MOLECULAR MECHANISMS OF TOXIC EFFECTS OF FOTEMUSTINE IN RAT HEPATOCYTES AND SUBCELLULAR RAT-LIVER FRACTIONS

Fotemustine is a clinically used DNA-alkylating 2-chloroethyl-substitu ted N-nitrosourea, which sometimes shows signs of haematotoxicity and reversible liver and renal toxicity as toxic side-effects, Mechanistic data on these side-effects are scarce and incomplete, In this study, firstly the cytotoxicity of fotemustine in freshly isolated rat hepato cytes was investigated and secondly the metabolism of fotemustine and possible mechanisms involved in the observed cytotoxicity, Fotemustine caused concentration- and time-dependent cytotoxic effects in rat hep atocytes. Extensive GSH-depletion and formation of GSSG were first obs erved, followed by lipid peroxidation and finally by cell death measur ed as LDH-leakage. 2-Chloroethyl analogues of fotemustine, which in co ntrast to fotemustine have no carbamoylating potency, were not toxic t o rat hepatocytes, The data suggest that the cytotoxicity of fotemusti ne is resulting from its reactive decomposition product, DEP-isocyanat e, GSH-conjugation of DEP-isocyanate was shown to protect against the cytotoxicity of fotemustine, however, only temporary and not completel y, Synthetical DEP-SG, the GSH-conjugate of DEP-isocyanate, was also t oxic to rat hepatocytes, albeit to a significantly lesser extent than fotemustine. In rat liver microsomes, no fotemustine-induced LPO was o bserved, suggesting that reactive decomposition products of fotemustin e do not directly cause peroxidation of cellular membranes, Fotemustin e did not affect the antioxidant enzymes superoxide dismutase, catalas e, GSH-peroxidase, GSSG-reductase and GSH S-transferases. Thus, direct effects on these antioxidant enzymes are not likely to explain the cy totoxic effects of fotemustine in hepatocytes. In conclusion, it is pr oposed that the cytotoxicity of fotemustine in rat hepatocytes is caus ed by rapid and extensive depletion of GSH by DEP-isocyanate, a reacti ve decomposition product of fotemustine, consequently hampering the en dogenous protection against its own toxicity, Knowledge of molecular m echanisms of the cytotoxicity of fotemustine may contribute to a more rational design of selective protection against toxic side-effects whi ch occur upon therapy of patients with fotemustine.