ENHANCEMENT BY O-6-BENZYL-N-ACETYLGUANOSINE DERIVATIVES OF CHLOROETHYLNITROSOUREA ANTITUMOR ACTION IN CHLOROETHYLNITROSOUREA-RESISTANT HUMAN-MALIGNANT MELANOCYTES

The exposure of cells to O-6-methylguanine or O-6-benzylguanine is kno wn to reduce the enzymatic activity of O-6-alkylguanine-DNA alkyltrans ferase, which leads to a sensitivity enhancement to chloroethylnitroso urea cytotoxic effects. The main disadvantage of the guanine derivativ es is their low water solubility, which makes their formulation diffic ult for clinical use in humans. To overcome this problem, water-solubl e O-6-alkylguanine-DNA alkyltransferase inhibitors have been synthesiz ed and their ability to increase the chloroethylnitrosourea potency in vitro and in vivo was evaluated. Four water-soluble molecules (O-6-me thyl-N-acetylguanosine; O-6-methyl-N-acetyldeoxyguanosine; O-6-benzyl- N-acetylguanosine, BNAG; and O-6-benzyl-N-acetyldeoxyguanosine, BNADG) were tested for sensitivity of M4Beu cells to roethyl]-N[2-(methylsul fonyl)ethyl]-N'-nitrosourea (cystemustine) based on the colony-forming ability of M4Beu melanoma cells. The cell sensitivity to cystemustine was increased by benzylated derivative pretreatment but not with meth ylated derivative pretreatment. Furthermore, BNAG or BNADG pretreatmen t followed by cystemustine was less cytotoxic than BNAG or BNADG given simultaneously and followed 24 hr later by BNAG or BNADG. Comparative studies performed with O-6-benzylguanine on the same model showed tha t this inhibitor was effective at lower concentrations than the corres ponding guanosine or deoxyguanosine analogs. Preliminary pharmacologic al assays were carried out in nude mice bearing the M4Beu tumor to det ermine whether the BNAG-cystemustine combination has greater antitumor activity than cystemustine alone. Simultaneous i.p. injection of 200 mg/kg of BNAG and 15 mg/kg of cystemustine followed by an i.p. injecti on 4 hr later of 200 mg/kg of BNAG led to a significant enhancement of inhibition of tumor growth.