Species differences in the hepatobiliary disposition of morphine-6-glucuronide mediated by hepatic transporters in rats and humans.

Morphine-6-glucuronide (M6G), the active metabolite of morphine, is currently in clinical development due to its higher analgesic activity. In humans, intravenously administered M6G was predominantly eliminated unchanged through the kidney, whereas it was excreted into the urine as parent drug as well as its metabolites morphine and M3G in normal rats. In bile-duct-cannulated rats, however, bile excretion of the parent drug was the main route of clearance. In the study, we investigated the mechanisms underlying the species differences in vivo disposition of M6G. In hepatocyte uptake assay, we showed that M6G uptake in rat hepatocytes was 75-fold higher than that in human hepatocytes. Hepatic uptake transporter phenotyping study identified M6G as a substrate for rat rOatplal, rOatpla4, rOatp1b2, as well as for human hOATP1B1 and hOATP1B3. Among these, rOatps exhibited significantly stronger uptake of M6G compared to hOATPs. Furthermore, M6G was not a substrate for the canalicular efflux transporters MDR1, hBCRP/rBcrp, hBSEP/rBsep, and hMRP2, but it was recognized by rMrp2. These findings aligned with the observation that M6G exhibited significant biliary excretion in the rat sandwich cultured hepatocyte (SCH) model, but not in the human SCH. Additionally, no species differences were observed in renal uptake mediated by OAT3. Overall, M6G underwent renal clearance in humans via glomerular filtration and active secretion primarily mediated by hOAT3. Although a portion of M6G was also eliminated through the kidney in rats, the majority was subjected to enterohepatic circulation mediated primarily by rOatps and rMrp2, leading to the formation of morphine and M3G, which were subsequently excreted in the urine. The marked difference in the uptake activities of sinusoidal transporters hOATPs/rOatps and the substrate specificity of canalicular transporters hMRP2/rMrp2 were critical factors underlying the species differences in the hepatobiliary disposition of M6G.