Cytotoxicity and sister chromatid exchanges in 9L cells treated with monofunctional and bifunctional nitrogen mustards.

To investigate the role of DNA interstrand crosslink formation on cytotoxicity and the induction of sister chromatid exchanges (SCEs), we treated 9L cells with the bifunctional mustard bis(2-chloroethyl)methylamine (HN2), which can alkylate DNA and form DNA interstrand crosslinks, or with two monofunctional mustards, bis(ethyl)-2-chloroethylamine and bis(methyl)-2-chloroethylamine, which can only alkylate DNA. On a molar basis, HN2 was 900-2400 times more cytotoxic and 471-686 times more effective at inducing SCEs than the monofunctional mustards. HN2 induced high levels of DNA interstrand crosslinks in 9L cells; no interstrand crosslinks were detected in cells treated with the monofunctional mustards. Comparison of the alkylating activity of each of the mustards by 4-(p-nitrobenzyl)pyridine reactivity showed only a 4-fold difference, which is not sufficient to account for the large differences in cell survival and induction of SCEs. We conclude that the effectiveness of HN2 at inducing cytotoxicity and SCEs results from the formation of DNA interstrand crosslinks.