Natural resistance to methotrexate in human melanomas.

Human melanomas are naturally resistant to methotrexate (MTX). The mechanism of intrinsic drug resistance has been explored in 3 melanoma cell lines not previously exposed to this agent. All 3 lines exhibited relative MTX resistance with ID50 values of greater than 1 microM. Drug uptake studies were performed over an extracellular concentration range of 0.1 to 10 microM MTX. The uptake was linear over the initial 10 min at all concentrations and subsequently reached plateau levels only at the 10 microM concentration. Lineweaver-Burke transformations yielded apparent Km (uptake) values of 1.4 to 5 microM, similar to data obtained from other human cell lines. The level of dihydrofolate reductase (DHFR) in the human melanoma cells ranged between 8.42 to 11.98 pmoles/mg protein. The melanoma DHFR levels are several fold higher than in MTX-sensitive human tumor lines and up to a hundred-fold higher than that measured in human brain tumor cells by our assay. The intrinsic resistance of these melanoma lines has therefore been attributed to elevated intracellular levels of DHFR.