Prolonged inhibition of O-6-methylguanine DNA methyltransferase in human tumor cells by O-6-benzylguanine in vitro and in vivo

We previously demonstrated that sustained depletion of methylguanine DNA me thyltransferase (MGMT) activity is required for optimal reversal of chloroe thylnitrosourea resistance in tumor cells. The purpose of this study was to design O-6-benzyl-guanine (BG) treatments that deplete MGMT activity in tu mor cells and xenograft tumors in a prolonged manner. When SF767 cells were treated with a bolus dose of BG (25 mu M for 1 h), >95% of MGMT activity w as depleted but 33% of the activity recovered within 24 h. In contrast, MGM T activity was completely depleted for 24 h when cells were pretreated with a low dose of BG (2.5 mu M) for 24 h, followed by the bolus dose and same low-dose treatment for 24 h. This combination regimen of pre- and post-trea tments with a bolus dose sensitized cells N,N'-bis(2-chloroethyl)-N-nitroso urea in vitro by similar to 2-fold more than the bolus dose alone. Similar BG treatment with Alzet micro-osmotic pumps produced sustained inhibition o f MGMT activity in vivo. In xenograft SF767 tumors, low-dose pre- and post- treatments (8 mg/kg over 24 h) combined with an i.p. bolus dose (80 mg/kg) of BG inhibited >95% of MGMT activity for 24 h after the bolus. The bolus d ose alone did not deplete MGMT for 24 h. These results demonstrate that com bination low-dose and bolus BG treatment is superior to the bolus dose alon e in depleting MGMT activity in a sustained manner in vitro and in vivo. Wh en combined with N, N'-bis(2-chloroethyl)-N-nitrosourea treatment, this BG regimen also should also produce greater antitumor activity than the single bolus dose evaluated clinically.