Structure and activity of specific inhibitors of thymidine phosphorylase to potentiate the function of antitumor 2′-deoxyribonucleosides

Abstract

A new class of 5-halogenated pyrimidine analogs substituted at the 6-position was evaluated as competitive inhibitors of thymidine phosphorylase (TPase). The most potent member of the series was 5-chloro-6-(2-iminopyrrolidin-1-yl)methyl-2,4(1 H,3 H)-pyrimidinedione hydrochloride (TPI), which has an apparent K i value of 1.7 × 10 −8 M. TPI selectively inhibited the activity of TPase, but not that of uridine phosphorylase, thymidine kinase, orotate phosphoribosyltransferase, or dihydropyrimidine dehydrogenase. In vitro inhibition studies of TPI using a thymidine analogue, 5-trifluoromethyl-2′-deoxyuridine (F 3dThd), as the substrate demonstrated that F 3dThd phosphorolytic activity was inhibited markedly by TPI (1 × 10 −6 M) in extracts from the liver, small intestine, and tumors of humans, from the liver and small intestine of cynomolgus monkeys, and from the liver of rodents, but not from the liver or small intestine of dogs or the small intestine of rodents, suggesting that the distribution of TPase differs between humans and animal species, and that TPI could contribute to the modulation of TPase in humans. When F 3dThd or 5-iodo-2′-deoxyuridine (IdUrd) was coadministered to mice with TPI at a molar ratio of 1:1, the blood levels of F 3dThd (or IdUrd) were about 2-fold higher than when F 3dThd (or IdUrd) was administered alone. In monkeys, the maximum concentration (C max) and the area under the concentration–time curve (AUC) after oral F 3dThd alone were 0.23 μg/mL and 0.28 μg · hr/mL, respectively, but markedly increased to 15.18 μg/mL (approximately 70-fold) and 28.47 μg · hr/mL (approximately 100-fold), respectively, when combined with equimolar TPI. Combined oral administration of TPI significantly potentiated the antitumor activity of F 3dThd on AZ-521 human stomach cancer xenografts in nude mice. In conclusion, TPI may contribute not only to inhibition of TPase-mediated biological functions but also to potentiation of the biological activity of various 2′-deoxyuridine and thymidine derivatives by combining with them.