-in-oil emulsion;

(c) adding a first non-solvent of the polymer, either synthetic oils or vegetable oils, in a ratio of first non-solvent to organic solvent of from about 1:5 wt/wt to about 3:1 wt/wt, to cause the polymer to precipitate onto the solid particles or around the water droplets to form embryonic microcapsules; and

(d) quenching with a second non-solvent of the polymer, to harden the microcapsules.

After the quenching step, the microsphere product is collected and dried under a vacuum to reduce the level of organic solvent in the final microsphere

product. By the process of this invention, a final microsphere product is produced that has a residual solvent level of less than one percent. By residual solvent is meant the level of residual second non- solvent that remains in the final microsphere product. There will also be a remaining amount of organic solvent that remains in the final microcapsule product, typi¬ cally ethylene chloride. The methylene chloride level is vacuum extractable to 0.1 % or less. As used herein, the ratios are based on a weight/weight ratio.

In the preferred embodiment of this invention, the general process of the invention involves the following steps:

(a) dissolving a polymeric coating material in an organic solvent;

(b) adding the core material as solid particles to form a suspension;

(c) adding a first non-solvent of the polymer, either synthetic oils or vegetable oils, in a ratio of first non-solvent to organic solvent of from about 1:5 wt/wt to about 3:1 wt/wt, to cause the polymer to precipitate onto the solid particles or around the water droplets to form embryonic microcapsules; and

(d) quenching with a second non-solvent of the polymer, to harden the microcapsules.

After the quenching step, the microsphere product is collected and dried under a vacuum to reduce the level of organic solvent in the final microsphere product. By the process of this invention wherein the core material is added as solid particles, a final microsphere product is produced that has a residual solvent level of less than one percent and a reduced water content.

The core material that may be used in this inven¬ tion can include any material that is not soluble or miscible with the polymeric coating material or the organic solvent for the polymer or the first or second non-solvent for the polymer.

The polymeric coating material that may be used in this invention may be either natural or synthetic polymers, or combinations thereof. The polymers may include cellulosic polymers, polyvinyl acetate, poly¬ vinyl alcohol, polyvinyl chloride, natural and synthetic rubbers, polyacrylates, polyorthoesters, and the like.

Specific examples include polystyrene, ethyl- cellulose, cellulose acetate, hydroxy propylmethyl cellulose, cellulose acetate, dibutylaminohydroxypropyl ether, polyvinyl butyral, polyvinyl formal, poly- (meth)acrylic acid ester, polyvinylacetal-diethylamino acetate, 2-methyl-5-vinyl pyridine, methacrylate- ethaσrylic acid copolymer, polycarbonate, polyesters, polypropylene, vinylchloride-vinylacetate copolymer, polysaccharides, and glycerol distearate.

Suitable polymers for use with a pharmaceutical core material include biodegradable polymers such as polyanhydrides and aliphatic polyesters including polylactide, polylactide-co-glycolide polyglycolide, polycaprolactone, polylactide-co-caprolactone, poly- hydroxybutyride, polyanhydride, polydioxanone, and copolymers thereof such as poly(lactide-co-glycolide) copolymer and polylactide homopolymer.

The organic solvent that may be used according to this invention to dissolve the polymeric coating material must be a material which will dissolve the polymeric coating material and which will not dissolve the core material to be encapsulated. If the core

material is a pharmaceutical compound, then the organic solvent must also be chemically inert with respect to any pharmaceutical compounds to be encapsulated.

The organic solvent can be selected from a variety of common organic solvents including halogenated aliphatic hydrocarbons, typically the C^ to c ₄ halo¬ genated alkanes, such as, for example, chloroform, methylene chloride, ethylene dichloride, ethylene chloride, ethyl acetate, methylchloroform and the like; aromatic hydrocarbon compounds, halogenated aromatic hydrocarbon compounds; cyclic ethers such as tetrahydro- furan and the like.

The first non-solvent for the polymer according to this invention may be selected from synthetic oils or vegetable oil compounds. Synthetic oils may include silicone oil, mineral oil, and petroleum oils. Vege¬ table oils may include sesame oil, peanut oil, soybean oil, corn oil, cotton seed oil, coconut oil, linseed oil, and other related oils. By non-solvent is meant a solvent that is miscible with the organic solvent but is not a solvent for the polymeric coating or core materi¬ al. The inventors have discovered that the amount and type of the first non-solvent for the polymer controls the level of residual solvent (second non-solvent) in the final microsphere product. According to this invention, the ratio of first non-solvent to organic solvent is from about 1:5 to about 3:1. Further, in one embodiment of this invention, by employing solid drug particles rather than aqueous solutions or suspensions of drug, no residual water is present in the final product.

The polymeric coating material is dissolved in the selected organic solvent prior to adding the core

material. The amount of polymeric coating material dissolved in the solvent is typically from about 5 to 50 percent.

The core material is then added to the polymer- solvent solution, preferably, as solid particles to form a suspension or as an aqueous solution to form a water- in-oil emulsion. The amount of core material added to the polymer-solvent solution is not critical, although the ratio of core material to polymer is important insofar as that at too high an amount of core material to polymer, microspheres will not form. Typically, the upper limit of the ratio of core material to polymer may be about 80 parts by weight core material to about 20 parts by weight polymer. There is no lower limit to the ratio at which the core material can be combined with the polymeric coating material, except that at very low loadings of core material in the microcapsules, the microcapsules would not be practically useful.

Typically in the process of this invention, core material is added to the polymer-solvent solution such that the core material comprises about 5 to 50 weight percent.

The first non-solvent, synthetic oil or vegetable oil, in an amount of the ratio of about 1:5 to about 3:0, first non-solvent to organic solvent, is then added slowly to the mixture of polymeric solvent solution with the added core material. The first non-solvent causes the polymeric material to precipitate out of the organic solvent onto the core material, thereby encapsulating the core material. The first non-solvent is added under carefully controlled conditions of temperature, rate, and stir speed. For example, temperature conditions may range from -20 degrees C to +30 degrees C; rate 5

g/minutes per g of batch; stir speed from 400 to 2500 rpm.

Following the isolation of the microspheres, the microspheres are treated in a quenching step to harden them. In this step, the microspheres are quenched with a second non-solvent. This second non-solvent hardens the microspheres and extracts the solvent from the microspheres and yet does not dissolve the microspheres (wall material or core material) .

The amount of second non-solvent added in this quenching step is typically from about 0.251/g of batch to about 1 1/g of batch. Suitable second non-solvents include heptane and aliphatic hydrocarbons such as hexanes.

After the solvent has been removed from the microspheres, the microspheres are isolated, such as by filtration or sieving, and are dried by exposure to air or by other conventional drying techniques, such as vacuum drying, drying over a dessicant, or the like. Preferably, the microspheres are dried under a vacuum at room temperature to further reduce the level of organic solvent in the final microsphere product.

As has been found from experimentation, in conven¬ tional phase-separation techniques, 5 to 10% residual solvent (heptane) is usually present in the finished product. By the process according to this invention, the residual solvent content is about one percent.

Thus, by the process of this invention a final microsphere product is produced that contains about one percent residual solvent. In the preferred embodiment, the inal microsphere product contains about one percent residual solvent with no residual water present. The microsphere product of the present invention is usually

made up of particles of a spherical shape although sometimes the microspheres may be irregularly shaped. The microspheres can vary in size, ranging from sub- micron to millimeter diameters. Preferably, εubmicron to 250 urn diameters are desirable for pharmaceutical formulations allowing administration of the microspheres with a standard syringe and needle. The microspheres find utility in a wide variety of applications depending upon the encapsulated core compound. Advantageously, the microsphere product, according to this invention, is an encapsulated pharmaceutical compound which can be administered to both humans and animals.

The core material that may be used in the process of this invention may include agricultural agents, such as insecticides, fungicides, herbicides, rodenticides, pesticides, fertilizers, and viruses for crop protection and the like; cosmetic agents, such as deodorants, fragrances, and the like; food additives such as flavors; and pharmaceutical agents.

Pharmaceutical compounds are the preferred core materials of the process according to this invention. These compounds may also include the non-toxic pharma¬ ceutically acceptable acid addition salts, such as hydrochloride, εulfate, phosphate, succinate, benzoate, acetate, pamoate, fu erate, mesylate, and the like. Among the pharmaceutical drugs which may be utilized are the following: contraceptive agents including estrogens such as diethyl stilbestrol, 17-beta-estradiol, estrone, ethinyl estradiol, mestranol, and the like; progestins such as norethindrone, norgestryl, ethynodiol diacetate, lynestrenol, medroxyprogesterone acetate, dimethi- sterone, megestrol acetate, chlormadinone acetate, norgestimate, norethisterone, ethisterone, melengestrol,

norethynodrel and the like; and spermicidal compounds such as nonylphenoxypolyoxyethylene glycol, benzethoniu chloride, chlorindanol and the like. Other biologically active agents which can be incorporated in the present microcapsules include gastrointestinal therapeutic agents such as aluminum hydroxide, calcium carbonate, magnesium carbonate, sodium carbonate and the like; non- steroidal antifertility agents; parasympathomimetic agents; psychotherapeutic agents; major tranquilizers such as chloropromazine HC1, clozapine, mesoridazine, metiapine, reserpine, thioridazine and the like; micro tranquilizers such as chlordiazepoxide, diazepam, meprobamate, temezepam and the like; rhinological decongestants, sedative-hypnotics such as codeine, phenobarbital, sodium pentobarbital, sodium secobarbital and the like; other steroids such as testosterone and testosterone propionate; sulfonamides; sympathomimetic agents; vaccines; vitamins and nutrients such as the essential amino acids, essential fats and the like; antimalarials such as 4-aminoquinolines, 8-amino- quinolines, pyrimethamine and the like; anti-migraine agents such as mazindol, phentermine and the like; anti- Parkinson agents such as L-dopa; anti-spasmodics such as atropine, methscopola ine bromide and the like; anti- spasmodics and anticholinergic agents such as bile therapy, digestants, enzymes and the like; anti-tussives such as dextromethorphan, noscapine and the like; bronchodilators, cardiovascular agents such as anti- hypertensive compounds, Rauwolfia alkaloids, coronary vasodilators, nitroglycerin, organic nitrates, penta- erythritotetranitrate and the like; electrolyte replace¬ ments such as potassium chloride, ergotalkaloids such as ergota ine with and without caffeine, hydrogenated ergot

alkaloids, dihydroergocristine methanesulfate, dihydro- ergocornine ethanesulfonate, dihydroergokroyptine methanesulfate and combinations thereof, alkaloids such as atropine sulfate. Belladonna, hyoscine hydrobromide and the like; analgetics; narcotics such as codeine, dihydrocodienone, meperidine, morphine and the like; non-narcotics such as εaliσylates, aspirin, acetamino¬ phen, d-propoxyphene and the like; antibiotics such as the cephalosporins, chloranphenical, gentamicin, Kanamycin A, Kanamycin B, the penicillins, ampicillin, streptomycin A, antimycin A, chloropamtheniol, metromi- dazole, oxytetracycline penicillin G, the tetracyclines, and the like; anti-cancer agent; anti-convulsants such as mephenytoin, phenobarbital, trimethadione; anti- emetics such as thietylperazine; antihistamines such as chlorophinazine, dimenhydrinate, diphenhydramine, perphenazine, tripelennamine and the like; anti-inflam¬ matory agents such as hormonal agents, hydrocortisone, prednisolone, prednisone, non-hormonal agents, allopuri- nol, aspirin, indomethacin, phenylbutazone and the like; prostaglandins; cytotoxic drugs such as thiotepa, chlorambucil, cyclophospha ide, melphalan, nitrogen mustard, methotrexate and the like; antigens of such microorganisms as Neisseria gonorrhea. Mvcobacterium tuberculosis. Herpes virus (hu onis, types 1 and 2), Candida albicans. Candida tropicalis. Trichomonas vaqinalis. Haemophilus vaginalis. Group B streptococcus E. coli. gtrep. mutans. Microplasma hominis. Heπophilus ducreyi. Granuloma inguinale, Lv phopathia venereum, Treponema pallidum. Brucella abortus,. Brucella πieliten- sis. Brucella suis. Brucella canis. Ca pylobacter fetus. Campylobacter fetus intestinalis. Leptospira pomona, Listeria moncvtoqenes. Brucella ovis. Equine herpes

virus 1, Equine arteritis virus, IBR-IBP virus, BVD-MB virus, Chlamvdia psittaci. Trichomonas foetus. Toxo- plasma σondii. Escherichia coli. Actinobaccillus e uuli. Salmonella abortus ovis. Salmonella abortus eσui. Pseudomonas aeruginosa. Corynebacterium equi. Corvnebac- terium pyogenes. Actinobaccilus seminis. Mvσoplasma bovigenitaliu _f Asperσillus fumjqatus, Absidia ramosa. Trypanosoma equiperdum. Babesia caballi. Clostridium tetani. and the like; antibodies which counteract the above microorganisms; and enzymes such as ribonuclease, neuramidinase, trypsin, glycogen phosphorylase, sperm lactic dehydrogenase, sperm hyaluronidase, adenoεsine- triphosphataεe, alkaline phoεphataεe, alkaline phospha- taεe esterase, a ino peptidase, trypsin chymotrypsin, amylase, muramidaεe, acrosomal proteinase, diesterase, glutamic acid dehydrogenase, succinic acid dehydroge¬ nase, betaglycophosphatase, lipase, ATP-ase alpha- peptate gama-glutamylotranspeptidase, εterol-3-beta-ol- dehydrogenase, DPN-di-aprorase.

Other compounds of particular interest are hormon- ally active polypeptides, especially luteininzing hormone-releasing hormone (LR-RH) polypeptides, ana¬ logues, and antagonists thereof; mammalian growth hormones, including human, bovine, equine, and sheep growth hormones; alpha, beta, gamma, and omega interferon; interleukin I and interleukin II; and erythropoietin.

Having generally described the invention, further understanding can be obtained by reference to certain specific examples which are provided herein for purpose of illustration only and are not intended to be limiting unless otherwise specified.

EXΑMPIJ5 I

This example describes the procedure for preparing microspheres, varying the ratio of the first non- solvent, silicone oil, to the organic εolvent, methylene chloride.

The polymeric coating material, a copolymer with a 50:50 molar ratio of lactide:glycolide units with inherent viscosity of 0.69 dl/g, was weighed and dissolved in methylene chloride (CH ₂ Cl ₂ ) . The encapsu¬ late, histrelin, was dispersed in the solvent/polymer mixture using a stirrer. With continued stirring, silicone oil waε added to the mixture. The ratio of the silicone oil to the methylene chloride εolvent was varied in these experiments and the resultε are shown in the following Table 1. The εilicone oil cauεed the polymer to phaεe separate, and depoεit as droplets of solvent-fluid polymer onto the surface of the water- εoluble microdropletε. These solvent-fluid polymer droplets then coalesced to form a continuous film around the water-soluble microdroplets.

The microspheres were then hardened by quenching by pouring the contents of this first mixture into a beaker containing heptane. The heptane/methylene chloride/- εilicone oil εolution waε removed by filtration. The microspheres were further washed with aliquots of heptane. The microspheres were dried at room tempera¬ ture under vacuum. The microsphereε obtained from this preparation were determined to have diameters ranging in size from 25 to 150 microns.

The following Table 1 gives the resultε of the teεtingε to εhow the effect of the ratio of eilicone oil to methylene chloride and the effect of the viεcosity of

the εilicone oil. The viεcoεity of the εilicone oil was studied at 200cε, 350cs, and 500cs. In three of the tests, histrelin was used as the encapsulate; with the other testε a placebo was used.

* Vacuum extractable to 0.8%

** Loaded encapsulate = histrelin

EXAMPLE II

■Silicone Oil 1:2.6 Ratio rwt/wt. of First Non-Solvent to Solvent

A 50:50 lactide:glycolide copolymer, 1.80 g, was disεolved in 15.7 g of methylene chloride and charged to a 100 ml flask. Histrelin, 200.4 mg, was suspended in 12.2 g of methylene chloride and added with εtirring to the polymer εolution. An additional 6.0 g of methylene chloride waε then added (total = 33.9 g) . Silicone oil

(350 cs), 13.13 g was added over 8.5 minutes. Then, the contents of the reaction flask were tranεferred to 2800 ml of n-heptane with εtirring at 21*C. Stirring was continued for 3 hours. The microεpheres were collected on stain ess-εteel sieves and allowed to air dry. The reεidual heptane content waε found to be 8.7% by weight. The hiεtrelin content was 7.76% by weight.

E AMP ; HT

Silicone Oil 1.9:1 Ratio of First Non-Solvent to Solvent

1.8 g of a 50:50 lactiderglycolide copolymer was dissolved in 15.0 g of methylene chloride. Histrelin, 201.5 mg, was suspended in 15.0 g of methylene chloride and sonicated briefly to disperse the drug. The disperεion waε then added with εtirring to the polymer εolution. An additional 12.67 g of methylene chloride waε added to the reaction flaεk. Silicone oil (350 cs) , 82.17 g, was added over 13 minutes. The contents of the reaction flaεk were tranεferred to 3250 ml of n-heptane with εtirring at 18.2*C. Stirring waε continued for 3 hourε. The microcapεuleε were collected on εtainless- εteel sieves and allowed to air dry. Reεidual heptane waε found to be 0.9% by weight. Hiεtrelin content was 8.35% by weight.

The foregoing invention has been deεcribed in some detail by way of illustration and example for purposes of clarity and understanding. It will be obvious that certain changes and modifications may be practiced within the scope of the invention, aε limited only by εcope of the appended claims.