Modulation of both endogenous folates and thymidine enhance the therapeutic efficacy of thymidylate synthase inhibitors

Plasma levels of folates and thymidine in mice are about 10-fold higher tha n in humans and may influence the therapeutic efficacy of thymidylate synth ase (TS) Inhibitors, such as 5-fluorouracil (5FU) and the antifolates pemet rexed (MTA) and raltitrexed (RTX). Therefore, we tested their therapeutic e fficacy in various murine tumor models, grown in mice on a normal and a fol ate-depleted diet, with high and low thymidine kinase (TK) levels. MTA and RTX were inactive against Colon-26-10 [doubling times gained by treatment; growth delay factor (GDF), 0.5 and 0.3, respectively], whereas 5FU was very active (GDF, >10; complete cures). Colon-26-10/F, grown In mice on a folat e-depleted diet, was more sensitive to RTX and MTA (GDF, 2.1 and 1.3, respe ctively) but not to 5FU (GDF, 1.2); however, leucovorin reversed the effect leading to cures. Folate depiction did not reverse resistance of Colon-26A and Colon-26G (low TK) to MTA and RTX, whereas leucovorin only enhanced th e 5FU effect in Colon-26A and Colon-26A/F. Folic acid at 15 mg/kg did not i mprove the therapeutic efficacy of MTA in folate-deficient mice. The folate -depleted diet decreased the reduced folates in Colon-26A/F and Colon-26-G/ F tumors less (4-5-fold; P < 0.01) than in Colon-26-10/F tumors (8-fold; P < 0.001). Folate depletion increased TS levels 2-3-fold in all of the model s and TK levels 6-fold (P < 0.01) in Colon-26G/F, explaining the lark of ac tivity of MTA and RTX in Colon-26G/F. In contrast, TK-deficient FM3A/TK tum ors were much more sensitive to RTX, MTA, and 5FU than parent FM3A tumors, which have comparable TS levels. The rate of thymidine phosphorylysis varie d considerably in all of the tumors without a clear relation to antitumor a ctivity. In conclusion, tumor folates may potentiate (5FU) or protect (antifolates). Murine tumor models should combine low folates and low thymidine rescue to optimize preclinical testing of antifolates.