Accumulation of O6-Methylguanine in Human DNA after Therapeutic Exposure to Methylating Agents and Its Relationship with Biological Effects

O6-Methylguanine has been measured in peripheral blood leukocytes of 14 patients during one or more cycles of treatment with procarbazine (daily treatment for 10 days) and in 12 patients during one or more cycles of treatment with dacarbazine (single dose per cycle). Adduct formation at levels up to about 0.4 fmole/μg DNA was detected in all procarbazine-and all but one dacarbazine-treated patients at some point after treatment. O6-Methylguanine accumulated during procarbazine treatment in a dose-related manner (mean rate of accumulation 2.8× 10-4fmole/μ g DNA per mg/ m2dose) and appeared to approach a plateau by the end of the cycle (above 600 mg/ m2cumulative dose). The average rate of O6-methylguanine formation 2 hr after dacarbazine treatment was 11± 8× 10-4fmole/μ g DNA per mg/ m2dose. Individuals examined on more than one treatment cycle with either drug showed broadly similar methylation responses. The rate of adduct accumulation showed a nonsignificant, negative correlation with the pretreatment lymphocyte levels of the repair enzyme O6-alkylguanine- DNA alkyltransferase (AGT) in the case of procarbazine and no correlation in the case of dacarbazine. No consistent lymphocyte AGT depletion was noted as a result of treatment with either drug. No correlation between O6-methylguanine formation and hematological toxicity was observed. In eight patients showing full remission after treatment with dacarbazine, the value of O6-methylguanine (averaged over all the cycles) was 0.252 ± 0.120 fmole/μg DNA while in four patients showing partial or no response it was 0.087 ± 0.110 fmole/μg DNA (p < 0.05). One of the two nonresponding procarbazine-treated patients showed very low levels of O6-methylguanine during a cycle of observation of DNA adduct formation. Human blood leukocytes are about 5-fold less susceptible than those of the rat to accumulation of O6-methylguanine during exposure to procarbazine. Furthermore, similar adduct levels were found in rat peripheral blood leukocytes and lymphocytes, bone marrow, and lymph nodes, suggesting that levels observed in human blood leukocytes may reflect those in the presumed target tissues for the leukemogenic (bone marrow) and the therapeutic (lymph nodes) effects of procarbazine. The possible use of these observations in the assessment of carcinogenic risks to humans exposed to procarbazine or environmental methylating agents such as dimethylnitrosamine is discussed.