Abstract 36: Novel anti-Sialyl-Tn monoclonal antibodies and antibody-drug conjugates (ADCs) demonstrate tumor specificity in vitro and in vivo antitumor efficacy
Authors: Prendergast Jillian M. , Eavarone David A. , Rao Patricia E. , Curtis Adam D. , Shopland Lindsay S. , Hoffert Todd A. , Stein Jenna , Behrens Jeff , Dransfield Daniel T. 
Published: July 2017

Abstract
Tumor-associated carbohydrate antigens (TACAs) historically have been challenging targets for antibody therapeutics. Sialyl-Tn (STn) is a cancer specific antigen that is expressed on the cell surface of carcinomas including ovarian, colon, prostate, and pancreatic tumors but is rarely present in normal tissue. STn expression has been linked to innate immune suppression, a chemoresistant phenotype, metastasis, and poor prognosis. Previous attempts to target this antigen in the clinic with synthetic glycan vaccines proved safe but lacked efficacy. We have developed highly selective humanized monoclonal antibodies and antibody drug conjugates (ADCs) targeting TACAs, such as STn. Remarkable sequence homology across all anti-STn mAbs was observed in both heavy and light chains, and hot spots for hypermutation were identified. These antibodies were selected using our glycan microarray that enriches for candidates whose binding is protein-independent, highly selective and demonstrates exceptional target affinity.
Lead humanized candidates demonstrated single digit nanomolar EC50s in ELISA/flow cytometric assays, STn selective cell internalization, and STn specific glycan binding on Siamab’s proprietary glycan array. STn binding sites in common tumor lines (ovarian, gastric and breast) were determined per cell and subsequent cytotoxicity assays in these lines demonstrated in vitro efficacy. Tumor microarray experiments revealed membranous staining in cancerous tissues of various indications. Binding studies of anti-STn antibodies to primary human cancer samples by flow cytometry demonstrated that both tumor and Myeloid-Derived Suppressor Cells (MDSC, both myeloid and granulocytic) express STn. In an OVCAR3 xenograft model, 30 days after the last anti-STn ADC dose was given, groups treated (Q7Dx4) exhibited mean tumor volumes below the Day 1 pre-treatment mean tumor volumes (155mm3). Flow cytometric analysis of tumors from these mice demonstrated that anti-STn ADC treatment reduces STn expression on the primary tumor in a dose-dependent manner (Q7Dx4 vs. single dose) compared to the isotype-ADC control. Our data demonstrates that high-affinity, STn-selective mAbs show promise as therapies for solid tumors and could also target MDSCs to promote antitumor immune responses.
Citation Format: Jillian M. Prendergast, David A. Eavarone, Patricia E. Rao, Adam D. Curtis, Lindsay S. Shopland, Todd A. Hoffert, Jenna Stein, Jeff Behrens, Daniel T. Dransfield. Novel anti-Sialyl-Tn monoclonal antibodies and antibody-drug conjugates (ADCs) demonstrate tumor specificity in vitro and in vivo antitumor efficacy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 36. doi:10.1158/1538-7445.AM2017-36