The tumor suppressive Hippo pathway is frequently dysregulated in cancer, resulting in the constitutive activation of YAP1 in a variety of solid cancers including hepatocellular cancer (HCC) and head and neck squamous cell carcinoma (HNSCC) among others. Here, we identified potent next-generation constrained ethyl (cEt) antisense oligonucleotides (ASOs) to selectively target either human or mouse YAP1 and evaluated these in several preclinical models of both HCC and HNSCC. In the genetically engineered mouse (GEM) model of HCC (Salvador KO mice) and in the carcinogen-induced models of HCC (diethylnitrosamine, DEN-induced HCC) systemic treatment with mouse YAP1 ASOs resulted in a marked reduction in YAP1 protein level in (~90% reduction) and strong regressions of established tumors. Interestingly, YAP1 ASO treatment also resulted in significant infiltration of T cells and myeloid lineage cells in the tumors in these immune-competent mouse models of HCC suggesting a role in immune activation. Consistent with this notion, YAP1 ASO treatment in combination with anti-PD1 antibody treatment led to enhanced antitumor activity. In HNSCC, loss of the tumor suppressor FAT1 is a frequent occurrence and results in activation and nuclear localization of YAP1. In a panel of human HNSCC cell lines, proliferation was strongly reduced by human YAP1 ASOs selectively in the cell lines harboring homozygous copy loss or nonsense mutations of FAT1 while minimal antiproliferative activity was observed in cells with wild type FAT1. Importantly, the nuclear localization of YAP1 appeared to be a key determining factor for the sensitivity of human HNSCC cells to YAP1 depletion by ASO. In vivo treatment of mice bearing human CAL33 tumors that harbor FAT1 mutation, YAP1 ASOs greatly reduced YAP1 expression in tumors and suppressed tumor growth in both subcutaneous and orthotopic models. Collectively, these results suggest a potential therapeutic use of YAP1 ASOs in the treatment of both HCC and HNSCC with YAP1 activation as a single agent or in combination with immuno-oncology drugs. Based on these finding broad human ASO clinical candidates screens were performed leading to the selection of ION-537, a potent ASO drug targeting human YAP1 has now advanced into human clinical trials in cancer patients with YAP1 activation.

Citation Format: A. Robert Macleod. The discovery and characterization of ION-537: A next generation antisense oligonucleotide inhibitor of YAP1 in preclinical cancer models [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 ND11.