7-methylguanine and Melanoma

Dacarbazine (DTIC) is a DNA-methylating drug used in the treatment of malignant melanoma. Among the DNA dducts induced by DTIC are N7-methylguanine (N7-meG) and O6-methylguanine (O6-meG). The latter adduct, in particular, may be important in the mutagenic as well as the cytotoxic activity of DTIC. Repair of O6-meG is carried out by the enzyme O6-alkylguanine-DNA-alkyltransferase (AGT) by a process which results in its autoinactivation. N7-meG is lost from DNA partly spontaneously and partly by enzymatic depurination followed by excision repair of the resulting apurinic site. The purpose of this study was to determine the in vivo kinetics of formation and repair of O6-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 administration of hydroxyurea. In addition, we examined the induction of mutations at the HPRT gene locus. Thirty-four patients with malignant melanoma received 1.0 g/m2 DTIC i.v. every 3 weeks. Hydroxyurea was added to the second and subsequent doses of DTIC in 19 patients. The concentrations of O6-meG, N7-meG, and AGT in peripheral blood lymphocytes were determined up to 24 h after each of the first two doses of DTIC. Mutations at the HPRT gene locus were determined using the T-cell clonal assay. Peak O6-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 levels 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 of O6-meG was observed at all time points after the second dose of DTIC (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 O6-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 hydroxyurea tended to be associated with lower levels of N7-meG (P = 0.08). There was no correlation between either O6-meG or N7-meG levels and the grade of neutropenia. On the basis of a limited series of blood samples analyzed, there was no firm evidence that chemotherapy with DTIC resulted in induction of HPRT mutations in lymphocytes. In

Topics: Antineoplastic Combined Chemotherapy Protocols; Dacarbazine; DNA Adducts; DNA Repair; Female; Guanine; Humans; Hydroxyurea; Hypoxanthine Phosphoribosyltransferase; Leukocytes; Male; Melanoma; Middle Aged; Mutation; O(6)-Methylguanine-DNA Methyltransferase

An ELISA method was used to determine N7-methylguanine (N7-MeGua) adducts in DNA of white blood cells from cancer patients treated with the methylating antitumor drug dacarbazine by i.v. administration. From four patients who received dacarbazine only, at dosages of 250, 400 or 800 mg/m2, the blood samples were collected before and several days after administration, and from one patient also during the first few hours. N7-MeGua levels increased rapidly during the first hour after treatment, hardly changed during the following 7 h and decreased during the next days (half-life of approximately 72 h). The adduct levels appeared to be dose-dependent. Blood cells were also analyzed from three patients who received dacarbazine (225 mg/m2) in combination with other chemotherapeutic drugs during three successive days. One of these patients showed a larger initial level of N7-MeGua and a cumulative increase with the dacarbazine dose compared to the other two patients. For several patients adduct levels were monitored during two chemotherapy cycles, for which quite similar data were obtained.

Topics: Adult; Antineoplastic Combined Chemotherapy Protocols; Dacarbazine; DNA; Enzyme-Linked Immunosorbent Assay; Female; Guanine; Humans; Leukocytes; Male; Melanoma; Middle Aged; Sarcoma