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SHORT COMMUNICATION

Rifampin Induces the in Vitro Oxidative Metabolism, but Not the in Vivo Clearance of Diclofenac in Rhesus Monkeys

Department of Drug Metabolism, Merck Research Laboratories, West Point, Pennsylvania

Effects of rifampin on in vitro oxidative metabolism and in vivo pharmacokinetics of diclofenac (DF), a prototypic CYP2C9 marker substrate, were investigated in rhesus monkeys. In monkey hepatocytes, rifampin markedly induced DF 4'-hydroxylase activity, with values for EC₅₀ of 0.2 to 0.4 µM and E_max of 2- to 5-fold over control. However, pretreatment with rifampin did not alter the pharmacokinetics of DF obtained after either i.v. or intrahepatic portal vein (i.pv.) administration of DF to monkeys. At the dose studied, plasma concentrations of rifampin reached 10 µM, far exceeding the in vitro EC₅₀ values. Under similar treatment conditions, rifampin was previously shown to induce midazolam (MDZ) 1'-hydroxylation in rhesus monkey hepatocytes (EC₅₀ and E_max values 0.2 µM and 2- to 3-fold, respectively), and markedly affected the in vivo pharmacokinetics of MDZ (>10-fold decreases in the i.pv. MDZ systemic exposure and its hepatic availability, F_h) in this animal species. In monkey liver microsomes, DF underwent, predominantly, glucuronidation, and, modestly, oxidation; the intrinsic clearance (CL_int = V_max/K_m) value for the glucuronidation pathway accounted for >95% (versus about 75% in human liver microsomes) of the total (glucuronidation + hydroxylation) intrinsic clearance value. In monkey hepatocytes, the hydroxylation also was a minor component (10%) relative to the glucuronidation, supporting the liver microsomal finding. Collectively, our results suggest that the oxidative metabolism is not the major in vivo clearance mechanism of DF in either untreated or rifampin-treated monkeys and, conceivably, also in humans, raising a question about the utility of DF as an in vivo CYP2C9 probe.