Abstract LB_A01: Rapidly cleared MCL1 inhibitor TTX-810 showing robust efficacy in preclinical tumor models with no effects on cardiomyocytes in vitro and in vivo
Authors: Rauh Ulrike , Kaushik Virendar , Pilari Sabine , Kosmidis Georgios , Serrano-Wu Michael , Wei Guo , Zhang Weiqun , Kaulfuss Stefan , Thede Kai , Poncet-Montange Guillaume , Daniels Douglas , Lemke Christopher T , Hubbard Brian , Golub Todd R. 
Published: December 2023

Abstract
MCL1, an anti-apoptotic protein frequently amplified in cancer, confers resistance to standard of care, thus highlighting its importance as a therapeutic target. However, the involvement of MCL1 in cardiomyocyte function raises concerns for safely targeting the protein in cancer patients. MCL1 inhibitors currently in clinical trials exhibit shortcomings, either due to insufficient potency or extended half-lives that heighten the risk of cardiotoxicity. This study systematically explores MCL1 inhibitor-induced cell dynamics in cancer cells and healthy cardiomyocytes. TTX-810, a potent, selective, and rapidly-cleared MCL1 inhibitor, was used to identify a treatment regimen that induces efficacy, but spares healthy cardiomyocytes. TTX-810 demonstrates potent and reversible binding to the BH3-binding groove of human MCL1 (KD = 0.3 nM). It inhibits the interaction of MCL1 with the pro-apoptotic effector protein BAK or BH3-only protein BIM in cells in a dose- and time-dependent manner, leading to activation of the intrinsic apoptotic cascade and cancer cell death. At suprapharmacologic concentrations and extended exposure durations TTX-810 does induce apoptosis in human iPSC-derived cardiomyocytes, aligning with the recognized mechanism of MCL1 inhibitor-related cardiotoxicity. In contrast, short-term TTX-810 exposure induces efficient cancer cell killing in vitro, and has no impact on cell viability, apoptosis induction, or troponin I release in human iPSC-derived cardiomyocytes, even at high concentration. Similarly, TTX-810 induces strong monotherapy efficacy with once weekly in vivo dosing in an aggressive AML cancer model that is resistant to current standard of care agents like Venetoclax or Azacytidine. Repeated once weekly dosing in dogs over 4-weeks had no impact on troponin I levels in blood, electrocardiogram parameters (PR, QRS, QT and calculated QTcV intervals), heart rate, nor cardiac tissue morphology. In conclusion, this study identifies an optimal TTX-810 treatment regimen that effectively targets cancer cells, while preserving healthy cardiomyocytes. The study also demonstrates that a minimum exposure time is sufficient for efficacy, and this effect is even more pronounced when combined with standard-of-care agents. Moreover, limiting the exposure time expands the TTX-810 therapeutic window with regard to cardiotoxicity, which was validated in GLP toxicology studies in dogs and positions TTX-810 for future clinical development.
Citation Format: Ulrike Rauh, Virendar Kaushik, Sabine Pilari, Georgios Kosmidis, Michael Serrano-Wu, Guo Wei, Weiqun Zhang, Stefan Kaulfuss, Kai Thede, Guillaume Poncet-Montange, Douglas Daniels, Christopher T Lemke, Brian Hubbard, Todd R. Golub. Rapidly cleared MCL1 inhibitor TTX-810 showing robust efficacy in preclinical tumor models with no effects on cardiomyocytes in vitro and in vivo [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr LB_A01.