Part I: The first cohort of 10 patients receives O6-benzylguanine (O6-BG) IV over 1 hour at dose level 1 beginning 6 hours prior to surgery. If at least 3 of 10 patients in the first cohort have detectable levels of O6-alkylguanine-DNA alkyltransferase (AGT), then a second cohort of 10 patients receives O6-BG as above at dose level 2. Dose escalation continues until at least 8 of 10 patients have undetectable AGT activity. At this point, 4 additional patients are accrued. If at least 11 of 14 patients at this dose level have undetectable levels of AGT, then this dose level constitutes the biologic modulatory dose of O6-BG. If less than 11 of 14 patients have undetectable levels of AGT, then 10 additional patients are treated at a higher dose. If at any time 3 or more patients at a dose level have detectable AGT activity, accrual is stopped at that dose level and patients are treated at the next higher dose level. (Part I closed to accrual effective 7/10/2000)