Thermal sensitisation by lonidamine of human melanoma cells grown at low extracellular pH

Purpose: This study tested the ability of lonidamine (LND), a clinically applicable inhibitor of monocarboxylate transporters (MCT), to thermally sensitise human melanoma cells cultured at a tumour-like extracellular pH (pH_e) 6.7.

Materials and methods: Human melanoma DB-1 cells cultured at pH_e 6.7 and pH_e 7.3 were exposed to 150?µM LND for 3?h, beginning 1?h prior to heating at 42?°C (2?h). Intracellular pH (pH_i) was determined using 2′,7′-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) and whole spectrum analysis. Levels of heat shock proteins (HSPs) were determined by immunoblot analysis. Cell survival was determined by colony formation.

Results: Treatment with LND at pH_e 6.7 reduced pH_i to 6.30?±?0.21, reduced thermal induction of HSPs, and sensitised cells growing at pH_e 6.7 to 42?°C. When LND was combined with an acute acidification from pH_e 6.7 to pH_e 6.5, pH_i was reduced to 6.09?±?0.26, and additional sensitisation was observed. LND had negligible effects on cells cultured at pH 7.3.

Conclusions: The results show that LND can reduce pH_i in human melanoma cells cultured at a tumour-like low pH_e so that the 42?°C induction of HSPs are abrogated and the cells are sensitised to thermal therapy. Cells cultured at a normal tissue-like pH_e 7.3 were not sensitised to 42?°C by LND. These findings support the strategy that human melanoma cells growing in an acidic environment can be sensitised to thermal therapy in vivo by exposure to an MCT inhibitor such as LND.