Abstract B197: Selective depletion of YAP1 with next-generation antisense oligonucleotides leads to immune cell infiltration and tumor regression in mouse models of HCC

The Hippo pathway plays important roles in maintaining tissue homeostasis. When dysregulated, however, it contributes to the development of a variety of cancers including hepatocellular carcinoma (HCC). YAP1 is the final regulator of the pathway and its causal role in promoting HCC growth has been demonstrated previously in multiple animal models. YAP1 is therefore considered an attractive drug target for HCC but as a transcription coactivator is difficult to inhibit by conventional approaches. We investigated how selective downregulation of YAP1 would affect the growth of established HCC using optimized Gen2.5 antisense oligonucleotides (ASOs) for mouse YAP1. Here, we describe the antitumor activity of YAP1 ASOs in 2 mouse models of HCC, which appears to be mediated by both tumor autonomous and immune modulatory functions of YAP1. In YAP1-activated Salvador KO mice with established HCC, systemic treatment of YAP1 ASO for 2 months at 75 mg/kg/week resulted in a marked reduction (>90%) in tumor YAP1 levels and regression of HCC tumors, which is in contrast to the tumors in the control ASO-treated animals that significantly increased in size. Proliferation of tumor cells was drastically reduced in YAP1 ASO-treated animals as demonstrated by low Ki-67 staining along with a decrease in Cyr61, a downstream YAP1 target. YAP1 ASOs were also tested in the diethylnitrosamine-induced HCC model. Although the model has been widely used in HCC research, lack of reliable biomarkers and high variability in tumor take between animals make it less ideal for an efficacy study. To overcome these limitations, we established a subcutaneous model by in vivo passaging tumors excised from the liver in either nude or immune-competent C57BL/6 mice. Activation of YAP1 in this model was demonstrated by strong nuclear localization of YAP1 protein in tumors. While the effect of YAP1 ASO on tumor was primarily delaying its growth in nude mice, the same YAP1 ASO induced tumor regression when DEN tumors were grown in immune competent mice, suggesting an immune component for activity. Strong infiltration of myeloid lineage cells was observed in tumors of YAP1 ASO-treated animals at the end of the 6 week study, suggesting potential activation of immune cells as a result of YAP1 depletion. Significant infiltration of T cells and myeloid lineage cells into YAP1 ASO-treated tumors was also observed at an early time point as demonstrated by increases in CD3 and F4/80 signals by IHC. Immune cell infiltration into tumors was more pronounced in the combination of ASOs for YAP1 and its paralog mouse TAZ than YAP1 ASO alone; however, the combination group exhibited notable signs of inflammation on the skin and in the liver. Importantly, when tumor-bearing animals were taken off the treatment after tumor regression was observed with YAP1 ASO, the antitumor response was sustained for more than 6 weeks with 2 out of 8 animals remaining tumor free in YAP1 ASO group. Further, safety of ASO-mediated YAP1 inhibition was confirmed in both normal mice with no notable toxicity and tumor-bearing mice with improvement in liver transaminases despite near complete depletion of YAP1 in the liver. Collectively, these results provide evidence for immune activation as a result of YAP1 downregulation and also suggest a potential therapeutic use of YAP1 ASO in the treatment of HCC with YAP1 activation. Citation Format: Youngsoo Kim, Joanna Schmidt, Minji Jo, Jing Yi, Tianyuan Zhou, Xiaolin Luo, Sun Young Yim, Ju-Seog Lee, Randy L. Johnson, Brett P. Monia, Gordon Mills, A. Robert MacLeod. Selective depletion of YAP1 with next-generation antisense oligonucleotides leads to immune cell infiltration and tumor regression in mouse models of HCC [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B197.