Purpose: The effects of HMA (3-amino-6-chloro-5-(1-homopiperidyl)-N- (diaminomethylene)pyrazinecarboxamide), an analog of amiloride, on the intracellular pH (pHi) of SCK tumor cells in vitro and on the thermosensitivity of tumors in vivo were investigated.
Methods and materials: The pHi of SCK tumor cells in vitro was measured with the BCEC fluorescence spectroscopy method. The 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 the in vivo-in vitro excision assay method.
Results: The pHi of SCK tumor cells in pH 7.5 and 6.6 medium was about 7.50 and 7.15, respectively. The presence of 10-50 microM of HMA lowered the pHi by 0.1-0.2 pH units both in pH 7.5 and 6.6 medium. Heating at 43 degrees C 120 min lowered the pHi by 0.2 and 0.3 pH units in pH 7.5 and 6.6 medium, respectively. When the cells were heated in the presence of 10-50 microM HMA, a marked decline in pHi occurred and and the decline in pHi resulting from 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 i.v. injected with 0.1 mg/kg of HMA and the tumors were heated 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 the SCK tumor at 42.5 degrees C for 1 h caused a tumor growth delay of 0.9 days. An i.v. injection of 1 mg/kg or 10 mg/kg of HMA prior to heating at 42.5 degrees C for 1 h caused a tumor growth delay 2.1 and 3.1 days longer, respectively, than that by heating alone. Such an enhancement of heat-induced tumor growth delay by HMA was due to increased cell killing, as determined with the in vivo-in vitro excision assay of clonogenic cells in the tumors.
Conclusion: HMA is a potent thermosensitizer, particularly in an acidic environment. Thermosensitization by HMA may occur preferentially in tumors relative to normal tissues since the intratumor environment is acidic.