Ambarish K. Singh

An integrated approach to the selection of optimal salt form for a new drug candidate

Abstract A general method was developed to select the optimal salt form for BMS-180431, a novel HMG-CoA reductase inhibitor and a candidate for oral dosage form development, in an expeditious manner at the onset of the drug development process. The physicochemical properties such as hygroscopicity, physical stability of crystal forms at different humidity conditions, aqueous solubility, and chemical stability of seven salts e.g., sodium, potassium, calcium, zinc, magnesium, arginine and lysine, were studied using a multi-tier approach. The progression of studies among different tiers was such that the least time-consuming experiments were conducted earlier, thus saving time and effort. A ‘go/no go’ decision was made after each tier of testing the salts, thus avoiding generation of extensive data on all available salt forms. The hygroscopicities of all BMS-180431 salts were evaluated at tier 1 and four salts (sodium, potassium, calcium and zinc) were dropped from consideration due to excessive moisture uptake within the expected humidity range of pharmaceutical manufacturing plants (30–50% R.H. at ambient temperature). The remaining three salts were subjected to the tier 2 evaluation for any change in their crystal structures with respect to humidity and the determination of their aqueous solubilities in the gastrointestinal pH range. The magnesium salt was dropped from further consideration due to humidity-dependent changes in its crystal structure and low solubility in water (3.7 mg/ml at room temperature). Arginine and lysine salts, which were resistant to any change in their crystalline structures under extremes of humidity conditions (6 and 75% R.H.) and had high aqueous solubilities (> 200 mg/ml), were elevated to tier 3 for the determination of their chemical stability. Based on solid state stability of these two salts under accelerated conditions (temperature, humidity, and presence of excipients), consideration of ease of synthesis, ease of analysis, potential impurities, etc., and input from the marketing group with respect to its preference of counter ion species, the arginine salt was selected for further development. The number of tiers necessary to reach a decision on the optimal salt form of a compound may depend on the physicochemical properties studied and the number of salts available. This salt selection process can be completed within 4–6 weeks and be easily adopted in the drug development program.

STEREOSPECIFIC ENZYMATIC HYDROLYSIS OF RACEMIC EPOXIDE : A PROCESS FOR MAKING CHIRAL EPOXIDE

Abstract Among various microbial cultures evaluated, Rhodotorula glutinis SC 16293 and Aspergillus niger SC 16311 catalyzed the stereospecific hydrolysis of the racemic epoxide, RS -1-{2′,3′-dihydrobenzo[ b ]furan-4′-yl}-1,2-oxirane, 1 to the corresponding R -diol, R -1-{2′,3′-dihydrobenzo[ b ]furan-4′-yl}-ethane-1,2-diol, 3 . The S -epoxide, S -1-{2′,3′-dihydrobenzo[ b ]furan-4′-yl}-1,2-oxirane, 2 remained unreacted in the reaction mixture. A reaction yield of 45–50% (theoretical maximum yield is 50%) and an enantiomeric excess (ee) of >95% were obtained for unreacted S -epoxide 2 using each culture. Addition of 10% methyl tert -butyl ether to an aqueous reaction mixture during hydrolysis by R. glutinis improved the ee of the unreacted S -epoxide 2 to >99% (yield 48%) and that of the R -diol 3 to 79%. Unlike R. glutinis , hydrolysis of racemic epoxide 1 in the presence of 10% methyl tert -butyl ether by A. niger showed an adverse effect and gave S -epoxide 2 in 54% yield and 49% ee.

Characterization of humidity-dependent changes in crystal properties of a new HMG-CoA reductase inhibitor in support of its dosage form development

Abstract Humidity-dependent changes in the crystal properties of the disodium salt of a new HMG-CoA reductase inhibitor (SQ-33600) were characterized using a combination of gravimetric, thermal, and spectral techniques. The drug substance was found to exhibit rapid moisture sorption and/or desorption, depending on the environmental conditions. Three crystalline solid hydrates and one liquid crystalline phase were identified, each having a definite stability over a range of humidity. The drug substance turned amorphous upon wet granulation, and the amorphous phase reconverted to crystalline hydrates upon exposure to 33–75% relative humidity. To avoid physical instability of dosage forms due to phase changes, manufacturing of solid dosage forms by dry processing below 52% relative humidity was recommended. The dissolution of drug from solid dosage forms was observed to be independent of the crystal form of the active.

Selective removal of a benzyl protecting group in the presence of an aryl chloride under gaseous and transfer hydrogenolysis conditions

Abstract Selective removal of a benzyl protecting group in the presence of an aryl chloride using Pd/C under gaseous and transfer hydrogenolysis conditions is described. The addition of chloride salts to the debenzylation reaction provides excellent selectivity.

A chemoselective, acid mediated conversion of amide acetal to oxazole: The key step in the synthesis of cardiovascular drug, ifetroban sodium

Abstract The cyclization of acetal amide was carried out with trimethylsilyl trifluoromethanesulfonate, followed by elimination using sodium methoxide to give 2,5-disubstituted oxazole, thus completing a new route to the cardivascular drug ifetroban sodium.