Hexamethylmelamine: Metabolic Activation and Cytotoxicity

Abstract

Hexamethylmelamine (HMM) is an investigational s-triazine antitumour agent with established antitumour activity against human malignancies. We have shown that HMM is metabolically activated by hepatic monooxygenases to reactive species which covalently bind to microsomal protein and to calf thymus DNA. The N-methylol intermediate in HMM demethylation covalently binds to protein and calf thymus DNA in the absence of activating systems (Ames et al, Cancer Res. 43: 500, 1983). When HMM is incubated for 1hr with continuous human tumour cell lines in culture, no inhibition of colony formation is observed at concentrations as high as 200µg/ml. In this same system, mitomycin C totally abolishes colony formation at concentrations of 0.4µg/ml. When HMM is similarly incubated in the presence of 9,000 x g rat liver supernatant activating system, colony formation is inhibited only when NADPH is added to incubation mixtures. Cyclophosphamide was used as a positive control for these studies. We have also determined that HMM is significantly metabolised in an NADPH dependent manner by rat kidney, lung and brain microsomal preparations, but not by tumour cells under identical conditions. Cytotoxicity appears to be dependent on conversion of HMM to alkylating metabolites by hepatic and possibly extrahepatic tissues rather than formation of such metabolites by tumour cells. Supported in part by RCDA CA0755 (MMA), and CA30250, DHHS.