Treatment and signaling contexts for the application of the imipridone ONC201 to prostate cancer cells

Prostate cancer is the most common non-cutaneous cancer in men and a significant cause of cancer mortality. In the majority of the cases localised disease is treated with surgery or radiotherapy and whilst androgen deprivation therapy can extend survival in the subset of cases that progress, metastatic disease is incurable. We have previously reported that oncogenic drivers of prostate cancer (e.g. PTEN loss or mutation), as well as therapy, lead to changes in the activation of prostate cancer cell stress signalling pathways and in particular the unfolded protein response (UPR) 1 . Here we assess whether the activation of the UPR in response to radiation mirrors the immunogenic/inflammatory response, which is known to characterize the initially acute apoptotic phase before resolving to a chronic level. Further we have evaluated the impact of combining radiation with ONC201, a novel anti-cancer small molecule in clinical trials that is known to activate the pro-apoptotic stress response signalling 2 . We find that ONC201 has a delayed cytotoxic effect as a single agent in prostate cancer cells, with significant dose-dependent reductions in cell viability occurring 72 hours post-treatment. However activation of multiple arms of the UPR occurs earlier and is detectable at 24 hours for ATF4, ATF6 and IRE1-XBP1 at the protein/mRNA levels in the absence of impacts on cell viability. This delayed impact on viability offers a time window within which to evaluate short-term administration of ONC201 as a ‘primer9 to enhance the cytotoxic response to radiation. We show that ‘priming9 does indeed enhance radiotherapy response as assessed by cell viability and clonogenic assays. Since PTEN-loss is a high-incidence event in the development and progression of prostate cancer and has been linked to prognosis, we have also evaluated the impact of altering PTEN expression in our models as well as the effect of knocking down core regulators of the UPR. Cumulatively this work represents a further progression to positioning ONC201 in an appropriate molecular context to have durable effects when combined with other therapies to treat prostate cancer. 1.Sheng, X. et al., EMBO Molecular Medicine 7, 788-801 (2015). 2.Allen, J. et al. Discovery and clinical introduction of first-in-class imipridone ONC201. Oncotarget (2016). doi:10.18632/oncotarget.11814 Citation Format: Francesca Amoroso, Adam Pickard, Kimberly Glass, Rohinton Tarapore, J. E. Allen, Ian G. Mills. Treatment and signaling contexts for the application of the imipridone ONC201 to prostate cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 839.