EVALUATION OF TC 99M MAG3 BIOCYTIN AS HEPATOBILIARY AGENT

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Summary of Work: In the course of developing a Tc-99m radiopharmaceutical that incorporates an avidinbiotin system to detect tumors, we have synthesized Tc-99m mercaptoacetyltriglycine-lysine- biotin (Tc-99m MAG3-biocytin). This radiopharmaceutical was shown to be a promising hepatobiliary agent because its hepatic uptake was not affected by elevated levels of bilirubin. In contrast, Tc-99m disofenin, a biliary agent of choice in clinic, shows a decreased hepatic uptake in the presence of high levels of bilirubin, thereby making it difficult to detect hepatic disease caused by hepatocyte dysfunction. During FY 97, we have tested the utility of this new hepatobiliary agent to investigate the potential benefits of synthetic inhibitors of ethanol- inducible cytochrome P450 2E1 (CYP2E1) in an acute hepatitis model caused by a single oral administration of carbon tetrachloride (CC14) in Balb/c mice. it was previously reported that CYP2E1 is responsible for the metabolism of alcohol, fatty acids, acetaminophen and CC14, and that synthetic inhibitors such as isopropyl-2-(1,3-dithioetane-2- ylidene)-2-[N-(4-methylthiazol-2-yl)-carbamoyl] acetate (YH439) and chlormethiazole suppress CYP2E1 gene transcription. For the studies on acute hepatitis mice, the animals were treated with one dose of CC14 (0.25 ml/kg) 18 h before hepatobiliary scintigraphy. For the studies on the protective effects of YH439, mice were pretreated with two doses of YH439 (50 mg/kg/day) at 48 h and 24 h and one dose of CC14 at 18 h before the scintigraphy. The whole-body scintigraphic data were acquired over a course of 30 min. Time-activity curves were generated by drawing circular regions of interest over the upper left lobe of the liver, and also the head and neck area for the blood background. The physical parameters, such as peak liver/heart ratio (Rmax), peak hepatic uptake time (Tmax), and hepatic half-clearance time (HCT) were then determined from time-activity curves. In normal mice, Tc-99m MAG3-biocytin was taken up rapidly by liver and then excreted via biliary system (Rmax 8.3, Tmax 128 s, HCT 397 s). In mice with CC14-induced hepatotoxicity, dynamic images showed impaired hepatic uptake with prolonged blood background radioactivity (Rmax 1.5, Tmax 74 s, HCT 747 s). This altered scintigraphic result was consistent with typical centrilobular necrosis in histopathology. In contrast, treatment of YH439 not only prevented the centrilobular necrosis but also significantly improved the kinetic parameters (Rmax 6.2, Tmax 97 s, HCT 377 s). This study demonstrated that the hepatotoxicity was induced by CYP2E1-mediated reaction and its protection by CYP2E1 inhibitor could be studied in vivo using external scintigraphic imaging methods.