USE OF RAT AND HUMAN IN-VITRO SYSTEMS TO ASSESS THE EFFECTIVENESS ANDENZYMOLOGY OF DEOXYGUANINE ANALOGS AS PRODRUGS OF AN ANTIVIRAL AGENT

BRL 55792, BRL 55791, and BRL 55039 are prodrugs of an active antivira l agent 9-(3-hydroxypropoxy) guanine (BRL 44385). The prodrugs were 6- deoxygenated analogs of BRL 44385, with ether groups substituted at th e g-position: BRL 55792 with an (isopropoxymethyloxy)propoxy group, BR L 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 d ealkylation. Metabolism was studied in rat liver in vitro systems to f ind a model to evaluate BRL 44385 production. Rat hepatocytes performe d both reaction steps and were used to assess which of the three prodr ugs demonstrated greatest production of the active drug. BRL 55792 dem onstrated greatest conversion in vitro, and this was in agreement with in vivo data. The production of BRL 44385 from BRL 55792 was also dem onstrated in human hepatocyte incubations, providing evidence that the se reactions can occur in humans, thereby increasing confidence that B RL 55792 would be a suitable prodrug for human therapy. Further experi ments were performed to investigate the enzymes involved in these conv ersions. The 6-oxidation step occurred in the cytosol. Use of allopuri nol 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 subcel lular fractions. Inhibition of this reaction by aminobenrotriazole and ketoconazole (P-450 inhibitors) indicated that it was mediated by cyt ochrome P-450.