Marked suppression of the activity of some, but not all, antifolate compounds by augmentation of folate cofactor pools within tumor cells

Folates have been co-administered with some antifolates to diminish host to xicity; however, the extent to which this will reduce antitumor activity is not known. To further clarify this issue, studies were undertaken to chara cterize and quantitate the impact of alterations in intracellular folate le vels on the activities of a variety of antifolates in L1210 murine leukemia cells. Intracellular folate cofactor levels increased almost in proportion to the increase in extracellular 5-formyltetrahydrofolate (5-CHO-THF) over a concentration range that encompassed physiological levels of 5-methyltet rahydrofolate. This resulted in a spectrum of increases in the ic(50) value s of antifolates upon continuous exposure to drugs [Lometrexol (DDATHF) (70 x) > trimetrexate (TMQ) (30x), multitargeted antifolate, LY231514 (ALIMTA) (30x) > Raltitrexed, Tomudex (ZD1693) (10x), 6R-2',5'-thienyl-5,10-dideazat etrahydrofolic acid (LY309887) (10x) > methotrexate (MTX) (6x) > (2S)-2-{o- fluoro-p-[N-(2,7-dimethyl-4-oxo-3-4-dihydroquinazolin-6-ylmethyl)-N-(prop-2 -ynyl)amino]benzamido}-4-(tetrazol-5-yl) butyric acid (ZD9331) (3x), N-alph a-(4-amino-4-deoxypteroyl)-N-delta-hemiphthaloyl-1-ornithine (PT523) (3x)]. Upon a 4-hr pulse exposure to drug, the ic(50) values for DDATHF and ALIMT A were increased > 180- and 5-fold, respectively, with only a 2.5-fold incr ease in the extracellular 5-CHO-THF level within the physiological range. T he reductions in drug sensitivities could be attributed to decreases in acc umulation of polyglutamate derivatives of ALIMTA and DDATHF. Hence, in thes e studies, natural folates diminished the activity of agents that undergo p olyglutamation by suppression of the formation of these active congeners at the level of folylpolyglutamate synthetase. For inhibitors of dihydrofolat e reductase, the suppressive effect of endogenous folates appears to be due to competition between the antifolate and dihydrofolate at the level of th e target enzyme. These data should be carefully considered in the design of regimens with antifolates, which incorporate co-administration of folates. (C) 2001 Elsevier Science Inc. All rights reserved.