SELECTIVE CELLULAR ACIDIFICATION AND TOXICITY OF WEAK ORGANIC-ACIDS IN AN ACIDIC MICROENVIRONMENT

The mean extracellular pH (pHe) within solid tumours has been found to be lower than in normal tissues. Agents which cause intracellular aci dification at low pHe might have selective toxicity towards cells in t umours. Weak acids (or their anions) with pKa values in the range of 4 6 have a higher proportion of molecules in the uncharged form at low pHe and can diffuse more rapidly into cells. The effects of organic ac ids including succinate, monomethyl succinate and malonate to acidify cells have been evaluated under conditions of different pHe in the aci dic range. These weak acids caused intracellular acidification of muri ne EMT-6 and human MGH-U1 cells in a concentration and pHe dependent f ashion. At concentrations of 10 mm and above, these acids also caused in vitro cytotoxicity to these cells at low pHe (< 6.5). The rate and extent of cellular acidification caused by these weak acids, and their cytotoxicity at low pHe, were enhanced by exposure to amiloride and 5 -(N-ethyl-N-isopropyl)amiloride (EIPA), agents which inhibit Na+/H+ ex change, and hence the regulation of intracellular pH. Acid dependent c ytotoxicity was also investigated in a murine solid tumour using the e ndpoints of growth delay and colony formation in vitro following treat ment in vivo. Agents were tested alone or with 15 Gy X-rays to select a population of hypoxic (and presumably acidic) cells. Achievable seru m concentrations of succinate were about 1 mm and no antitumour activi ty of succinate was detected when used in this way. It is concluded th at weak acids are selectively taken up into cells, and can cause selec tive cellular acidification and toxicity, at low pHe in culture. Weak acids that are normal cellular metabolites are not toxic in vivo, but weak acids carrying cytotoxic groups offer the potential for selective uptake and toxicity under the conditions of low pHe that exist in man y solid tumours.