We have previously shown that exposure of cells in culture to O6-methylguanine significantly reduces their level of the repair protein, O6-alkylguanine-DNA-alkyltransferase (AGT), thus rendering cells more sensitive to the cytotoxic effects of chemotherapeutic chloroethylating agents. Experiments were carried out in mice to determine whether the AGT content of tissues and tumors could be reduced by in vivo treatment with O6-methylguanine. There was a dose-dependent decrease in AGT activity in liver tissues of CD-1 mice to 24% of basal levels after four hourly intraperitoneal injections of O6-methylguanine (110 mg/kg). Although the decline in AGT activity in the liver was reversible, the activity remained at 75% of basal levels for up to 25 h after the final injection. The effect of O6-methylguanine treatment on AGT activity was measured in mouse tissues as well as human colonic carcinoma tumors (HT29 and BE) grown in Swiss athymic nude mice. The activity in the liver, kidney, and spleen of these mice decreased to 33%-35% of control levels, whereas the activity in HT29 tumors was likewise diminished to 25% of control levels after four hourly injections of O6-methylguanine (100 mg/kg). There was no enhancement of the tumoricidal effectiveness of chloroethylating agents on the HT29 tumor after O6-methylguanine treatment, probably due to a disproportionately higher level of AGT in human tissue than in murine tissue. However, these studies suggest that O6-methylguanine can be given in vivo to examine the role of the AGT protein in protecting against the toxic and carcinogenic effects of alkylating agents.

The DNA-repair protein O6-alkylguanine-DNA alkyltransferase is known to protect tumor cells from the antitumor effects of carmustine (BCNU). This repair protein was inactivated in Copenhagen rat prostate tumors by treatment with O6-benzylguanine in attempts to increase the effectiveness of BCNU therapy. The alkyltransferase activity in the liver, kidney, lung, and prostate of Copenhagen rats was 66, 37, 65, and 122 fmol/mg protein, respectively. The activity in the Dunning R3327G rat prostate tumor was found to be 129 and 126 fmol/mg protein from intact and castrated animals, respectively. The level of this protein remained low in the tissues and tumors of rats for up to 24 h and slowly began to rise at 36 h following an i.p. injection of 80 mg/kg O6-benzylguanine. Animal survival and body weight as well as tumor volumes were monitored in rats bearing prostate tumors in the flank area that had received no treatment, O6-benzylguanine alone, BCNU alone (5.5-60 mg/kg), or 80 mg/kg O6-benzylguanine 1 h prior to BCNU (5.5 mg/kg). When O6-benzylguanine was combined with BCNU therapy, there was a regression in tumor growth that was not observed in animals treated with an equal dose of BCNU alone. A similar regression in tumor growth was observed in animals treated with a higher dose of BCNU alone (45 mg/kg); however, this regimen was more toxic than O6-benzylguanine plus BCNU (5.5 mg/kg) as determined by animal weight loss. The mean weight loss observed in animals treated with BCNU alone and in those given the combination was 24% and 6%, respectively. Histopathology revealed that animals receiving either BCNU alone or the combination had a decrease in all types of bone marrow cells, a loss of intestinal crypts, and a decreased number of lymphocytes in the spleen. The enhancement of the antitumor effect on BCNU by pretreatment with O6-benzylguanine supports a role for this therapy in the treatment of prostate cancer.