ANTINEOPLASTIC ACTIVITY OF SEQUENCED ADMINISTRATION OF O-6-BENZYLGUANINE, STREPTOZOTOCIN, AND 1,3-BIS(2-CHLOROETHYL)-1-NITROSOUREA IN-VITROAND IN-VIVO

The purpose of this study was to evaluate the anti-tumor activity of s equenced administration of O-6-benzylguanine (BG), streptozotocin (STZ ), and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in vitro and in viv o. We measured the recovery of O-6-methylguanine DNA methyltransferase (MGMT) and BCNU cytotoxicity in the human glioma SF767 cell line, and anti-tumor activity against xenografts following exposure to BG, STZ or the combination of BG + STZ combined with BCNU. In SF767 cells, the combination of BG (10 mu M) + STZ (0.05 mM) produced sustained inhibi tion of MGMT activity for at least 24 hr, and a greater potentiation o f BCNU cytotoxicity than either agent alone. The combined treatment of BG + STZ increased BCNU-induced cell kill by 0.5 to 1.0 log over BG o r STZ alone. The maximally tolerated doses of the combination of BG STZ + BCNU administered to nude mice i.p. were the following: BG (80 m g/kg), STZ (100 mg/kg), and BCNU (15 mg/kg). Utilizing these doses of BG and STZ, the depletion and repletion profile of MGMT activity in SF 767 xenografts was measured. STZ at 100 mg/kg did not affect xenograft MGMT activity. Subsequent to BG treatment, xenograft MGMT activity wa s inactivated completely for 12 hr, and the tumors gradually recovered approximately 40% of control activity by 24 hr. The combination of BG + STZ produced sustained inhibition of MGMT activity for 24 hr in the xenografts with complete recovery of MGMT activity by 48 hr. Administ ration of the combination of BG + BCNU to nude mice bearing SF767 tumo r resulted in significant inhibition of tumor growth for 23 days. Howe ver, the addition of STZ to this combination provided no greater anti- tumor activity than that observed with BG + BCNU. The three-drug combi nation of BG, STZ, and BCNU produced no more than 2.4 to 13.0% weight loss with occasional lethal toxicity. Collectively, these data suggest that prolonged depletion of MGMT might be required for optimal revers al of BCNU resistance both in vitro and in vivo.