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Background SBT6290 is a novel therapeutic comprised of a selective TLR8 agonist conjugated to a Nectin-4-specific monoclonal antibody, designed for systemic delivery and tumor-localized activation of myeloid cells. Nectin-4 is a cell surface adhesion molecule that is overexpressed in multiple solid tumor types including triple negative breast, head and neck, lung, and urothelial cancers, with limited expression in normal tissues. Many solid tumors, including those expressing Nectin-4, are resistant to immunotherapy due to immune-suppressive mechanisms, loss of HLA, low neoantigen availability, and/or minimal T cell infiltrates. These tumors, however, are often replete with myeloid cells. Activation of these cells has emerged as a promising approach in overcoming resistance mechanisms to current cancer immunotherapies. TLR8 is highly expressed in myeloid cell types prevalent in human tumors, including conventional DCs and macrophages. Agonism of TLR8 in human myeloid cells activates a broad spectrum of anti-tumor immune mechanisms, including proinflammatory cytokine production, repolarization of suppressive myeloid cells, and the priming of CTL responses. Here, we show that SBT6290 potently activates human myeloid cells in a Nectin-4-dependent manner and that a mouse surrogate confers single agent anti-tumor activity in preclinical studies. These data support the development of SBT6290 for the treatment of patients with Nectin-4-expressing tumors. Methods SBT6290 activity was characterized in vitro using co-culture systems consisting of human immune cells and Nectin-4-expressing tumor cells. The in vivo efficacy of the SBT6290 surrogate was evaluated as a single agent in mouse tumor models expressing Nectin-4. Results Studies with human immune cells show that SBT6290 potently induces multiple anti-tumor immune activities including proinflammatory cytokine and chemokine production, inflammasome activation, direct activation of DCs and indirect T and NK cell cytolytic activity. This activity requires the presence of Nectin-4 expressing tumor cells and the engagement of Fc gamma receptors on the surface of the myeloid cells by the conjugate to facilitate delivery of SBT6290 into myeloid cells. Notably, SBT6290 is >100 fold more potent than the free, unconjugated TLR8 agonist. Systemic administration of a SBT6290 surrogate in mice results in robust single agent efficacy in tumor models intrinsically resistant to checkpoint blockade, including the EMT6 model engineered to express human Nectin-4. Conclusions The preclinical data described here show the potential for SBT6290 to drive robust, single agent anti-tumor responses and support the clinical development of SBT6290 for patients with Nectin-4 expressing tumors.

SBT6050, a novel therapeutic comprised of a potent toll-like receptor (TLR) 8 agonist conjugated to a HER2-directed monoclonal antibody that binds an epitope distinct from trastuzumab, is designed for systemic delivery and tumor-localized activation of human myeloid cells in the presence of moderate and high HER2-expressing tumor cells. Many solid tumors, including those expressing HER2, are refractory to immunotherapy due to immune-suppressive mechanisms, loss of HLA, low neoantigen availability, and/or minimal T cell infiltrates. These tumors frequently contain abundant populations of tumor-associated myeloid cells. Activation of these cells through TLR agonism has emerged as a promising approach in overcoming resistance mechanisms to current cancer immunotherapies. Unlike other endosomal TLRs such as TLR7 and TLR9, TLR8 is highly expressed in human myeloid cells known to be prevalent in human tumors such as conventional DCs and macrophages. TLR8's restricted myeloid cell expression removes the risk of inducing T cell death or tumor cell proliferation as described for other innate immune activators such as STING. Agonism of TLR8 in human myeloid cells activates a broad spectrum of anti-tumor immune mechanisms, including proinflammatory cytokine production, repolarization of suppressive myeloid cells and the priming of CTL responses. These functions cannot be replicated by a potent TLR7 agonist or with clinical agents such as resiquimod that agonize TLR7 and only weakly engage TLR8. Here we present data demonstrating the superiority of SBT6050 at activating human myeloid cells compared to HER2 antibody conjugates that use either a selective TLR7 agonist or resiquimod. In vitro studies with human immune cells show that SBT6050 potently induces multiple anti-tumor immune activities, including proinflammatory cytokine and chemokine production, inflammasome activation, direct activation of DCs and indirect T and NK cell cytolytic activity. Our data indicate the favorable profile of SBT6050 is likely due to efficient engagement of TLR8 in conjugate form and TLR8's unique expression profile, and not due to differences in the potency of the small molecule payloads. Using an SBT6050 mouse surrogate we show in vivo evidence for intratumoral TLR agonist activation of several myeloid driven anti-tumor pathways leading to curative single agent efficacy in both a T cell-excluded syngeneic tumor model and a xenograft model lacking T, B and NK cells. Collectively, these data demonstrate that SBT6050 displays an attractive functional profile unachievable with agonist-based antibody conjugates directed against other innate immune receptors. These data also highlight the potential for SBT6050 to be clinically active as a monotherapy. SBT6050 is projected to enter the clinic in 2020 for patients with moderate or high HER2-expressing tumors. Citation Format: Michael R. Comeau, Ty Brender, Monica Childs, Jamie Brevik, Damion Winship, Heather Metz, Jenny Chang, Jeffrey Adamo, Ben Setter, Hengyu Xu, Li-Qun Fan, Brenda Stevens, Sean W. Smith, Phil Tan, Robert DuBose, Yvette Latchman, Peter Baum, Valerie Odegard. SBT6050, a HER2-directed TLR8 ImmunoTAC™therapeutic, is a potent human myeloid cell agonist that provides opportunity for single agent clinical activity [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4537.

3110 Background: Solid tumors are replete with myeloid cells which, when activated, drive potent anti-tumor responses. Clinical development of systemically administered myeloid cell agonists, however, 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. A systemically delivered myeloid cell agonist with tumor-localized activity has the potential to overcome challenges encountered with other innate immune/myeloid cell agonists in clinical development. Methods: SBT6050 is a novel therapeutic comprised of a potent toll-like receptor (TLR) 8 agonist payload conjugated to a HER2-directed monoclonal antibody. Delivery of the payload into the endosome of human myeloid cells, where TLR8 resides, requires the co-engagement of HER2 on tumor cells and Fc gamma receptor on human myeloid cells. Thus, SBT6050 is designed for systemic delivery and tumor-targeted activation of human myeloid cells. Results: Studies with human immune cells show that SBT6050 potently induces, in a HER2-dependent manner, multiple anti-tumor immune activities due to its direct activation of myeloid cells and the subsequent induction of T and NK cell cytolytic activity. SBT6050 is designed to activate human myeloid cells only in the presence of HER2-positive tumor cells with moderate (2+ by IHC) or high (3+ by IHC) expression levels. Tumor-localized activity has been demonstrated in mouse models using a SBT6050 mouse surrogate. Systemic delivery results in robust single agent efficacy in multiple mouse tumor models, even those engineered to lack T cells, without accompanying peripheral cytokine production. Trastuzumab and SBT6050 bind to distinct epitopes on HER2 and enhanced activity is observed when the two agents are combined. Conclusions: SBT6050 is a systemically administered, tumor-targeted myeloid cell agonist that demonstrates single agent efficacy in multiple mouse tumor models without peripheral cytokine production. A first-in-human study with SBT6050 is expected to begin this year for patients with HER2-expressing solid tumors.

Mouse models of cancer are an invaluable biomedical research tool in which characterization of the abundance, distribution and co-localization of immuno-oncology targets is critical for the advancement of disease understanding. Nanostring Technologies' Digital Spatial Profiling (DSP) technology allows for the simultaneous analysis of 10s to 100s of proteins from discrete regions of interest (ROI), providing a spatial context to multiplexed data. The purpose of this study is to validate an antibody panel designed to characterize key tumor and immunology markers on the DSP platform. DSP technology utilizes barcoded oligo tags that are covalently linked to detection reagents, in this case, antibodies. The workflow for a typical DSP experiment consists of incubating a formalin-fixed, paraffin-embedded (FFPE) tissue with a cocktail of oligo-labeled primary antibodies and three fluorescently labeled visualization markers. The visualization markers and tissue morphology guide the selection of ROIs. UV light then illuminates each ROI, releasing the oligo tags through a UV photocleavable linker. Tags are collected through microcapillary aspiration and then deposited into a microtiter plate for further analysis on the standard nCounter® platform. In order to validate antibodies for this technology, the specificity and sensitivity of each antibody was evaluated for proper immunohistochemistry (IHC) staining pattern on appropriate tissues as well as signal-to-noise ratios of positive counts above background. Panels will also undergo an evaluation of single vs. multiplex interactions to ensure there are no deleterious effects of multiplexing antibodies. Immunohistochemical analysis of antibodies chosen for panel displays indistinguishable staining patterns on mouse control tissues for unconjugated primary antibodies and oligo-conjugated primary antibodies. Visualization markers allow for an ROI selection strategy that utilizes a range of circular ROIs (e.g., 50-650um) as well as custom masking of areas of interest. Key immuno-oncology targets including B7-H3, beta-catenin, CD19, CD25, CD3, CD4, histone H3, Ly-6G, pan-cytokeratin, PD-1, PD-L1, F4/80, Ki-67, STAT3, etc., have been validated thus far. Ongoing validation efforts will continue to expand this antibody panel. Nanostring Technologies' DSP platform is applicable not only to human tissue but also to mouse preclinical samples. Antibody specificity, sensitivity and interaction within a cocktail have been validated for this application. DSP technology allows for the multiplexing of important mouse immuno-oncology markers so that researchers can evaluate the abundance as well as the spatial distribution of these key targets simultaneously. Citation Format: Sarah Warren, Heather Metz, Kristi Barker, JingJing Gong, Alison VanSchoiack, Quoc Huynh, Chris Merritt, Lucas Dennis, Yan Liang, Joseph M. Beechem. Validation of digital spatial profiling of key immuno-oncology targets for mouse FFPE preclinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3858.

Nanostring Technologies’ Digital Spatial Profiling (DSP) technology allows for the simultaneous analysis of 10’s to 100’s of proteins from discrete regions of interest (ROI), providing a morphological context to multiplexed protein analysis. The purpose of this study is to validate an antibody panel designed to characterize key immunology and tumor markers on the DSP platform. IHC was performed on human FFPE tissues and human cell line pellets to evaluate binding specificity in both unconjugated and oligo-conjugated antibodies. The dynamic range of antibodies was tested using positive and negative FFPE cell pellets at different ratios. Interaction screens were performed to evaluate potential effects of multiplexing antibodies. A tissue microarray (TMA) containing normal and cancer tissues was employed to assess assay robustness. IHC analysis of antibodies displayed indistinguishable staining patterns for both unconjugated and oligoconjugated antibodies. Mixed-proportion cell pellet assays revealed strong correlations between positive counts and positive cell numbers in an ROI. For example, CD3 displayed a LOD of 4% when assayed using cell pellet mixtures containing CD3+ CCRF-CEM cells and CD3− HEK293T cells. Interaction studies showed similar values for antibodies alone or in combination (R2 > 0.8). TMA hierarchical clustering analysis demonstrated expected patterns for immune and tumor cells across different tissues. These results demonstrate that indexing oligo conjugation does not interfere with antibody specificity and that these conjugated antibodies are robust reagents for quantification of protein abundance. Continued work on the DSP platform will expand the library of antibodies accessible for profiling.