This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Faessel, H. M. Articles by Greco, W. R. Search for Related Content PubMed PubMed Citation Articles by Faessel, H. M. Articles by Greco, W. R.

Super in Vitro Synergy between Inhibitors of Dihydrofolate Reductase and Inhibitors of Other Folate-requiring Enzymes: The Critical Role of Polyglutamylation¹

Agouron Pharmaceuticals Inc., San Diego, California 92121 [R. C. J., T. J. B.]; Grace Cancer Drug Center [H. M. F., H. K. S., Y. M. R., W. R. G.] and Department of Biomathematics [H. M. F., W. R. G.], Roswell Park Cancer Institute, New York State Department of Health, Buffalo, New York 14263; and Department of Biochemistry, University of South Alabama, College of Medicine, Mobile, Alabama 36688 [M. G. N.]

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, , 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 ± SE of 1.50 ± 0.25 at 2.3 µM folic acid to 146 ± 20 at 78 µM 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 µM 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--glutamate synthetase and the effect of the DHFR inhibitor in decreasing the protection by folic acid of cells to the other antifolates.

¹ Supported by NIH Grants RR10742, CA65761, and CA16056 and American Cancer Society Grant DHP178.

² To whom requests for reprints should be addressed, at Department of Biomathematics, Roswell Park Cancer Institute, Elm & Carlton Streets, Buffalo, NY 14263.

Received 12/19/97. Accepted 5/11/98.

This article has been cited by other articles:

				R. G. Azrak, C. L. Frank, X. Ling, H. K. Slocum, F. Li, B. A. Foster, and Y. M. Rustum The mechanism of methylselenocysteine and docetaxel synergistic activity in prostate cancer cells. Mol. Cancer Ther., October 1, 2006; 5(10): 2540 - 2548. [Abstract] [Full Text] [PDF]

				J. Afeltra, E. Dannaoui, J. F. G. M. Meis, J. L. Rodriguez-Tudela, and P. E. Verweij In Vitro Synergistic Interaction between Amphotericin B and Pentamidine against Scedosporium prolificans Antimicrob. Agents Chemother., October 1, 2002; 46(10): 3323 - 3326. [Abstract] [Full Text] [PDF]