EFFECTS OF HMA, AN ANALOG OF AMILORIDE, ON THE THERMOSENSITIVITY OF TUMORS IN-VIVO

Purpose: The effects of HMA peridyl)-N-(diaminomethylene)pyrazinecarbo xamide), an analog of amiloride, on the intracellular pH (pHi) of SCK tumor cells in vitro and on the thermosensitivity of tumors in vivo we re investigated. Methods and Materials: The pHi of SCK tumor cells in vitro was measured with the BCECF fluorescence spectroscopy method. Th e effect of HMA on the thermosensitivity of SCK tumors grown SC in the legs of A/J mice was assessed by the tumor growth delay method and th e in vivo-in vitro excision assay method. Results: The pHi of SCK tumo r cells in pH 7.5 and 6.6 medium was about 7.50 and 7.15, respectively . The presence of 10-50 mu M of HMA lowered the pHi by 0.1-0.2 pH unit s both in pH 7.5 and 6.6 medium. Heating at 43 degrees C for 120 min l owered the pHi by 0.2 and 0.3 pH units in pH 7.5 and 6.6 medium, respe ctively. When the cells were heated in the presence of 10-50 mu M HMA, a marked decline in pHi occurred and the decline in pHi resulting fro m the combination of heat and HMA was more pronounced in pH 6.6 medium than in pH 7.5 medium. Heating the SCK tumors grown SC in the legs of A/J mice at 43.5 degrees C for 1 h resulted in a growth delay of 3.7 days. When the host mice were IV injected with 0.1 mg/kg of HMA and th e tumors were heated 20 min later, the tumor growth was delayed by 8.2 days, which was 4.5 days longer than that by heating alone. Heating t he SCK tumor at 42.5 degrees C for 1 h caused a tumor growth delay of 0.9 days. An IV injection of 1 mg/kg or 10 mg/kg of HMA prior to heati ng at 42.5 degrees C for 1 h caused a tumor growth delay 2.1 and 3.1 d ays longer, respectively, than that by heating alone. Such an enhancem ent of heat-induced tumor growth delay by HMA was due to increased cel l killing, as determined with the in vivo-in vitro excision assay of c lonogenic cells in the tumors. Conclusion: HMA is a potent thermosensi tizer, particularly in an acidic environment. Thermosensitization by H MA may occur preferentially in tumors relative to normal tissues since the intratumor environment is acidic.