2000 Volume 15 Issue 3 Pages 265-274
In 1993, eighteen acute deaths occurred in Japanese patients who received a clinical dose of the new oral antiviral drug, sorivudine (SRV: 1-β-D-arabinofuranosyl-(E)-5-(2-bromovinyl) uracil), for herpes zoster. That was only within 40 days after SRV was approved by the Japanese government for clinical use. All patients who died had been receiving one of 5-fluorouracil (5-FU) prodrugs for anticancer chemotherapy. None of patients who received SRV alone or SRV and anti-cancer drugs other than 5-FU or its prodrugs, died or suffered from toxic symptoms. A toxicokinetic study using rats orally given simultaneously with SRV and the 5-FU prodrug, tegafur (FT: 1-(2-tetrahydrofuryl)-5-fluorouracil), once a daily, strongly suggested that the patients died from a marked increase in tissue 5-FU levels by the mechanismbased inhibition of hepatic dihydropyrimidine dehydrogenase (DPD), with (E)-2-(5-bromovinyl) uracil (BVU), a metabolite of SRV by gut flora. DPD is a rate-limiting enzyme in the catabolism of 5-FU in the rat and the human. All rats showed a marked increase in the 5-FU level in plasma and tissues such as intestines and bone marrow from the first day of the experiment and died within 10 days after the repeated administration of SRV and FT. Before the death, the rats showed severe diarrhea with bloody flux and marked decreases in white blood cell and platelet counts as had been reported for the patients.
In the presence of NADPH, DPD isolated from the untreated rat liver and human recombinant DPD were completely inactivated and radiolabeled by [14C] BVU. Two radioactive peptides were isolated by HPLC from a tryptic digest of the radiolabeled human DPD. Amino acid sequencing indicated that the two peptides were derived from a common regional sequence of the DPD, which was in the potential pyrimidine-binding domain located at positions 665-686 of the enzyme subunit. Only the Cys residue at a position of 671 in the DPD was found to be radiolabeled and unidentified by the amino acid sequencing of the tryptic fragments. MALDI-TOF MS analysis indicated that the SH of Cys671 formed a sulfide bond by the reaction with the allyl bromide type of reactive metabolite, 5-(2-bromovinylydeny) uracil, yielded in the DPD subunit. The sulfide bond formation appears to make it impossible for the DPD to interact with the 5-FU through the substrate-binding domain. Therefore, the patients who died from the drug-drug interactions were most likely to become extremely poor metabolizers for 5-FU by the mechanism-based inhibition of DPD with BVU from SRV.
