TY - JOUR T1 - <strong>New pharmacokinetic parameters of imaging substrates quantified from rat liver compartments</strong> JF - Drug Metabolism and Disposition JO - Drug Metab Dispos DO - 10.1124/dmd.121.000546 SP - DMD-AR-2021-000546 AU - Catherine M Pastor AU - Kim LR Brouwer Y1 - 2021/01/01 UR - http://dmd.aspetjournals.org/content/early/2021/10/20/dmd.121.000546.abstract N2 - Hepatobiliary imaging is increasingly used by pharmacologists to quantify liver concentrations of transporter-dependent drugs. However, liver imaging does not quantify concentrations in extracellular space, hepatocytes, and bile canaliculi. Our study compared the compartmental distribution of two hepatobiliary substrates Gadobenate dimeglumine (BOPTA, 0.08 liver extraction ratio, ER) and Mebrofenin (MEB, 0.93 ER) in a model of perfused rat liver. A gamma counter placed over livers measured liver concentrations. Livers were pre-perfused with Gadopentetate dimeglumine (DTPA) to measure extracellular concentrations. Concentrations coming from bile canaliculi and hepatocytes were calculated. Transporter activities were assessed by concentration ratios between compartments and pharmacokinetic parameters that describe the accumulation and decay profiles of hepatocyte concentrations. The high liver concentrations of MEB relied mainly on hepatocyte and bile canaliculi concentrations. In contrast, the three compartments contributed to the low liver concentrations obtained during BOPTA perfusion. Non-linear regression analysis of substrate accumulation in hepatocytes revealed that cellular efflux is measurable ~4 min after the start of perfusion. The hepatocyte to extracellular concentration ratio measured at this time-point was much higher during MEB perfusion. BOPTA transport by Mrp2 induced an aquaporin-mediated water transport, while MEB transport did not. BOPTA clearance from hepatocytes to bile canaliculi was higher than MEB clearance. MEB did not efflux back to sinusoids while BOPTA basolateral efflux contributed to the decrease in hepatocyte concentrations. In conclusion, our ex-vivo model quantifies substrate compartmental distribution and transport across hepatocyte membranes and provides an additional understanding of substrate distribution in the liver. Significance Statement When transporter-dependent drugs target hepatocytes, cellular concentrations are important to investigate. Low concentrations on cellular targets impair drug therapeutic effects, while excessive hepatocyte concentrations may induce cellular toxicity. With a gamma counter placed over rat perfused livers, we measured substrate concentrations in the extracellular space, hepatocytes, and bile canaliculi. Transport across hepatocyte membranes was calculated. The study provides an additional understanding of substrate distribution in the liver. ER -