Objective: The effect of combining lonidamine (1-[(2,4-dichlorophenyl)methyl]-1H-indazole-3-carboxylic acid) with MIBG (meta-iodobenzylguanidine), on intracellular pH (pHi), thermal induction of heat shock proteins (HSPs) and thermal sensitization of DB-1 human melanoma cells was investigated. Human melanoma cells grown at low pH are sensitized to 42°C hyperthermia by exposure to lonidamine, an inhibitor of H+-linked monocarboxylate transporters (MCT). MIBG, an inhibitor of mitochondrial respiration, increases tumor pO2, reduces tumor extracellular pH, and radiosensitizes tumors.

Methods: DB-1 cells cultured at pH 6.7 and pH 7.3 were exposed to inhibitors for 3 h, starting 1 h prior to a 42°C (2 h) treatment. pHi was determined using BCECF and whole spectrum analysis, HSP levels determined by immunoblot analysis and survival determined by colony formation.

Results: Treatment of cells grown at pH 6.7 with lonidamine (100 µM) or MIBG (350 µM) decreased the pHi from 6.63(±0.09) to 6.35(±0.10) and 6.20(±0.19) respectively. The combination of lonidamine and MIBG reduced pHi to 6.27 (±0.15). Under these conditions, the 42°C-induction of HSPs was blocked, and the cells were sensitized to thermal killing. However, treatment of cells cultured at pH 7.3 with the inhibitors either alone or combined only minimally reduced pHi and did not block thermal induction of HSPs and did not sensitize to 42°C. Increasing the concentration of lonidamine to 150 µM alone or combined with MIBG further reduced pHi in cells cultured at pH 6.7 (pHi<6.0). However, the combined treatment also reduced pHi in cells cultured at pH 7.3 to a level (6.38±0.14) that sensitizes to 42°C.

Conclusions: These in vitro results indicate that thermal sensitization may be achieved using either lonidamine alone or MIBG alone depending on the drug concentrations. Although xenograft studies are required for verification in vivo, these results support the strategy that human melanoma can be sensitized to thermal therapy in vivo by exposure to an MCT inhibitor such as lonidamine and/or a respiratory inhibitor such as MIBG. (Supported by grant no. PO1 CA56690 from NCI, NIH, DHHS).