There is increasing experimental evidence to indicate that O6-methyldeoxyguanosine (O6-MedG) formation in DNA is a critical cytotoxic event following exposure to certain antitumor alkylating agents and that the DNA repair protein O6-alkylguanine-DNA-alkyltransferase (ATase) can confer resistance to these agents. We recently demonstrated a wide interindividual variation in the depletion and subsequent regeneration of ATase in peripheral blood lymphocytes of patients treated with 24-h continuous infusion of 1-p-carboxyl-3,3-dimethylphenyltriazene (CB10-277) for metastatic melanoma. We have now measured the formation of O6-MedG in the DNA of peripheral leukocytes of nine patients receiving this treatment regimen. This lesion could be detected in DNA within 1 h and a progressive increase in adduct levels occurred during the CB10-277 infusion and for 24 h after completion. Considerable interindividual variation was observed in the peak O6-MedG levels, with values ranging from 3.0 to 23.8 mumol O6-MedG/mol deoxyguanosine (mean, 12.3 +/- 6.4 mumol O6-MedG/mol deoxyguanosine) following the first treatment cycle, possibly as a consequence of differences in the capacity of patients to metabolize CB10-277 to a methylating agent. There was, nevertheless, a clear temporal relationship between the progressive formation of leukocyte O6-MedG and lymphocyte ATase depletion. Repeated-measures regression showed that this was statistically significant (P < 0.001) during the CB10-277 infusion. A significant inverse correlation was also seen between pretreatment lymphocyte ATase activity and peak O6-MedG levels in leukocyte DNA (r = -0.73) and the area under the leukocyte O6-MedG concentration-time curve (r = -0.76). Metabolism of CB10-277 to a methylating agent could be one factor that combines with DNA repair capacity to determine clinical response, because the two responses observed in this series occurred in the two patients with the highest leukocyte O6-MedG levels and also the lowest pretreatment ATase activity. Hematological toxicity developed in the same two patients.