Redox cycling by motexafin gadolinium enhances cellular response to ionizing radiation by forming reactive oxygen species

Purpose: To examine the mechanism of radiation enhancement by motexafin gad olinium (Gd-Tex) in vitro. Methods and Materials: Oxidation of ascorbate and NADPH by Gd-Tex was evalu ated in a neutral buffer. Growth inhibition of human uterine cancer cell li ne MES-SA was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetra zolium bromide (MTT) dye. Clonogenic assays were used to measure radiation response in MES-SA, A549 human lung carcinoma, E89, a CHO cell line variant deficient in glucose-6-phosphate dehydrogenase activity, and murine lympho ma cell lines LYAR and LYAS. Results: Gd-Tex catalyzed the oxidation of NADPH and ascorbate under aerobi c conditions, forming hydrogen peroxide. Decreased viability was observed i n MES-SA cells incubated with Gd-Tex in media containing NADPH or ascorbate . Gd-Tex and ascorbate increased fluorescence in dichlorofluorescin acetate -treated cultures. Synergistic effects on the aerobic radiation response in MES-SA and A549 were seen using Gd-Tex in combination with L-buthionine-(S ,R)-sulfoximine (BSO). Incubation with Gd-Tex in the presence of ascorbate increased the aerobic radiation response of E89 and the apoptosis-sensitive B-cell line (LYAS). Conclusions: Gd-Tex sensitizes cells to ionizing radiation by increasing ox idative stress as a consequence of futile redox cycling. Optimization of th e concentration of ascorbate (or other reducing species) may be required wh en evaluating Gd-Tex activity in vitro. (C) 2001 Elsevier Science Inc.