Abstract:
The present invention provides novel crystalline polymorphs of enantiopure Erdosteine, referred to as Form I and Form II, and processes for the preparation thereof.

Description:
This application is a U.S. national stage of PCT/EP2008/009499 filed on Nov. 10, 2008, which claims priority to and the benefit of European Application No. 07022263.3 filed on Nov. 16, 2007, the contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates in general to Erdosteine, in particular to enantiopure Erdosteine in the form of crystalline polymorphs and to processes for the preparation thereof. 
     BACKGROUND OF THE INVENTION 
     Erdosteine, (chemical name 2-[N-3-(2-oxotetrahydrothienyl)]acetamido)-thioglycolic acid) of formula I was first disclosed in FR 2,502,153 and U.S. Pat. No. 4,411,909. 
     
       
                 
         
             
             
         
      
     
     Racemic R,S-Erdosteine is used in the therapy of diseases of the respiratory tract thanks to its mucolytic properties. The racemate exists in a single solid-state form, whose physical characterization is reported in table 1 below. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Characterization of Racemic Erdosteine 
               
             
          
           
               
                   
                 X-ray powder 
                   
                   
               
               
                   
                 diffraction pattern 
                 Infrared absorption 
               
               
                   
                 2theta (deg) 
                 peaks 
                 DSC heating curve 
               
               
                   
                 (FIG. VII) 
                 (FIG. VIII) 
                 (FIG. IX) 
               
               
                   
                   
               
               
                   
                 18.2 
                 1796 cm −1   
                 Melting peak in 
               
               
                   
                 19.7 
                 1740 cm −1   
                 range 141-168° C. 
               
               
                   
                 20.1 
                 1686 cm −1   
                 Onset at 156° C. 
               
               
                   
                 21.5 
                 1675 cm −1   
                 Small shoulder at 
               
               
                   
                 25.9 
                 1609 cm −1   
                 155° C. 
               
               
                   
                 27.4 
                 1561 cm −1   
               
               
                   
                 28.4 
                 1162 cm −1   
               
               
                   
                 31.3 
               
               
                   
                 32.8 
               
               
                   
                 36.7 
               
               
                   
                   
               
             
          
         
       
     
     A drawback associated to the use of racemic Erdosteine is its poor flowability and solubility, which negatively affects manufacturing processes of finished pharmaceutical forms and bioavailability. In fact, when a powder does not flow easily, glidants need to be added in order to prepare solid pharmaceutical forms such as tablets or capsules. 
     Other important properties of solid pharmaceutical forms are the solubility and the dissolution rate in aqueous fluids (in particular in gastric juices); these properties influence the dosage and the bioavailability of active principles. 
     In the pharmaceutical field, polymorphs of active principles often allow to improve the manufacturing process of finished pharmaceutical forms. Both flowability and solubility depend in fact on the solid state form of the active principle, which is in turn influenced by the conformation and orientation of the molecules in the unit crystalline cell. The unit cell defines a particular polymorph and may give rise to a specific thermal behaviour, X-ray crystallographic pattern and infrared absorption. Each polymorph may thus possess specific solid-state properties like flowability and dissolution rate. 
     Thus, it would be desirable to provide polymorphs of Erdosteine and processes for the preparation thereof, in order to overcome its poor flowability and solubility. 
     DESCRIPTION OF THE INVENTION 
     It has now been found that pure Erdosteine enantiomers, i.e. R-Erdosteine (formula 2 below) and S-Erdosteine (Formula 3 below) exhibit a polymorphic behaviour, each giving rise to two different polymorphs, herein after referred to as Form I and Form II (polymorph I and II), characterized by a specific thermal behaviour, X-ray diffraction pattern and infrared absorption. 
     
       
                 
         
             
             
         
      
     
     Thus, in one embodiment, the invention provides a crystalline polymorph of substantially enantiomerically pure Erdosteine, referred to as Form I, characterized by the following physical properties:
         X-ray powder diffraction pattern with peaks at about 17.0, 20.1, 22.8, 23.4, 27.3, 28.0, 29.1, 30.6, 33.2±0.2 degrees two-theta, substantially as depicted in  FIG. 1 ;   infrared absorption spectrum with peaks at about 1740, 1692, 1683, 1607 and 1576 cm-1, substantially as depicted in  FIG. 2 ;   melting point at about 112-134° C. (onset at 123° C.), with a DSC thermogram substantially as depicted in  FIG. 3 .       

     These data are reported in Table 2. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Characterization of Erdosteine Form I 
               
             
          
           
               
                   
                 X-ray powder 
                   
                   
               
               
                   
                 diffraction pattern 
                 Infrared absorption 
               
               
                   
                 2theta (deg) 
                 peaks 
                 DSC heating curve 
               
               
                   
                 (FIG. 1) 
                 (FIG. 2) 
                 (FIG. 3) 
               
               
                   
                   
               
               
                   
                 17.0 
                 1740 cm −1   
                 Melting peak in 
               
               
                   
                 20.1 
                 1692 cm −1   
                 range 112-134° C. 
               
               
                   
                 22.8 
                 1683 cm −1   
                 Onset at 123° C. 
               
               
                   
                 23.4 
                 1607 cm −1   
               
               
                   
                 27.3 
                 1576 cm −1   
               
               
                   
                 28.0 
               
               
                   
                 29.1 
               
               
                   
                 30.6 
               
               
                   
                 33.2 
               
               
                   
                   
               
             
          
         
       
     
     Enantiomerically pure R- or S-Erdosteine Form I can be prepared by means of a process which comprises the following steps:
         a) providing a solution of enantiomerically pure R- or S-Erdosteine in an organic solvent, preferably acetone;   b) adding gaseous or aqueous ammonia to precipitate enantiomerically pure R- or S-Erdosteine ammonium salt;   c) recovering the ammonium salt;   d) dissolving the ammonium salt in water;   e) acidifying the resulting water solution with hydrochloric acid;   f) cooling the acidified water solution so as to precipitate enantiomerically pure R- or S-Erdosteine Form I;   g) recovering enantiomerically pure R- or S-Erdosteine Form I.       

     According to a preferred embodiment of the invention, the process is conducted at a temperature between 0 and 50° C., more preferably at room temperature and enantiomerically pure Erdosteine Form I is recovered in step f) by cooling the solution to a temperature of about 0 to 10° C. 
     In a second embodiment, the invention provides a crystalline form of substantially enantiomerically pure R- or S-Erdosteine, referred to as Form characterized by the following physical properties:
         X-ray powder diffraction pattern with peaks at about 18.3, 24.7, 28.9, 31.3, 32.3, 32.7, 36.0, 36.3, 37.2±0.2 degrees two-theta, substantially as depicted in  FIG. 4 ;   infrared absorption spectrum with peaks at about 1708, 1692, 1644, 1538 and 1189 cm-1, substantially as depicted in  FIG. 5 ;   melting point at about 103-142° C. (onset at 126° C.), with a DSC thermogram substantially as depicted in  FIG. 6 .       

     These data are reported in Table 3. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Characterization of Erdosteine Form II 
               
             
          
           
               
                   
                 X-ray powder 
                   
                   
               
               
                   
                 diffraction pattern 
                 Infrared absorption 
               
               
                   
                 2theta (deg) 
                 peaks 
                 DSC heating curve 
               
               
                   
                 (FIG. 4) 
                 (FIG. 5) 
                 (FIG. 6) 
               
               
                   
                   
               
               
                   
                 18.3 
                 1708 cm −1   
                 Melting peak in 
               
               
                   
                 24.7 
                 1692 cm −1   
                 range 103-142° C. 
               
               
                   
                 28.9 
                 1644 cm −1   
                 Onset at 126° C. 
               
               
                   
                 31.3 
                 1538 cm −1   
               
               
                   
                 32.3 
                 1189 cm −1   
               
               
                   
                 32.7 
               
               
                   
                 36.0 
               
               
                   
                 36.3 
               
               
                   
                 37.2 
               
               
                   
                   
               
             
          
         
       
     
     In another aspect, the present invention provides a process for preparing crystalline enantiomerically pure R- or S-Erdosteine Form II, comprising the following steps:
         a) providing a solution of enantiomerically pure or enriched R- or S-Erdosteine in aqueous diluted ammonia;   b) acidifying the solution by addition of concentrated hydrochloric acid, optionally in the presence of a crystal seed of a previously prepared sample of R- or S-Erdosteine Form II;   c) cooling the solution;   d) recovering enantiomerically pure Erdosteine Form II.       

     Preferably, the process is conducted at a temperature between 0 and 50° C., more preferably at room temperature. 
     According to a preferred embodiment, enantiomerically pure Erdosteine Form II is recovered in step c) by cooling the solution to a temperature of about 0 to 10° C. 
     The polymorphs of the invention possess advantageous chemical-physical properties in respect of the racemic amorphous form, in particular in terms of flowability and dissolution rate. These advantages will be clearer by considering the following comparison between Form I of S-Erdosteine and the amorphous racemic form. The test for the evaluation of flowability showed that racemic Erdosteine does not flow through the funnel. Conversely, Form I has a mean flowing time of about 35 seconds, which allows to handle and transfer the powder more easily, and to avoid the use or reduce the amount of glidants in the finished pharmaceutical formulation. 
     This comparison is reported in Table 4. 
     
       
         
               
             
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Flowability comparison 
               
             
          
           
               
                   
                 Flowing time (mean value) 
               
               
                   
                   
               
             
          
           
               
                   
                 Racemic 
                 ∞ 
               
               
                   
                 Erdosteine 
               
               
                   
                 Erdosteine Form I 
                 35.6″ 
               
               
                   
                   
               
             
          
         
       
     
     A dissolution rate test was performed in order to define the Intrinsic Dissolution Rate of Erdosteine Form I and Racemic Erdosteine. Six thin 150 mg tablets of S-erdosteine Form I and six 150 mg tablets of racemic Erdosteine were prepared by compression with a 10 ton press and their dissolution profile in 0.1M hydrochloric acid was evaluated. 
     The two profiles are reported in  FIG. 10 , superimposed on the same graph. As can be clearly seen, the dissolution profile of Form I is very different from that of racemic, non-polymorphic Erdosteine. In fact, after 15 minutes more than 80% Form I is dissolved, while more that 25 minutes are necessary to dissolve 40% racemic Erdosteine. The higher solubility of Form I allows to obtain an improved pharmaceutical form in terms of dosage and bioavailability. 
     The invention is now illustrated in greater detail in the examples below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a characteristic X-ray powder diffraction spectrum of S-Erdosteine and R-Erdosteine Form I; 
         FIG. 2  is a characteristic infrared (IR) absorption spectrum of S-Erdosteine and R-Erdosteine Form I; 
         FIG. 3  is a characteristic DSC thermogram of S-Erdosteine and R-Erdosteine Form I; 
         FIG. 4  is a characteristic X-ray powder diffraction spectrum of S-Erdosteine and R-Erdosteine Form II; 
         FIG. 5  is a characteristic infrared (IR) absorption spectrum of S-Erdosteine and R-Erdosteine Form II; 
         FIG. 6  is a characteristic DSC thermogram of S-Erdosteine and R-Erdosteine Form II; 
         FIG. 7  is a characteristic X-ray powder diffraction spectrum of R,S-Erdosteine; 
         FIG. 8  is a characteristic infrared (IR) absorption spectrum of IRS-Erdosteine; 
         FIG. 9  is a characteristic DSC thermogram of R,S-Erdosteine; 
         FIG. 10  is a comparison between the dissolution of S-Erdosteine Form I and Racemic Erdosteine. 
     
    
    
     EXAMPLES 
     Example 1 
     Preparation of S-Erdosteine Form I 
     Enantiomerically pure S-Erdosteine (300 g) is dissolved in acetone (5 L). 28% Aqueous ammonia (85 mL) is added, thus precipitating the ammonium salt of the desired product, which is filtered and dried under vacuum. The resulting solid is dissolved in water (1 L) and acidified to pH 3 with the addition of 37% hydrochloric acid. S-Erdosteine Form I thus obtained is filtered and dried under vacuum to obtain 250 g of pure product. 
     Example 2 
     Preparation of S-Erdosteine Form II 
     Enantiomerically pure S-Erdosteine (60 g) is slurried in water (60 mL) and 28% aqueous ammonia is added until pH 5.7. The clear solution is acidified to pH 3 by addition of 37% hydrochloric acid, until the pH reaches about 4. The obtained slurry is cooled at 0° C. for 6 hours and S-Erdosteine Form II thus obtained is filtered and dried under vacuum to obtain 28 g of pure product.