Metoclopramide (METO) is increasingly prescribed for conditions previously treated with cisapride, but its metabolic enzymology and drug interactions are poorly understood. To characterize the metabolism of METO in humans, we developed an HPLC assay to measure METO and its metabolites in human liver microsomes (HLMs) and recombinant CYP450s. We also assessed the ability of METO to inhibit the CYP450 system, using isoform-specific substrate reaction probes of CYP 1A2, 2C9, 2D6, 2E1, and 3A4. Injection of supernatant from incubations containing METO, NADPH regenerating system, and HLMs resulted in one major peak (Ml) and minor peaks that had spectral characteristics similar to METO. The formation of these metabolites was dependent on NADPH, HLMs, substrate, protein concentration, and time of incubation. M1 formation followed Michaelis-Menten kinetics (Km=72 μM; Vmax=736 pmol/min/mg protein). The metabolite was formed at the highest rate by CYP2D6 (V=4.5 pmol/min/pmol P450) and to a lesser extent by CYP1A2 (0.97 pmol/min/pmol P450). Quinidine (1μM) inhibited Ml formation from METO (25μM) by 37%. METO is a potent inhibitor of CYP2D6, with negligible effect on other isoforms tested. Further inhibition of CYP2D6 was observed when METO was preincubated with HLMs and NADPH before the substrate probe was added (Ki=0.96 μM; Kinact=0.02 min-1), suggesting mechanism-based inhibition. The elimination of METO may be slow in poor metabolizers of CYP2D6 or in patients taking inhibitors of this isoform. METO may also reduce the clearance of CYP2D6 substrates.
|Original language||English (US)|
|Journal||Clinical Pharmacology and Therapeutics|
|State||Published - Dec 1 2001|
ASJC Scopus subject areas
- Pharmacology (medical)