Pronounced radiosensitization of cultured human cancer cells by COX inhibitor under acidic microenvironment

Purpose: To demonstrate the influence of pH on the cytotoxicity and radiosensitization by COX (cyclooxygenase) -1 and -2 inhibitors using established human cancer cells in culture. Methods and Materials: Nonselective COX inhibitor, ibuprofen (113), and selective COX-2 inhibitor, SC-236, were used to determine the cytotoxicity and radiosensitization at varying pit of culture media, Human colon carcinoma cell line (HT-29) was exposed to the drug alone and in combination with radiation at different pH of the cell culture media. The end point was clonogenic ability of the single-plated cells after the treatment. Results: Cytotoxicity and radiosensitization of IB increased with higher drug concentration and longer exposure time. The most significant radiosensitization was seen with IB (1.5 mM) for 2-h treatment at pit 6.7 before irradiation. The dose-modifying factor as defined by the ratio of radiation doses required to achieve the same effect on cell survival was 1.8 at 10% survival level. In contrast, SC-236 (50 muM for 2-8 h) showed no pH-dependent cytotoxicity. There was modest increase in the cell killing at lower doses of radiation. Conclusion: An acidic pH was an important factor affecting the increased cytotoxicity and radiosensitization by ibuprofen. Radiation response was enhanced at shoulder portion of the cell survival curve by selective COX-2 inhibitor. (C) 2002 Elsevier Science Inc.