Kinetic models to improve the interpretation of the hepatic transport of 99mTc-mebrofenin

Abstract

Imaging probes are needed to assess the interplay between liver uptake and biliary excretion, which controls the clearance of many compounds. 99mTc-mebrofenin (MEB) is a radiopharmaceutical for hepatobiliary scintigraphy (HBS) transported through the hepatocyte by OATP transporters at the sinusoidal (blood-liver) interface, and the MRP2 transporter at the canalicular (liver-bile) interface. Our aim was to develop a simple method to separately measure the activity of these two transporter systems in vivo. 99mTc-MEB was injected into control rats (CTL, n = 5) and after a bolus of rifampicin (RIF, 40 mg/kg IV, n = 5), an OATP/MRP2 inhibitor. Planar dynamic acquisitions were performed during 40 min. ROI were drawn over the heart, liver and intestine to obtain time-activity curves (TACs). Three quantification methods were tested: (i) calculation of the transfer rate constants between regions using integration plot analysis, (ii) estimation of the area under the TAC ratio (AUCR) between regions and (iii) compartmental modelling considering a mean volume for organs and a dual blood input function for the liver (hepatic artery and portal vein). (i) RIF significantly decreased the blood-to-liver transfer rate constant (0 to 2 min, linear part) (CTL = 1.5 ± 0.2 min−1, RIF = 0.3 ± 0.0 min−1). However, the liver-to-intestine transfer rate constant could not be accurately determined with this method. (ii) AUCRliver/blood (0 to 2 min) was significantly decreased by RIF (CTL = 2.0 ± 0.2, RIF = 0.8 ± 0.1). A significant decrease was also observed for AUCRintestine/liver (10 to 25 min, CTL = 1.4 ± 0.2, RIF = 0.6 ± 0.2). (iii) These results were confirmed by compartmental modeling, which identified a significant decrease for RIF-treated rats in k1 (blood to hepatocyte transport, CTL = 7.4 ± 1.6 min−1, RIF = 0.9 ± 0.1 min−1), k2 (hepatocyte to blood backflux, CTL = 0.15 ± 0.08 min−1, RIF = 0.01 ± 0.01 min−1) and k3 (hepatocyte to intrahepatic bile-duct transport, CTL = 0.15 ± 0.08 min−1, RIF = 0.05 ± 0.02 min−1) but not for k5 (intrahepatic bile-duct to intestine, CTL = 0.12 ± 0.03 min−1, RIF = 0.64 ± 0.88 min−1). 99mTc-MEB HBS can study the interplay between sinusoidal and canalicular transporters in mediating hepatobiliary clearance in vivo. AUCR is a simple parameter to describe the respective activity of both transporter systems. Compartmental modeling enriched the interpretation of the data and revealed the importance of an additional transport system responsible for the backflux of 99mTc-MEB from liver to blood, most likely MRP3.