Comparison of single- versus double-bolus treatments of O-6-benzylguanine for depletion of O-6-methylguanine DNA methyltransferase (MGMT) activity invivo: Development of a novel fluorometric oligonucleotide assay for measurement of MGMT activity

Previous studies have demonstrated that optimal reversal of 1,3-bis(2-chlor oethyl)-1-nitrosourea (BCNU) resistance requires complete inactivation of t he DNA repair protein O-6-methylguanine DNA methyltransferase (MGMT) for at least 24 h following BCNU administration. In preparation for clinical tria ls at this institution, this study was undertaken to compare the efficacy o f a conventional single-bolus dose versus double-bolus dose treatments with O-6-benzylguanine (BG) in depleting MGMT activity in vivo. In xenograft hu man glioma SF767 tumors, a single 30-mg/kg bolus dose of BG completely inhi bited MGMT activity for at least 8 h, but approximately 50% of the basal MG MT activity recovered within 24 h. To sustain the MGMT depletion for 24 h, a second bolus injection of BG at escalating doses was administered 8 h aft er the first dose. Second bolus doses of 5, 10, and 15 mg/kg BG attenuated the MGMT recovery in a dose-dependent manner compared with the single 30-mg /kg BG dose alone. When the 15-mg/kg BG dose was administered 8 h after the 30-mg/kg initial dose, MGMT activity was completely inactivated in the tum or xenografts for 24 h. This double-bolus BG treatment also depleted MGMT a ctivity in normal murine tissues, including the liver, kidney, lung, brain, spleen, and bone marrow; and the kinetics of MGMT recovery varied among th ese tissues. When combined with BCNU treatment, the double-bolus BG treatme nt would be expected to produce greater antitumor activity in future trials than the conventional single-bolus BG treatment.