High-performance liquid chromatography method for the separation of isomers of 9β-methylcarbacyclin (ciprostene) and the analysis of pharmaceutical samples

An high-performance liquid chromatography method was developed that separates (5Z)-9 beta-methyl-6 alpha-carbaprostaglandin I2 (9 beta-methylcarbacyclin; ciprostene) from several closely related isomers: (i) a double bond isomer at C-5, (ii) an epimer at C-15, and (iii) combinations of i and ii. The method uses a Zorbax ODS column with a mobile phase consisting of acetonitrile-buffer pH 2.5 at a ratio of 45:55 for determination of purity and 40:60 for optimum separation of isomers. Detection is measurement of absorbance at 200 nm. Peak heights are measured relative to butylparaben which is used as an internal standard. The method is linear, free from interferences from degradation of ciprostene and has a relative standard deviation of 0.5-1% for solutions at 60 micrograms/ml. This assay has been applied to bulk powders and solutions of the drug. Stressed chemical stability studies have shown that the C-15 allylic secondary alcohol group undergoes epimerization under acidic conditions in solution and oxidation in the solid and solution state under thermal stress or in the presence of oxidants.