Influence of Polyamine Depletion Caused by α-Difluoromethylornithine, an Enzyme-activated Irreversible Inhibitor of Ornithine Decarboxylase, on Alkylation- and Carbamoylation-induced Cytotoxicity in 9L Rat Brain Tumor Cells in Vitro¹

Using a colony-forming efficiency assay, we studied the effect of polyamine depletion on the cytotoxicity of four nitrosoureas with different capacities to alkylate and/or carbamoylate biomolecules. 9L rat brain tumor cells were treated with 10 mm α-difluoromethylomithine for 48 hr before a 1-hr treatment with nitrosoureas. The cytotoxicity of 2-[3-(2-chloroethyl)-3-nitrosoureido]-d-glucopyranose, which alkylates and subsequently cross-links DNA but does not carbamoylate, was significantly increased by depletion of intracellular polyamines; the dose enhancement ratio of 1.3 is identical to that found for 1,3-bis(2-chloroethyl)-1-nitrosourea and 1-(2-chloroethyl)-3-trans-4-methylcyclohexyl-1-nitrosourea in previous studies. Addition of exogenous putrescine to polyamine-depleted 9L cells 24 hr before treatment prevented this phenomenon. In contrast, the cytotoxicity of 1,3-bis(trans-4-hydroxycyclohexyl)-1-nitrosourea, which carbamoylates only, was significantly decreased in polyamine-depleted cells. This compound alone reduced intracellular polyamine levels. Polyamine depletion did not affect the cytotoxicity of the monoalkylating nitrosoureas N-ethyl-N-nitrosourea and N-methyl-N-nitrosourea. Thus, polyamine depletion apparently potentiates the cytotoxicity only of chloroethylnitrosoureas that alkylate and cross-link.