Interaction of Chemotherapeutic Agents with Methotrexate and 5-Fluorouracil and Its Effect on de Novo DNA Synthesis

The transport of methotrexate is known to be affected by corticosteroids and vincristine in L1210 leukemia cells. The deoxyuridine suppression test was used to measure the metabolic consequences of using these drugs with the antimetabolites, methotrexate and 5-fluorouracil, in both L1210 leukemia cells and normal human marrow cells. The deoxyuridine suppression test can be utilized as a sensitive measure of methotrexate and 5-fluorouracil biological activity in producing defective de novo DNA synthesis. The deoxyuridine suppression test was found to detect changes in biological activity equal to 20 ng (0.044 nmole) of methotrexate and 200 ng (1.94 nmoles) of 5-fluorouracil. Hydrocortisone and prednisone, but not dexamethasone or prednisolone, decreased the methotrexate effect to one-half in both LI210 and human cells as measured by the deoxyuridine suppression test. 5-Fluorouracil biological activity was not affected by any steroid studied. Vincristine produced variable results, but on the average it decreased the methotrexate effect in human marrow. Vincristine consistently decreased the methotrexate effect in L1210 systems. Cephalosporin, 75 μg/ml (0.214 μmole), had no effect. In parallel studies, hydrocortisone decreased the uptake of methotrexate, but not folic acid, in human and L1210 cells. The deoxyuridine suppression test warrants further investigation as a method of screening drugs for interaction with antagonists of de novo DNA synthesis. This study extends earlier evidence of drug interaction with methotrexate in a murine system to human cells and demonstrates that there is a metabolic consequence, reduced potency of methotrexate, as a result of reduced transport produced by certain corticosteroids.