These results indicate that 100 mg/m2 of O6-BG can maintain tumor AGT levels less than 10 fmol/mg protein for at least 18 hours after treatment, a time interval in which bis(2-chloroethyl)nitrosourea (BCNU)-induced chloroethyl adducts are fully converted into interstrand cross-links. A 100-mg/m2 dose of O6-BG will be used in combination with BCNU in another phase I trial designed to determine the maximal-tolerated dose of BCNU.

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.

1. A comparison was made of the binding of 5alpha-dihydrotestosterone (17beta-hydroxy-5alpha-androstan-3-one) and cyclic AMP in the rat prostate gland. Distinct binding mechanisms exist for these compounds, and cyclic AMP cannot serve as a competitor for the 5alpha-dihydrotestosterone-binding sites and vice versa. In contrast with the results obtained with 5alpha-dihydrotestosterone, very small amounts of cyclic AMP are retained in nuclear chromatin and the overall binding of this cyclic nucleotide is not markedly affected by castration. 2. Androgenic stimulation does not lead to major increases in the adenylate cyclase activities associated with any subcellular fraction of the prostate gland. Accordingly, changes in the concentration of cyclic AMP in the prostate gland after hormonal treatment are likely to be small, but these were not measured directly. 3. When administered to whole animals in vivo, small amounts of non-degraded cyclic AMP are found in the prostate gland but sufficient to promote an activation of certain carbohydrate-metabolizing enzymes in the cell supernatant fraction. The stimulatory effects of cyclic AMP were not evident with cytoplasmic enzymes engaged in polyamine synthesis or nuclear RNA polymerases. These latter enzymes were stimulated solely by the administration of testosterone. 4. By making use of antiandrogens, a distinction can be drawn between the biochemical responses attributable to the binding of 5alpha-dihydrotestosterone but not of cyclic AMP. Evidence is presented to suggest that the stimulation of RNA polymerase, ornithine decarboxylase and S-adenosyl-l-methionine decarboxylase is a consequence of the selective binding of 5alpha-dihydrotestosterone. Only the stimulation of glucose 6-phosphate dehydrogenase can be attributed to cyclic AMP or other metabolites of testosterone. 5. Overall, this study indicates that the formation of cyclic AMP is not a major feature of the androgenic response and affects only a restricted number of biochemical processes. Certainly, cyclic AMP cannot be considered as interchangeable with testosterone and its metabolites in the control of the function of the prostate gland. This difference is additionally emphasized by the failure of cyclic AMP to restore the morphology of the prostate gland in castrated animals; morphological restoration only follows the administration of androgens.

O6-Benzyl-2'-deoxyguanosine is a potential antitumor drug modulator that is intended to reduce or eliminate O6-alkylguanine-DNA alkyltransferase activity in tumors prior to treatment with genotoxic chemotherapeutic alkylating agents. The rationale for using this compound instead of the more active O6-benzylguanine and its substituted benzyl derivatives at the benzyl ring is its greater solubility in aqueous media and potential pharmacologic advantage. Metabolism and disposition of O6-benzyl-2'-deoxyguanosine was determined in adult male Sprague-Dawley rats following an ip injection of 100 mg/kg. Under these conditions, the compound was partially metabolized to yield a glucuronic acid conjugate, which was secreted exclusively in the bile. Removal of the 2'-deoxyribose or the benzyl group to yield O6-benzylguanine and 2'-deoxyguanosine, respectively, occurred to a lesser extent. Metabolism accounted for the clearance of at least 58% of the total dose and took place primarily in the liver. Direct excretion of unchanged drug, mainly in urine, accounted for the remainder of the dose. Analysis of venous blood showed the presence of O6-benzyl-2'-deoxyguanosine and O6-benzylguanine at concentrations which are considered to be effective in depleting alkyltransferase activity. Levels of the nucleoside reached a maximum of 45 microM at 2 h, while those of O6-benzylguanine peaked to 20 microM at 4 h and remained at that level for at least 4 more hours. Transport of O6-benzyl-2'-deoxyguanosine in C6 glioma cells increased linearly with the extracellular concentration of the drug up to 600 microM. Intracellular levels of the drug reached 1.2 pmol per microM of extracellular compound per 10(6) cells as soon as 30 s after exposure and remained as high for at least 1 h. Such levels indicate that entrapment of the nucleoside inside cells by either phosphorylation or other means is probably not an important feature for this drug.(ABSTRACT TRUNCATED AT 250 WORDS)