Effect of O⁶-Benzylguanine on the Sensitivity of Human Tumor Xenografts to 1,3-Bis(2-chloroethyl)-1-nitrosourea and on DNA Interstrand Cross-Link Formation

Abstract

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 µm 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.