Metabolism and Disposition of O6-Benzyl-2'-deoxyguanosine in Sprague-Dawley Rats

Abstract

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)

Potentiation of BCNU antitumor efficacy by 9-substituted O 6-benzylguanines. Effect of metabolism

“…The metabolism and disposition of dBG in SpargueDawley rats has been previously described [3]. Following i.p.…”

“…The insolubility and rapid clearance of BG and BOG have stimulated development of additional inhibitors, with ED 50 s compa- Early tests with dBG, one of the most soluble derivatives of BG, but having about one tenth of its ability to inactivate MGMT in vitro, have shown that this compound is actually more potent than BG in enhancing BCNU eciency against tumors in the athymic mouse model [1]. This has been explained by its ecient metabolic conversion to BG and BOG [3,4]. Evidently, systemic administration of more water soluble prodrugs that yield BG either in the tumor or even in the circulation may be a more eective method of sensitizing tumors to chemotherapy than administration of BG itself.…”

“…The sample volume was again reduced to 1 ml by rotatory evaporation. Samples were analyzed for metabolites retaining their label by reverse phase chromatography according to published procedures [3]. BG and the parent metabolite were determined by their UV absorption (254 nm) and by¯uores-cence (ex: 305, em: 370).…”

“…BG and other radioactive metabolites were also quantitated when applicable by collecting fractions at 0.5-min intervals and counting for tritium with a scintillation counter. The same experiment was repeated for [8-3 H] BGS using SpragueDawley rats in which the bile duct was cannulated to collect bile according to the method previously described [3].…”

“…Since dBG is ten times less active against O 6 -methylguanine-DNA methyltransferase (MGMT) than O 6 -benzylguanine (BG) in cell-free systems [2], it has been proposed that metabolic activation of dBG to a more potent MGMT inactivator might account for its unexpected potency in the athymic mouse model. dBG is metabolized to yield BG, and although the latter is not apparent in urine or bile, it is the prominent metabolite in the circulation after dBG administration [3,4]. Further studies demonstrated that in rats, a major fraction of dBG was converted to BG over a period of 6 h after its administration [4].…”