Investigating the in vitro stereoselective metabolism of m-nisoldipine enantiomers: characterization of metabolites and cytochrome P450 isoforms involved.

m-Nisoldipine, as a novel 1,4-dihydropyridine calcium ion antagonist, was presented as a couple of enantiomers [(-), (+)-m-nisoldipine]. In this report, the in vitro metabolism of m-nisoldipine enantiomers was investigated in rat liver microsomes (RLM) by the combination of two liquid chromatography mass spectrometric techniques for the first time. The metabolites were separated and assayed by ultra-high performance liquid chromatography coupled to quadrupole time-of-flight mass spectrometry and further identified by comparison of their mass and chromatographic behaviors with reference substances. A total of 18 metabolites of (-)-m-nisoldipine and 16 metabolites of (+)-m-nisoldipine were detected, respectively, which demonstrated that (+)-m-nisoldipine is more metabolically stable than (-)-m-nisoldipine. In addition, the identified metabolic pathways of m-nisoldipine enantiomers were involved in dehydrogenation, oxidation and ester hydrolysis. Afterwards, based on high-performance liquid chromatography coupled to triple quadrupole linear ion trap mass spectrometry, various selective cytochrome P450 (CYP) enzyme inhibitors were employed to evaluate CYP isoforms. The results indicated that the inhibitors of CYP1A1/2, CYP2B1/2, 2D and 2C11 had no obvious inhibitory effects, yet the inhibitor of CYP 3A had a significant inhibitory effect on metabolism of m-nisoldipine enantiomers. This showed that CYP 3A might primarily metabolize m-nisoldipine in RLM.