Effect of a hepatic activation system on the antiproliferative activity of hexamethylmelamine against human tumor cell lines.
Incubation of hexamethylmelamine for 1 h with human tumor cell lines in culture did not inhibit colony formation at concentrations up to the limit of drug solubility (200 micrograms/ml). When 1-h incubations were carried out in the presence of a 9,000 g rat liver supernatant preparation and an NADPH-generating system, hexamethylmelamine markedly reduced colony formation. Cyclophosphamide inhibition of colony formation was also dependent on the presence of a 9,000 g supernatant preparation and an NADPH-generating system in incubation mixtures. A 1-h incubation of N-methylolpentamethylmelamine (a DNA-alkylating metabolite formed during N-demethylation of hexamethylmelamine) with human tumor cell lines reduced colony formation in the absence of the liver-activating system. Substantial NADPH-dependent N-demethylation of hexamethylmelamine was observed with rat liver, lung, and kidney microsomal preparations. In contrast, little or no HMM metabolism was observed with tumor cells, tumor cell homogenates, or NADPH-fortified tumor cell microsomal preparations. NADPH-dependent formation of cytotoxic metabolites is a prerequisite for antiproliferative activity of hexamethylmelamine against these human tumor cell lines. In vivo activity of hexamethylmelamine against some tumors may require metabolism by normal cells and subsequent transport of active species to the tumor site.