Investigations on Leucovorin Rescue after Methotrexate Treatment of Human Osteosarcoma Cells in vitro
High-dose methotrexate (MTX) therapy with subsequent Leucovorin (LV) rescue in the treatment of osteosarcoma is based on the assumption that the cells of this tumour have a highly impaired active membrane transport system for folates. In normal cells this system is shared by MTX. It is assumed, that in contrast to normal tissues the tumour cells cannot be rescued by the MTX-antidote folinic acid (LV) from the cytotoxicity of this drug. Based on therapeutic regimens used in the clinic 3 osteosarcoma lines and 1 fibroblast line were exposed to MTX and/or LV in various dosages and time schedules. The cells were grown in monolayers and the effect on the cell growth was evaluated. It turned out that the cytotoxic effect of MTX (10-7-10-4M) can be overcome by relatively low doses of LV (10-8-10-5M). Studies on MTX transport showed that MTX uptake in osteosarcoma cells is saturable and highly temperature dependent thus indicating a carrier mediated active transport system. These results are not consistent with the above concept and they should be taken into consideration when designing new therapeutic regimens.