23P Prolonged pemetrexed pretreatment increases efficiency of ionizing radiation combination therapy and correlates with the persistence of treatment-induced DNA damage in lung cancer cells.

pools could sensitize cancer cells to subsequent treatment with cisplatin. Methods: NSCLC A549 cells were treated with pemetrexed for 72 hours. In addition, 24 hours of cisplatin treatment was initiated at day 1, 2 or 3 resulting in either simultaneous pemetrexed application or pemetrexed pretreatment for 24 or 48 hours, respectively. Cell growth and colony formation as well as senescence induction were quantified after treatment. Cell cycle distribution and DNA damage induction was quantified by flow cytometry. Results: Extended pemetrexed pretreatment for 48 hours prior to cisplatin treatment maximally delayed long-term cell growth and significantly reduced the number of recovering clones. Moreover, apoptosis and senescence were augmented and recovery from treatment-induced DNA damage was delayed. Interestingly, a resistant cell population was identified that displayed an epithelialto-mesenchymal transition and which had a stem cell phenotype. Conclusions: Prolonged pemetrexed pretreatment optimizes the anticancer efficiency of pemetrexed–cisplatin combination therapy, therefore, this study warrants further investigations to elucidate whether such an adaptation could enhance the effectiveness of the standard clinical treatment regimen. In addition, a subpopulation of therapy resistant cells with EMT and cancer stem cell features was identified. Legal entity responsible for the study: N/A Funding: Supported by the Bernese Cancer League and the Swiss Cancer Research (KFS-3530–08–2014) to TMM Disclosure: All authors have declared no conflicts of interest.