Abstract 944: exoSTING: An engineered exosome therapeutic that selectively delivers STING agonist to the tumor resident antigen-presenting cells resulting in improved tumor antigen-specific adaptive immune response

Background: The Stimulator of Interferon Genes (STING) pathway is an attractive target in immuno-oncology. Selective activation of the STING pathway in antigen presenting cells (APCs) is essential for eliciting a potent and specific anti-tumor immune response. STING is ubiquitously expressed in normal cells including T cells and endothelial cells. Direct intra-tumoral administration of a free STING agonist results in activation of the STING pathway in all cells, resulting in loss of viability of immune cells, tissue damage, and systemic immune activation due to vascular leakage. Exosomes are an efficient natural messenger system that delivers macromolecules between cells. Leveraging exosome biology, we have developed a novel engineered exosome therapeutic, exoSTING, to selectively target the STING pathway in tumor resident APCs. Results: exoSTING is composed of exosomes engineered to express high levels of Protein X (PrX), a transmembrane glycoprotein naturally occurring on exosomes, and loaded with a STING agonist. Following intra-tumoral injection in checkpoint refractory B16F10 tumors, exoSTING selectively activates STING in APCs, leading to anti-tumor immunity with greater than 100-fold improvement in potency compared to free STING agonist. The activity of exoSTING is substantially diminished in the PrX knock out exosomes, highlighting the importance of this surface glycoprotein for preferential activation of APCs. In contrast to the free STING agonist, exoSTING is retained within the injected tumor, minimizing systemic exposure. Furthermore, exoSTING administration preserves the viability of T cells and APCs, reduces collateral tissue damage, and does not induce systemic cytokine production, resulting in a broader therapeutic window in contrast to free STING agonist. exoSTING produced an increased systemic tumor-specific T cell response as demonstrated by elimination of non-injected abscopal tumors. The specificity of exoSTING activity was demonstrated using a STING knockout mouse (Tmem173gt/J). T cells and myeloid cells play a critical role in exoSTING mediated anti-tumor immunity, in contrast to NK cells which are not required. exoSTING treatment results in significant induction of PD-L1 expression (P Conclusion: exoSTING is an engineered exosome therapeutic that leverages exosome biology and specifically targets the STING pathway in APCs, resulting in greater potency with preserved viability of T cells and APCs, greater systemic tumor antigen specific immune response, reduced systemic cytokine production, and enhanced efficacy. Citation Format: Su Chul Jang, Raymond J. Moniz, Chang Ling Sia, Rane A. Harrison, Damian Houde, Nikki Ross, Ke Xu, Nuruddeen Lewis, Raymond Bourdeau, Christine McCoy, Tong Zi, Agata Villiger-Oberbek, Scott Estes, Jorge Sanchez-Salazar, Kevin Dooley, William K. Dahlberg, Sriram Sathyanarayanan, Kyriakos D. Economides. exoSTING: An engineered exosome therapeutic that selectively delivers STING agonist to the tumor resident antigen-presenting cells resulting in improved tumor antigen-specific adaptive immune response [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 944.