Hyperthermic sensitization by the radical initiator 2,2'azobis (2-amidinopropane) dihydrochloride (AAPH). I. In vitro studies

AAPH (2,2'azobis-(2-amidinopropane dihydrochloride)) is a water-soluble, heat-labile azo compound which undergoes thermal decomposition to produce carbon-centred free radicals. These carbon-centred radicals might be directly cytotoxic or may react with oxygen to produce potentially cytotoxic alkoxyl and peroxyl radicals. The rate of free radical production as a result of AAPH thermal decomposition increases with increasing temperature. We have evaluated the efficacy of AAPH as a heat sensitizer for Chinese hamster V79 cells by the clonogenic assay. AAPH (50 mM) was not cytotoxic to V79 cells at 37°C for exposures up to 3 h. In contrast, AAPH (50 mM) was found to markedly sensitize cells exposed to 42, 43 and 45°C. For a 75 min exposure to 42°C alone, cell survival was reduced to 9×10^-1; however, a 75 min exposure at 42°C + AAPH resulted in survival of 5.5×10^-4. For 43 and 45.5°C heating, cell survival was potentiated by AAPH at the 1% survival level by 4.1 and 1.4-fold, respectively. AAPH was also found to sensitize both hypoxic cells and thermotolerant cells. These findings would encourage in vivo evaluation of AAPH (or analogues) as a temperature-dependent heat sensitizer. AAPH represents a new class of heat sensitizers which may have use in unravelling the mechanism(s) of heat killing and may have utility in local hyperthermia treatment.