We investigated the cytotoxic effects of nitrosoureas with and without a 42-hr preincubation with the ornithine decarboxylase (EC 22.214.171.124) inhibitor α-difluoromethylornithine (DFMO, 1 mM) in a MER+ (methylation excision repair positive) human cell line. DFMO combined with a chloroethyl nitrosourea [1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) or 1-(2-chloroethyl)-1-nitrosourea (CNU)] yielded increased toxicity with D37 ratios of 1.9 and 3.3 respectively. There was no enhanced toxicity with the monofunctional nitrosourea 1-ethyl-1-nitrosourea (ENU). BCNU or CNU did not induce DNA-DNA interstrand crosslinks in cells with or without a DFMO pretreatment. DNA single-strand breakage was not increased by addition of DFMO. BCNU-induced DNA-protein crosslinking was decreased in cells pretreated with DFMO. These findings are similar to those in MER- cells in that the chloroethyl carbonium alkylating species is required for the enhanced cytotoxicity seen with DFMO. The ability to form DNA interstrand crosslinks, however, does not appear to be necessary for this toxicity enhancement.
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