Molecular Pharmacology, Vol 19, 87-91, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Inhibition of Methotrexate Polyglutamate Accumulation in Cultured Human Cells

¹ The McGill Centre for Human Genetics and the Medical Research Council Genetics Group, The Penny Cole Hematology Research Laboratory and the McGill Cancer Centre, McGill University-Montreal Children’s Hospital Research Institute, and Department of Pediatrics and Biology, McGill University, Montreal, Quebec, Canada

Incubation of confluent monolayers of human diploid fibroblasts in 1 µM [³H]methotrexate results in the accumulation of increasing levels of nonexchangeable intracellular methotrexate (MTX). This accumulation is due to metabolism of MTX to poly(-glutamyl) derivatives of progressively longer polyglutamate chain length and retention of the latter in a nonexchangeable state. This metabolism is unaffected by the addition of 10 µM folic acid (pteroylglutamic acid) to the incubation medium. In contrast, the addition of 10 µM 5-formyltetrahydrofolate or 5-methyltetrahydrofolate results in cessation of synthesis of MTX polyglutamates and therefore cessation of accumulation of high intracellular total MTX levels. Direct evidence of depletion of the exchangeable intracellular pool of MTX derivatives suggests that the reduced folates act by competing with MTX for transport into cells. We cannot rule out that inhibition of polyglutamate accumulation may be due in part to competition of these reduced folates for the polyglutamate synthetase enzyme. Inhibition of polyglutamate synthesis by folinic acid and other reduced folates may play a role in the "rescue" of tissues from MTX toxicity.

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ACKNOWLEDGMENTS We thank C. M. Baugh and M. G. Nair, Department of Biochemistry, University of South Alabama, for the kind provision of methotrexate polyglutamates. A.-M. Audet and Y. Guillemot were responsible for pilot studies from which this work derived. We thank Claire Pepin and Judy St. James for help in the preparation of the manuscript.