Studies on the Mechanism of CB 3717 Induced Hepatotoxicity
CB 3717 is a folate-based inhibitor of thymidylate synthetase currently undergoing clinical evaluation. However, its assessment has been hindered by the frequent occurrence of hepatotoxicity. In an attempt to elucidate the mechanism of this toxicity, studies have been performed in mice using a therapeutic but non-toxic dose (100mg/kg i.p.). Experiments with 14C-CB 3717 demonstrated that hepatic uptake of CB 3717 is rapid and extensive (at t = 1.33hr, hepatic 14C-CB 3717 = 2.2mM). In view of the known ability of organic anions to bind to and inhibit hepatic glutathione-s-transferases (EC 188.8.131.52, GSH-T) (Kaplowitz, Am. J. Physiol. 239: G439-G444, 1980) the inhibition of GSH-T by CB 3717 was examined in vitro and weak noncompetitive inhibition demonstrated (Ki = 400 µM). In vivo, pretreatment (2hr) with CB 3717 slightly reduced the paracetamol induced depletion of liver GSH, suggesting that inhibition of GSH-T by CB 3717 may have pathological significance. Indeed, pretreatment (2hr) of mice with CB 3717 increased the whole animal toxicity of paracetamol (LD50: paracetamol alone 476mg/kg; CB 3717 + paracetamol 283mg/kg). However, a more interesting observation is that CB 3717 alone induces a moderate (30-40$) and sustained (0.5 - >4hr) depletion of liver GSH. This effect of CB 3717, plus its weak inhibition of GSH-T may make patients more susceptible to hepatotoxic insults. If this is so, the concurrent administration of a nucleophile should reduce the hepatotoxicity of CB 3717 in man.