Differential expression of uridine phosphorylase in tumors contributes to an improved fluoropyrimidine therapeutic activity

Abrogation of uridine phosphorylase (UPase) leads to abnormalities in pyrimidine metabolism and host protection against 5-fluorouracil (5-FU) toxicity. We elucidated the effects on the metabolism and antitumor efficacy of 5-FU and capecitabine (N ⁴-pentyloxycarbonyl-5′-deoxy-5- fluorocytidine) in our UPase knockout(UPase ^-/-) model. Treatment with 5-FU (85 mg/kg) or capecitabine (1,000 mg/kg) five days a week for four weeks caused severe toxicity and structural damage to the intestines of wild-type (WT) mice, but not in UPase ^-/- animals. Capecitabine treatment resulted in a 70% decrease in blood cell counts of WT animals, with only a marginal effect in UPase ^-/- mice. UPase expressing colon 38 tumors implanted in UPase ^-/- mice revealed an improved therapeutic efficacy when treated with 5-FU and capecitabine because of the higher maximum tolerated dose for fluoropyrimidines achievable in UPase ^-/- mice. ¹⁹F-MRS evaluation of capecitabine metabolism in tumors revealed similar activation of the prodrug in UPase ^-/- mice compared with WT. In WT mice, approximately 60% of capecitabine was transformed over three hours into its active metabolites, whereas 80% was transformed in tumors implanted in UPase ^-/- mice. In UPase ^-/- mice, prolonged retention of 50dFUR allowed a proportional increase in tumor tissue. The similar presence of fluorinated catabolic species confirms that dihydropyrimidine dehydrogenase activity was not altered in UPase ^-/- mice. Overall, these results indicate the importance of UPase in the activation of fluoropyrimidines, the effect of uridine in protecting normal tissues, and the role for tumor-specific modulation of the phosphorolytic activity in 5-FU or capecitabine-based chemotherapy.