(S)-Ornidazole Disodium Phosphate

Secnidazole is a long-lasting nitroimidazole antimicrobial agent that is used as racemic mixture in clinical settings. We developed and validated an enantioselective high-performance liquid chromatography method to determine secnidazole enantiomers in rat plasma. Secnidazole enantiomers and S-(-)-ornidazole (internal standard) were extracted from 50 μL of rat plasma using diethyl ether-dichloromethane (3:2, v/v). Baseline resolution (Rs=2.45) was achieved within 7.0 min on a Chiral-AGP column (150 mm × 4.0mm, 5 μm) at 20°C. The mobile phase consisted of 10mM ammonium acetate-methanol (96:4, v/v) and was delivered at a flow rate of 0.5 mL/min with ultraviolet detection at 318 nm. The method was linear over the concentration range 0.500-100 μg/mL for both enantiomers. The lower limit of quantification was 0.500 μg/mL for both enantiomers. The relative standard deviation values for intra- and inter-day precision were 0.8-8.6 and 1.8-8.2% for S-(+)-secnidazole and R-(-)-secnidazole, respectively. The relative error values of accuracy ranged from -7.8 to 1.1% for S-(+)-secnidazole and from -7.3 to -0.1% for R-(-)-secnidazole. The method was successfully used to determine the pharmacokinetic properties of secnidazole enantiomers in rats after administration of the racemate and individual enantiomers. The pharmacokinetic results indicate that the disposition of secnidazole enantiomers is not stereoselective and chiral inversion and enantiomer-enantiomer interaction do not occur in rats.

A rapid, sensitive, and enantioselective method was developed and validated for determination of ornidazole enantiomers in human plasma by liquid chromatography-tandem mass spectrometry. Ornidazole enantiomers were extracted from 100μl of plasma using ethyl acetate. Baseline chiral separation (Rs=2.0) was obtained within 7.5min on a Chiral-AGP column (150mm×4.0mm, 5μm) using an isocratic mobile phase of 10mM ammonium acetate/acetic acid (100/0.01, v/v). Stable isotopically labeled R-(+)-d5-ornidazole and S-(-)-d5-ornidazole were synthesized as internal standards. Acquisition of mass spectrometric data was performed in multiple reaction monitoring mode via positive electrospray ionization, using the transitions of m/z 220→128 for ornidazole enantiomers, and m/z 225→128 for d5-ornidazole enantiomers. The method was linear in the concentration range of 0.030-10.0μg/ml for each enantiomer. The lower limit of quantification for each enantiomer was 0.030μg/ml. The relative standard deviation values of intra- and inter-day precision were 1.8-6.2% and 1.5-10.2% for R-(+)-ornidazole and S-(-)-ornidazole, respectively. The relative error values of accuracy ranged from -4.5% to 1.2% for R-(+)-ornidazole and from -5.4% to -0.8% for S-(-)-ornidazole. The validated method was successfully applied to a stereoselective pharmacokinetic study of ornidazole after oral administration of 1000mg racemic ornidazole.