Brequinar potentiates 5-fluorouracil antitumor activity in a murine model colon 38 tumor by tissue-specific modulation of uridine nucleotide pools.

Modulation of pyrimidine metabolism or the metabolic fate of 5-fluorouracil by a number of different agents has permitted a significant increase in the response rate to this agent, particularly for colorectal cancers. Brequinar, a noncompetitive inhibitor of mitochondrial dihydroorotate dehydrogenase has been shown to achieve a tumor-specific modulation of the therapeutic effect of 5-fluorouracil. A selective decrease of uridine nucleotide pools in Colon tumor 38 compared to normal tissues of C57/BL6 mice was observed after Brequinar administration. This effect was achieved with very low nontherapeutic doses of Brequinar (8 to 27% of the maximum tolerated dose in this model). Pretreatment with Brequinar 4 and 24 h prior to administration of [3H]fluorouracil significantly increased incorporation of the fluoropyrimidine into Colon 38 tumor RNA, while minimal effects were seen in normal tissues of C57/BL6 mice. Brequinar (15, 30, and 50 mg/kg) was administered 4 h prior to fluorouracil (85 mg/kg) on a weekly basis in Colon 38-bearing mice. All combinations potentiated 5-fluorouracil antitumor activity and the lowest dose of Brequinar (15 mg/kg) showed a reduced toxicity (weight loss) compared to the same dose of 5-fluorouracil as a single agent. When Brequinar preceded fluorouracil by 24 h, greater toxicity and less antitumor activity were observed. A comparison of the optimal Brequinar-fluorouracil regimen with a previously optimized N-(phosphonoacetyl)-L-aspartic acid-fluorouracil combination in Colon 38 tumor indicated that Brequinar-fluorouracil was more effective and less toxic.