FORMATION AND LOSS OF O-6-METHYLDEOXYGUANOSINE IN HUMAN-LEUKOCYTE DNA FOLLOWING SEQUENTIAL DTIC AND FOTEMUSTINE CHEMOTHERAPY

There is increasing evidence to indicate that O-6-methyldeoxyguanosine (O-6-MedG) formation in DNA is a critical cytotoxic event following exposure to certain anti-tumour alkylating agents and that the DNA repair protein O-6-alkylguanine-DNA alkyltransferase (ATase) can confer resistance to these agents. We recently demonstrated a wide inter-individual variation in the depletion and subsequent regeneration of ATase in human peripheral blood lymphocytes following sequential DTIC (400 mg m(-2)) and fotemustine (100 mg m(-2)) treatment, with the nadir ATase activity occurring approximately 4 h after DTIC administration. We have now measured the formation and loss of O-6-methyldeoxyguanosine (O-6-MedG) in the DNA of peripheral leucocytes of eight patients receiving this treatment regimen. O-6-MedG could be detected within 1 h and maximal levels occurred approximately 3-5h after DTIC administration. Following the first treatment cycle, considerable inter-individual variation was observed in the peak O-6-MedG levels, with values ranging from 0.71 to 14.3 mu mol of O-6-MedG per mol of dG (6.41 +/- 5.53, mean +/- s.d.). Inter- and intra-individual variation in the extent of O-6-MedG formation was also seen in patients receiving additional treatment cycles. This may be a consequence of inter-patient differences in the capacity for metabolism of DTIC to release a methylating intermediate and could be one of the determinants of clinical response. Both the pretreatment ATase levels and the extent of ATase depletion were inversely correlated with the amount of O-6-MedG formed in leucocyte DNA when expressed either as peak levels (r = -0.59 and -0.75 respectively) or as the area under the concentration-time curve (r = -0.72 and -0.73 respectively). One complete and one partial clinical response were seen, and these occurred in the two patients with the highest O-6-MedG levels in the peripheral leucocyte DNA, although the true significance of this observation has yet to be established.