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A study was made of the in vivo effects of equitoxic doses of AT-125 and 5-FU combination, being administered either simultaneously (% ILS 152) or with a 6-h pretreatment with AT-125 (% ILS 184). To examine the biochemical basis for the scheduled synergism, measurements were made of the concentration of PRPP, the specific activities of CPS II, cytidine, thymidine, uridine, deoxyuridine kinases, and fluorinated nucleotide formation in P388 tumors and the small intestine. Two hours after in vivo simultaneous treatment of mice bearing tumors the concentration of PRPP increased 9- and 6-fold above baseline in the tumor and the small intestine, respectively. In the AT-125 pretreatment arm the concentration of PRPP increased 18- and 7-fold above baseline in the tumor and the small intestine, respectively. CPS II activity was reduced to 28%–18% of control in the tumors in the simultaneous and pretreatment groups, respectively, whereas it remained unchanged in the small intestine. Specific activities of cytidine kinase (5.5±1), thymidine kinase (4.0±1.6), uridine kinase (35.6±6.5), and deoxyuridine kinase (2.4±1.1) nmol/mg protein/h remained unchanged with treatment. In concert with the increased intratumor concentration of PRPP, fluorinated nucleotide formation was proportionally increased in the treatment arms. These results indicate the importance of drug scheduling of the above two agents in treating P388 leukemia.