INACTIVATION MECHANISM OF HUMAN DIHYDROPYRIMIDINE DEHY DROGENASE BY 5-(2'-BROMOVINYL)URACIL, A METABOLITE OF THE ANTIVIRAL, SORIVUDINE

Abstract

An enzymatic study was performed to clarify the mechanism of 18 acute deaths in patients who had received the new oral antiviral drug, sorivudine (SRV), during anticancer chemotherapy with 5-fluorouracil (5-FU) prodrugs. Human dihydropyrimidine dehydrogenase (hDPD), playing a key role in the liver as the rate-limiting enzyme in catabolism of 5-FU, was expressed in E. coli, purified, and incubated in the presence of NADPH with SRV or (E)-5-(2-bromovinyl)uracil (BVU), a metabolite of SRV. hDPD was rapidly and irreversibly inactivated by BVU, but not by SRV. Radioactivity of [14C]BVU was incorporated into hDPD in the presence of NADPH in a manner reciprocal to the enzyme inactivation. In the absence of NADPH, hDPD was not inactivated by BVU, nor radiolabeled with [14C]BVU. Thus, as we demonstrated previously with studies using the rat, the acute deaths were strongly suggested to be attributable to markedly elevated tissue 5-FU levels, which were responsible for irreversible inhibition of hDPD by covalent binding of a reduced form of BVU as a suicide inactivator. A significant linear correlation (r=0.58, p=0.005) was observed between liver-DPD activity and levels of the immuno-detectable protein with anti-hDPD-anti-serum, indicating that the antibody could be a valuable tool for the DPD deficiency screening.