Abstract:
Pharmaceutical forms of pimobendan for oral administration comprising citric acid, whereby a constant, satisfactory resorption is ensured even when there are major pH fluctuations in the gastrointestinal tract.

Description:
This is a continuation of application Ser. No. 07/907,003, filed Jul. 1, 1992, now abandoned, which is a continuation of application Ser. No. 07/644,161, filed Jan. 22, 1991, now abandoned. 
    
    
     The invention relates to pharmaceutical forms of pimobendan for oral administration. Pimobendan is 4,5dihydro-6-[2-(4-methoxyphenyl)-1H-benzimidazol-5-yl]-5-methyl-3(2H)-pyridazinone which was described in European Patent No. 8391; pimobendan is a substance having cardiotonic, hypotensive and antithrombotic activities. 
     Unlike other substances having different structures mentioned in this patent specification, the resorption of pimobendan when administered orally is prone to considerable inter- and intra-individual fluctuations if the active substance is incorporated in known or conventional pharmaceutical forms for oral administration. The reason for this is that pimobendan is characterized by a low solubility in aqueous media and a very highly pH-dependent solubility. 
     Depending on the buffer system used, about 100 to 300 mg/l dissolve at a pH between 1 and 3 (corresponding to a 0.01 to 0.03% solution), but at pH 5 only about 1 mg/l will dissolve in water (corresponding to a 0.0001% solution or 1 ppm). 
     In in vivo tests on humans with pimobendan packed into hard gelatin capsules, one test subject showed no blood level of pimobendan, a second test subject showed a very low blood level and a third showed higher blood levels, but overall the blood levels of pimobendan fluctuated very considerably from one individual to another and the levels were too low. These unsatisfactory resorption characteristics can be explained primarily by the high pH-dependency of the solubility of pimobendan in aqueous media and by fluctuating pH conditions in the gastrointestinal tracts of the test subjects. It is known that the pH of the gastric juices, particularly in patients who have been fasting, can fluctuate between 1 and 6, but in patients who have not fasted it is more frequently between 3 and 5 than 1 to 2. 
     It was therefore obvious to increase the solubility of pimobendan by the simultaneous administration of an acid. In vitro tests showed, however, that pimobendan in 0.1 N hydrochloric acid (pH value 1.1) dissolved in a quantity of only 100 mg/l (corresponding to a 0.01% solution). In a fumaric acid solution, pH 2.27, only 50 mg/l dissolve (corresponding to a 0,005% solution), in a 20% (by weight) tartaric acid solution, pH 1.2, 960 mg/l dissolve (corresponding to a 0.096% solution), and in a 40% tartaric acid solution, pH 0.7, only 3.9 g/l dissolve (corresponding to a 0.39% solution). None of these levels is sufficient, or the addition of acid required is no longer practicable for dissolving a sufficient quantity of the active substance and thereby ensuring reliable resorption, even if such quantities of these acids are administered orally simultaneously with the active substance. 
     Surprisingly, the applicants have now succeeded in overcoming the low solubility and high pH dependency of the solubility of pimobendan and ensuring a very satisfactory and more constant resorption, even if there are considerable pH fluctuations in the gastrointestinal tract, by intimately mixing the pimobendan with citric acid in a ratio by weight of at least or less than 1:5 and subsequently processing it with conventional excipients to form a powder, pellets or granules for oral administration. The granules, powder or pellets may also be compressed with suitable excipients to form tablets which may, if desired, also be covered with a flavor-masking coating. 
     Citric acid is a safe and well tolerated excipient which increases the solubility of pimobendan by a factor of 100, compared with artificial gastric juice (pH 1.2). Thus, 7.6 g/l will dissolve in an aqueous solution of pH 1.4 containing 20% by weight of citric acid, whilst in an aqueous solution of pH 1.0 containing 40% by weight of citric acid as much as 12.1 g of pimobendan will dissolve per liter. However, these quantities of dissolved pimobendan are sufficient to ensure adequate resorption of the active substance even in patients who, when given conventional pimobendan preparations by oral route, showed no blood levels or very low and sharply fluctuating blood levels of pimobendan. 
     Citric acid is difficult to process into solid preparations. To avoid the formation of salts of pimobendan with citric acid, which would increase the hygroscopicity of the formulation, it is at first sight obvious to process the active substance and acid in two separate granulates. However, it has been found (see Examples 1b to 1d and 2a) that when separated in this way the citric acid cannot fully develop its solubilizing activity. However, it has been found that by intimately mixing pimobendan with citric acid to form a powder mixture which is subsequently processed into granules, a pellet or tablets, it is possible to obtain preparations with small amounts of citric acid which ensure adequate dissolution and sufficiently high blood levels. Technically, this can be achieved for example by nonaqueous granulation, e.g. by granulation with alcohol or by the use of suitable granulating methods which make it possible to add the granulating liquid in accurately metered amounts, with simultaneous drying. Another possibility is the preparation of granules by dry granulation, these granules containing the active substance and citric acid intimately mixed. Owing to the hygroscopic properties of the citric acid, care must be taken to ensure that the preparation forms disintegrate rapidly in the release medium; in the case of tablets, this is achieved by the addition of disintegrants, e.g. Amberlite IRP 88 (methacrylic resin with exchangeable protons), Crospovidone (crosslinked polyvinylpyrrolidone) and microcrystalline cellulose, which will simultaneously improve the poor compression properties of citric acid. 
     A weight ratio of pimobendan to citric acid of between 1:10 and 1:20 is preferred. The upper limit is defined by the ability of the preparations to be swallowed. 
     The prevention of sharply fluctuating blood levels (both inter- and intra-individually) by the addition of citric acid can be explained as follows: when the intimate mixture of active substance and citric acid comes into contact with gastric juice, an acidic microsphere is formed around the particles owing to the high rate of dissolution of the citric acid. This microsphere is always acidic, irrespective of the pH of the gastrointestinal juices, and ensures that the finely divided active substance will reliably dissolve and therefore be freely available for resorption. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the change in the plasma pimobendan concentration after oral administration to dogs of a tablet containing 5 mg pimobendan without citric acid. 
     FIG. 2 shows the change in the plasma pimobendan concentration after oral administration to dogs of a tablet containing 5 mg. pimobendan and 50 mg of citric acid. 
     FIG. 3 shows the change in the plasma pimobendan concentration after oral administration to humans of a capsule containing 5 mg. pimobendan and 230 mg of citric acid. 
     FIG. 3 also shows the change in the plasma pimobendan concentration after oral administration to humans of a capsule containing 5 mg. pimobendan and 207 mg of citric acid. 
     FIG. 4 shows the change in the plasma pimobendan concentration after oral administration to humans of a tablet containing 2.5 mg. pimobendan and 50 mg of citric acid. 
     FIG. 4 also shows the change in the plasma pimobendan concentration after oral administration to humans of a capsule containing 2.5 mg. pimobendan and 209 mg of citric acid. 
     FIG. 5a shows the change in the plasma pimobendan concentration after oral administration to human test subject a of a tablet containing 5 mg. pimobendan without citric acid. 
     FIG. 5b shows the change in the plasma pimobendan concentration after oral administration to human test subject b of a tablet containing 5 mg. pimobendan without citric acid. 
     FIG. 5c shows the change in the plasma pimobendan concentration after oral administration to human test subject c of a tablet containing 5 mg. pimobendan without citric acid. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Solubility tests have shown, for pH values between 1 and 6, that the active substance dissolves out of this intimate mixture virtually irrespective of the pH. In addition, the active substance also forms supersaturated solutions with the citric acid which remain stable for hours. This ensures a high level of resorption in any case, even in patients with abnormally high pH levels in their gastrointestinal juices. Of the many acids tested for this purpose, citric acid has unexpectedly proved outstanding; apart from acting as an acid, it serves as a solubilizing agent and also as a stabilizer for the active substance solution obtained. An important prerequisite for the dissolving of the active substance independently of the local physiological pH value is the intimate mixing of the pimobendan with the citric acid. For this, it is necessary that both substances be present in powder form or as very small crystals, so that they will make contact with each other over a large surface area. 
     Tests on dogs after oral administration of a form according to Example 1a containing 5 mg of pimobendan, by comparison with a form according to Example 1b containing 5 mg of pimobendan and 50 mg of citric acid, showed that the pimobendan plasma level was approximately trebled by the form containing the citric acid compared with the form containing no citric acid. The tests were each carried out on 5 experimental animals. The mean curve values found are shown in FIGS. 1 and 2. The plasma level values are shown in nanograms per milliliter as a function of time. 
     Human trials using oral pimobendan forms according to Examples 3b and 3c (capsules) on 11 test subjects gave the mean curves for the plasma levels shown in FIG. 3. The maxima occurred at 1 to 1.5 hours after administration. In addition to the capsule formulations according to Examples 3b and 3c, the plasma level curves of the tablet form according to Example 2b and the capsule form according to Example 4 were each tested on 11 test subjects. It was found that the tablet containing only 50 mg of citric acid according to Example 2b is bioequivalent to the capsule formulation containing 209 mg of citric acid according to Example 4. The plasma levels were obtained by high pressure liquid chromatographic methods, resulting in the mean curves for the plasma levels shown in FIG. 4 (mean values±standard deviations). 
     By way of a comparison, a tablet formulation according to Example 3a was administered orally, i.e. a formulation with no citric acid. This resulted in the curves for the plasma levels shown in FIGS. 5a to c on three test subjects. If FIGS. 5a to c are compared with FIG. 4, the superiority of the citric acid preparation, which is made apparent by the reduced fluctuations in plasma levels, over a preparation without citric acid becomes very clear. 
     It goes without saying, that instead of using pimobendan, one of its possible enantiomers can also be used with equal success. 
     As a further illustration, by way of example, of the object of this invention, reference is made to the following Examples of oral preparations. In these Examples: 
     
         ______________________________________Amberlite IRP 88 =        methacrylic resin with exchangeable H.sup.+Collidone 25 =        polyvinylpyrrolidone, average molecular        weight 29,000Avicel =     microcrystalline cellulosePolyplasdone XL =        crosslinked polyvinylpyrrolidone =        polyvinylpolypyrrolidoneCompritol 888 =        glyceryl monobehenateTween 80 =   polyoxyethylene-(20)-sorbitan mono-        oleateExplotab =   sodium carboxymethyl starchAerosil 130 V =        highly dispersed, X-ray amorphous        silicon dioxide.______________________________________ 
    
     EXAMPLE 1 
     Tablets Containing 5Mg of Pimobendan 
     a) Tablets without citric acid 
     Composition 
     1 tablet contains (mg) 
     
         ______________________________________(1) Pimobendan       5.0(2) Microcrystalline cellulose                58.0(3) Sec. calcium phosphate                72.0(4) Corn starch      54.0(5) Amberlite IRP 88 10.0(6) Magnesium stearate                1.0                200.0______________________________________ 
    
     Preparation 
     Some of the corn starch is dissolved in water with heating and the mixture of ingredients (1) to (4) is granulated therewith. (5) and (6) are added to the dried granules. Tablets 8 mm in diameter and weighing 200 mg are compressed from the finished mixture. 
     Measurement of rate of dissolution: 
     According to USP XXIII, paddle method, 150 rpm, in McIlvaine buffer, pH 5.5. 
     x from 3 individual measurements in each case. 
     Results 
     After 5 min.: 8.5% 
     10 min.: 10.2% 
     15 min.: 10.7% 
     20 min.: 10.8% 30 min.: 10.8% pimobendan dissolved. 
     b) Tablets containing 50 mg of citric acid 
     Composition 
     1 tablet contains (mg) 
     
         ______________________________________(1) Pimobendan       5.0(2) Citric acid      50.0(3) Microcrystalline cellulose                42.0(4) Collidone 25     0.5(5) Sec. calcium phosphate                52.0(6) Corn starch      39.5(7) Amberlite IRP 88 10.0(8) Magnesium stearate                1.0                200.0______________________________________ 
    
     Preparation 
     Some of the corn starch is dissolved in water with heating and ingredient (1), some of (3), (5) and some of (6) are granulated therewith. (2) and the remainder of (3 ) and (6 ) are granulated with the aqueous solution of (4). The granules are dried and mixed together. (7) and (8) are added to the mixture of dried granules to form the final mixture. This is then compressed to form tablets 8 mm in diameter and weighing 200 mg. 
     Note 
     Active substance and acid are presented in separate granules for ease of manufacture but are mixed together. 
     Measurement of rate of dissolution: as in Example 1a. 
     Results 
     After 5 min.: 7.7% 
     10 min.: 19.2% 
     15 min.: 34 % 
     20 min.: 40.6% 
     30 min.: 43% pimobendan dissolved. 
     c) Tablets containing 103 mg of citric acid 
     Composition 
     1 tablet contains (mg) 
     
         ______________________________________(1) Pimobendan       5.0(2) Citric acid      103.0(3) Microcrystalline cellulose                35.0(4) Collidone 25     1.0(5) Sec. calcium phosphate                31.5(6) Corn starch      81.5(7) Amberlite IRP 88 10.0(8) Magnesium stearate                3.0                270.0______________________________________  Preparation 
    
     Analogous to Example 1b. 
     Tablets: 9 mm diameter, 270 mg in weight. 
     Note 
     The active substance and acid are present in separate granulates for ease of manufacture but are mixed together. 
     d) Tablets containing 206 mg of citric acid 
     Composition 
     1 tablet contains (mg) 
     
         ______________________________________(1) Pimobendan      5.0(2) Citric acid     206.0(3) Avicel          50.0(4) Collidone 25    2.0(5) Sec. calcium phosphate               63.0(6) Corn starch     46.0(7) Amberlite IRP 88               20.0(8) Magnesium stearate               3.0               395.0______________________________________ 
    
     Preparation 
     Analogous to Example 1b. 
     Tablets: 11 mm in diameter, weight 395 mg. 
     Note 
     The active substance and acid are present in separate granulates for ease of manufacture but are mixed together. 
     Measurement of rate of dissolution: analogous to Example 1a. 
     Result 
     After 5 min.: 23.8% 
     10 min.: 59% 
     15 min.: 67% 
     30 min.: 69% pimobendan dissolved. 
     EXAMPLE 2 
     Tablets Containing 2.5 Mg of Pimobendan 
     a) Tablets containing 103 mg of citric acid 
     Composition 
     1 tablet contains (mg) 
     
         ______________________________________(1) Pimobendan          2.5(2) Corn starch         23.0(3) Microcrystalline cellulose                   26.0(4) Anhydrous calcium phosphate                   31.5(5) Polyplasdone XL     59.0(6) Citric acid, fine particles (anhydrous)                   103.0(7) Compritol 888       5.0                   250.0______________________________________ 
    
     Preparation 
     (1) to (4) are granulated with aqueous starch solution. The other tablet ingredients are added to the dry granules to make the final mixture. From this, tablets are compressed measuring 9 mm in diameter and weighing 250 mg. 
     Note 
     The active substance and acid occur separately, for ease of manufacture, but are subsequently mixed together. 
     Measurement of speed of dissolution: analogous to Example 1a. 
     Results 
     After 5 min.: 18.7% 
     10 min.: 20.5% 
     20 min.: 21.8% 
     30 min.: 22.2% 
     60 min.: 22.7% pimobendan dissolved. 
     b) Tablets containing 50 mg of citric acid 
     Composition 
     1 tablet contains (mg) 
     
         ______________________________________(1) Pimobendan           2.5(2) Anhydrous powdered citric acid                    50.0(3) Avicel PH 101        13.0(4) Anhydrous calcium hydrogen phosphate                    15.0(5) Undried corn starch  6.0(6) Collidone 25         0.5(7) Insoluble polyvinylpyrrolidone                    59.0(8) Compritol 888        3.0(9) Magnesium stearate   1.0                    150.0______________________________________ 
    
     Preparation 
     (6) is dissolved in ethanol and the mixture of ingredients (1) to (5) is granulated therewith. (7) to (9) are added to the dry granules to form the mixture ready for compression. This mixture is compressed to form tablets measuring 8 mm in diameter. 
     Note 
     Active substance and acid are present together in the same granulate. 
     Measurement of rate of dissolution: analogous to Example 1a. 
     Result 
     After 15 min.: 71.1% 
     30 min.: 85% pimobendan dissolved. 
     EXAMPLE 3 
     Capsules Containing 5 Mg of Pimobendan 
     a) Capsules without citric acid 
     Composition 
     1 capsule contains (mg) 
     
         ______________________________________Pimobendan        5.0Lactose           90.25Corn starch       36.0Tween 80          0.5Explotab          8.0Magnesium stearate             0.25             140.0______________________________________ 
    
     Preparation 
     The individual powders are intensively mixed together and packed into size 4 hard gelatin capsules (140 mg per capsule). 
     b) Capsules containing 230 mg of citric acid 
     Composition 
     1 capsule contains (mg) 
     
         ______________________________________(1) Pimobendan         5.0(2) Citric acid        230.45(3) Collidone 25       3.78(4) Magnesium stearate 0.77                  240.00  mg______________________________________ 
    
     Preparation 
     (1) and (2) are intensively mixed together and granulated with an alcoholic solution of (3). (4) is added to the dried granulate. The final mixture thus obtained is packed into size 1 hard gelatin capsules (240 mg per capsule). 
     Note 
     Active substance and acid are present together in one and the same granulate. 
     Measurement of rate of dissolution: analogous to Example 1a. 
     Result 
     After 5 min: 100% pimobendan dissolved. 
     c) Capsules containing207 mg of citric acid composition: 
     1 capsule contains (mg) 
     
         ______________________________________(1) Pimobendan, finely ground                5.0(2) Citric acid      206.5(3) Microcrystalline cellulose                40.0(4) Aerosil 130 V    11.0(5) Collidone 25     4.0(6) Magnesium stearate                1.5                268.0______________________________________ 
    
     Preparation 
     (1) is triturated with (2). (3) and (4) are added to the triturated material. The mixture is granulated with an alcoholic solution of (5). (6) is mixed into the dry granulate. The finished mixture is packed into size 1 capsules (268 mg per capsule). 
     Note 
     Active substance and acid are present in one and the same granulate. 
     Measurement of rate of dissolution: analogous to Example 1a. 
     Result 
     After 5 min.: 84.1% 
     10 min.: 90.2% 
     15 min.: 91.7% 
     30 min.: 92.5% pimobendan dissolved. 
     EXAMPLE 4 
     Capsules Containing 2.5 Mg of Pimobendan 
     Capsules Containing 209 Mg of Citric Acid 
     Composition 
     1 capsule contains (mg) 
     
         ______________________________________(1) Pimobendan       2.5(2) Powdered citric acid                209.0(3) Microcrystalline cellulose                40.0(4) Silicon dioxide  11.0(5) Polyvinylpyrrolidone                4.0(6) Magnesium stearate                1.5                268.0______________________________________ 
    
     Preparation 
     Analogous to Example 3c. 
     Note 
     Active substance and acid are present in one and the same granulate. 
     Measurement of rate of dissolution: analogous to Example 1a. 
     Result 
     After 15 min.: 96.5% 
     30 min.: 99.1% pimobendan dissolved. 
     EXAMPLE 5 
     Film Coated Tablet Containing 2.5 Mg of Pimobendan 
     Tablets Containing 50 Mg of Citric Acid 
     Composition 
     1 tablet contains (mg) 
     
         ______________________________________(1) Pimobendan           2.5(2) Powdered anhydrous citric acid                    50.0(3) Avicel PH 101        13.0(4) Anhydrous calcium hydrogen phosphate                    15.0(5) Undried corn starch  6.0(6) Collidone 25         0.5(7) Insoluble polyvinylpyrrolidone                    59.0(8) Compritol 888        3.0(9) Magnesium stearate   1.0                    150.0______________________________________ 
    
     Preparation 
     The preparation is as described in Example 2b, but the finished mixture is compressed into biconvex tablets. These are coated with 5 mg of hydroxypropylmethylcellulose per tablet. 
     Note 
     Active substance and acid are together in the same granulate. 
     Measurement of rate of dissolution: analogous to Example 1a. 
     Results 
     After 10 min.: 76.8% 
     30 min.: 86.1% pimobendan dissolved.