Paradoxical response of malignant melanoma to methotrexate in vivo and in vitro.

Methotrexate (MTX) shows consistent cytotoxicity for melanoma cells in vitro but it is ineffective in clinical use at equivalent concentrations in vivo. This apparent paradox has been investigated by cell culture techniques and results quantified by cell number. In an in vitro model of high dose MTX therapy followed by leucovorin rescue (HD-MTX-LCR) there was survival of both melanoma and choriocarcinoma cell lines but not of an acute lymphocytic leukaemia cell line. The 70H metabolite of MTX was identified by HPLC in plasma samples of melanoma patients treated by HD-MTX-LCR, in which MTX concentrations approximately 10(-5) M were maintained for 24 h. However, metabolism per se is unlikely to account for the lack of response to MTX clinically. In vitro 70H MTX (10(-7) - 10(-6) M) was two orders of magnitude less cytotoxic for melanoma than MTX (10(-9) - 10(-8) M). The cellular accumulation of [3H]-MTX, using a rapid gradient centrifuge technique for separation of melanoma cells from medium, was reduced in the presence of 70H-MTX. The results suggest that reduced cellular uptake of MTX combined with biochemical rescue of tumour cells may partially explain the paradoxical lack of clinical response of melanoma to the drug.