Abstract:
The present invention provides a novel stable amorphous form of carvedilol dihydrogen phosphate and the process for its preparation that involves reaction of carvedilol base with ortho phosphoric acid in the presence of stabilizer in a suitable solvent or mixture of solvents followed by concentration and isolation. An alternate process for preparation of amorphous form of carvedilol dihydrogen phosphate involves addition of stabiliser to the solution of stable amorphous or crystalline carvedilol dihydrogen phosphate in a suitable solvent or mixture of solvents followed by concentration and isolation. The novel stable amorphous form of carvedilol dihydrogen phosphate is highly stable.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a novel stable amorphous form of carvedilol dihydrogen phosphate that is highly stable and process for its preparation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Carvedilol has structure as shown in formula 1. Carvedilol is disclosed in U.S. Pat. No. 4,503,067 (assigned to Boehringer Mannheim, GmbH, Germany) and it is chemically known as (±)-1-(9H-carbazol-4-yloxy)-3-[[2(2-methoxyphenoxy)ethyl]amino]-2-propanol. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0003]    Carvedilol is a racemic mixture of R(+) and S(−) enantiomers. Both enantiomers are nonselective β-adrenergic blocking agent with α 1  blocking activity while S(−) enantiomer also has non-selective β-adrenoreceptor blocking activity. Carvedilol is used for treatment of hypertension, congestive heart failure and angina. 
         [0004]    There are several patents and patent applications that are directed to crystalline salts and solvates thereof and also to their preparation. 
         [0005]    The product U.S. Pat. No. 4,503,067 (Boehringer Mannheim) describes salts of carvedilol with acids such as hydrochloric acid, hydrobromic acid, phosphoric acid, sulphuric acid, acetic acid, citric acid, maleic acid or benzoic acid. 
         [0006]    The U.S. Pat. No. 6,515,010 covers carvedilol methane sulfonate. 
         [0007]    Another U.S. Pat. No. 7,056,942 assigned to Teva discloses crystalline carvedilol hydrochloride hydrate. 
         [0008]    The patent application US 2005/240,027 (assigned to SmithKline Beecham) disclose crystalline carvedilol dihydrogen phosphate hemihydrate, carvedilol dihydrogen phosphate dihydrate, carvedilol dihydrogen phosphate, carvedilol dihydrogen phosphate methanol solvate and carvedilol hydrogen phosphate. 
         [0009]    In the patent application US 2005/277,689 (assigned to GlaxoSmithKline) crystalline salt, forms or solvate of carvedilol selected from the group consisting of carvedilol mandelate, lactate, maleate, sulfate, glutarate, mesylate, phosphate, citrate, hydrogen bromide, oxalate, hydrochloride, benzoate and corresponding forms or solvates thereof are described. 
         [0010]    The patent application US 2005/261,355 (assigned to SB Pharmco) covers carvedilol hydrobromide monohydrate and carvedilol hydrobromide. 
         [0011]    The patent application US 2005/148,779 (GlaxoSmithKline) claims crystalline carvedilol monocitrate monohydrate 
         [0012]    We have filed an Indian patent application 119/KOL/2007 dated 31 Jan. 2007 in Indian Patent Office on amorphous form of carvedilol dihydrogen phosphate and the process for its preparation. The amorphous form obtained by the process described in this application has glass transition temperature below 60° C. The amorphous solids in general having low glass transition temperature not only create complications during formulation but also has stability related problems on storage. The glass transition temperature is the temperature, below which the physical properties of amorphous materials vary in a manner similar to those of a crystalline phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). A materials glass transition temperature is the temperature below which molecules have little relative mobility. Thus, there exist a need to develop stable amorphous form of carvedilol dihydrogen phosphate which have improved characteristics. 
         [0013]    The synthetic exploration of novel crystalline forms, amorphous form and/or solvates thereof of a pharmaceutically active compound provides a possibility to a organic chemist, to obtain a new form or solvate that has improved characteristics such as bulk density, particle size, stability, solubility in aqueous solution and ease of processing in the formulation for preparing suitable pharmaceutical dosages. 
         [0014]    In case of carvedilol dihydrogen phosphate, the patent as well publication literature lack in providing information on stable amorphous form of carvedilol dihydrogen phosphate, thereby indicating poor attention given by the synthetic chemists for its isolation and preparation. The present invention is directed to the stable amorphous form of carvedilol dihydrogen phosphate and to also to the development of process for its preparation. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention provides a stable amorphous form of carvedilol dihydrogen phosphate and process for its preparation. The process for the preparation of stable amorphous form of carvedilol dihydrogen phosphate comprises of:
       (a) preparation of a solution of carvedilol base in a suitable solvent or mixture of solvents,   (b) addition of stabiliser,   (c) addition of ortho phosphoric acid,   (d) removal of solvent, and   (e) isolation of solid.       
 
         [0021]    Another process for the preparation of stable amorphous form of carvedilol dihydrogen phosphate comprises of:
       (a) preparation of a solution of stable amorphous or crystalline carvedilol dihydrogen phosphate in a suitable solvent or mixture of solvents,   (b) addition of stabiliser,   (c) removal of solvent, and   (d) isolation of solid.       
 
         [0026]    Accordingly, the present invention provides a thermodynamically stable amorphous form of carvedilol dihydrogen phosphate having glass transition temperature in the range between 50° C.-176° C. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1 : PXRD of stable amorphous form of carvedilol dihydrogen phosphate obtained by using 5% PVPK-30 as stabiliser. 
           [0028]      FIG. 2 : FT-IR spectrum of stable amorphous form of carvedilol dihydrogen phosphate obtained by using 5% PVPK-30 as stabiliser. 
           [0029]      FIG. 3 : Differential Scanning calorimetry (DSC) of stable amorphous form of carvedilol dihydrogen phosphate obtained by using 5% PVPK-30 as stabiliser. 
           [0030]      FIG. 4 : Thermal Gravimetric Analysis (TGA) of stable amorphous form of carvedilol dihydrogen phosphate obtained by using 5% PVPK-30 as stabiliser. 
           [0031]      FIG. 5 : Stability of stable amorphous form of carvedilol dihydrogen phosphate obtained by using 5% PVPK-30 as stabiliser at 15 days, 1 month and 2 month at 40° C.±2° C. (RH 75%). 
           [0032]      FIG. 6 : PXRD of amorphous form of carvedilol dihydrogen phosphate without stabiliser. 
           [0033]      FIG. 7 : FT-IR spectrum of amorphous form of carvedilol dihydrogen phosphate without stabiliser. 
           [0034]      FIG. 8 : Differential Scanning calorimetry (DSC) of stable amorphous form of carvedilol dihydrogen phosphate obtained by using 5% PVPK-30 as stabiliser. 
           [0035]      FIG. 9 : TGA of amorphous form of carvedilol dihydrogen phosphate obtained without stabiliser. 
           [0036]      FIG. 10 : PXRD of amorphous form of carvedilol dihydrogen phosphate without stabiliser after 1 month at 40° C.±2° C. (RH 75%). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    The inventors of the present patent application have surprisingly and unexpectedly found that a novel stable amorphous form of carvedilol dihydrogen phosphate can be prepared which is highly stable. 
         [0038]    Accordingly, the present invention provides a thermodynamically stable novel form of carvedilol dihydrogen phosphate that is highly stable. 
         [0039]    The novel stable amorphous form of carvedilol dihydrogen phosphate of the present invention is characterized by X-ray diffractogram as shown in  FIG. 1 , FT-IR spectrum as shown in  FIG. 2 , DSC as shown in  FIG. 3  and TGA as shown in  FIG. 4 . The amorphous carvedilol obtained without stabiliser having glass transition temperature 55-56° C. while the stable amorphous form of the present invention has glass transition temperature in the range between 76 to 185° C. The phosphorous content of the stable amorphous form of the present invention is between 18-20% on anhydrous basis. 
         [0040]    The amorphous form of carvedilol dihydrogen phosphate without stabiliser was prepared according to the method described in application 119/KOL/2007 dated 31 Jan. 2007 files in Indian Patent Office which are described in examples 8 and 9 in the experimental section. The amorphous form of carvedilol dihydrogen phosphate obtained without stabiliser is characterized by X-ray diffractogram as shown in  FIG. 6 , FT-IR spectrum as shown in  FIG. 7 , DSC as shown in  FIG. 8  and TGA as shown in  FIG. 9 . 
         [0041]    The PXRD of amorphous form of carvedilol dihydrogen phosphate without stabiliser after 1 month at 40° C.±2° C. (RH 75%) is shown in  FIG. 10 . It shows appearance of several sharp peaks indicating slow conversion of amorphous form in to crystalline form. 
         [0042]    The PXRD of stable amorphous form of carvedilol dihydrogen phosphate obtained with stabiliser 5% PVPK-30 after 15 days, 1 month and 2 month at 40° C.±2° C. (RH 75%) is shown in  FIG. 5 . It does not show sharp peaks that can arise from crystalline form, thus indicating that the amorphous form of carvedilol dihydrogen phosphate obtained with stabiliser is highly stable and does not undergo conversion in to crystalline form. 
         [0043]    Thus, it has been unexpectedly found that the stable amorphous form of carvedilol dihydrogen phosphate can be obtained in a simple and reproducible manner. Therefore, the most preferred embodiment of the present invention is to provide highly stable novel amorphous form of carvedilol dihydrogen phosphate. 
         [0044]    In an another preferred embodiment, the present invention provide a process for the preparation of stable amorphous form of carvedilol dihydrogen phosphate comprises of:
       (a) preparation of a solution of carvedilol base in a suitable solvent or mixture of solvents,   (b) addition of stabiliser,   (c) addition of ortho phosphoric acid,   (d) removal of solvent, and   (e) isolation of solid.       
 
         [0050]    Generally, the aqueous or ortho phosphoric acid is added to the solution of carvedilol base in a suitable solvent. The addition may be performed at a temperature 0-50° C., preferably at 25-30° C. The quantity of carvedilol base and ortho phosphoric acid may be molar equivalent or ortho phosphoric acid may be in slight molar excess. 
         [0051]    The present invention further provide an another process for the preparation of stable amorphous form of carvedilol dihydrogen phosphate comprises of:
       (a) preparation of a solution of stable amorphous or crystalline carvedilol dihydrogen phosphate in a suitable solvent or mixture of solvents,   (b) addition of stabiliser,   (c) removal of solvent, and   (d) isolation of solid.       
 
         [0056]    The term “suitable solvent” in either of the processes described above includes any protic or aprotic polar solvent or mixture of solvents in which carvedilol is soluble, for example, lower alkanol, dihydroxy solvents, lower aliphatic ketone, and water miscible cyclic and acyclic ethers, cyclic and acyclic amides and nitriles. 
         [0057]    The examples of lower alkanol include methanol, ethanol, n-propanol and isopropanol. Dihydroxy solvent such as ethylene glycol and diethylene glycol can also be used. The examples of aliphatic ketone include those acetone, 2-butanone and 4-methyl pentanone. The examples of cyclic ethers include tetrahydrofuran and 1,4-dioxane. The examples of cyclic and acyclic amides include formamide, acetamide, propanamide and 2-methyl pyrrolidinone. The examples of nitrile solvents include acetonitrile and propionitrile. Other polar aptotic solvents like diethylene glycol dimethyl ether, dimethyl sulfoxide, sulfolane can also be useful solvents. The process may also be carried out by using one or two solvents. The preferred suitable solvent is lower alkanol such as methanol, ethanol, propanol and isopropanol and the most preferred solvent is methanol or ethanol. 
         [0058]    In either of the process variety of the compounds can be used as stabiliser such as polyvinyl pyrrolidone K-30 (PVPK K-30); 5% Eudragit RSPO, 5% Klucef EXF, 5% Polyethylene glycol-6000 (5% PEG-6000), 5% Polyethylene glycol-4000 (5% PEG-4000), 5% Ethyl cellulose, 5% Hydroxypropyl methyl cellulose (HPFC E-5) and the like. 
         [0059]    The quantity of stabiliser can be used in the range 0.01 to 0.1 weight by weight, most preferably 0.05 times weight by weight with respect to carvedilol base or amorphous/crystalline carvedilol dihydrogen phosphate. Stabiliser can added to the solution at temperature 0-250° C., preferably at 50-75° C., most preferably at 50-55° C. 
         [0060]    The removal of the solvent before isolation of solid in either of the processes described above is accomplished by techniques which include distillation at atmospheric pressure, distillation under reduced pressure, evaporation, spray drying, freeze drying and agitation thin film evaporation. Moreover, the product obtained may be further dried to achieve the desired moisture values or desired residual solvent values because of the higher glass transition temperature which is in the range of 70-176° C., preferably is in the range of 80-90° C. 
         [0061]    The thermodynamically stable forms are always preferred for the formulation because of their advantages. Due to high thermodynamic stability the stable amorphous form of carvedilol dihydrogen phosphate of the present invention is useful for pharmaceutical formulation. The effective amount of stable amorphous form of carvedilol dihydrogen phosphate can be used to prepare pharmaceutical composition in association with one or more non toxic pharmaceutically acceptable carriers and/or diluents thereof, and if desired, other active ingredients, which may be administered orally, intravascularly, intraperitoneally, subcutaneously, intramuscularly or topically for the treatment of hypertension, congestive heart failure and angina in a mammal in need thereof. 
         [0062]    The present invention is illustrated by the following representative examples and are not intended to limit the scope of the invention. 
       EXAMPLES 
       [0063]    The powder X-ray diffraction spectrum is measured using Philips (PAN alytical X′ pert pro) diffractogram (copper anti cathode) and expressed in terms of inter planar distance d, Bragg&#39;s angle 2 theta, intensity and relative intensity (expressed as a percentage of the most intense peak). The scanning parameters included: measurement range: 3-40 degrees two theta; continuous scan. 
         [0064]    The FTIR spectra were obtained using a Perkin-Elmer, Spectrum-100 instrument. 
         [0065]    The thermogravimetric analysis (TGA) was done using Perkin Elmer Pyris 1 TGA instrument. 
       Preparation of Stable Amorphous Carvedilol Dihydrogen Phosphate with Stabiliser 
     Example 1 
     Preparation of Stable Amorphous Carvedilol Dihydrogen Phosphate Using 5% PVPK-30 as Stabiliser 
       [0066]    Mixture of 100 g of carvedilol base and 2000 ml methanol was heated at 50-55° C. to get clear solution. To the solution, 274 g of 88% ortho phosphoric acid was added dropwise. To the solution, 5 g of PVPK-30 was added. The solvent was distilled off under reduced pressure. Residual solid was scratched to get 120 g of free flowing stable amorphous solid. 
       Example 2 
     Preparation of Stable Amorphous Carvedilol Dihydrogen Phosphate Using 5% Eudragit RSPO as Stabiliser 
       [0067]    Mixture of 10 g of carvedilol base and 200 ml methanol was heated at 50-55° C. to get clear solution. To the solution, 2.74 g of 88% ortho phosphoric acid was added dropwise. To the solution, 0.5 g of 5% Eudragit RSPO was added. The solvent was distilled off under reduced pressure. Residual solid was scratched to get 119 g of free flowing stable amorphous solid. 
       Example 3 
     Preparation of Stable Amorphous Carvedilol Dihydrogen Phosphate Using 5% Klucef EXF as Stabiliser 
       [0068]    Mixture of 10 g of carvedilol base and 200 ml methanol was heated at 50-55° C. to get clear solution. To the solution, 2.74 g of 88% ortho phosphoric acid was added dropwise. To the solution, 0.5 g of 5% Klucef EXF was added. The solvent was distilled off under reduced pressure. Residual solid was scratched to get 122 g of free flowing stable amorphous solid. 
       Example 4 
     Preparation of Stable Amorphous Carvedilol Dihydrogen Phosphate Using 5% PEG-6000 as Stabiliser 
       [0069]    Mixture of 10 g of carvedilol base and 200 ml methanol was heated at 50-55° C. to get clear solution. To the solution, 2.74 g of 88% ortho phosphoric acid was added dropwise. To the solution, 0.5 g of 5% PEG-6000 was added. The solvent was distilled off under reduced pressure. Residual solid was scratched to get 117 g of free flowing stable amorphous solid. 
       Example 5 
     Preparation of Stable Amorphous Carvedilol Dihydrogen Phosphate Using 5% Ethyl Cellulose as Stabiliser 
       [0070]    Mixture of 10 g of carvedilol base and 200 ml methanol was heated at 50-55° C. to get clear solution. To the solution, 2.74 g of 88% ortho phosphoric acid was added dropwise. To the solution, 0.5 g of 5% ethyl cellulose was added. The solvent was distilled off under reduced pressure. Residual solid was scratched to get 120 g of free flowing stable amorphous solid. 
       Example 6 
     Preparation of Stable Amorphous Carvedilol Dihydrogen Phosphate Using 5% PEG-4000 as Stabiliser 
       [0071]    Mixture of 10 g of carvedilol base and 200 ml methanol was heated at 50-55° C. to get clear solution. To the solution, 2.74 g of 88% ortho phosphoric acid was added dropwise. To the solution, 0.5 g of 5% PEG-4000 was added. The solvent was distilled off under reduced pressure. Residual solid was scratched to get 121 g of free flowing stable amorphous solid. 
       Example 7 
     Preparation of Stable Amorphous Carvedilol Dihydrogen Phosphate Using 5% HPMC E-5 as Stabiliser 
       [0072]    Mixture of 10 g of carvedilol base and 200 ml methanol was heated at 50-55° C. to get clear solution. To the solution, 2.74 g of 88% ortho phosphoric acid was added dropwise. To the solution, 0.5 g of 5% HPMC E-5 was added. The solvent was distilled off under reduced pressure. Residual solid was scratched to get 118 g of free flowing stable amorphous solid. 
       Example 8 
     Preparation of Stable, Amorphous Carvedilol Dihydrogen Phosphate Using Spray Dryer 
       [0073]    Carvedilol base (10 g) was dissolved in methanol (500 mL). To the solution, ortho phosphoric acid (2.9 g, 88% aqueous solution,) was added. The solution was subjected to spray drying to afford 5 g of stable amorphous carvedilol dihydrogen phosphate melting at 62-63° C. having 4.1% moisture content. 
       Example 9 
     Conversion of Crystalline Carvedilol Dihydrogen Phosphate into Stable Amorphous Carvedilol Dihydrogen Phosphate 
       [0074]    Crystalline carvedilol dihydrogen phosphate (10 g) was dissolved in methanol (750 ml) and the solution was spray dried to afford 4.7 g of stable amorphous carvedilol dihydrogen phosphate. 
       Example 10 
     Preparation of Stable Amorphous Carvedilol Dihydrogen Phosphate 5% PVPK-30 as Stabiliser 
       [0075]    Mixture of 10 g of stable amorphous carvedilol dihydrogen phosphate obtained in example 8 and 200 ml methanol was heated at 50-55° C. to get clear solution. To the solution, 2.74 g of 88% ortho phosphoric acid was added dropwise. To the solution, 0.5 g of 5% PVPK-30 was added. The solvent was distilled off under reduced pressure. Residual solid was scratched to get 9.2 g of free flowing stable amorphous solid. 
       Example 11 
     Preparation of Stable Amorphous Carvedilol Dihydrogen Phosphate Using 5% PVPK-30 as Stabiliser 
       [0076]    Mixture of 10 g of crystalline carvedilol dihydrogen phosphate hemihydrate and 200 ml methanol was heated at 50-55° C. to get clear solution. To the solution, 2.74 g of 88% ortho phosphoric acid was added dropwise. To the solution, 0.5 g of 5% PVPK-30 was added. The solvent was distilled off under reduced pressure. Residual solid was scratched to get 9.1 g of free flowing stable amorphous solid.