Metabolism of 5'-ether prodrugs of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil in rats.

1-beta-D-Arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU) is a selective antiherpesviral agent that has been shown to be metabolically stable in mice. However, E-5-(2-bromovinyl)uracil (BVU) is the major metabolite found after oral dosing in animals other than mice. When BV-araU was given orally to germ-free rats, only small amounts of BVU were found in the plasma, suggesting an important role of enterobacteria in the formation of BVU. Then, the metabolism of BV-araU prodrugs was studied in specific-pathogen free rats to select oral prodrugs of BV-araU with enhanced metabolic stability. 5'-O-Ethyl BV-araU (Et-BV-araU) gave about a 2-fold higher BV-araU blood concentration 3 and 6 hr after administration than after oral dosing of BV-araU, while the level of BVU was lower. Other aliphatic alkyl prodrugs also gave a lower level of BVU, but did not give the same elevation in blood concentration of BV-araU as did Et-BV-araU. Dosing of 5'-O-acetyl BV-araU resulted in blood concentrations of BV-araU and BVU similar to those after oral administration of BV-araU. 5'-O-Aromatic alkyl prodrugs showed poor bioavailability. A nearly 2-fold higher urinary recovery rate was seen for Et-BV-araU than for BV-araU or 5'-O-acetyl BV-araU. The conversion of Et-BV-araU to BV-araU was demonstrated in vitro using rat liver extract in the presence of co-factors, although the reaction was slow. The 5'-O-aliphatic alkyl prodrugs were completely resistant to degradation by enterobacteria, whereas the esters were partially degraded to BVU. Et-BV-araU may be a useful oral prodrug of BV-araU due to its increased metabolic stability and bioavailability.