Modulation of Mammalian O6-Alkylguanine-DNA Alkyltransferase in vivo by O6-Benzylguanine and its Effect on the Sensitivity of a Human Glioma Tumor to l-(2-chloroethyl)-3-(4-methylcyclohexyl)-l-nitrosourea

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Abstract
Experiments were carried out in mice and hamsters to determine whether the activity of the DNA repair protein, O 6-alkylguanine-DNA alkyltransferase, in tissues and tumors was reduced by treatment with O 6-benzylguanine in vivo. Following intraperitoneal injection of O 6-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 O 6-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 O 6-benzylguanine. The efficient reduction of alkyltransferase activity by O 6-benzylguanine was in sharp contrast to the inability of O 6-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 O 6-methylguanine compared to a more than 90% reduction in activity in animals after treatment with O 6-benzylguanine. The sensitivity of SF767 cells to meCCNU after treatment with O 6-benzylguanine was increased substantially. Furthermore, treatment of nude mice carrying SF767 tumor with 60 mg/kg of O 6-benzylguanine prior to either 7.5 or 15 mg/kg of meCCNU led to significant inhibition of tumor growth. These studies indicate mat O 6-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. Our studies also suggest that as a modulator of alkyltransferase activity, O 6-benzylguanine may have utility as an adjuvant in nitrosourea therapy for human malignancies.

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