Molecular Pharmacology, Vol 19, 491-495, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Transient Induction of Phenotypic Resistance in Human Lymphoblastoid Cells following Sequential Use of Two Antifolates

¹ Departments of Medicine and Microbiology-Immunology, Duke University Medical Center, Durham, North Carolina 27710

This paper demonstrates an unexpected antagonism of cytoxicity with the use of two antifolates. Cell killing depended on the order of administration, as sequential in vitro exposures, to a lipid-soluble antifolate, metoprine (DDMP), and methotrexate (MTX). This was manifested in greatly increased survival of cultures following a program of 4 days of exposure of human lymphoblastoid (WIL-2) cells to 5 µM DDMP, 2 days of recovery in medium without drug, then 4 days of exposure to either 5 µM DDMP or 1 µM MTX. By contrast, 1 µM MTX in the first cycle, regardless of the drug used in the second cycle, gave no survivors. Serial measurement of dihydrofolate reductase levels showed that DDMP led to increased levels of free dihydrofolate reductase during the recovery period which were not inhibited by a second exposure to drug. These results emphasize the importance of sequence and timing when drugs are used in multiagent protocols, even when the drugs have a common enzyme target.

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ACKNOWLEDGMENTS The authors wish to thank Mr. Seaton Bowers of the Burroughs Wellcome Company for his valuable technical assistance and Dr. David Duch, also of Burroughs Wellcome Company, for his help in this project.