Comparison of Cytochrome P-450-dependent Metabolism and Drug Interactions of the 3-hydroxy-3-methylglutaryl-CoA Reductase Inhibitors Lovastatin and Pravastatin in the Liver

In an in vitro study, the cytochrome P-450 3A (CYP3A)-dependent metabolism and drug interactions of the 3-hydroxy-3-methylglutaryl-Co A reductase inhibitors lovastatin and pravastatin were compared. Lovastatin was metabolized by human liver microsomes to two major metabolites: 6'beta-hydroxy [Michaelis-Menten constant (Km): 7.8 +/- 2.7 microM] and 6'-exomethylene lovastatin (Km,10.3 +/- 2.6 microM). 6'beta-Hydroxylovastatin formation in the liver was inhibited by the specific CYP3A inhibitors cyclosporine (Ki, 7.6 +/- 2.3 microM), ketoconazole (Ki, 0.25 +/- 0.2 microM), and troleandomycin (Ki, 26.6 +/- 18.5 microM). Incubation of pravastatin with human liver microsomes resulted in the generation of 3'alpha,5'beta, 6'beta-trihydroxy pravastatin (Km, 4,887 +/- 2,185 microM) and hydroxy pravastatin (Km, 20,987 +/- 9,389 microM). The formation rates of 3'alpha,5'beta,6'beta-trihydroxy pravastatin by reconstituted CYP3A enzymes were (1,000 microM pravastatin) 1.9 +/- 0.6 pmol.min-1.pmol CYP3A4 and 0.06 +/- 0.04 pmol.min-1.pmol CYP3A5, and the formation rates of hydroxy pravastatin were 0.12 +/- 0.02 pmol.min-1.pmol CYP3A4 and 0.02 +/- 0.004 pmol.min-1.pmol CYP3A5. The specific CYP3A inhibitors cyclosporine, ketoconazole, and troleandomycin significantly inhibited hydroxy pravastatin formation by human liver microsomes, but only ketoconazole inhibited 3'alpha, 5'beta,6'beta-trihydroxy pravastatin formation, suggesting that other CYP enzymes are involved in its formation. It is concluded that, compared with lovastatin [CLint formation 6'beta-hydroxylovastatin (microl.min-1.mg-1): 199 +/- 248, 6'-exomethylene lovastatin: 138 +/- 104)], CYP3A-dependent metabolism of pravastatin [CLint formation 3'alpha,5'beta, 6'beta-trihydroxy pravastatin (microl.min-1.mg-1): 0.03 +/- 0.03 and hydroxy pravastatin: 0.02 +/- 0.02] is a minor elimination pathway. In contrast to lovastatin, drug interactions with pravastatin CYP3A-catalyzed metabolism cannot be expected to have a clinically significant effect on its pharmacokinetics.