The combined action among polyglutamylatable and nonpolyglutamylatable antifolates, directed against dihydrofolate reductase (DHFR), glycinamide ribonucleotide formyltransferase (GARFT), 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase (AICARFT), and thymidylate synthase (TS), in human ileocecal HCT-8 cells was examined in a 96-well plate growth inhibition assay (96-h continuous drug exposure). An interaction parameter, alpha, was estimated for each of 95 experiments by fitting a seven-parameter model to data with weighted nonlinear regression. In a representative experiment, raising the folic acid concentration in the medium dramatically increased the Loewe synergy for the combination of trimetrexate (TMTX) and the GARFT inhibitor AG2034 (from a mean alpha +/- SE of 1.50 +/- 0.25 at 2.3 microM folic acid to 146 +/- 20 at 78 microM folic acid). Enhancements were also found for combinations of TMTX with the GARFT inhibitors AG2032, Lometrexol, and LY309887, the AICARFT inhibitor AG2009, and the TS inhibitors LY231514 and Tomudex but not with the GARFT inhibitor LL95509 or with the TS inhibitors AG337, ZD9331, and BW1843U89. Replacing TMTX with methotrexate in two-drug mixtures decreased the intensity of Loewe synergy. Examination of isobolograms at different effect levels revealed informative reproducible changes in isobol patterns. No two-drug combinations among inhibitors of GARFT, AICARFT, and TS exhibited Loewe synergy at either 2.3 or 78 microM folic acid. Thus, the ideal requirement for the folic acid-enhanced synergy is that a nonpolyglutamylatable DHFR inhibitor be combined with a polyglutamylatable inhibitor of another folate-requiring enzyme. A hypothesis to explain this general phenomenon involves the critical role of folylpoly-gamma-glutamate synthetase and the effect of the DHFR inhibitor in decreasing the protection by folic acid of cells to the other antifolates.
