The role of human CYP2C8 and CYP2C9 variants in pioglitazone metabolism in vitro.

The cytochrome P450 enzyme CYP2C8 appears to have a major role in pioglitazone metabolism. The present study was conducted to further clarify the role of individual CYPs and of the CYP2C8/9 polymorphisms in the primary metabolism of pioglitazone in vitro. Pioglitazone (2-400 microM) was incubated with isolated cytochrome P450 enzymes or human liver microsomes, some of them carrying either the CYP2C83/3 genotype (and also the CYP2C92/2 genotype) or the CYP2C81/1 genotype (five samples each). The formation of the primary pioglitazone metabolite M-IV was monitored by HPLC. Enzyme kinetics were estimated assuming a single binding site. Mean intrinsic clearance of pioglitazone to the metabolite M-IV was highest for CYP2C8 and CYP1A2 with 58 pmol M-IV/min/nmol CYP P450/microM pioglitazone each, 53 for CYP2D61, 40 for CYP2C191, and 34 for CYP2C92, respectively. CYP2A6, CYP2B6, CYP2C91, CYP2C93, CYP2E1, CYP3A4 and CYP3A5 did not form quantifiable amounts of M-IV. CYP2C81/1 microsomes (25 +/- 4 pmol M-IV/min/mg protein/muM pioglitazone) showed lower intrinsic clearance of pioglitazone than CYP2C83/3 microsomes (35 +/- 9, p = 0.04). In all samples, metabolite formation showed substrate inhibition, while pioglitazone did not inhibit CYP2C8-mediated paclitaxel metabolism. CYP2C8, CYP1A2 and CYP2D6 are major CYPs forming M-IV in vitro. The higher activity of CYP2C83/CYP2C92 microsomes may result from a contribution of CYP2C92, or from differences in CYP2C8 expression. The evidence for substrate-specific inhibitory effects of pioglitazone on CYP2C-mediated metabolism needs to be tested in further studies.