Molecular dosimetry for sister-chromatid exchange induction and cytotoxicity by monofunctional and bifunctional alkylating agents

The induction of sister-chromatid exchanges (SCEs) and cytotoxicity in 9L cells treated with monofunctional and bifunctional alkylating agents has been investigated. Three classes of monofunctional and bifunctional agents were studied: nitrosoureas, mustards and epoxides. Independent of class the bifunctional agents were 55–630-fold more effective at inducing SCEs and 300–2400-fold more effective at inducing cellular cytotoxicity than the corresponding monofunctional agents. Comparing the induction of SCEs and cytotoxicity by these agents showed that these two cellular responses to DNA damage are highly correlated. The extent of DNA alkylation in cells treated with 1-ethyl-1-nitrosourea (ENU) or 1-(2-chloro-ethyl)-1-nitrosourea (CNU) was similar indicating that the increased effectiveness of CNU to induce SCEs and cytotoxicity was not due to increased DNA alkylation. Molecular dosimetry calculations indicate that for CNU and ENU treatment of 9L cells there are 116 and 8500 alkylations per SCE induced and 2.6 × 10⁴ and 4.6 × 10⁶ alkylations at the dose required to reduce survival of 9L cells by 90%. Comparison of the DNA alkylation products produced by CNU and ENU treatment of 9L cells suggests that the formation of the intrastrand crosslink N⁷-bis(guanyl)ethane the interstrand crosslink 1-(3-deoxycytidyl)-2-(1-deoxyguanosinyl)ethane by CNU is responsible for the increased effectiveness of CNU treatment at both induction of SCEs and cytotoxicity.