Indomethacin lowers the threshold thermal exposure for hyperthermic radiosensitization and heat-shock inhibition of ionizing radiation-induced activation of NF-κB

Purpose: It is well established that salicylate and several other non-steroidal anti-inflammatory agents (NSAID), including indomethacin, can activate the heat-shock response, albeit at high concentrations. This is significant since heat shock significantly alters the cellular cytotoxic response to ionizing radiation (IR). It was previously shown that heat shock, as well as NSAIDs, inhibits IR-induced activation of NF-κB and that NF-κB protects against IR-induced cytotoxicity. Hence, it is hypothesized that pretreatment with indomethacin before heating will lower the temperature and heating times required to inhibit the activation of NF-κB and induce significant hyperthermic radiosensitization. Materials and methods: Experiments were performed in HeLa cell lines and the DNA-binding activity was determined by EMSA. Cellular radiosensitivity was determined by clonogenic assay. Results: HeLa cells pretreated with indomethacin showed a decrease in the temperature-time combination necessary to inhibit IR-induction of NF-κB DNA binding. In addition, clonogenic cell survival assays using identical conditions showed an indomethacin dose-dependent enhancement of hygerthermic radiosensitization. Thus, similar concentrations of indomethacin both lowered the threshold thermal exposure to inhibit activation of NF-κB DNA-binding and increased the sensitivity of tumour cells to hyperthermic radiosensitization-induced cytotoxicity. In HeLa cells treated with N-α-tosylphenylalanyl-chloromethyl ketone (TPCK), a serine protease inhibitor that blocks activation of NF-κB, an increase in radiosensitivity was observed. Interestingly, no additional cell killing was observed when heat shock was added to cells treated with TPCK before IR, suggesting a possible common cytotoxic pathway. Conclusions: The results demonstrate that indomethacin lowers the temperature-time conbination necessary to induce several physiological processes associated with the heat-shock response. Furthermore, NSAID may be potential adjuvants in improving the clinical effectiveness of hyperthermia in radiation therapy.