Cytostatic potential of novel agents that inhibit the regulation of intracellular pH

Cells within the acidic extracellular environment of solid tumours maintain their intracellular PH (PH) through the activity of membrane-based ion exchange mechanisms including the Na+/H+ antiport and the Na+-dependant of exchanger, inhibition of these regulators mechanisms has been proposed as an approach to tumour therapy. Previously available inhibitors of these exchangers were toxic (e.g. 4,4-diisothiocyanstilbene-2,2-disulphonic acid). and/or non-specific (e.g. 5-N-ethyl-N-sopropyl amiloride). Using two human (MCF7, MDA-MB231 and one murine (EMT6) breast cancer lines, we evaluated the influence of two new agents, cariponde (an inhibitor of the Na+/H+ antipoint) and S3705 (an inhibitor of the Na+-dependent Cl-/HCO- exchanger) on the regulation of intracellular PH (pHi) The cytotoxicity of the two agents was assessed by using clonogenic assays. Our results suggest that cariponde has similar efficacy and potency to 5-N-ethyl-N-isopropyl amiloride for inhibition of Na+/H+ exchange S3705 is more potent and efficient than 4,4-diisothiocyanstilbene-2,2-disulphonic acid in inhibiting Na+-dependent Cl /HCO3- exchange. The agents inhibited the growth of tumour cells when they were incubated at low PHe (7.0 - 6.8), but were non-toxic to cells grown at doses that inhibited the regulation of pHi. Our results indicate that cariporide and S3705 are selective cytostatic agents under two in vitro conditions that reflect the slightly acidic microenvironment found in solid tumours.