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 (Cmax) 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.

A novel finding of nalbuphine-6-glucuronide, an active opiate metabolite, possessing potent antinociceptive effects: Synthesis and biological evaluation.

Nalbuphine, a partial agonist/antagonist opioid analgesic, is structurally related to morphine. It is equipotent to morphine and has no serious side effects. In the past few decades, studies focusing on morphine metabolism have indicated that one of its sugar-conjugated metabolites, morphine-6-glucuronide, exerts a higher analgesic effect than its parent drug. Considering that nalbuphine is a morphine analog that follows a similar metabolic scheme, nalbuphine glucuronides were synthesized in this study and their potential analgesic effects were assessed. Nalbuphine-3-glucuronide (N3G) and nalbuphine-6-glucuronide (N6G) were synthesized based on Schmidt's glycosylation with OPiv protections on the glycosyl donor. In a pharmacodynamic study, paw pressure and cold-ethanol tail-flick tests were conducted in rats to evaluate the analgesic response after intracisternal and intraperitoneal administrations of nalbuphine, N3G, or N6G. The antinociceptive response was evaluated for each compound by calculating the area under the curve and the duration spent at greater than 50% maximum possible analgesia. In conclusion, intracisternal administration of N6G exhibited a stronger analgesic response than nalbuphine in the pain tests after both cold and mechanical stimuli, but N3G had no obvious effect. Similar to that of morphine, the glucuronide metabolite of nalbuphine at the 6-O-position exerted at least three-fold higher antinociceptive potency and five-fold longer analgesic duration than nalbuphine.