Abstract 1951: SMAC mimetic and BET inhibitor - a promising combination for solid cancer treatment
Authors: Traexler Paula-Elena , Arnold Dominik , Ebner Florian , Slavic-Obradovic Ksenija , Jacob Robin , Thanh Ha Pham Thi , Aichinger Martin , Baum Anke , Wernitznig Andreas , Gerlach Daniel , Impagnatiello Maria-Antonietta , Santoro Valeria , Olt Sabine , Scharn Dirk , Ruzicka Regina , House Reniqua P. , Murphy Mary Y. , Reiser Ulrich , Engelhardt Harald , Zinzalla Vittoria , Laux Thorsten , Solca Flavio , Tontsch-Grunt Ulrike 
Published: July 2021

Abstract
SMAC mimetics (SMACm), drugs that mimic natural antagonists of Inhibitor of Apoptosis (IAP) are in clinical trials for hematological malignancies and solid cancers. The clinical benefit for patients in monotherapy was so far dismal. Hence, current clinical evaluation of SMACm focuses on combinations, in particular with checkpoint inhibitors and/or radiation therapy. The BET family protein BRD4 is a “reader” of epigenetic information and binds to acetylated chromatin to act as a key regulator of transcription. BET inhibitors (BETi) were tested in numerous clinical trials as novel treatment option for hematological and solid cancers. Like SMACm, monotherapy with BETi showed only moderate clinical activity stressing the importance of combination trials. BI 891065 a monovalent, oral SMACm, with a favorable safety profile allowing continuous dosing, is in Clinical Phase I (NCT03166631, NCT04138823). BI 894999 is a very potent and selective oral BETi, administered by intermittent dosing in Clinical Phase I (NCT02516553). We studied the combinatorial effect of both compounds in vitro across a large number of human tumor cell lines (lung, colorectal, pancreatic, and gastric cancer) in proliferation assays using Bliss synergy analysis and IncuCyte ZOOM® live cell imaging, and in vivo in three independent models. Of the 60+ cancer cell lines, around 30% showed synergy when treated with the BI 891065 + BI 894999 combination, irrespective of indication. Efficacy correlated with downmodulation of the key apoptosis regulator XIAP by BI 894999. In most of the cell lines tested, the synergistic effect was blocked by addition of the TNFα scavenger Enbrel and/or the apoptotic pathway inhibitor zVAD. In vivo efficacy was tested in two PDAC xenograft models (one immunocompetent and one immunodeficient) and one CRC model at clinically relevant doses. Daily oral application of both compounds was well tolerated and did not lead to drug-drug interactions. Target engagement markers were modulated as expected (cIAP1 degradation for SMACm and Hexim1 induction for BETi) and not compromised by the combination. Tumor growth inhibition in the BxPC3 pancreas model achieved 22% TGI for 50 mg/kg BI 891065, 70% TGI for 2 mg/kg BI 894999, and 96% TGI for the combination. In the Pan02 pancreas model 9% TGI for 50 mg/kg BI 891065, 30% TGI for 4 mg/kg BI 894999 and 92% TGI for the combination were recorded. Evaluation of the cellular composition of the tumor microenvironment in this immunocompetent model showed distinct changes evident of a reduced immuno-suppressive milieu upon combination treatment. Combination effects on tumor growth in the CRC model LoVo were far less pronounced. These preclinical in vitro and in vivo data are highlighting the potential of a SMACm/BETi combination for the treatment of solid cancers. The identification of patient selection markers for this combination will be necessary for advancing this concept into pivotal clinical trials.
Citation Format: Paula-Elena Traexler, Dominik Arnold, Florian Ebner, Ksenija Slavic-Obradovic, Robin Jacob, Ha Pham Thi Thanh, Martin Aichinger, Anke Baum, Andreas Wernitznig, Daniel Gerlach, Maria-Antonietta Impagnatiello, Valeria Santoro, Sabine Olt, Dirk Scharn, Regina Ruzicka, Reniqua P. House, Mary Y. Murphy, Ulrich Reiser, Harald Engelhardt, Vittoria Zinzalla, Thorsten Laux, Flavio Solca, Ulrike Tontsch-Grunt. SMAC mimetic and BET inhibitor - a promising combination for solid cancer treatment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1951.