5-Ethynyl-2(1H)-pyrimidinone: Aldehyde oxidase-activation to 5-ethynyluracil, a mechanism-based inactivator of dihydropyrimidine dehydrogenase

Abstract 5-Ethynyluracil is a potent mechanism-based inactivator of dihydropyrimidine dehydrogenase (DPD, EC 1.3.1.2) in vitro (Porter et al., J Biol Chem 267 : 5236–5242, 1992) and in vivo (Spector et al., Biochem Pharmacol , 46 : 2243–2248, 1993. 5-Ethynyl-2(1 H )-pyrimidinone was rapidly oxidized to 5-ethynyluracil by aldehyde oxidase. The substrate efficiency ( k cat / K m ) was 60-fold greater than that for N -methylnicotinamide. In contrast, xanthine oxidase oxidized 5-ethynyl-2(1 H )-pyrimidinone to 5-ethynyluracil with a substrate efficiency that was only 0.02% that of xanthine. Because 5-ethynyl-2(1 H )-pyrimidinone did not itself inactivate purified DPD in vitro and aldehyde oxidase is predominately found in liver, we hypothesized that 5-ethynyl-2(1 H )-pyrimidinone could be a liver-specific inactivator of DPD. We found that 5-ethynyl-2(1 H )-pyrimidinone administered orally to rats at 2 μg/kg inactivated DPD in all tissues studied. Although 5-ethynyl-2(1 H )-pyrimidinone produced slightly less inactivation than 5-ethynyluracil, the two compounds showed fairly similar patterns of inactivation of DPD in these tissues. At doses of 20 μg/kg, however, 5-ethynyl-2-pyrimidinone and 5-ethynyluracil produced equivalent inactivation of DPD. Thus, 5-ethynyl-2(1 H )-pyrimidinone appeared to be an efficient, but not highly liver-selective prodrug of 5-ethynyluracil.