Radiosensitization of non-small-cell lung cancer cells and xenografts by the interactive effects of pemetrexed and methoxyamine

Background and purpose The anti-folate pemetrexed is a radiosensitizer. In pre-clinical models, pemetrexed is more effective along with the base-excision-repair inhibitor methoxyamine. We tested whether methoxyamine enhances pemetrexed-mediated radiosensitization of lung adenocarcinoma cells and xenografts. Materials and methods A549 and H1299 cells were evaluated for cell cycle distribution by flow cytometry, radiosensitization by clonogenic assay, and DNA repair by neutral comet assay and repair protein activation. H460 cells were included in some studies. Xenografts in nude mice received drug(s) and/or radiation, and tumor growth was monitored by caliper and in vivo toxicity by animal weight. Results Exposure to pemetrexed/methoxyamine for 24 (H1299, H460) or 48 (A549) hours before irradiation resulted in accumulation of cells near the radiosensitive G1/S border; dose-enhancement factors of 1.62 ± 0.19, 1.97 ± 0.25, and 1.67 ± 0.30, respectively; reduction of mean inactivation dose by 32%, 30%, and 46%, respectively; and significant reductions of SF2 and SF4 (p < 0.05). Radiosensitization was associated with rapid DNA double-strand-break rejoining and increased levels of DNA–PKcs. Both tumor-growth rate and tumor-growth delay were significantly improved by adding methoxyamine to pemetrexed pre-irradiation (p < 0.0001); no mice lost weight during treatment. Conclusions Addition of methoxyamine to pemetrexed and fractionated radiotherapy may improve outcome for patients with locally advanced non-squamous non-small-cell lung cancer.