Patent Publication Number: US-2005131040-A1

Title: Process for the preparation of crystalline losartan potassium

Description:
FIELD OF THE INVENTION  
      The present invention relates to a process for the preparation of crystalline losartan potassium and crystalline hydrate losartan potassium, comprising the crystallization of losartan potassium from an aprotic solvent.  
     TECHNOLOGICAL BACKGROUND  
      Losartan potassium, namely {2-butyl-5-chloro-3-[2′-(2H-tetrazol-5-yl)-biphenyl-4-ylmethyl]-3H-imidazol-4-yl}-methanol potassium salt, of formula (I) is a known angiotensin II antagonist.  
                 
 
      Angiotensin II antagonists are known medicaments used in the treatment of hypertension, anxiety, glaucoma and cardiac attacks. The synthesis of losartan in the free acid form (formula II) or as tetrazolyl-protected derivative is disclosed in U.S. Pat. No. 5,138,069 and WO 93/10106. The advantages provided by pharmaceutical products in the crystalline form in terms of easiness of processes for the preparation of related medicaments are well known. Crystalline compounds are in fact known to be more suited to the formulation of galenic forms, thanks both to their flowability in the form of powders or granulates, and to the surface properties of the crystals which promote adhesion, for example during the preparation of tablets. Furthermore, the solubility of crystalline compounds in aqueous solutions, in particular in the gastric juices, can also be significantly different than that of the corresponding amorphous compounds. There is therefore the need to discriminate between the crystalline and the amorphous forms of biologically active compounds, so as to fulfil the various pharmaceutical requirements.  
      A number of crystalline and amorphous forms of losartan potassium are known from WO 95/17396 and WO 03/048135. According to WO 95/17396, crystalline losartan potassium is prepared by salification of acid losartan with an alkali hydroxide. The losartan potassium aqueous solution is then added to a isopropanol-cyclohexane azeotropic mixture under reflux. Water is then removed by azeotropic distillation of the resulting water-isopropanol-cyclohexane ternary mixture, which boils at 64° C. When the solution is anhydrous, the head temperature raises to 69° C. and losartan potassium crystallizes.  
      U.S. Pat. No. 5,859,258 discloses another crystallization process which comprises dissolution of losartan potassium in isopropanol-water, distillation of the binary azeotrope to an approx. 2.6% water content, precipitation by addition of a losartan potassium suspension in cyclohexane, subsequent distillation of the ternary azeotrope to a water content ranging from 0.02 to 0.11%, and finally drying crystalline losartan potassium under vacuum at a temperature of approx. 45-50° C.  
      The known methods for the preparation of crystalline losartan potassium involve operations which require remarkable precision in the solvents ratios (which ratios are difficult to control on an industrial scale), as well as complex, cumbersome procedures.  
      There is therefore the need for a process for the preparation of crystalline losartan potassium, which is simpler, easier and fulfils the requirements for the production on an industrial scale.  
     SUMMARY OF THE INVENTION  
      An efficient process has been found for a simple, efficient preparation of losartan potassium in the crystalline form I, known from WO 95/17396 (in the following: crystalline form), and in the crystalline hydrate form having PXRD pattern with the characteristic peaks at about 5.7, 8.9, 13.3, 17.5, 20.0 and 21.1±0.2 degrees 2θ as reported in claim  31  of WO 03/048135 (in the following: crystalline hydrate form). 
    
    
     DETAILED DISCLOSURE OF THE INVENTION  
      An object of the invention is a process for the preparation of losartan potassium in the crystalline form or in the crystalline hydrate form, as defined above, comprising the reaction of a dispersion of acid losartan of formula (II), in an organic aprotic solvent, with a potassium basic salt, in the presence of water.  
                 
 
      An organic aprotic solvent is preferably a solvent selected from the group comprising acetone, toluene, acetonitrile and ethyl acetate, more preferably ethyl acetate or toluene, in particular ethyl acetate.  
      A potassium basic salt is for example potassium hydroxide, potassium carbonate or potassium bicarbonate, preferably potassium bicarbonate.  
      The molar ratio of potassium salt to compound of formula (II) ranges from about 0.8 to 1.5, and is preferably about 1.0.  
      The weight ratio of water to potassium basic salt ranges from about 0.1 to 5.0, preferably from about 1.0 to 3.0.  
      The weight ratio of organic aprotic solvent to compound of formula (II) ranges from about 4:1 to 15:1, preferably from about 9:1 to 11:1.  
      According to a preferred embodiment of the invention, losartan potassium in the crystalline hydrate form is obtained by a process comprising: 
          a) salification reaction to obtain losartan potassium;     b) cooling of the losartan potassium solution to precipitate losartan potassium crystalline hydrate;     c) filtration of the mixture to isolate crystalline hydrate losartan potassium;     d) washing of the losartan potassium crystalline hydrate with a solvent in which losartan potassium crystalline hydrate is insoluble; and     e) drying of the losartan potassium crystalline hydrate.        

      According to a further preferred embodiment of the invention, losartan potassium in the crystalline form is obtained by a process comprising: 
          a) salification reaction to obtain losartan potassium;     a′) azeotropic distillation to remove water;     b) cooling of the losartan potassium solution to precipitate crystalline losartan potassium;     c) filtration of the mixture to isolate crystalline losartan potassium;     d) washing of the crystalline losartan potassium with a solvent in which crystalline losartan potassium is insoluble; and     e) drying of the crystalline losartan potassium.        

      Each of the single steps a), b), c), d) and e), respectively, in both preferred embodiments of the invention above, can be similarly performed.  
      According to step a), the reaction is carried out at a temperature ranging from about 15° C. to the reflux temperature of the reaction mixture, preferably approx. from 40 to 80° C.  
      After the azeotropic distillation of step a′), the water content of the product is equal to or lower than 1%.  
      In step b), the losartan potassium solution is cooled to a temperature lower than 0° C., preferably from −2° C. to −5° C.  
      In step d), the solvent can be for example a solvent selected from the group comprising toluene, acetone, acetonitrile and ethyl acetate, in particular the same organic aprotic solvent previously used in step a) above, more preferably ethyl acetate.  
      In step e), drying is carried out preferably under vacuum at a temperature ranging from about 40 to 55° C.  
      The following examples further illustrate the invention.  
     EXAMPLE 1  
     Preparation of Losartan Potassium, Crystalline Hydrate Form  
      A suspension of {2-butyl-5-chloro-3-[2′-(2H-tetrazol-5-yl)-biphenyl-4-ylmethyl]-3H-imidazol-4-yl}-methanol (II) (4.2 g) in ethyl acetate (60 ml) is added with a solution of potassium bicarbonate (1 g) in water (2.8 g), at 50° C. The resulting clear solution is cooled to −2/−5° C. and the precipitate which forms is filtered, washed with 10 ml of ethyl acetate and dried under vacuum at 60° C., thereby obtaining 4.3 g of losartan potassium crystalline hydrate.  
      NMR: ( 1 H, DMSO, 300 mHz): δ 0.80 (3H, t, J=10. CH 3 ), 1.25 (2H, sext, J=10. CH 3 CH 2 ), 1.45 (2H, quin, J=10. CH 3 CH 2 CH 2 ), 2.45-2.55 (2H, m, CH 3 CH 2 CH 2 CH 2 ), 4.25 (2H, d, J=3, CH 2 OH), 5.15-5.25 (3H, m, CH 2 Ar and OH), 6.88 (d, 2H, J=12, ArH), 7.08 (d, 2H, J=12, ArH), 7.23-7.36 (3H, m, ArH), 7.50-7.55 (1H, ArH).  
     EXAMPLE 2  
     Preparation of Losartan Potassium, Crystalline Hydrate Form  
      A suspension of {2-butyl-5-chloro-3-[2′-(2H-tetrazol-5-yl)-biphenyl-4-ylmethyl]-3H-imidazol-4-yl}-methanol (II) (4.2 g) in ethyl acetate (60 ml) is added with a solution of potassium hydroxide (0.5 g) in water (0.5 g), at 50° C. The resulting clear solution is cooled to −2/−5° C. and the precipitate which forms is filtered, washed with 10 ml of ethyl acetate and dried under vacuum at 60° C., thereby obtaining 4.2 g of losartan potassium crystalline hydrate.  
     EXAMPLE 3  
     Preparation of Losartan Potassium, Crystalline Form  
      A suspension of {2-butyl-5-chloro-3-[2′-(2H-tetrazol-5-yl)-biphenyl-4-ylmethyl]-3H-imidazol-4-yl}-methanol (II) (4.2 g) in ethyl acetate (60 ml) is added with a solution of potassium bicarbonate (1 g) in water (2.8 g), at 50° C. When the mixture turns to a clear solution, 30 ml of solvent are distilled off, then the solution is cooled to −2/−5° C. The resulting precipitate is filtered, washed with 10 ml of ethyl acetate and dried under vacuum at 60° C., thereby obtaining 4.3 g of losartan potassium crystalline form.  
     EXAMPLE 4  
     Preparation of Losartan Potassium, Crystalline Form  
      A suspension of {2-butyl-5-chloro-3-[2′-(2H-tetrazol-5-yl)-biphenyl-4-ylmethyl]-3H-imidazol-4-yl}-methanol (II) (4.2 g) in toluene (60 ml) is added with a solution of potassium bicarbonate (1 g) in water (2.8 g), at 50° C. 30 ml of solution are distilled off. The mixture is cooled to −2/−5° C. and the resulting precipitate is filtered, washed with 10 ml of toluene and dried under vacuum at 60° C., thereby obtaining 4.4 g of losartan potassium, crystalline form.