RT Journal Article SR Electronic T1 Extrapolation of Diclofenac Clearance from in Vitro Microsomal Metabolism Data: Role of Acyl Glucuronidation and Sequential Oxidative Metabolism of the Acyl Glucuronide JF Journal of Pharmacology and Experimental Therapeutics JO J Pharmacol Exp Ther FD American Society for Pharmacology and Experimental Therapeutics SP 969 OP 978 DO 10.1124/jpet.102.038992 VO 303 IS 3 A1 Kumar, Sanjeev A1 Samuel, Koppara A1 Subramanian, Ramaswamy A1 Braun, Matthew P. A1 Stearns, Ralph A. A1 Chiu, Shuet-Hing Lee A1 Evans, David C. A1 Baillie, Thomas A. YR 2002 UL http://jpet.aspetjournals.org/content/303/3/969.abstract AB Diclofenac is eliminated predominantly (∼50%) as its 4′-hydroxylated metabolite in humans, whereas the acyl glucuronide (AG) pathway appears more important in rats (∼50%) and dogs (>80–90%). However, previous studies of diclofenac oxidative metabolism in human liver microsomes (HLMs) have yielded pronounced underprediction of human in vivo clearance. We determined the relative quantitative importance of 4′-hydroxy and AG pathways of diclofenac metabolism in rat, dog, and human liver microsomes. Microsomal intrinsic clearance values (CLint =V max/K m) were determined and used to extrapolate the in vivo blood clearance of diclofenac in these species. Clearance of diclofenac was accurately predicted from microsomal data only when both the AG and the 4′-hydroxy pathways were considered. However, the fact that the AG pathway in HLMs accounted for ∼75% of the estimated hepatic CLint of diclofenac is apparently inconsistent with the 4′-hydroxy diclofenac excretion data in humans. Interestingly, upon incubation with HLMs, significant oxidative metabolism of diclofenac AG, directly to 4′-hydroxy diclofenac AG, was observed. The estimated hepatic CLint of this pathway suggested that a significant fraction of the intrahepatically formed diclofenac AG may be converted to its 4′-hydroxy derivative in vivo. Further experiments indicated that this novel oxidative reaction was catalyzed by CYP2C8, as opposed to CYP2C9-catalyzed 4′-hydroxylation of diclofenac. These findings may have general implications in the use of total (free + conjugated) oxidative metabolite excretion for determining primary routes of drug clearance and may question the utility of diclofenac as a probe for phenotyping human CYP2C9 activity in vivo via measurement of its pharmacokinetics and total 4′-hydroxy diclofenac urinary excretion.