METHYL DNA-ADDUCTS, DNA-REPAIR, AND HYPOXANTHINE-GUANINE PHOSPHORIBOSYL TRANSFERASE MUTATIONS IN PERIPHERAL WHITE BLOOD-CELLS FROM PATIENTSWITH MALIGNANT-MELANOMA TREATED WITH DACARBAZINE AND HYDROXYUREA

Dacarbazine (DTIC) is a DNA-methylating drug used in the treatment of malignant melanoma, Among the DNA dducts induced by DTIC are N7-methyl guanine (N7-meG) and O-6-methylguanine (O-6-meG). The latter adduct, i n particular, may be important in the mutagenic as well as the cytotox ic activity of DTIC, Repair of O-6-meG is carried out by the enzyme O- 6-alkylguanine-DNA-alkyltransferase (AGT) by a process which results i n its autoinactivation, N7-meG is lost from DNA partly spontaneously a nd partly by enzymatic depurination followed by excision repair of the resulting apurinic site, The purpose of this study was to determine t he in vivo kinetics of formation and repair of O-6-meG and N7-meG and the changes in AGT in peripheral WBCs with repeated doses of DTIC, and to determine the effects on these processes of concomitant administra tion of hydroxyurea, In addition, we examined the induction of mutatio ns at the HPRT gene locus, Thirty-four patients with malignant melanom a received 1.0 g/m(2) DTIC i.v. every 3 weeks, Hydroxyurea was added t o the second and subsequent doses of DTIC in 19 patients. The concentr ations of O-6-meG, N7-meG, and AGT in peripheral blood lymphocytes wer e determined up to 24 h after each of the first two doses of DTIC, Mut ations at the HPRT gene locus were determined using the T-cell clonal assay, Peak O-6-meG levels were detected 1 and 4 h after the first and second dose of DTIC, respectively. AGT concentrations declined to 56. 7% (range, 40.3-76.9%) and 55.0% (range, 45.4-58.9%) of pretreatment l evels 24 h after the first and second doses of DTIC, respectively, and were still approximately 25% below their initial levels just prior to administration of the second dose of DTIC. An increase in formation o f O-6-meG was observed at all time points after the second dose of DTI C (P = 0.0001), which was not affected by cotreatment with hydroxyurea (P > 0.5). There was a negative correlation between pretreatment AGT levels and the O-6-meG concentration at 24 h after therapy (r = -0.554 , P = 0.014). N7-meG levels peaked at 6 h after DTIC therapy and were not significantly influenced by the cycle number, Cotreatment with hyd roxyurea tended to be associated with lower levels of N7-meG (P = 0.08 ). There was no correlation between either O-6-meG or N7-meG levels an d the grade of neutropenia, On the basis of a limited series of blood samples analyzed, there was no firm evidence that chemotherapy with DT IC resulted in induction of HPRT mutations in lymphocytes. In conclusi on, repeated administrations of DTIC resulted in higher concentrations of O-6-meG, probably due to reduction in cellular AGT, Hydroxyurea di d not significantly influence the kinetics of O-6-meG, and N7-meG addu ct formation. There was no significant induction of HPRT gene mutation s with DTIC, This study suggests that sequencing of DTIC doses should be evaluated using the time course of cellular AGT depletion and DNA a dduct formation to achieve higher cytotoxic efficiency.