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The Mechanism of Transport of the Multitargeted Antifolate (MTA) and Its Cross-resistance Pattern in Cells with Markedly Impaired Transport of Methotrexate¹

Departments of Medicine and Molecular Pharmacology and the Albert Einstein Comprehensive Cancer Center, Albert Einstein College of Medicine, Bronx, New York 10461

MTA (LY231514) is an antifolate that targets multiple folate-dependent enzymes. In this report, MTA transport was characterized in wild-type L1210 cells and variants with impaired membrane transport or polyglutamation. MTA influx via the reduced folate carrier was somewhat faster (30%) than that for methotrexate (MTX). Unlike MTX, MTA was rapidly polyglutamated in L1210 cells; hence, a folylpoly--glutamate synthetase-deficient L1210 variant was used to assess net transport and efflux properties. The MTA transmembrane gradient for exchangeable drug was 2.5 times greater than the MTX gradient, attributable primarily to an efflux rate constant 40% that of MTX. No MTA was bound to dihydrofolate reductase. When grown with folic acid, MTX-resistant L1210 variants with mutations in the reduced folate carrier demonstrated cross-resistance to MTA, markedly reduced MTA accumulation, and only a slightly decreased intracellular folate cofactor pool as compared to L1210 cells. However, when 5-formyltetrahydrofolate was the growth substrate, these MTX-resistant cells were less resistant or negligibly resistant to MTA, accumulated more MTA, and had a lower folate pool as compared to L1210 cells. MTA activity and the intracellular folate pool in L1210 cells were inversely related. These data indicate that MTA polyglutamation in L1210 cells is favored by both the generation of high intracellular drug levels and high MTA affinity for FPGS relative to MTX. Cells resistant to MTX because of impaired transport may retain appreciable sensitivity to MTA because of a concurrent reduction in tetrahydrofolate cofactor transport resulting in cellular folate depletion, which diminishes endogenous folate suppression of MTA polyglutamation.

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