O6-benzylguanine potentiates the in vivo toxicity and clastogenicity of temozolomide and BCNU in mouse bone marrow.

The effects of treatment of mice with O6-benzylguanine (O6-BeG) on the levels of O6-alkylguanine-DNA alkyltransferase (ATase) in the hematopoietic compartment and on the in vivo sensitivity of hematopoietic progenitor cells to the toxic and clastogenic effects of the antitumor agents 1,3-bis(2-chloroethyl)-nitrosourea (BCNU) and temozolomide were studied. When the overall effects of BCNU alone or with O6-BeG pretreatment were compared, dose potentiating factors of 4.17 for marrow cellularity, 4.57 for granulocyte macrophage-colony forming cells (GM-CFC) and 8.25 for colony forming unit-spleen (CFU-S) in O6-BeG pretreated versus nonpretreated animals were observed. A similar trend of dose potentiation was observed for temozolomide, although it was of lower magnitude: 1.20 for marrow cellularity, 1.63 for GM-CFC, and 1.68 for CFU-S. When the clastogenic effects of BCNU and temozolomide were examined in the mouse bone marrow micronucleus assay, a significantly (P < .05 to .001) higher frequency of micronuclei formation was observed in mice that received O6-BeG pretreatment compared with mice that received no pretreatment. These data suggest that the use of O6-BeG as a tumor-sensitizing agent before treatment of patients with O6-alkylating agents may lead to more severe hematological toxicity and possibly to an increased incidence of secondary leukemias as a result of elevated mutation frequencies in these patients.