IN-VITRO TOXICITY OF ZAMIFENACIN (UK-76,654) AND METABOLITES IN PRIMARY HEPATOCYTE CULTURES

1. We compared the sensitivities of primary hepatocytes from rat, dog and monkey to zamifenacin and two major metabolites, the methylenediox y ring-opened catechol, UK-80,178 and its methylated product, UK-82,20 1. Toxicity was determined both via neutral red uptake and enzyme leak age data. 2. Canine hepatocytes were most sensitive to the cytotoxic e ffects of zamifenacin during 24-h exposure. Significant decreases in m edium concentrations of zamifenacin in the presence of primary hepatoc ytes verified cellular uptake during the initial 2-h incubation. All t hree cell types were much more sensitive to UK-82,201 than to the cate chol metabolite or parent drug. 3. The rapid onset of cytotoxicity ind icated by elevations of alanine aminotransferase (ALT), aspartate amin otransferase (AST) and other markers in the medium after UK-82,201 exp osure, the delayed but substantial cytotoxic response to the parent dr ug which was suggestive of biotransformation to a reactive moiety, in vivo and in vitro drug metabolism results and subacute toxicology data suggest that dog may more effectively transform zamifenacin into UK-8 2,201, which is relatively hepatotoxic. 4. Because the catechol was ge nerally less toxic than the O-methylated product, species that elimina te zamifenacin primarily as the catechol or its conjugate may be less affected by the potential hepatotoxicity of the methylated product. Ou r studies show that dog is the most sensitive species due to metabolis m of the common catechol metabolite. The low incidence of potential he patotoxicity in the clinic points to rare but important differences in the metabolism of Zamifencin. We conclude that the findings in dog we re not predictive of subsequent effects in man.