Abstract 3271: A systemically administered, conditionally active TLR8 agonist for the treatment of HER2-expressing tumors

Clinical development of systemically administered myeloid cell agonists has been hindered by acute toxicities due to peripheral activation of the targeted cell types. Intratumoral administration, the route of delivery typically used for innate immune/myeloid cell agonists, is limited by tumor accessibility and a dependence on abscopal responses. Here, we describe an example of a new composition class, termed ImmunoTAC, that allows for systemic delivery of an immune modulator with activity localized to tumor sites and secondary lymphoid structures. Agonism of TLR8 in human myeloid cells drives anti-tumor immunity by inducing direct macrophage killing of tumor cells, repolarizing suppressive myeloid cell populations to a pro-immunogenic phenotype, and inducing dendritic cells to drive tumor-specific CTL responses. SBT6050 is an ImmunoTAC comprised of a novel, potent TLR8 agonist conjugated to a HER2-directed monoclonal antibody. SBT6050 is designed to activate human monocytes and macrophages only in the presence of HER2pos tumor cells with moderate (2+ by IHC) or high (3+ by IHC) expression levels. This activity is dependent upon the ability of the Fc domain of the antibody to engage Fcγ receptors on the surface of the myeloid cells. Additionally, SBT6050 potently activates conventional dendritic cells that, in turn, drive a Th1/CTL program in T cells. Systemic delivery of a SBT6050 surrogate in mice shows robust single agent efficacy in tumor models intrinsically resistant to checkpoint blockade, such as the previously characterized HER2+ CT26 syngeneic model. The intratumoral immune response is characterized by robust activation of tumor-associated myeloid cells, infiltration of neutrophils, persistent increases in local cytokine and chemokine production, decreases in Treg infiltrate, and the generation of a potent, neo-Ag specific anti-tumor CTL response. Mice cured with single-agent treatment are resistant to tumor rechallenge, demonstrating induction of long-lived immunological memory. In contrast to observations with small molecule TLR8 agonists, ImmunoTAC does not induce peripheral cytokine production or associated CRS-like toxicity in mice, consistent with the localized activity of the molecule. Silverback’s lead candidate, SBT6050, is currently in preclinical development for patients with moderate or high HER2-expressing tumors. More broadly, the data presented here describe a novel therapeutic modality that allows for systemic administration of immune modulators with tissue-localized activity. Citation Format: Kara Moyes, Ty Brender, Sean W. Smith, Hengyu Xu, Ben Setter, Li-Qun Fan, Rebecca Brunette, Justin Killebrew, Phil Tan, Craig Coburn, Peter Baum, Valerie Odegard. A systemically administered, conditionally active TLR8 agonist for the treatment of HER2-expressing tumors [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 3271.