title: Application of a loading wash-out method for investigating the hepatocellular efflux of a hepatically-generated metabolite, morphine-3-glucuronide.
abstract: Previous studies using the rat isolated perfused liver demonstrated that the hepatic disposition of morphine-3-glucuronide is membrane permeability-rate limited, and that the movement of the metabolite across hepatic sinusoidal and canalicular membranes is partly via carrier-mediated transport systems. As a consequence of the membrane permeability-limitation, the biliary excretion of hepatically-generated morphine-3-glucuronide is much more efficient than that of morphine-3-glucuronide reaching the liver via the vasculature. We have quantitated the cellular efflux kinetics (cell-to-blood and cell-to-bile) of morphine-3-glucuronide in the rat isolated perfused liver using a loading wash-out design. In the 'loading' phase, morphine was infused into the liver (2.7 microM) and the biliary excretion and sinusoidal efflux of morphine-3-glucuronide was assessed under steady-state conditions. Subsequently, the infusion was stopped and the concentration vs time profile of morphine-3-glucuronide in outflow perfusate (the wash-out phase) was determined. A physiologically-based pharmacokinetic model was used to determine the rate-constants for the movement of hepatically-generated morphine-3-glucuronide into the sinusoidal and canalicular spaces of the liver, and the associated membrane permeability terms. The mean (+/-s.d.) rate constants for the biliary excretion and sinusoidal efflux of morphine-3-glucuronide were determined to be 0.160 +/- 0.043 and 0.169 +/- 0.068 min(-1), respectively, and the corresponding membrane permeability parameters were 1.12 and 1.18 mL min(-1), respectively. The sinusoidal membrane permeability term was significantly less than hepatic blood flow in the rat. The volume of distribution of hepatically-generated morphine-3-glucuronide (207.5 +/- 74.8 mL) was found to be approximately 50-times the intracellular space of the rat liver, suggesting that hepatically-generated morphine-3-glucuronide accumulates within hepatocytes. The results indicate that hepatically-generated morphine-3-glucuronide undergoes intracellular accumulation, probably as a consequence of poor membrane permeability.
authors.affiliation: Centre for Pharmaceutical Research, School of Pharmacy and Medical Sciences, University of South Australia, Adelaide, Australia. allan.evans@unisa.edu.au
authors.forename: A M
authors.initials: AM
authors.lastname: Evans
authors.forename: J
authors.initials: J
authors.lastname: O'Brien
authors.forename: R L
authors.initials: RL
authors.lastname: Nation
mesh_terms: D000465:Algorithms; D000818:Animals; D019540:Area Under Curve; D001646:Bile / Q000378:metabolism; D005260:Female; D066298:In Vitro Techniques; D008099:Liver / Q000737:chemistry / Q000166:cytology / Q000378:metabolism*; D008954:Models, Biological; D009022:Morphine Derivatives / Q000737:chemistry / Q000493:pharmacokinetics*; D010101:Oxygen Consumption / Q000187:drug effects; D010477:Perfusion; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D009022:Morphine Derivatives; C018108:morphine-3-glucuronide