IN-VITRO METABOLISM OF SIMVASTATIN IN HUMANS [SBT]IDENTIFICATION OF METABOLIZING ENZYMES AND EFFECT OF THE DRUG ON HEPATIC P450S

Simvastatin (SV) is a lactone prodrug used for the treatment of hyperc holesterolemia. Upon incubation of SV with liver microsomal preparatio ns from human donors, four major metabolic products were formed (3'-hy droxy SV, 6'-exomethylene SV, 3',5'-dihydrodiol SV, and the active hyd roxy acid, SVA), together with several minor unidentified metabolites. The 3',5'-dihydrodiol SV, a new metabolite, was inactive as an inhibi tor of HMG-CoA reductase. Kinetic studies of SV metabolism in human li ver microsomes suggested that the major NADPH-dependent metabolites (3 '-hydroxy SV, 6'-exomethylene SV, and 3',5'-dihydrodiol SV) were forme d with relatively high intrinsic clearances, consistent with the exten sive metabolism of SV observed in vivo. Based on four different in vit ro approaches, namely 1) correlation analysis, 2) chemical inhibition, 3) immunoinhibition, and 4) metabolism by recombinant human P450, it is concluded that CYP3A is the major enzyme subfamily responsible for the metabolism of SV by human liver microsomes. Both CYP3A4 and CYP3A5 were capable of catalyzing the formation of 3',5'-dihydrodiol, 3'-hyd roxy, and 6'-exomethylene metabolites. However, CYP3A4 exhibited highe r affinity (> 3 fold) for SV than CYP3A5. Also, the studies indicated that CYP2D6, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP1A2, and CYP2E1 did n ot play significant roles in the metabolism of SV in vitro. Over the c oncentration range of 0-40 mu M, SV inhibited the activity of CYP3A, b ut not the activities of CYP2C8/9, CYP2C19, or CYP2D6 in human liver m icrosomes. The inhibition of hepatic midazolam 1'-hydroxylase, a CYP3A marker activity, by SV was competitive with a K-i value of <similar 1 0 mu M was > 30-fold less potent than ketoconazole and itraconazole as an inhibitor of CYP3A. Under the same conditions, SVA, the hydrophili c hydroxy acid form of SV, did not inhibit CYP3A, CYP2C8/9, CYP2C19, o r CYP2D6 activities. The results suggested that the in vivo inhibitory effects of SV on the metabolism of CYP3A substrates likely would be l ess than those of ketoconazole and itraconazole at their respective th erapeutic concentrations. In addition, metabolic activities mediated b y the other P450 enzymes tested are unlikely to be affected by SV.