A Simple And Quantitative Method To Address The Synchronicity Between Uptake And Efflux Transporters In The Rat Liver Using Tc-99m-mebrofenin Scintigraphy

Complete text of publication follows: Aim/Introduction: Imaging probes are needed to noninvasively and quantitatively assess the interplay between liver uptake and biliary secretion, which controls the clearance of many endogenous compounds and xenobiotics. ^99mTc-mebrofenin is a readily available radiopharmaceutical for hepatobiliary scintigraphy. Previous studies performed in vitro and in transporter-deficient mice have shown that the hepatobiliary transport of ^99mTc-mebrofenin involves organic anion-transporting polypeptide (OATP) transporters at the sinusoidal (blood-liver) interface, and the efflux transporter MRP2 at the canalicular (liver-bile) interface. The aim of our study was to develop a simple quantification method to describe the activity of these transport systems in vivo. Materials and Methods: ^99mTc-mebrofenin was injected into control rats (A_inj=42±6 MBq; n=5) and immediately after a bolus of rifampicin (40 mg/kg IV, A_inj=42±3 MBq; n=5), a potent OATP/MRP2 inhibitor. Planar dynamic acquisitions were performed with a clinical Siemens Symbia gamma-camera during 40 minutes. Regions of interest were drawn over the heart (blood-pool), 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 modeling considering a mean volume for organs and a dual blood input function for the liver (hepatic artery and portal vein). Outcome parameters were compared using the Mann-Whitney test (p rifampicin=0.33±0.03 min^-1). However, the liver-to-intestine transfer-rate constant could not be accurately determined with this method. AUCR_liver/blood estimated from 0 to 2 minutes was significantly decreased by rifampicin (control=1.98±0.22 vs rifampicin=0.81±0.05). A significant decrease was also observed for AUCR_{intestine/liver} estimated from 10 to 25 minutes (control=1.36±0.20 vs rifampicin=0.64±0.16). These results were confirmed by compartmental modeling, which identified a significant decrease for rifampicin-treated rats in k1 (blood-to-hepatocyte exchange, control=7.37±1.62 min^-1 vs rifampicin=0.88±0.09 min^-1), k2 (hepatocyte-to-blood backflux, control=0.15±0.08 min^-1 vs rifampicin=0.01±0.01 min^-1) and k3 (hepatocyte-to intrahepatic bile-duct exchange, control=0.15±0.08 min^-1 vs rifampicin=0.05±0.02 min^-1) but not for k4 (intrahepatic bile-duct to intestine, control=0.12±0.03 min^-1 vs rifampicin=0.64±0.88 min^-1). Conclusion: ^99mTc-mebrofenin planar scintigraphy offers a readily available method to study the interplay between sinusoidal and canalicular transporters in mediating hepato-biliary 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 back-flux of ^99mTc-mebrofenin from liver to blood, most likely MRP3