Aqueous coating composition and process for preparing solid pharmaceutical preparation

Abstract: An aqueous coating composition comprising hydroxypropyl methyl cellulose trimellitate typically having a mean particle size of up to 10 µm and a plasticizer is applied to a solid pharmaceutical preparation to form a coating film having acid resistance and solubility at about pH 4. The coated preparation has an improved bioavailability.

Description: This invention relates to an aqueous coating composition suitable for use in forming an acid-resistant protective coating on a solid pharmaceutical preparation and a process for preparing a coated solid pharmaceutical preparation using the same. BACKGROUND Enteric coatings are widely used for various purposes including the protection of acid-labile medicaments from gastric acid and the protection of gastric mucosa from irritative or attacking medicaments. Known enteric coating compositions include cellulose compounds such as cellulose acetate phthalate, cellulose acetate trimellitate, hydroxypropyl methyl cellulose phthalate, hydroxypropyl methyl cellulose acetate succinate, and carboxymethyl ethyl cellulose; vinyl compounds such as polyvinyl alcohol acetate phthalate; and acrylic compounds such as methacrylic acid-ethyl acrylate copolymers. These bases are used in coating after the polymers are dissolved in organic solvents or aqueous latexes or water dispersions of the polymers are formed. However, since these commercially available enteric polymers have a dissolution pH in the range of 5 to 7, they suffer from a decline of bioavailability. More particularly, when the polymer is applied to a medicament whose absorption site should be in the upper portion of the small intestine, the enteric preparation (in the form of polymer-coated tablets or granules) will travel past the absorption site before it is sufficiently dissolved to release the medicament. As a solution to this problem, JP-A 8-133989 and 8-301790 (which has an US equivalent; US 5,700,929) propose hydroxypropyl methyl cellulose acetate maleate and hydroxypropyl methyl cellulose trimellitate respectively, having dissolution pH down at about 4. Because of the low dissolution pH, these polymers, apart from the concept of prior art enteric coating, are expected to find an application as an acid-resistant protective coating composition specialized for protecting medicaments from strong acidity. These polymers, however, are used as solutions in organic solvents which have the danger of explosion and fire, are hazardous to the safety and hygiene of workers, and cause environmental pollution as a result of release to the atmosphere. There is also a possibility that a minor amount of solvent be left in the coated solid preparation. It would be desirable to provide a low-pH dissolving coating in a new and more convenient way. One aspect of the invention is to provide an aqueous coating composition which enables the use in an aqueous form of hydroxypropyl methyl cellulose trimellitate which is a low pH dissolution type base. Another aspect of the invention is to provide a process for preparing a coated solid pharmaceutical preparation using the same. The inventors have found that an aqueous coating composition consisting essentially of hydroxypropyl methyl cellulose trimellitate (sometimes referred to as HPMCT) having a mean particle size of up to 10 µm dispersed in water in the presence of a plasticizer selected from the group consisting of triethyl citrate, glycerol triacetate, acetylated monoglyceride and phthalic esters is effective in forming an acid resistant protective coating suitable for use on a solid pharmaceutical preparation e.g. by spraying and drying the composition thereon. The thus coated solid preparation has good bioavailability. With respect to aqueous enteric coating compositions, JP-B 56-12614 discloses a method of forming a coating composition by dispersing enteric cellulose derivative powder having a mean particle size of up to 100 µm in water containing a compatible gelling agent having a boiling point of at least 100°C; and JP-B 58-55125 discloses the use of triethyl citrate as the gelling agent in this method. Additionally, JP-A 59-167521 and 59-190925 disclose a method of forming a coating composition by dispersing enteric cellulose derivative powder having a mean particle size of up to 10 µm in triethyl citrate-containing water. However, the cellulose derivatives used in the prior art enteric coating compositions have a monobasic carboxylic acid introduced into a cellulose skeleton by way of ester or ether bonds, whereas HPMCT has a dibasic carboxylic acid introduced into a cellulose skeleton. Thus the prior art cellulose derivatives and the HPMCT differ in affinity to plasticizers, etc. It is the inventors' discovery that an aqueous coating composition obtained by blending HPMCT (different from the cellulose derivatives used in the prior art enteric coating compositions) with a plasticizer exhibits good disintegration at pH 4.0 to 4.5 as demonstrated later in Examples and allows the HPMCT to exert its effect to a full extent. In a first aspect, the invention provides an aqueous coating composition in the form of a dispersion consisting essentially of hydroxypropyl methyl cellulose trimellitate having a mean particle size not above 10 µm dispersed in water and a plasticizer selected from the group consisting of triethyl citrate, glycerol triacetate, acetylated monoglyceride and phthalic esters, said dispersion containing about 3 to 15% by weight of hydroxypropylmethyl cellulose trimellitate and the amount of said plasticizer being 25 to 50% by weight based on hydroxypropylmethyl cellulose trimellitate. The aqueous coating composition is suitable for use as a coating on a solid pharmaceutical preparation. In a second aspect, the invention provides a process for preparing a coated solid pharmaceutical preparation comprising the steps of spraying an aqueous coating composition onto a solid pharmaceutical preparation and drying the composition to form a coating of HPMCT on the surface of the solid pharmaceutical preparation, said aqueous coating composition being in the form of a dispersion consisting essentially of hydroxypropylmethyl cellulose trimellitate having a mean particle size of up to 10 µm dispersed in water and a plasticizer selected from the group consisting of triethyl citrate, glycerol triacetate, acetylated monoglyceride and phthalic esters, said dispersion containing about 3 to 15% by weight of hydroxypropylmethyl cellulose trimellitate and the amount of said plasticizer being 25 to 50% by weight based on hydroxypropylmethyl cellulose trimellitate. The hydroxypropyl methyl cellulose trimellitate (HPMCT) used herein can be synthesized by any well-known process as disclosed in JP-A 8-301790 (and its US equivalent US 5,700,929). After synthesis, the HPMCT may be ground by any well-known method, for example, in an air blast pulverizer, ball mill or vibrating mill. The HPMCT should have a mean particle size of up to 10 µm, and more preferably up to 7 µm. If the mean particle size exceeds 10 µm, sometimes a large amount of the coating solution must be applied in order to form an acid resistant coating. The lower limit of the mean particle size is not critical although a particle size of 2 µm or more is recommended for practical use. With a particle size of less than 2 µm, it sometimes becomes difficult to disperse the HPMCT into primary particles by ordinary agitation, failing to achieve the desired effect. The aqueous coating composition has the HPMCT dispersed in water in the presence of a plasticizer. The aqueous coating composition or dispersion contains about 3 to 15%, preferably about 5 to 10% by weight of the HPMCT. A low HPMCT concentration of less than 3% by weight may take a long time in coating. A high HPMCT concentration in excess of 15% by weight may be inconvenient to apply because, for example, clogging of a spray gun can occur due to the agglomeration of HPMCT particles in the dispersion. The plasticizers used herein are triethyl citrate, glycerol triacetate (or triacetin), acetylated monoglyceride, and phthalic esters such as dimethyl phthalate, diethyl phthalate, and dibutyl phthalate. An appropriate amount of the plasticizer added is typically about 20 to 60%, more preferably about 25 to 50% by weight based on the HPMCT. Amounts of less than 20% by weight may tend to result in film formation not fully taking place, failing to produce a fully acid resistant coated preparation. Amounts of more than 60% by weight may sometimes give rise to the problem that granules or tables stick together during coating or storage. The aqueous coating composition may be obtained by dispersing finely divided HPMCT in an aqueous solution containing a plasticizer such as triethyl citrate under ordinary agitation. The solution should preferably be at a temperature of 40°C or lower in order to prevent the HPMCT from agglomerating. Most often, dispersion is carried out at room temperature. If desired, the plasticizer may be added after the HPMCT has been dispersed. When the coating composition is prepared, additives that can be used in drugs, for example, surfactants and protective colloids may be added in order to improve the stability of the coating composition or water dispersion. Similarly, talc, titanium oxide and silicon dioxide may be added for the purpose of preventing the pharmaceutical preparation (in the form of granules or tablets) from sticking during coating or storage. Other additives such as pigments, dyes, flavors and perfumes may also be added, if desired. A water-soluble film coating agent or acrylic aqueous latex may be added for the purpose of controlling or altering the solubility and moisture permeability of a coating film. The solid pharmaceutical preparation to be coated with the coating composition includes tablets, granules, fine powders and capsules. Any desired one of well-known methods may be used in coating the solid pharmaceutical preparation with the coating composition. For example, use may be made of a pan coating apparatus, pneumatic drum coating apparatus, fluidized bed coating apparatus, agitating fluidized bed coating apparatus, or tumbler coating apparatus. The coating composition may be sprayed over the solid pharmaceutical preparation in such an apparatus through a spray gun, followed by drying. The spray gun preferably used herein may incorporate a twin-fluid atomizing nozzle; well-known as such. The amount of the film coated on the surface of the solid pharmaceutical preparation by such coating operation is generally about 3 to about 40% by weight based on the weight of the solid pharmaceutical preparation although the coating amount (or coverage) varies with the type, shape, size, and surface state of the solid pharmaceutical preparation and the characteristics of drug and additives in the solid pharmaceutical preparation. Before the coating composition is applied, a film coating agent such as hydroxypropyl methyl cellulose (HPMC) may be previously applied to the solid pharmaceutical preparation to form a protective film thereon for the purpose of protecting the solid pharmaceutical preparation from water that can contact the surface of the solid pharmaceutical preparation in the coating process. After the coating composition is applied, wax or a film coating agent as above may be overcoated for the purpose of improving the stability or compression resistance of the coated pharmaceutical preparation. EXAMPLE Examples are given below by way of illustration and not by way of limitation. All parts and % are by weight. Example 1 (1) Preparation of tablets A mixture of 70 parts of lactose, 26 parts of cornstarch, and 4 parts of polyvinyl pyrrolidone (PVP K-30 by BASF) was granulated with the aid of water and dried. To this tablet-forming powder was added 0.5 part of magnesium stearate, followed by thorough mixing. Using a rotary tablet machine, the powder was compressed into tablets having a diameter of 8 mm and a weight of 200 mg. The tablets had a hardness of about 10 kg and a disintegration time of about 5 minutes. (2) Preparation of coating solution With stirring, 28.8 g of triethyl citrate was dissolved in 1246.6 g of purified water at room temperature. With stirring, 96 g of finely divided hydroxypropyl methyl cellulose trimellitate (HPMCT) was dispersed in the solution, yielding a water dispersion of HPMCT for coating. It is noted that 28.8 g of triethyl citrate corresponded to 30% of the weight of HPMCT and that HPMCT was finely divided using an air blast pulverizer model IDS-2 (Nippon Pneumatic Industry K.K.). The HPMCT had the following degree of substitution and mean particle size. methoxyl DS 1.44 (14.9%) hydroxypropoxyl MS 0.18 (4.6%) trimellitate DS 0.56 (35.9%) mean particle size ∼4.5 µm Note that DS represents a degree of substitution, MS represents a molar substitution, and the mean particle size is as measured by dry laser light diffractometry (Helos & Rodos by Nippon Bunko K.K.). A film dimensioned 1 cm x 1 cm x 100 µm thick was formed from HPMCT in an organic solvent. The dissolution pH of this film in Mcllvaine buffer solution was measured according to a disintegration test as prescribed in the 13th revision of the Japanese Pharmacopoeia, by filling auxiliary cylinders with the film, and feeding buffer solutions having slightly different pH. The pH at which the film was dissolved in about 30 minutes was 3.9. (3) Coating operation A coating apparatus with a pneumatic drum having a diameter of 30 cm (by Shin-Etsu Chemical Co., Ltd.) was charged with 1.2 kg of the tablets obtained in (1), which were coated with the water dispersion obtained in (2) under the following conditions. Coating conditions Spray gun: one twin-fluid atomizing nozzle Spray pressure: 200 kPa Drying air temperature: 80°C Spraying speed: 12 g/min Coating amount: 8% of HPMCT Spraying was continued until the coating amount reached 8% based on the tablet weight of HPMCT. During the coating process, samples were taken out when the coating amount was 5%, 6% and 7% of HPMCT. The thus coated tablets (including the finally coated tablets and intermediately sampled tablets) were dried at 60°C for one hour before they were subject to the following tests. (4) Examination of coated tablets Acid resistance test According to a disintegration test procedure as prescribed in the 13th revision of the Japanese Pharmacopoeia, disintegration tests using No. 1 and No. 2 solutions were carried out each on six coated tablets. The results are shown in Table 1. Disintegration at different pH Like the acid resistance test, the disintegration time was measured on six finally coated tablets (8% HPMCT coverage) using Mcllvaine buffer solutions having different pH values instead of No. 2 solution. An average disintegration time is shown in Table 1. Example 2 The procedure of Example 1 was repeated except that HPMCT having a mean particle size of about 8.5 µm was used. The test results of the coated tablets are shown in Table 1. Example 3 The procedure of Example 1 was repeated except that a water dispersion for coating was obtained by dissolving 38.4 g (40% based on HPMCT) of triethyl citrate in 1237.0 g of purified water at room temperature under agitation and dispersing 96 g of finely divided HPMCT therein under agitation. The test results of the coated tablets are shown in Table 1. Example 4 The procedure of Example 1 was repeated except that a water dispersion for coating was obtained by dissolving 24.0 g (25% based on HPMCT) of triethyl citrate in 1251.4 g of purified water at room temperature under agitation and dispersing 96 g of finely divided HPMCT therein under agitation. The test results of the coated tablets are shown in Table 1. Example 5 The procedure of Example 1 was repeated except that a water dispersion for coating was obtained by dissolving 48.0 g (50% based on HPMCT) of triethyl citrate in 1227.4 g of purified water at room temperature under agitation and dispersing 96 g of finely divided HPMCT therein under agitation. The test results of the coated tablets are shown in Table 1. Disintegration time (min) E1 E2 E3 E4 E5 Acid resistance test No. 1 solution, HPMCT coverage 5% unchanged 1 tablet failed unchanged 2 tablets failed unchanged No. 1 solution, HPMCT coverage 6% unchanged unchanged unchanged unchanged unchanged No. 1 solution, HPMCT coverage 7% unchanged unchanged unchanged unchanged unchanged No. 1 solution, HPMCT coverage 8% unchanged unchanged unchanged unchanged unchanged No. 2 solution, HPMCT coverage 8% 8 7 9 8 8 Disintegration at different pH pH 3.5 >120 >120 >120 >120 >120 pH 4.0 14 13 16 13 15 pH 4.5 9 8 10 8 10 The invention enables aqueous coating of HPMCT which is a low pH dissolution type base, without sacrificing the advantages characteristic of HPMCT including acid resistance and dissolution at about pH 4. A coating film having an improved bioavailability can be formed on a solid pharmaceutical preparation. Although some preferred embodiments-have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described in the Examples.

Claims: An aqueous coating composition in the form of a dispersion consisting essentially of hydroxypropylmethyl cellulose trimellitate having a mean particle size of up to 10 µm dispersed in water and a plasticizer selected from the group consisting of triethyl citrate, glycerol triacetate, acetylated monoglyceride and phthalic esters, said dispersion containing about 3 to 15% by weight of hydroxypropylmethyl cellulose trimellitate and the amount of said plasticizer being 25 to 50% by weight based on hydroxypropylmethyl cellulose trimellitate. An aqueous coating composition according to claim 1 suitable for use in coating a solid pharmaceutical preparation. An aqueous coating composition according to claim 1 or claim 2, wherein the dispersion contains 5 to 10% by weight of hydroxypropylmethyl cellulose trimellitate. An aqueous coating composition according to any one of claims 1-3, wherein the plasticizer is a phthalic ester, being dimethyl phthalate, diethyl phthalate or dibutyl phthalate. A process for preparing a coated solid pharmaceutical preparation comprising the steps of: spraying an aqueous coating composition onto a solid pharmaceutical preparation and drying the composition to form a coating of hydroxypropylmethyl cellulose trimellitate on the surface of the solid pharmaceutical preparation, said aqueous coating composition being in the form of a dispersion consisting essentially of hydroxypropylmethyl cellulose trimellitate having a mean particle size of up to 10 µm dispersed in water and a plasticizer selected from the group consisting of triethyl citrate, glycerol triacetate, acetylated monoglyceride and phthalic esters, said dispersion containing about 3 to 15% by weight of hydroxypropylmethyl cellulose trimellitate and the amount of said plasticizer being 25 to 50% by weight based on hydroxypropylmethyl cellulose trimellitate. A process according to claim 5 wherein the aqueous coating composition is as defined in any one of claims 2 to 4. A process according to claim 5 or claim 6 further including the preliminary step of preparing the aqueous coating composition, wherein the hydroxypropylmethyl cellulose trimellitate is dispersed at a temperature of 40°C or lower.

Inventors: KOKUBO HIROYASU, NISHIYAMA YUICHI, KOKUBO, HIROYASU, NISHIYAMA, YUICHI, Kokubo, Hiroyasu, Shin-Etsu Chemcial Co., Ltd., Nishiyama, Yuichi, Shin-Etsu Chemcial Co., Ltd.
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1999
Main classification: A61K9
IPC codes: A61K9, A61K47