Abstract:
The invention relates to the β polymorph of Aztreonam, which contains less than 2.5% by weight residual solvent and to a process of making said polymorph.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/470,788, filed May 15, 2003, the content of which is incorporated herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to a crystalline, anhydrous β polymorph of [3S-[3α(Z),4β]]-3-[[(2-amino-4-thiazolyl)[(1-carboxy-1-methylethoxy)imino]acetyl]amino]-4-methyl-2-oxo-1-azetidine-sulfonic acid (Aztreonam), having less than 2.5% by weight residual solvent and to a method for making said polymorph.  
       BACKGROUND OF THE INVENTION  
       [0003]     U.S. Pat. No. 4,946,838, assigned to E. R. Squibb &amp; Sons, Inc., discloses a crystalline anhydrous form of the antibacterial agent, [3S-[3α(Z),4β]]-3-[[(2-amino-4-thiazolyl)[(1-carboxy-1-methylethoxy)imino]acetyl]amino]-4-methyl-2-oxo-1-azetidinesulfonic acid (Aztreonam). This crystalline, anhydrous form is designated as the β polymorph. According to the &#39;838 patent, its main advantages compared to the α-polymorph are higher thermal stability, flowability and lower hygroscopicity. The new polymorph is characterized by X-ray powder diffraction.  
         [0004]     Polymorph β can be obtained by crystallization of α-Aztreonam from anhydrous solvents such as alcohols, most preferably ethanol or by the precipitation of the Aztreonam salt with an organic amine in the presence of HCl. (see U.S. Pat. No. 4,826,973). U.S. Pat. No. 4,826,973, assigned to E. R. Squibb &amp; Sons, Inc., discloses, in Table II, that the crystals of the β-form contain 0-1% water and 2.5% ethanol. This relatively high solvent content makes this form unsuitable for pharmaceutical formulations.  
       SUMMARY OF THE INVENTION  
       [0005]     The invention provides a crystalline, anhydrous β polymorph of [3S-[3α(Z),4β]]-3-[[(2-amino-4-thiazolyl)[(1-carboxy-1-methylethoxy)imino]acetyl]amino]-4-methyl-2-oxo-1-azetidine-sulfonic acid (Aztreonam), having less than 2.5% by weight residual solvent. Preferably, the β polymorph of Aztreonam has less than 1% by weight residual solvent. Most preferably, the β polymorph of Aztreonam has less than 0.5% by weight residual solvent.  
         [0006]     The invention further provides a process for preparing a crystalline, anhydrous β polymorph of Aztreonam, which entails dissolving Aztreonam in an organic solvent at a temperature of between about −60° C. to about 5° C. in the presence of a base to form a clear solution, and adding acid to the solution while stirring efficiently until a precipitate is obtained. Preferably, the solution is seeded with β Aztreonam, while the efficient stirring continues. Preferably the precipitate is reslurried in ethanol.  
         [0007]     Preferably, the obtained β Aztreonam polymorph contains less than 2.5% by weight residual solvent. Most preferably, the β polymorph of Aztreonam has less than 1% by weight residual solvent. Most preferably, the β polymorph of Aztreonam has less than 0.5% by weight residual solvent. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]     The invention provides a crystalline, anhydrous β polymorph of Aztreonam, having less than 2.5% by weight residual solvent. Preferably, the β polymorph of Aztreonam has less than 1% by weight residual solvent. Most preferably, the β polymorph of Aztreonam has less than 0.5% by weight residual solvent. The term “solvent” means organic solvent, such as a C 1-6  alcohol, preferably absolute C 1-6  alcohol, e.g., ethanol, methanol, isopropanol, etc. Preferably the organic solvent is absolute ethanol. The solvent may be hydrated or anhydrous.  
         [0009]     The term “residual solvent” refers to the solvent used in the reaction.  
         [0010]     Applicants observed that the crystallization of β-Aztreonam (as described in the patents mentioned above) is different from the “normal” crystallization process. The precipitation of its crystals starts spontaneously in the coincidence of two independent factors. One factor is the concentration of Aztreonam in the solution, the other is the water content of the solution. The former should be high enough and the latter should be low enough in order to induce crystallization of this polymorph. These two parameters are gradually and uncontrollably approaching their limit-value parallel with the dissolution of crude Aztreonam. The precipitation is usually very quick after reaching this critical point. The effect of temperature on crystallization was also observed. In particular, at higher temperature precipitation becomes considerably faster. Because of this rapid growth of crystals, the formation of agglomerates occurs spontaneously as well. The agglomerates are suspected of being responsible for the high solvent content of this polymorph.  
         [0011]     Based on these observations, Applicants aimed to modify the crystallization of β-Aztreonam, i.e., to decrease the rate of precipitation and thus hinder the formation of agglomerates. The precipitation rate was successfully decreased by using a low crystallization temperature, preferably 5° C. or less. When temperatures of 5° C. or lower are used, crystallization is slow enough to obtain a low residual solvent content in the final product. The time of crystallization is typically 16-24 h at 5° C. and it is even longer at lower temperatures.  
         [0012]     The other important factor that affects the residual solvent content is the rate of stirring. In the absence of stirring or using slow and inefficient stirring, a high solvent content was detected in the product. For example, efficient stirring in a 1 L glass reactor is achieved at a rate of at least 500 rpm (more preferably 700 rpm) using a large diameter paddle stirrer.  
         [0013]     The β polymoph of Aztreonam made according to the invention, contained less than 0.5% solvent, as recommended by the ICH guidelines. Its water content is typically 0.5%. In summary, the good drying ability is attributed to the low temperature of crystallization and the efficient stirring which enable the slow growth of crystals (crystallization completes in 16-24 h). These three factors result in the lack of agglomerates, which are believed to be responsible for trapping solvent molecules.  
         [0014]     The β polymorph of Aztreonam having less than 2.5% solvent, may be made by dissolving Aztreonam, preferably a polymorph of Aztreonam, most preferably the α polymorph, in an organic solvent at a temperature of between about −60° C. to about 5° C., preferably from about −40° C. to about 5° C., more preferably from about −25° C. to about 5° C., and most preferably from about −5° C. to about 5° C., in the presence of a base to form a clear solution, and adding acid while efficiently stirring the solution until a precipitate is obtained. Preferably, the solution is seeded with β Aztreonam while the efficient stirring is continued. Preferably, the precipitate is reslurried in ethanol.  
         [0015]     The phrase “efficient stirring” means turbulent stirring such that the peripheral velocity of the stirrer is preferably 25 m/s or greater, more preferably 100 m/s or greater, and most preferably 400 m/s or greater. The stirring rate depends on the reactor volume. For example, at least 500 rpm is turbulent stirring for a reactor volume of 1 liter.  
         [0016]     The starting material may be any known polymorph of Aztreonam such as the α, β, δ or γ aztreonam polymorphs.  
         [0017]     Preferably, the organic solvent used to dissolve the polymorph of Aztreonam is a C 1-6  alcohol, preferably absolute C 1-6  alcohol, such as ethanol, methanol, isopropanol etc., most preferably, the solvent is absolute ethanol.  
         [0018]     The base used in the process is selected from the group consisting of triethylamine, n-butylamine and 2,4,6-trimethyl pyridine. Preferably, the base is gaseous or aqueous triethylamine. The base is preferably added in an amount of 1.1-1.2 moles per mole of the α polymorph of Aztreonam.  
         [0019]     The acid used in the process is preferably concentrated hydrochloric acid. It is preferably added in an amount sufficient to adjust the pH of the solution to between about 1.8-3.2, more preferably to between about 2.2-2.8.  
         [0020]     After the acid is added, the solution is seeded with ˜1% β-Aztreonam with efficient stirring for a time sufficient to precipitate the β polymorph of Aztreonam.  
       EXAMPLES  
     Example 1  
       [0021]     In a glass reactor, 18 g α-Aztreonam is added to a mixture of 240 ml absolute ethanol and 11.5 ml ethanolic ammonia (3.65 M, 1.2 equiv.) at 0° C. with stirring. After 10-30 min the insoluble material is filtered off (filter is washed with 30 ml chilled absolute ethanol) and the solution is acidified with 3.6 ml aq. HCl (37%, 1.22 equiv.). The solution is immediately seeded with 0.18 g β-Aztreonam and the hazy solution is stirred efficiently (500 rpm) overnight at this temperature. After filtration the product is dried at 38° C. in an air-circulating oven. Yield: 13.7 g; Residual ethanol: 0.95%  
       Example 2  
       [0022]     In a glass reactor, 18 g α-Aztreonam is added to a mixture of 240 ml absolute ethanol and 11.5 ml ethanolic ammonia (3.65 M, 1.2 equiv.) at 0° C. with stirring. After 10-30 min the insoluble material is filtered off (filter is washed with 30 ml chilled absolute ethanol) and the solution is acidified with ethanolic HCl (3.6 M) to pH 2.4. The solution is immediately seeded with 0.18 g β-Aztreonam and the hazy solution is stirred efficiently (500 rpm) overnight at this temperature. After filtration the product is dried at 38° C. in an air-circulating oven. Yield: 14.5 g; Residual ethanol: 0.78%  
       Example 3  
       [0023]     In a glass reactor, 75.6 g α-Aztreonam is added to a mixture of 1050 ml absolute ethanol and 30 ml triethylamine (1.2 equiv.) at 0° C. with stirring. After 10-30 min the insoluble material is filtered off and the solution is acidified with ethanolic HCl (3.6 M) to pH 2.0. The solution is immediately seeded with 0.8 g β-Aztreonam and the hazy solution is stirred efficiently (600 rpm) overnight at this temperature. After filtration the product is dried at 38° C. in an air-circulating oven. Yield: 58.7; Residual ethanol: 0.18%  
       Example 4  
       [0024]     8.6 kg Aztreonam crude dry, 115 L ethanol absolute and 5.5 L ethanolic ammonia was charged at 0° C. into a 400-liter reactor. The solution was stirred at 0° C. for 1 hr. The solution was acidified to pH=2.9 with 1.5 L HCl 32% and seeded. The stirring was maintained for 16 hours at 0° C. The suspension was filtrated and washed with absolute ethanol. 11.9 kg of wet material was obtained. The wet material (11.9-Kg) was re-slurried twice with 95% ethanol at 45° C. in a 160-liter reactor. 6.4 Kg of wet material was obtained. The solid was dried in a vacuum oven. The polymorph of the dry sample was the β polymorph of Aztreonam containing 0.496% residual ethanol.