We have previously shown that O6-benzylguanine can be used to deplete cells of the DNA repair protein O6-alkylguanine-DNA alkyltransferase and to enhance the sensitivity of human glioma (SF767) and colon tumor (HT29) cells to the cytotoxic effects of alkylnitrosoureas. In the present study, the combination of O6-benzylguanine and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) was evaluated in vitro to determine the number of DNA interstrand cross-links formed and in vivo to compare the therapeutic index with that of BCNU alone. The number of DNA interstrand cross-links, as measured by alkaline elution, was increased in HT29 cells treated with 10 microM O6-benzylguanine for 2 h prior to BCNU exposure compared to cells treated with BCNU only. The number of single strand breaks was not increased by prior exposure to O6-benzylguanine. To evaluate the therapeutic index, HT29 and SF767 cells were grown as xenografts in nude mice and the tumor growth rate after treatment with BCNU alone was compared with the rate after treatment with O6-benzylguanine and BCNU. Treatment was administered i.p. when tumors reached 100-200 mm3. For animals bearing HT29 xenografts that were treated with 60 mg/kg O6-benzylguanine 1 h prior to 20 mg/kg BCNU, the average time for tumor volume to increase by 200% was 25 days, compared to 10 days for animals treated with 20 mg/kg BCNU alone. For animals bearing SF767 xenografts, the tumor growth of controls was not significantly different from that of animals treated with O6-benzylguanine alone or BCNU alone up to the maximally tolerated dose (50 mg/kg). For these 3 groups, the average time for tumors to reach 300 mm3 was 9-12 days. However, when animals were treated with 80 mg/kg O6-benzylguanine 1 h prior to receiving 20 mg/kg BCNU tumor size did not increase for at least 21 days. Our studies demonstrate that the therapeutic index of BCNU can be increased when given in combination with O6-benzylguanine.

Experiments were carried out in mice and hamsters to determine whether the activity of the DNA repair protein, O6-alkylguanine-DNA alkyltransferase, in tissues and tumors was reduced by treatment with O6-benzylguanine in vivo. Following intraperitoneal injection of O6-benzylguanine, there was a rapid and complete loss of alkyltransferase activity in both livers and kidneys of mice and hamsters. The activity in mouse tissues was slowly restored, reaching pretreatment activities at 16 hr and 72 hr after injection of O6-benzylguanine at 10 mg/kg or 126 mg/kg, respectively. The activity in hamster liver was restored at a significantly lower rate, reaching less than 20% pretreatment activity 72 hr after treatment with 100 mg/kg of O6-benzylguanine. The efficient reduction of alkyltransferase activity by O6-benzylguanine was in sharp contrast to the inability of O6-methylguanine to bring about similar reductions. Activities dropped to about 55% of pretreatment activities in several mouse organs 4 hr after treatment with 126 mg/kg of O6-methylguanine compared to a more than 90% reduction in activity in animals after treatment with O6-benzylguanine. The sensitivity of SF767 cells to meCCNU after treatment with O6-benzylguanine was increased substantially. Furthermore, treatment of nude mice carrying SF767 tumor with 60 mg/kg of O6-benzylguanine prior to either 7.5 or 15 mg/kg of meCCNU led to significant inhibition of tumor growth. These studies indicate that O6-benzylguanine is a suitable compound for use in experiments to examine the role of the alkyltransferase protein in vivo in counteracting the effects of alkylating agents.(ABSTRACT TRUNCATED AT 250 WORDS)