In vivo administration of O-6-benzylguanine does not influence apoptosis or mutation frequency following DNA damage in the murine intestine, but doesinhibit P450-dependent activation of dacarbazine

Clinically relevant cancer chemotherapeutic alkylating agents such as temoz olomide and dacarbazine induce apoptosis and are mutagenic via the formatio n of O-6-alkylguanine adducts in DNA, The DNA repair protein O-6-alkylguani ne-DNA alkyltransferase (AGT) functions by dealkylating such adducts and ca n thus prevent apoptosis and mutagenesis. In attempts to maximize the clini cal effectiveness of these alkylating agents, inhibitors of AGT such as O-6 -benzylguanine (BeG) have been developed. We show here that within murine s mall intestinal crypt cells, BeG administration does not alter the apoptoti c response to the direct-acting methylating agents N-methyl-N-nitrosurea (M NU), temozolomide and N-methyl-N' -nitro-N-nitrosoguanidine. Furthermore, w e show that BeG pretreatment fails to elevate the mutation frequency at the murine Dlb-1 locus following exposure to MNU. Consistent with these result s, we show that intestinal AGT activity is effectively abolished by adminis tration of 100 mg/kg temozolomide, even in the absence of BeG. In contrast, pretreatment with BeG transiently abolished the apoptotic response to the methylating prodrug dacarbazine. Activation of dacarbazine to its reactive intermediate has previously been shown to be cytochrome p450 dependent and me show here that pretreatment of mice with the cytochrome P450 inhibitor m etyrapone also inhibits dacarbazine-induced apoptosis, Thus BeG increases n either the prevalence of apoptosis nor mutation frequency in the murine sma ll intestine, but is capable of inhibiting P450-dependent prodrug activatio n. The positive implication from this study is that BeG treatment may not e xacerbate the toxic and mutagenic effects of methylating agents within norm al cells, although it may engender other adverse reactions through the supp ression of cytochrome P450-dependent processes.