Abstract 2019: Enhancing the effect of immunotherapy by inhibiting tumor promoting effect of HDAC6 (2019)

Abstract Histone deacetylases (HDAC) are recognized for diverse functions beyond remodeling of chromatin landscape. It may range from regulating the cellular-health to immune-diseases like cancer, positioning the HDAC inhibitors (HDACi) at a crucial junction of immunotherapy. The toxicity among pan-HDACi has led to the development of selective inhibitors, which helped to understand the roles of specific HDACs in immune responses. For example, HDAC6 is reported to promote the pro-tumorigenic STAT3 pathway. By using specific HDAC6i, the downstream immune-modulatory pathways of STAT3, such as PDL1, could be targeted. HDAC6 has been associated with numerous structural functions, including cellular motility, shape, antigen processing, and intracellular transport through the acetylation of tubulin and cortactin, among others. This function suggests that HDAC6 could also a regulator of cellular processes involved in metastasis. In this respect, we found that the selective HDAC6i NextA was able to reduce the primary tumor growth and the appearance of spontaneously metastatic nodules in the murine Triple Negative Breast Cancer (TNBC) tumor model 4T1. Additionally, the in vitro use of NextA in multiple murine and human breast cancer cell lines was found to reduce invasion and modulate multiple EMT-specific genes without exhibiting excessive cytotoxicity. PDL1 expression was also reduced, as described previously in melanoma models. Given that 4T1 is a TIL-infiltrated tumor, we tested the efficacy of αPD-1 immune checkpoint inhibitors (ICI) and found a lower dose of αPD-1 to be more effective than the higher doses to reduce primary and secondary tumor growth. However, the expression of IFNγ &amp; PD-L1 were enhanced with the monotherapy. In an in vitro setting, we were able to nullify the upregulation of PD-L1 by αPD-1 with either NextA or IFNγ neutralization. To apply this insight in vivo, we tested the combination of NextA and αPD-1 to find a significant reduction in tumor growth, both in primary &amp; secondary nodules. Analysis of the effector molecules revealed a reduction in intra-tumoral PD-L1 and IDO1 along with a moderate level of IFNγ, as well as a reduction in several key EMT signature genes, including cMYC, MMP9, vimentin, and twist. We also identified E-cadherin, a negative regulator of the metastatic process, as a novel target of NextA that was found to be upregulated in vivo. This was corroborated by the in vitro observations in several murine and human breast cancer cell lines, irrespective of hormonal receptor status. In conclusion, we propose that combining HDAC6i along with ICI such as αPD-1 may offer a novel avenue to enhance the efficacy of immunotherapy, as well as alter the tumor-intricate physiology, without incurring unnecessary toxicity. Note: This abstract was not presented at the meeting. Citation Format: Debarati Banik, Melissa Beaty, Erical Palmer, Maria Del Mar Gracia Hernandez, Satish K. Noonepalle, Prathima Vembu, Alan P. Kozikowski, Alejandro Villagra. Enhancing the effect of immunotherapy by inhibiting tumor promoting effect of HDAC6 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2019.