Metabolism of the active Carfentanil metabolite, 4- Piperidinecarboxylic acid, 1- (2- hydroxy- 2- phenylethyl) - 4- [(1- oxopropyl) phenylamino] -, methyl ester in vitro

Carfentanil, a µ-opioid receptor (MOR) agonist with an analgesic potency 10,000 times that of morphine, is extensively metabolized to norcarfentanil (M1), 4-Piperidinecarboxylic acid, 1-(2-hydroxy-2-phenylethyl)-4-[(1-oxopropyl)phenylamino]-, methyl ester (M0 in this article), and other low abundant metabolites in human hepatocytes and liver/lung microsomes. M0 possessed comparable MOR activity to carfentanil, and accounted for approximately 12 % of the total carfentanil metabolite formation in human liver microsomes (HLMs). Little is known about the subsequent elimination of M0. This study investigated its metabolic pathway in HLMs, separation and preliminary identification of metabolites by liquid chromatography-tandem mass spectrometry, and possible involvement of cytochrome P450 enzymes in M0 metabolism with kinetic analysis. M0 produced 9 metabolites via N-dealkylation (M1), oxidation (M3, M6–9), N-dealkylation followed by oxidation (M2 and M4), and glucuronidation (M5). Formation of the major metabolite M1 fitted typical Michaelis-Menten kinetics. Recombinant human CYP3A5 showed the highest activity toward M1 formation followed by CYP3A4 and CYP2C8, while M8 was primarily formed by CYP3A4 followed by CYP2C19 and CYP2C8. These findings reveal the main involvement of CYP3A5 and 3A4 in human hepatic elimination of M0 with a kinetic profile similar to carfentanil which may inform development of treatment protocols for carfentanil exposure.