Unexpected genetic toxicity to rodents of the N',N'-dimethyl analogues of MNU and ENU

Lijinsky and his colleagues have reported that the N',N'-dimethyl analogues of ENU and MNU [N',N'-dimethyl-N-ethyl-N-nitrosourea (DMENU) and trimethylnitrosourea (TMNU), respectively] are carcinogenic to rats despite their extreme hydrolytic stability which would reduce or preclude generation of alkylating species analogous to those formed upon hydrolysis of ENU and MNU. Lijinsky and his colleagues were unable to rationalize those activities of DMENU and TMNU despite extensive experimentation. We therefore decided to study this problem further. Whichever mode is accepted for the generation of electrophilic/mutagenic/carcinogenic reactive species from ENU and MNU, blocking of the free-NH2 group with methyl groups (-NMe2) should ablate or abolish activity. Consistent with this DMENU and TMNU gave negative results in the NBP alkylation test while the parent compounds gave an instantaneous deep blue coloration. Studies of the rate of hydrolysis of these four compounds revealed ENU and MNU to have half-lives of 8 min, while the alkylated analogues (DMENU and TMNU) had half-lives of 25 and 41 days, respectively. Hydrolysis of ENU and MNU, to yield the alkylating species, proceeds either via proton abstraction from the -NH2 group or by attack by water on the carbon of the carbonyl group. Methylation will inhibit both of these pathways, the first absolutely (no -NH2 protons) and the second partially, via steric inhibition. The slow hydrolysis observed for DMENU and TMNU suggests that the latter route of hydrolysis is applicable. Studies with strain TA1535 of Salmonella typhimurium (without S9 mix) confirmed the potent mutagenic activity for ENU and MNU (approximately 300-fold increase in revertants at 2,000 micrograms/plate and approximately 180-fold increase in revertants at 150 micrograms/plate respectively). In contrast, the methylated analogues showed only weak mutagenic activity (approximately 3-fold) at approximately 100-fold higher dose-levels. Addition of S9 mix did not affect the mutagenicity of DMENU or TMNU. To this point, hypothesis and data coincide. ENU and MNU are potent micronucleus-inducing agents to the mouse bone marrow, and given the above data, it was expected that DMENU and TMNU would show weak or no activity in that assay. In fact, the methylated analogues were as effective as ENU and MNU as clastogens to the mouse bone marrow. Four possible reasons for this conflict of theory and data are explored. The speculative explanation we favour for