Abstract

BRL 55792, BRL 55791, and BRL 55039 are prodrugs of an active antiviral agent 9-(3-hydroxypropoxy) guanine (BRL 44385). The prodrugs were 6-deoxygenated analogs of BRL 44385, with ether groups substituted at the 9-position: BRL 55792 with an (isopropoxymethyloxy)propoxy group, BRL 55791 with a (methoxymethyloxy)propoxy group, and BRL 55039 with an ethoxypropoxy group. Conversion of the prodrugs to BRL 44385 had been demonstrated in vivo in the rat and involved 6-oxidation followed by dealkylation. Metabolism was studied in rat liver in vitro systems to find a model to evaluate BRL 44385 production. Rat hepatocytes performed both reaction steps and were used to assess which of the three prodrugs demonstrated greatest production of the active drug. BRL 55792 demonstrated greatest conversion in vitro, and this was in agreement with in vivo data. The production of BRL 44385 from BRL 55792 was also demonstrated in human hepatocyte incubations, providing evidence that these reactions can occur in humans, thereby increasing confidence that BRL 55792 would be suitable prodrug for human therapy. Further experiments were performed to investigate the enzymes involved in these conversions. The 6-oxidation step occurred in the cytosol. Use of allopurinol and menadione (xanthine and aldehyde oxidase inhibitors) indicated that these conversions were catalyzed exclusively by xanthine oxidase in the rat but mainly by aldehyde oxidase in humans. The dealkylation reaction was detected in hepatocytes but not in homogenates or subcellular fractions. Inhibition of this reaction by aminobenzotriazole and ketoconazole (P-450 inhibitors) indicated that it was mediated by cytochrome P-450.