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

A new class of 5-halogenated pyrimidine analogs substituted at the 6-positi on was evaluated as competitive inhibitors of thymidine phosphorylase (TPas e). The most potent member of the series was 5-chloro-6-(2-iminopyrrolidin- 1-yl)methyl-2,4(1H,3H)-pyrimidinedione hydrochloride (TPI), which has an ap parent K-i value of 1.7 x 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 inh ibition studies of TPI using a thymidine analogue, 5-trifluoromethyl-2'-deo xyuridine (F(3)dThd), as the substrate demonstrated that F(3)dThd phosphoro lytic activity was inhibited markedly by TPI (1 x 10(-6) M) in extracts fro m the liver, small intestine, and tumors of humans, from the liver and smal l intestine of cynomolgus monkeys, and from the liver of rodents, but not f rom the liver or small intestine of dogs or the small intestine of rodents, suggesting that the distribution of TPase differs between humans and anima l species, and that TPI could contribute to the modulation of TPase in huma ns. When F(3)dThd or 5-iodo-2'-deoxyuridine (IdUrd) was coadministered to m ice with TPI at a molar ratio of 1:1, the blood levels of F(3)dThd (or IdUr d) were about 2-fold higher than when F(3)dThd (or IdUrd) was administered alone. In monkeys, the maximum concentration (C-max) and the area under the concentration-time curve (AUC) after oral F(3)dThd alone were 0.23 mu g/mL and 0.28 mu g . hr/mL, respectively, lour markedly increased to 15.18 mu g /mL (approximately 70-fold) and 28.47 mu g . hr/mL (approximately 100-fold) , respectively, when combined with equimolar TPI. Combined oral administrat ion of TPI significantly potentiated the antitumor activity of F(3)dThd on AZ-521 human stomach cancer xenografts in nude mice. In conclusion, TPI may contribute not only to inhibition of TPase-mediated biological functions b ut also to potentiation of the biological activity of various 2'-deoxyuridi ne and thymidine derivatives by combining with them. (C) 2000 Elsevier Scie nce Inc.