The human hepatic metabolism of simvastatin hydroxy acid is mediated primarily by CYP3A, and not CYP2D6

Abstract

Aims: To identify the cytochrome P450 (CYP) isoforms responsible for the metabolism of simvastatin hydroxy acid (SVA), the most potent metabolite of simvastatin (SV).

Methods: The metabolism of SVA was characterized in vitro using human liver microsomes and recombinant CYPs. The effects of selective chemical inhibitors and CYP antibodies on SVA metabolism were assessed in human liver microsomes.

Results: In human liver microsomes, SVA underwent oxidative metabolism to three major oxidative products, with values for Km and Vmax ranging from about 50 to 80 microM and 0.6 to 1.9 nmol x min(-1) x mg(-1) protein, respectively. Recombinant CYP3A4, CYP3A5 and CYP2C8 all catalysed the formation of the three SVA metabolites, but CYP3A4 was the most active. CYP2D6 as well as CYP2C19, CYP2C9, CYP2A6, CYP1A2 did not metabolize SVA. Whereas inhibitors that are selective for CYP2D6, CYP2C9 or CYP1A2 did not significantly inhibit the oxidative metabolism of SVA, the CYP3A4/5 inhibitor troleandomycin markedly (about 90%) inhibited SVA metabolism. Quercetin, a known inhibitor of CYP2C8, inhibited the microsomal formation of SVA metabolites by about 25-30%. Immunoinhibition studies revealed 80-95% inhibition by anti-CYP3A antibody, less than 20% inhibition by anti-CYP2C19 antibody, which cross-reacted with CYP2C8 and CYP2C9, and no inhibition by anti-CYP2D6 antibody.

Conclusions: The metabolism of SVA in human liver microsomes is catalysed primarily (> or = 80%) by CYP3A4/5, with a minor contribution (< or = 20%) from CYP2C8. CYP2D6 and other major CYP isoforms are not involved in the hepatic metabolism of SVA.

Figure 1

Effects of chemical inhibitors of CYP (A) and anti-CYP antibodies (B) on the metabolism of simvastatin hydroxy acid (SVA) by human liver microsomes pooled from 10 subjects, and effects of anti-P450 antibodies on the formation of M3 in liver microsomes from individual subjects and in recombinant CYP3A4 and CYP2C8 (C). For figure 1C: Anti-2C (▪); Anti-2D6 (); and Anti-3A (). Results (mean ± SD) were based on triplicate determinations. All incubations were carried out in human liver microsomes in the absence or presence of inhibitors. Control activities (in the absence of inhibitors/antibodies) were 0.8, 0.4 and 0.3 nmol min⁻¹ mg⁻¹ protein for the formation of M1 (▪), M2 () and M3 (), respectively. TAO, Troleandomycin; QC, quercetin; QD, quinidine; SZ, sulfaphenazole; FF, furafylline; HM1, HM2, HM3, HM4, HM5 and HM6, human liver microsomes from subjects 1, 2, 3, 4, 5 and 6, respectively; 3A4 and 2C8, CYP3A4 and CYP2C8, respectively.