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.