Abstract 2072: Selinexor in combination with chemotherapy or idelalisib elicits a synergistic cytotoxic effect in primary CLL cells
Authors: Todaro Maria , Griggio Valentina , Salvetti Chiara , Riganti Chiara , Landesman Yosef , Boccadoro Mario , Vitale Candida , Coscia Marta 
Published: August 2019

Despite the therapeutic efficacy of new targeted drugs in chronic lymphocytic leukemia (CLL), treatment of high-risk patients remains an unmet clinical need. Tumor suppressor proteins (TSPs) and growth regulatory proteins (e.g. p53, p21, FoxO3a - the effector of Akt signal transduction, and IκB - the endogenous inhibitor of NF-κB) bind the nuclear export protein XPO1 and are carried through the nuclear pore complex into the cell cytoplasm. XPO1 overexpression plays a pathogenic role in CLL by mislocalizing TSPs and other cargoes in the cytoplasm. Selinexor is an oral inhibitor of XPO1, which is active as single agent in different hematologic malignancies. The purpose of this study was to evaluate the in vitro cytotoxic effects of selinexor, used in combination with chemotherapy drugs or with the PI3K inhibitor, idelalisib, against primary CLL cells. Specifically, this study aimed at identifying combination regimens that might overcome single agent resistance. Purified CLL cells from 29 patients were exposed to selinexor (Sel) in combination with fludarabine (F-ara-A), bendamustine (Ben) or idelalisib (Ide). Drug concentrations and exposure times were selected based on data obtained in an initial cohort of 5 patients. In selected experiments, the M2-10B4 murine stromal cell (SC) line was used in co-culture with CLL primary cells. Cell viability was analysed by Annexin-V/propidium Iodide immunostaining and flow cytometry. Combination index (CI) was determined using Calcusyn software. NF-kB and Akt activity was tested through an ELISA assay. After 72-hour culture, a significant decrease in CLL cell viability was observed when samples were treated with drug combinations (i.e. Sel 0.1 µM + F-ara-A 1 µM/F-ara-A 10 µM/Ben 30 µM/Ben 50 µM/Ide 0.1 µM/Ide 1 µM/ Ide 10 µM; Sel 1 µM + F-ara-A 1 µM/Ben 30 µM/Ben 50 µM) compared to the single compounds. The majority of combinations exerted synergistic cytotoxic effects. Interestingly, the addition of selinexor was effective in restoring the fludarabine sensitivity of CLL cells showing in vitro resistance to fludarabine. Sel was also capable to overcome the SC-mediated drug resistance, as shown by the significantly increased and synergistic cytotoxicity exerted by drug combinations, compared to single agents, toward leukemic cells co-cultured with SC. From the molecular standpoint, preliminary data showed that Sel significantly potentiated the inhibitory effect exerted by single-agent idelalisib on NF-kB and Akt activity. Our data demonstrate that the combination of Sel with chemotherapy or idelalisib has synergistic cytotoxic effects, also counteracting intrinsic or SC-mediated drug resistance. In vivo experiments in the Eμ-TCL-1 mouse model are currently ongoing to further corroborate the efficacy of selected drug combinations.

Citation Format: Maria Todaro, Valentina Griggio, Chiara Salvetti, Chiara Riganti, Yosef Landesman, Mario Boccadoro, Candida Vitale, Marta Coscia. Selinexor in combination with chemotherapy or idelalisib elicits a synergistic cytotoxic effect in primary CLL cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2072.