Exchangeable Intracellular Methotrexate Levels in the Presence and Absence of Vincristine at Extracellular Drug Concentrations Relevant to Those Achieved in High-Dose Methotrexate-Folinic Acid “Rescue” Protocols

Studies were undertaken to (a) assess intracellular methotrexate (MTX) levels at extracellular drug concentrations comparable to those achieved in high-dose MTX-folinic acid rescue protocols and (b) establish whether there is a rationale for the use of vincristine in these regimens-The data indicate that only low levels of exchangeable MTX (intra-cellular MTX in excess of the tightly bound fraction) accumulated in Ehrlich ascites tumor cells at high extracellular MTX concentration. For instance, the exchangeable steady-state intracellular MTX level was —6.5 μm when the extracellular drug concentration was 85 μm. Over the interval of these experiments, exchangeable intracellular MTX did not exceed ~10 μm even when extracellular MTX was raised to 250 μm. These exchangeable intracellular MTX concentrations are comparable to those levels required experimentally to suppress (a) tetrahydrofolate synthesis from dihydrofolate and (b) tetrahydrofolate-dependent purine, pyrimidine, and amino acid synthesis in these cells in vitro. Vincristine (10 μm) augmented net MTX accumulation when the extracellular MTX level was 10,100, or 250 μm. The limited capacity of cells to accumulate exchangeable intracellular MTX and the apparent role for this intracellular MTX component in achieving the metabolic effects of this agent may account for the necessity for high MTX blood levels in the treatment of some tumors and may be the basis, in part, for the enhanced chemotherapeutic efficacy of high-dose MTX regimens. These studies provide a rationale for the combined use of vincristine and MTX in high-dose MTX protocols. The addition of vincristine may permit the achievement of the level of exchangeable intracellular MTX that is required to critically inhibit tetrahydrofolate synthesis without an increase in the extracellular MTX concentration. This may permit a reduced MTX dose, diminishing the excretory load on the kidney and minimizing nephrotoxicity due to deposition of MTX in the renal tubule and interstitium. While the data indicate that the ratio of the concentration of exchangeable intracellular MTX to the extracellular drug concentration may be very low under steady-state conditions at high extracellular drug levels, further studies are required to establish that these steady-state gradients for MTX represent nonequilibrium conditions.