Pyrimethamine induces apoptosis of melanoma cells via a caspase and cathepsin double-edged mechanism

The unresponsiveness of metastatic melanoma to conventional chemotherapeutic and biological agents is largely due to the development of resistance to apoptosis. Pyrimethamine belongs to the group of antifolate drugs, and in addition to antiprotozoan effects, it exerts a strong proapoptotic activity, which we recently characterized in human T lymphocytes. However, no data regarding pyrimethamine anticancer activity are available thus far. To this end, we examined the in vitro effects of pyrimethamine on apoptosis, cell cycle distribution, and cell proliferation of human metastatic melanoma cell lines. The in vivo antitumor potential of pyrimethamine was evaluated in a severe combined immunodeficiency (SCID) mouse xenotransplantation model. Our data indicate that pyrimethamine, when used at a clinically relevant concentration, induced apoptosis in metastatic melanoma cells via the activation of the cathepsin B and the caspase cascade (i.e., caspase-8 and caspase-9) and subsequent mitochondrial depolarization. This occurred independently from CD95/Fas engagement. Moreover, pyrimethamine induced a marked inhibition of cell growth and an S-phase cell cycle arrest. Results obtained in SCID mice, injected s.c. with metastatic melanoma cells and treated with pyrimethamine, indicated a significant inhibitory effect on tumor growth. In conclusion, our results suggest that pyrimethamine-induced apoptosis may be considered as a multifaceted process, in which different inducers or regulators of apoptosis are simultaneously implicated, thus permitting death defects of melanoma cells to be bypassed or overcome. On these bases, we hypothesize that pyrimethamine could represent an interesting candidate for the treatment of metastatic melanoma.