Disposition and metabolism of O6-alkylguanine-DNA alkyltransferase inhibitor in nude mice bearing human melanoma.

Tumor resistances to chloroethylnitrosourea (CENU) are mainly due to O6-alkylguanine-DNA alkyltransferase (AGT). Our laboratory has synthesized a new water-soluble AGT inhibitor. O6-benzyl-N-acetylguanosine (BNAG). We have shown that this compound is able to deplete AGT activity on M4Beu human melanoma cells and to enhance the antitumor power of CENU N'-[2-chloroethyl]-N-[2-(methylsulfonyl)ethyl]-N'-nitrosourea (cystemustine) towards the M4Beu melanoma grafted on nude mice. With a view to determining the best combination BNAG/CENUs conditions, we have studied the distribution and metabolism of BNAG in nude mice bearing M4Beu human melanoma. BNAG, labelled with carbon-14 on the benzyl group, was administered by single i.v. dose of 40 mg/kg. Blood analysis showed that the main radioactive compound was unchanged molecule, and only a small part was found as hippuric acid resulting from the metabolic cleavage of the benzyl group. BNAG and hippuric acid were mainly eliminated in the urine. Unchanged BNAG blood kinetics showed three phases: blood epuration (t1/2 (1) = 13 min), reabsorption and elimination (t1/2 (2) = 1.7 hr). This kinetic profile is probably due to an enterohepatic cycle. BNAG is distributed in several tissues (kidney, liver, skin, duodenum, colon, tumor) but not in the central nervous system, suggesting a poor blood-brain crossing. Because an important part of the administered dose is not metabolized, high unchanged BNAG level remains in most tissues, including M4Beu tumor, and AGT depletion can occur several hours after dosing.