Background: Translational control of oncoprotein expression has been implicated in the pathogenesis of multiple solid tumors. The eukaryotic translation initiation factor 4A (eIF4A) is an RNA helicase that catalyzes the unwinding of secondary structure in the 5′-untranslated region (5′-UTR) of mRNAs to facilitate translation initiation. eIF4A is an essential gene in many cancer cell lines and multiple oncogenes or anti-apoptotic proteins that contain structured 5’-UTRs have been shown to require enhanced eIF4A activity for translation. Zotatifin (eFT226) is a selective eIF4A inhibitor that has been shown previously to inhibit tumor growth in several immunocompromised mouse models and is currently in Phase I clinical trials as a single agent (NCT04092673). In this study we investigated the effects of Zotatifin both as a single agent and in combination with standard-of-care chemotherapy on tumor progression in several syngeneic triple-negative breast cancer (TNBC) mouse models. Methods: The activity of Zotatifin was assessed as single agent in eight different syngeneic p53-null mouse TNBC models across several intrinsic molecular subtypes. Synergism of Zotatifin with chemotherapy was evaluated in three responsive models. In addition, a Zotatifin-resistant model was developed following prolonged drug treatment. Drug response mechanisms and the effect on the tumor immune microenvironment were determined by tandem mass tag mass spectrometry, reverse phase protein array (RPPA) and mass cytometry. Results: Zotatifin treatment slowed tumor growth in six out of eight syngeneic TNBC models without obvious toxicity. Mass spectrometry of acutely treated tumors revealed that Zotatifin inhibited proliferative and stem cell signaling pathways including E2F targets, G2/M checkpoints, and NOTCH signaling, and induced proteins involved in Interferon-α and Interferon-γ responses. Mass cytometry analyses showed that Zotatifin suppressed the infiltration of M2 macrophages and neutrophils and promoted the infiltration of conventional dendritic cells to the tumor microenvironment. Interestingly, combination therapy of Zotatifin with chemotherapy significantly slowed tumor growth and improved survival. However, drug resistance emerged after prolonged treatment. The resistant tumors strikingly displayed a mesenchymal histology and an increased activity of the PI3K-AKT-mTOR pathway as determined by RPPA analysis. Combination treatment of Zotatifin with the mTOR inhibitor Everolimus prolonged survival. Conclusions: eIF4A is a therapeutic vulnerability in TNBC. Zotatifin inhibits tumor progression both through tumor-intrinsic mechanisms and by targeting the tumor immune microenvironment. Synergism between Zotatifin and chemotherapy supports the clinical potential of combination therapies.

Citation Format: Na Zhao, Jeffrey Mark Rosen. The RNA helicase EIF4A is a therapeutic vulnerability in triple-negative breast cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P145.