A dual system platform for drug metabolism: Nalbuphine as a model compound

Reaction phenotyping is a method commonly used for characterizing drug metabolism. It determines the drug metabolic pathways and ratios by measuring the metabolized fractions of individual enzymes. However, currently published results have focused on cytochrome P450s (CYPs), while not considering phase II metabolism. Therefore, the morphinan analgesic, nalbuphine, primarily metabolized in the liver via CYPs and UDP-glucuronosyltransferases (UGTs), was selected as a model drug to establish a dual-phase platform to elucidate its comprehensive metabolic pathway. Enzyme kinetics was studied using 8 common recombinant (r)CYPs, 10 rUGTs, and pooled human liver microsomes. The overall fraction of nalbuphine metabolized by each isozyme was evaluated by determining parent drug depletion. Finally, in vitro-in vivo correlation was validated in animal studies. Fluconazole, a specific UGT2B7 inhibitor, was administered orally to rats to determine the pharmacokinetic effects on nalbuphine and nalbuphine metabolites. Seventy-five percent and 25% of nalbuphine was metabolized by UGTs and CYPs, respectively. UGT2B7, UGT1A3, and UGT1A9 were primarily responsible for nalbuphine glucuronidation; only UGT2B7 produced nalbuphine-6-glucuronide. CYP2C9 and CYP2C19 were the two CYP isozymes that produced 3'-hydroxylnalbuphine and 4'-hydroxylnalbuphine. In vivo, the maximum serum concentration (C_max) and area under the curve (AUC) of nalbuphine increased 12.4-fold and 13.2-fold, respectively, with fluconazole co-administration. A dual system platform for drug metabolism was successfully established in this study and was used to generate a complete metabolic scheme for nalbuphine. This platform has been verified by in vivo evaluations and can be utilized to study drugs that undergo multisystem metabolism.