Received for publication July 3, 2006.
Revised August 19, 2006.
Accepted for publication August 22, 2006.

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

Abstract

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 - 0.4 µM and E_max of 2 - 5-fold over control. However, pretreatment with rifampin did not alter the pharmacokinetics of DF obtained following either intravenous (i.v.) or intra-hepatic 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 - 3-fold, respectively), and markedly affected the in vivo pharmacokinetics of midazolam, MDZ (>10-fold decreases in the i.pv. MDZ systemic exposure and its hepatic availability, Fh) 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% (vs. about 75% in human liver microsomes) of the total (glucurondation + 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 on the utility of DF as an in vivo CYP2C9 probe.

Key words: CYP induction, CYP2C, glucuronidation, in vivo probes