Abstract 1066: Imipridone ONC201 efficacy in refractory multiple myeloma via Erk1/2 inhibition and pro-apoptotic Bim upregulation: single agent and combinatorial approaches
Authors: Tu Yongsheng , He Jin , Liu Huan , Davis Richard Eric , Orlowski Robert Z. , Prabhu Varun Vijay , Allen Joshua , Yang Jing 
Published: July 2017

Abstract
Disease relapse and drug resistance occurs in the majority of multiple myeloma (MM) patients despite improvements offered by new treatments. There is an urgent unmet need for new therapies that can overcome drug resistance and prolong patient survival after failure of standard-of-care. ONC201, founding member of the impridone class of compounds, has robust preclinical efficacy in a variety of tumor types. A first-in-human ONC201 clinical study has demonstrated exceptional safety, therapeutic pharmacokinetics and evidence of tumor engagement that has led to several Phase I/II advanced cancer studies. Given the pronounced sensitivity of B-cell lymphomas to ONC201, we assessed the efficacy of ONC201 in MM preclinical models. We treated human MM cell lines and patient-derived cells isolated from bone marrow aspirates with ONC201 for 72 hours. ONC201 treatment decreased MM cell viability in CellTiter-Glo assays, with IC50 values that were 1 μM to 1.5 μM, even in high risk MM cell line RPMI8226. Consistent with prior reports, the efficacy of ONC201 was independent of TP53 status, as MM.1S or NCI-H929 cells with wild-type TP53 and OPM-2 or RPMI8226 with mutated TP53 had a similar sensitivity towards ONC201. Additionally, ONC201 was equally effective in paired MM cell lines with TP53 wild type or knockout. Western blot analysis showed increased apoptosis, cleavage of caspase-9, caspase-3, and PARP. We also found that ONC201 induced expression of the pro-apoptotic protein Bim in MM cells, which can occur downstream of ERK inactivation. shRNA knockdown of Bim expression in MM cells abrogated ONC201-induced apoptosis in annexin-V binding assays. Phosphorylation of Bim at Ser69 by Erk1/2 has been shown to promote proteasomal degradation of Bim. In accordance with this mechanism, we observed that ONC201 treatment reduced levels of phosphorylated Erk1/2, an indicator of Erk1/2 kinase activity, and downregulated Bim pSer69. In addition, ONC201 induced apoptosis in dexamethasome-, bortezomib-, and carfilzomib-resistant MM cell lines with the same efficacy as in wild-type cells. As a rational strategy to increase the efficacy of ONC201 by enhancing its inhibition of proteasome-mediated Bim degradation, we tested combinations of ONC201 with proteasome inhibitors. Synergistic reduction in cell viability and enhanced Bim expression and PARP cleavage was observed with ONC201 in combination with bortezomib or carfilzomib in MM cells. The combination of ONC201 and bortezomib enhanced the levels of Bim and cleaved PARP in MM cells. Overall, these findings demonstrate that ONC201 inhibits the Erk1/2 signaling pathway and induces Bim expression to induce apoptosis in MM regardless of p53 status and resistance to standard-of-care therapies. Our studies provide a strong rationale for clinical trials of ONC201 as a single agent or in combination with approved drugs in relapsed/refractory MM.
Citation Format: Yongsheng Tu, Jin He, Huan Liu, Richard Eric Davis, Robert Z. Orlowski, Varun Vijay Prabhu, Joshua Allen, Jing Yang. Imipridone ONC201 efficacy in refractory multiple myeloma via Erk1/2 inhibition and pro-apoptotic Bim upregulation: single agent and combinatorial approaches [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1066. doi:10.1158/1538-7445.AM2017-1066