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Formation of DNA Adducts and Induction of lacI Mutations in Big Blue Rat-2 Cells Treated with Temozolomide

Implications for the Treatment of Low-Grade Adult and Pediatric Brain Tumors¹

Laboratory of Molecular Therapeutics, Brain Tumor Research Center of the Department of Neurological Surgery, University of California, San Francisco, California 94143-0555

Temozolomide (TMZ) is a chemotherapeutic agent used in the treatment of high-grade brain tumors. Treatment of patients with alkylating chemotherapeutic agents has been established to increase their risk for acute myelogenous leukemia. The formation of DNA adducts and induction of mutations are likely to play a role in the etiology of therapy-related acute myeloid leukemia. To evaluate this issue for TMZ, we have measured the formation of DNA adducts and induction of lacI mutations in Big Blue Rat-2 cells treated with TMZ. Treatment of Big Blue Rat-2 cells with either 0, 0.5, or 1 mM TMZ resulted in lacI mutant frequencies of 9.1 ± 2.9 x 10^-5, 48.9 ± 12 x 10^-5, and 89.7 ± 40.3 x 10^-5, respectively. Comparison of the mutant frequencies demonstrated that 0.5 and 1 mM TMZ treatments increased the mutant frequencies by 5.3- and 9.8-fold and that this increase was significant (P < 0.001). Sequence analysis of the lacI mutants from the TMZ treatment group demonstrated that they were GCAT transitions at non-CpG sites, which is significantly different from the mutation spectrum observed in the control treatment group. Treatment of Big Blue Rat-2 cells with various concentrations of TMZ produced a linear increase in the levels of N7-methylguanine and O⁶-methylguanine. The lacI mutation spectrum induced by TMZ treatment is consistent with these mutations being produced by O⁶-MeG. This study establishes TMZ has significant mutagenic potential and suggests that careful consideration in the use of TMZ for the treatment of low-grade adult and pediatric brain tumors should be given.

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