Unconjugated Antibody Research Articles

CD117-Amanitin Antibody Drug Conjugates Effectively Deplete Human and Non-Human Primate HSCs: Proof of Concept As a Targeted Strategy for Conditioning Patients for Bone Marrow Transplant

Background: Genotoxic conditioning prior to allogeneic and autologous bone marrow transplantation (BMT) limits the use of these potentially curative treatments due to risks of regimen-related morbidities and mortality, including risks of organ toxicity, infertility, and secondary malignancies. CD117, which is specifically expressed on hematopoietic stem cells (HSCs) and progenitors is rapidly internalized and is an ideal target for an antibody drug conjugate (ADC) based approach to conditioning. We have previously shown that a single dose of an anti-CD117 ADC depleted >95% of bone marrow HSCs in a humanized mouse model and reduced disease burden while extending survival in an AML tumor model (Hartigan et al., Blood 2017 130:1894). The aim of this translational study was to develop a potent anti-CD117 ADC highly effective in eliminating host HSCs with a short half-life and minimal adverse side effects in a non-human primate (NHP) model.Methods: Three different DNA-damaging cytotoxic payloads and amanitin (AM) were site specifically conjugated to an anti-CD117 antibody. The ADCs were titrated and evaluated for in vitro cytotoxicity using human bone marrow CD34+ cells. The ADCs were administered in ascending doses to humanized NSG mice. HSC depletion and immunophenotype of the human cells in the peripheral blood was determined by flow cytometry. For amanitin conjugates, NHP HSC depletion was evaluated in male cynomolgus monkeys in single ascending doses (3/group). HSC content in the bone marrow was monitored by flow cytometry and colony-forming unit (CFU) analysis on day 7 or 14 and 56 post dosing. Hematology and clinical chemistries were evaluated throughout the two-month study.Results: Of the toxins evaluated, only anti-CD117 conjugated with the RNA polymerase II inhibitor amanitin resulted in >90% depletion of human HSCs in humanized NSG mice at 0.3 mg/kg. The AM-conjugates also demonstrated a broad therapeutic window in this model (therapeutic index of >120). As a proof-of-concept for the depletion of HSCs in large animals, a single i.v. dose escalation study was performed with the cross-reactive anti-CD117-AM in NHP. On-target, dose-dependent decreases in phenotypic HSCs and CFUs were observed in the bone marrow at day 7 post anti-CD117-AM dosing with >95% HSC depletion observed with a single dose of 0.3 mg/kg (Fig. 1). In the periphery, a dose-dependent transient decrease in reticulocytes was observed at day 4 with a neutrophil and monocyte nadir at day 18. The depth and duration of the depletion was also dose-dependent. The anti-CD117-AM induced depletion was on target and amanitin dependent as the unconjugated antibody and isotype-AM had no effect. Notably, white blood cell and lymphocyte counts were stable through day 56, demonstrating that this strategy will spare the adaptive immune system. Thrombocytopenia occurred 4-8 days post infusion and was dose-dependent, transient and reversible. This also occurred with the isotype-AM, suggesting the effect was off-target.Because the half-life of the anti-CD117-AM was 5 days, a second dose escalation study with anti-CD117-AM engineered to have a short half-life (~18 h) was performed in NHPs. The short half-life anti-CD117-AM demonstrated similar potency on all cell parameters at 0.3 mg/kg and was well tolerated at the effective dose. As expected, the short half-life anti-CD117-AM was rapidly cleared with a half-life of 15-18 h. In both studies, a transient dose dependent elevation of liver enzymes was observed in groups treated with the highest doses of isotype-AM, anti-CD117-AM, and the short half-life anti-CD117-AM.Conclusions: Anti-CD117-AM exhibited potent elimination of NHP HSCs and progenitors in vivo. The potency of short half-life anti-CD117-AM was comparable, providing a model for target cell depletion and rapid clearance prior to BMT. Both ADCs were well tolerated at the efficacious doses. This strategy preserves the adaptive immune system with delayed onset of neutrophil nadir (18 days), potentially shortening the period of neutropenia. Targeted depletion of hematopoietic cell subtypes with limited off-target effects could provide a significant improvement in standard-of-care approaches to patient preparation prior to HSC transplant. [Display omitted] DisclosuresPearse:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. McDonough:Magenta Therapeutics: Employment, Equity Ownership. Proctor:Magenta Therapeutics: Employment, Equity Ownership. Panwar:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Sarma:Magenta Therapeutics: Employment, Equity Ownership. McShea:Magenta Therapeutics: Employment, Equity Ownership. Kien:Magenta Therapeutics: Employment, Equity Ownership. Dushime:Magenta Therapeutics: Employment, Equity Ownership. Adams:Magenta Therapeutics: Employment, Equity Ownership. Hyzy:Magenta Therapeutics: Employment, Equity Ownership. Brooks:Magenta Therapeutics: Employment, Equity Ownership. Palchaudhuri:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties; Harvard University: Patents & Royalties. Li:Magenta Therapeutics: Employment, Equity Ownership. Kallen:Magenta Therapeutics: Employment, Equity Ownership. Sawant:Magenta Therapeutics: Employment, Equity Ownership. McDonagh:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Boitano:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Cooke:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties.

Targeting CD45 with an Amanitin Antibody-Drug Conjugate Effectively Depletes Human HSCs and Immune Cells for Transplant Conditioning

IntroductionBone Marrow Transplant (BMT) is a potentially curative treatment for malignant and non-malignant blood disorders and has demonstrated impressive outcomes in autoimmune diseases. Prior to BMT, patients are prepared with high-dose chemotherapy alone or with total body irradiation, and both are associated with early and late morbidities, such as infertility, secondary malignancies and organ toxicity; and substantial risk of mortality. This greatly limits the use of BMT in malignant and non-malignant conditions. To address these issues, we are developing antibody drug conjugates (ADCs) targeting hematopoietic stem cells (HSCs) and immune cells to more safely condition patients for BMT.ResultsTo enable simultaneous HSC and immune cell depletion for BMT we investigated targeting human CD45, a protein expressed exclusively on nearly all blood cells including HSCs. Antibody discovery campaigns identified several antibodies with sub-nanomolar affinities for human and non-human primate (NHP) CD45. We then created anti-CD45 ADCs with drug payloads including DNA-damaging, tubulin-targeting and RNA polymerase-inhibiting molecules. An ADC developed with alpha-amanitin (an RNA polymerase II inhibitor) enabled potent in vitro killing of primary human CD34+ HSCs and immune cells (40-120 picomolar IC50s).With this anti-CD45 amanitin ADC (CD45-AM), we explored depletion of HSCs and immune cells in vivo using humanized NSG mice. A single dose of 1 or 3 mg/kg CD45-AM enabled >95% depletion of human CD34+ cells in the bone marrow as assessed 7 or 14 days post-administration (Figure, n = 3/group, p values < 0.05); >95% depletion of human B-, T- and myeloid cells was observed in the periphery and bone marrow (Figure, p values < 0.05). Control non-targeting isotype matched-ADCs and anti-CD45 antibody not bearing a toxin had minimal effect on either HSC or immune cells.In hematopoietic malignancies, an anti-CD45 ADC would ideally reduce disease burden and enable BMT. In a model of acute lymphoblastic leukemia (REH cell line, n = 10 mice/group), and 3 patient-derived models of FLT3+NPM1+ acute myeloid leukemia (n = 4-5 mice/group per model), a single dose of 1 mg/kg CD45-AM more than doubled the median survival and several mice survived disease-free (p values < 0.001).Anti-CD45 antibodies have been investigated for BMT conditioning in patients as naked antibodies that rely on Fc-effector function to deplete lymphocytes (Biol Blood Marrow Transplant. 2003 9(4): 273-81); or as radioimmunotherapy (Blood. 2006 107(5): 2184-2191). These agents demonstrated infusion-related toxicities likely due to effector function elicited by the wild-type IgG backbone. To address this issue, we created anti-CD45 antibodies with reduced Fc-gamma receptor binding that prevented cytokine release in vitro and in humanized mice. As BMT will likely require fast clearing ADCs to avoid depleting the incoming graft, we also created fast-half-life CD45-AM variants with a t½ of 8-15 hours in mice. To determine the safety and pharmacokinetic properties of regular and fast half-life Fc-silent variants in an immune-competent large animal we tested these in cynomolgus monkeys. Single doses (3 mg/kg, iv, n = 3/group) of fast and regular half-life Fc-silent unconjugated anti-CD45 antibodies were both well tolerated in cynomolgus monkeys and displayed pharmacokinetic properties suitable for BMT.ConclusionThese results demonstrate that targeting CD45 with an amanitin ADC results in potent in vitro and in vivo human HSC and immune cell depletion. This new CD45-AM ADC also significantly reduced disease burden in multiple leukemia models. Our results indicate Fc-silencing may avoid infusion-related toxicities observed with previous CD45 mAbs. An alpha-amanitin ADC targeted to CD45 may be appropriate for preparing patients for BMT since we hypothesize it may i) be non-genotoxic; ii) effectively deplete both HSC and immune cells; iii) avoid bystander toxicity, due to amanitin's poor cell permeability as a free toxin; and iv) kill cycling and non-cycling cells, the latter being necessary for effective HSC depletion. As our anti-CD45 ADCs are cross-reactive, we are currently investigating their HSC and immune cell depletion activity in vivo in NHPs to enable further preclinical development of these transplant conditioning agents. [Display omitted] DisclosuresPalchaudhuri:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties; Harvard University: Patents & Royalties. Pearse:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Proctor:Magenta Therapeutics: Employment, Equity Ownership. Hyzy:Magenta Therapeutics: Employment, Equity Ownership. Aslanian:Magenta Therapeutics: Employment, Equity Ownership. McDonough:Magenta Therapeutics: Employment, Equity Ownership. Sarma:Magenta Therapeutics: Employment, Equity Ownership. Brooks:Magenta Therapeutics: Employment, Equity Ownership. Bhat:Magenta Therapeutics: Employment. Ladwig:Magenta Therapeutics: Employment, Equity Ownership. McShea:Magenta Therapeutics: Employment, Equity Ownership. Kallen:Magenta Therapeutics: Employment, Equity Ownership. Li:Magenta Therapeutics: Employment, Equity Ownership. Panwar:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Dushime:Magenta Therapeutics: Employment, Equity Ownership. Sawant:Magenta Therapeutics: Employment, Equity Ownership. Adams:Magenta Therapeutics: Employment, Equity Ownership. Falahee:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Lamothe:Magenta Therapeutics: Employment, Equity Ownership. Gabros:Magenta Therapeutics: Employment, Equity Ownership. Kien:Magenta Therapeutics: Employment, Equity Ownership. Gillard:Magenta Therapeutics: Employment, Equity Ownership. McDonagh:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Boitano:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Cooke:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties.

HDP101, a Novel B-Cell Maturation Antigen (BCMA)-Targeted Antibody Conjugated to α-Amanitin, Is Active Against Myeloma with Preferential Efficacy Against Pre-Clinical Models of Deletion 17p

Background:Deletion (del) of 17p involving the p53 tumor suppressor (TP53) remains an adverse prognostic factor in multiple myeloma (MM) despite the use of novel agents as well as high-dose chemotherapy with autologous stem cell rescue. Genomic TP53 deletion can cause haploinsufficiency of nearby genes, such as RNA polymerase II subunit A (POLR2A), which ia also located on 17p13.1. We therefore hypothesized that del 17p could reduce POLR2A expression and enhance sensitivity to a-Amanitin, a potent and specific inhibitor of POLR2A and RNA polymerase III.Methods:Pre-clinical studies were performed using HDP101, a monoclonal antibody-drug conjugate (ADC) targeting BCMA linked to a-Amanitin, along with unconjugated BCMA antibody, a-Amanitin, and a non-targeting control ADC in myeloma cell line models. The latter included H929, MM1.S, and MOLP-8 TP53 wild-type (WT) lines and isogenic cells in which TP53 had been knocked out (KO) using CRISPR/Cas9 genome editing techniques. To further model del 17p and POLR2A haploinsufficiency, POLR2A expression was knocked down using shRNAs.Results:Analysis of the Multiple Myeloma Research Foundation CoMMpassSM database revealed that del 17p13 by SeqFISH was associated with a significant reduction in POLR2A expression by RNASeq (26.05431 fragments per kilobase of transcript per million mapped reads (FPKM) with WT vs. 19.2983 FPKM with del 17p13; p<0.0001). Also, patients within the lower quartile of POLR2A expression, which included those with and without del 17p, had an inferior overall survival (p<0.0011) and a trend towards a worse progression-free survival, suggesting that low POLR2A levels by themselves are an adverse feature. The POLR2A inhibiting anti-BCMA/a-Amanitin conjugate HDP101 induced a time- and dose-dependent reduction in myeloma cell viability post 96-hours of drug exposure starting at concentrations as low as in the picomolar range. This reduction with HDP101 was much greater than that seen with controls, which included a-Amanitin alone, the anti-BCMA antibody without a-Amanitin, or an anti-digoxigenin antibody conjugated to a-Amanitin. When myeloma cells were co-cultured with HS-5 human marrow stromal cells, only a-Amanitin and, to a much greater extent, HDP101 induced a loss of cell viability in myeloma cells, while the stromal cells were spared by HDP101. Loss of cell viability due to HDP101 was associated with induction of apoptosis as judged by the appearance of an increased population of cells that had a sub-G0/G1 DNA content, and that stained positively with Annexin V. Moreover, HDP101 treatment caused the appearance of cleaved fragments of Caspase 9 and 3, and loss of the mitochondrial trans-membrane potential. H929, MM1.S, and MOLP-8 TP53 KO cells were more sensitive to both HDP101 and, less potently to a-Amanitin than were their isogenic TP53 WT parental cells. As H929 cells expressed high levels of BCMA regardless of TP53 or POLR2A status, these were further examined for their sensitivity to HDP101. Notably, the preferential impact upon TP53 KO cells was associated with increased expression of Activating transcription factors -4 and -6, suggesting enhanced induction of endoplasmic reticulum stress, and at least two arms of the unfolded protein response. Interestingly, shRNA-mediated knockdown (KD) of POLR2A expression alone was sufficient to increase baseline levels of apoptosis in both H929 TP53 WT and KO cells, with a greater impact in the latter, supporting the promise of this target. Importantly, KD of POLR2A expression to further model del 17p was also associated with enhanced sensitivity to HDP101 in both the TP53 WT and KO cells compared to the control treatments. Evaluation of HDP101 in primary samples and with in vivo models is underway, and will be presented at the Annual Meeting. These studies were supported by a Leukemia & Lymphoma Society Specialized Center of Research (SCOR-12206-17).Conclusions:Our preliminary data support the possibility that del 17p myeloma may have a therapeutic vulnerability to the POLR2A inhibitor a-Amanitin through loss of TP53, and that this sensitivity is further enhanced by decreased POLR2A expression, which is common among del 17p patients. Moreover, they suggest that HDP101 is a novel potent and specific therapeutic that could show enhanced activity in the clinic especially against high-risk multiple myeloma, where effective therapies are still needed to improve patient outcomes. DisclosuresPahl:Heidelberg Pharma AG: Employment. Orlowski:Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees; Janssen Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Consultancy; Genentech: Consultancy; Poseida: Research Funding; BioTheryX, Inc: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Millenium Pharmaceuticals: Consultancy, Research Funding.

Murine DX5+NKT Cells Display Their Cytotoxic and Proapoptotic Potentials against Colitis-Inducing CD4+CD62Lhigh T Cells through Fas Ligand.

Introduction It has been previously shown that immunoregulatory DX5+NKT cells are able to prevent colitis induced by CD4+CD62Lhigh T lymphocytes in a SCID mouse model. The aim of this study was to further investigate the underlying mechanism in vitro. Methods CD4+CD62Lhigh and DX5+NKT cells from the spleen of Balb/c mice were isolated first by MACS, followed by FACS sorting and cocultured for up to 96 h. After polyclonal stimulation with anti-CD3, anti-CD28, and IL-2, proliferation of CD4+CD62Lhigh cells was assessed using a CFSE assay and activity of proapoptotic caspase-3 was determined by intracellular staining and flow cytometry. Extrinsic apoptotic pathway was blocked using an unconjugated antibody against FasL, and activation of caspase-3 was measured. Results As previously shown in vivo, DX5+NKT cells inhibit proliferation of CD4+CD62Lhigh cells in vitro after 96 h coculture compared to a CD4+CD62Lhigh monoculture (proliferation index: 1.39 ± 0.07 vs. 1.76 ± 0.12; P = 0.0079). The antiproliferative effect of DX5+NKT cells was likely due to an induction of apoptosis in CD4+CD62Lhigh cells as evidenced by increased activation of the proapoptotic caspase-3 after 48 h (38 ± 3% vs. 28 ± 3%; P = 0.0451). Furthermore, DX5+NKT cells after polyclonal stimulation showed an upregulation of FasL on their cell surface (15 ± 2% vs. 2 ± 1%; P = 0.0286). Finally, FasL was blocked on DX5+NKT cells, and therefore, the extrinsic apoptotic pathway abrogated the activation of caspase-3 in CD4+CD62Lhigh cells. Conclusion Collectively, these data confirmed that DX5+NKT cells inhibit proliferation of colitis-inducing CD4+CD62Lhigh cells by induction of apoptosis. Furthermore, DX5+NKT cells likely mediate their cytotoxic and proapoptotic potentials via FasL, confirming recent reports about iNKT cells. Further studies will be necessary to evaluate the therapeutical potential of these immunoregulatory cells in patients with colitis.

Abstract 4266: Computational exploration of mechanistic determinants of ADC pharmacokinetics using QSP modeling strategies

Abstract The pharmacokinetics of antibody drug conjugate (ADC) therapeutics typically show a discrepancy between the PK of total antibody (conjugated and unconjugated antibody) and that of conjugated antibody, carrying one or more payload molecules. This discrepancy is often attributed strictly to deconjugation (Kamath, 2014), however recent evidence suggests that the underlying mechanisms may be more complex. This work employs a computational quantitative systems pharmacology (QSP) approach that generates a hypotheses to better understand the impact of drug antibody ratio (DAR) and the resulting changes in molecular properties on overall PK and relative payload disposition as observed in preclinical and clinical studies. Our work establishes the benefit of using computational models to design novel ADCs and to optimize the discovery and development of existing ADCs to better enable clinical trial design.

Abstract 3858: Validation of digital spatial profiling of key immuno-oncology targets for mouse FFPE preclinical models

Abstract 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.

Abstract 5652: Validation of novel InSituPlex™ technology utilizing standard chromogenic IHC and multiplexed tyramide-based immunofluorescence

Abstract Introduction: The demands of today's immuno-oncology research has driven the development of more robust tools to interrogate the tumor microenvironment, specifically using multiplex immunofluorescence (mIF). Multiplex immunofluorescence allows investigators to visualize multiple biomarkers in tissue sections while preserving spatial context. The current gold standard of immunohistochemistry is “brown” staining using DAB with a hematoxylin counterstain. In recent years, multiplex immunofluorescence has been accepted in the field using tyramide molecules (TSA) that permanently fix fluorescent dyes to the tissue. Consequently, tissue integrity is compromised and cannot be utilized in additional forms of analysis. In addition, high-level plexing using TSA requires the use of highly specialized and costly imaging equipment. Here we validate a novel technology, InSituPlex™, utilizing DNA barcoding of antibodies and the detection of markers using simple DNA hybridization techniques. InSituPlex allows for high-leveling of multiplexing, preservation of tissue integrity for additional analyses, and implementation on a variety of commonly used imaging platforms. Methods: Performance of the InSituPlex technology was assessed using 15 immuno-oncology relevant markers (CD3, CD4, CD8, CD20, CD25, CD68, CD163, FoxP3, Granzyme B, Ki67, PD1, PD-L1, pan-cytokeratin, Sox10, and TTF1). Each marker was benchmarked by performing a single-plex DAB chromogenic IHC and an immunofluorescence assay. Antibodies to be used for InSituPlex testing were first conjugated to unique DNA barcodes and the resulting conjugates were assessed utilizing the previously described assays to ensure equivalent performance of the InSituPlex conjugate to unconjugated antibodies. The performance of the complete InSituPlex assay was evaluated using InSituPlex HRP labelled imaging strands and the newly barcoded antibodies. InSituPlex vs. DAB/TSA performance was compared across platforms. All staining was performed manually and on the Leica BOND RX autostainer. Images were acquired across multiple imaging platforms, including the Leica VERSA whole slide scanner, and image analysis was performed using HALO from Indica Labs. Results: All antibodies performed equally well across all the staining methodologies as assessed qualitatively. Quantitatively, cell counts were performed on serial sections. Statistical analysis comparing InSituPlex to standard chromogenic and immunofluorescent assays across all 15 antibodies did not yield any significance (p-value &amp;gt; 0.1). Conclusions: Detection of relevant immune and cancer related markers utilizing InSituPlex technology is equivalent to standard chromogenic IHC and immunofluorescent assays. Citation Format: Katir Patel, Courtney Hebert, Jamie Buell, Sean Downing. Validation of novel InSituPlex™ technology utilizing standard chromogenic IHC and multiplexed tyramide-based immunofluorescence [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 5652.

Activated matriptase as a target to treat breast cancer with a drug conjugate.

The antitumor effects of a novel antibody drug conjugate (ADC) was tested against human solid tumor cell lines and against human triple negative breast cancer (TNBC) xenografts in immunosuppressed mice. The ADC targeting activated matriptase of tumor cells was synthesized by using the potent anti-tubulin toxin, monomethyl auristatin-E linked to the activated matriptase-specific monoclonal antibody (M69) via a lysosomal protease-cleavable dipeptide linker. This ADC was found to be cytotoxic against multiple activated matriptase-positive epithelial carcinoma cell lines in vitro and markedly inhibited growth of triple negative breast cancer xenografts and a primary human TNBC (PDX) in vivo. Overexpression of activated matriptase may be a biomarker for response to this ADC. The ADC had potent anti-tumor activity, while the unconjugated M69 antibody was ineffective in a mouse model study using MDA-MB-231 xenografts in mice. Treatment of a human TNBC (MDA-MB-231) showed potent anti-tumor effects in combination with cisplatin in mice. This ADC alone or in combination with cisplatin has the potential to improve the treatment outcomes of patients with TNBC as well as other tumors overexpressing activated matriptase.

Analytical Validation of Digital Spatial Profiling - a novel approach for multiplexed characterization of protein distribution and abundance in FFPE tissue sections

Abstract 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 &amp;gt; 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.

A Two-Step Immunocapture LC/MS/MS Assay for Plasma Stability and Payload Migration Assessment of Cysteine–Maleimide-Based Antibody Drug Conjugates

Plasma stability assessment under physiological temperature is an essential step for developing and optimizing antibody drug conjugate (ADC) molecules, especially those with cleavable linkers. The assessment of plasma stability often requires monitoring multiple analytes using a combination of bioanalytical assays for free payloads, conjugated payloads (or conjugated antibodies), total antibodies, and payloads that have migrated from antibodies to plasma constituent proteins. Bioanalytical assays are needed in early drug discovery to quickly screen diverse ADC candidates of different antibody constructs, linker variants, and antibody anchor sites. To improve the sensitivity and selectivity of LC/MS/MS-based assays for the assessment, immunocapture has been widely used for extracting ADCs and unconjugated antibodies from plasma samples. In this study, a novel two-step immunocapture LC/MS/MS assay was described to allow the quantification of conjugated payloads, total antibodies, and migrated payloads forming adducts with albumin in the plasma samples for stability assessment. A target antigen immobilized on magnetic beads was used to exhaustively extract the ADC and antibody-associated species. The remaining supernatant was then extracted further with anti-albumin beads for recovering the albumin-associated adducts for quantification. The method was optimized for higher efficiency and cost-effectiveness using microwave enhanced papain-based enzymatic cleavage for measuring conjugated payloads of ADCs and lysyl endopeptidase cleavage in the total antibody assay. A maleimide linker-based ADC with a proprietary payload, TAK-001, was used to demonstrate the streamlined workflow of the ADC stability assessment. The method could provide valuable evaluation of the stability of the ADC as well as the quantitative assessment of the albumin adducts formed from the linker-payload migration in mouse and human plasma. Furthermore, the method should be readily adaptable for other ADCs using thiol-maleimide conjugation chemistry.

Biophysical Properties and Heating-Induced Aggregation of Lysine-Conjugated Antibody-Drug Conjugates

The commercially available antibody-drug conjugate (ADC) product, Kadcyla® is synthesized using a 2-step reaction, wherein the linker is conjugated to native lysines on the mAb in step 1, followed by drug conjugation to the linker-modified antibody in step 2. In our study, we synthesized a lysine-conjugated ADC (Syn-ADC) on the same trastuzumab scaffold as Kadcyla® using a 1-step reaction. Mass spectrometry of both products revealed a subpopulation of Kadcyla® containing free linkers conjugated to the mAb, but not conjugated to the drug, which were absent in the 1-step reaction ADC product. Differential scanning calorimetry thermograms showed that the drug and linker conjugation significantly reduced the thermal stability and energies of activation for the denaturation of the CH2 domain of the ADCs. The heating induced aggregation events started as early as ∼57°C and ∼45°C for Kadcyla® and Syn-ADC, respectively, compared with 71°C for Herceptin®. The colloidal stability measurements clearly showed that the hydrophobic drug payload on ADCs significantly reduced the repulsive interprotein interactions when compared to the unconjugated antibody under formulation buffer conditions (pH 6.0). Attaching hydrophobic drug and linker moieties onto the antibody lowered the thermal and colloidal stabilities and increased the aggregation propensity of the ADCs.

MP65-09 NEAR INFRARED PHOTOIMMUNOTHERAPY USING ANTI-CD47 ANTIBODY FOR HUMAN BLADDER CANCER

You have accessJournal of UrologyBladder Cancer: Basic Research & Pathophysiology III1 Apr 2018MP65-09 NEAR INFRARED PHOTOIMMUNOTHERAPY USING ANTI-CD47 ANTIBODY FOR HUMAN BLADDER CANCER Bernhard Kiss, Kelly McKenna, Robert Ertsey, Nynke Sjoerdtje Van den Berg, Kathleen Mach, Eben Rosenthal, and Joseph Liao Bernhard KissBernhard Kiss More articles by this author , Kelly McKennaKelly McKenna More articles by this author , Robert ErtseyRobert Ertsey More articles by this author , Nynke Sjoerdtje Van den BergNynke Sjoerdtje Van den Berg More articles by this author , Kathleen MachKathleen Mach More articles by this author , Eben RosenthalEben Rosenthal More articles by this author , and Joseph LiaoJoseph Liao More articles by this author View All Author Informationhttps://doi.org/10.1016/j.juro.2018.02.2075AboutPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookTwitterLinked InEmail INTRODUCTION AND OBJECTIVES Near infrared photoimmunotherapy (PIT) is a new molecular-targeted cancer therapy that uses a specific photosensitizer based on a near-infrared (NIR) phthalocyanine dye, IRDye700DX, conjugated to a monoclonal antibody. CD47 is a cell surface protein that elicits a ″don′t eat me signal″ to prevent macrophage engulfment and is expressed on over 80% of bladder cancer tumors but absent on the luminal cell layer of normal bladder urothelium. Thus, targeting CD47 for PIT has the potential to selectively induce cell death in CD47 expressing tumor cells as well as increase phagocytosis of tumor cells. METHODS The anti-CD47 antibody (B6H12) was conjugated to IRDye700DX NHS ester and purified using a dialysis spin column. Anti-CD47-IRDye700 concentration was determined with a Coomassie Plus protein assay. The molecular weight of the conjugated antibody was verified by SDS-page and IRDye700 conjugation verified using the 700nm fluorescence channel of a PEARL imager. Binding of anti-CD47-IRDye700DX to CD47 was verified by ELISA. Three different human urothelial cell lines (UMUC3, HT-1376 and 639V) were used for in vitro NIR-PIT experiments. 639V cells were stably transfected with GFP using a lentiviral vector. After plating in a 96well plate and incubation with 10 µg/ml of anti-CD47-IRDye700 or unconjugated anti-CD47 antibody (1h on ice), cells were irradiated with a red light-emitting diode (LED) at 670-710nm wavelength at different energy levels. The cytotoxic effects of NIR-PIT with anti-CD47-IRDye700DX were determined by propidium iodide staining and flow cytometry. To evaluate macrophage activity, a phagocytosis assay using human macrophages and analyzed by flow cytometry was performed. RESULTS Cancer-cell death significantly increased with anti-CD47-IRDye700DX in a light-dose dependent manner in all 3 human bladder cancer cell lines. Significant cancer-cell death was first observed with 2J exposure and when exposed to 40J of NIR light over 90% of 639V and UMUC3 cells and over 50% of HT1376 died. Phagocytosis of cancer-cells incubated with anti-CD47-IRDye700DX and irradiated with 8J was significantly increased compared to anti-CD47-antibody alone (p=0.0002). There was no cytotoxicity associated with anti-CD47 antibody alone with NIR light, with NIR light alone or with anti-CD47-IRDye700DX without NIR light. CONCLUSIONS NIR-PIT with anti-CD47-IRDye700 shows significantly increased cancer-cell cytotoxicity and significantly increased cancer-cell phagocytosis making this combination a highly promising novel approach for molecular targeted bladder cancer therapy. © 2018FiguresReferencesRelatedDetails Volume 199Issue 4SApril 2018Page: e862 Advertisement Copyright & Permissions© 2018MetricsAuthor Information Bernhard Kiss More articles by this author Kelly McKenna More articles by this author Robert Ertsey More articles by this author Nynke Sjoerdtje Van den Berg More articles by this author Kathleen Mach More articles by this author Eben Rosenthal More articles by this author Joseph Liao More articles by this author Expand All Advertisement Advertisement PDF downloadLoading ...

EV-103 study: A phase 1b dose-escalation and dose-expansion study of enfortumab vedotin in combination with immune checkpoint inhibitor (CPI) therapy for treatment of patients with locally advanced or metastatic urothelial cancer.

TPS532 Background: Immune checkpoint inhibitors (CPIs) are approved for patients (pts) with locally advanced or metastatic urothelial cancer (la/mUC) who progress on prior platinum or are ineligible for first line cisplatin. However, only a minority of pts respond to CPI monotherapy and a combination approach may be beneficial. Enfortumab vedotin (EV), an antibody-drug conjugate (ADC), delivers the microtubule-disrupting agent monomethyl auristatin E (MMAE) to tumors expressing Nectin-4, which is overexpressed in mUC. Preliminary results from an ongoing phase 1 study of EV monotherapy in pts with mUC (Petrylak ASCO 2017) showed that EV was well tolerated in pts with mUC. EV showed a 41% (29 out of 71 pts) objective response rate (ORR) across all doses (24 wks median response duration). In pts with liver metastasis the ORR was 47% (9 out 19 pts) across all doses. Preclinical data show that an MMAE ADC, but not the unconjugated antibody, increases tumor immunogenicity by inducing immunogenic cell death, antigen presentation and tumor immune infiltration (Gardai 2015). Based on encouraging phase 1 study results and potential enhanced immune response, combining EV with a CPI may result in improved response rates with longer durability. Methods: This phase 1b study (NCT03288545) evaluates the safety/activity of EV + CPI in pts with la/mUC who have not previously received a CPI. The study has 2 parts: dose escalation (EV + pembrolizumab) and dose expansion (EV + CPI [pembrolizumab or atezolizumab]). Pts must have la/m transitional cell carcinoma of the urothelium, be eligible for treatment with a CPI, and be either ineligible for first-line cisplatin-based chemotherapy or have disease progression during or following treatment with ≥1 platinum-containing regimen. The primary objective is to assess the safety/tolerability of EV + CPI. Secondary objectives include determining a recommended dose of EV + CPI in pts with la/mUC, antitumor activity as measured by ORR, disease control rate, duration of response, PFS, and OS. Response is assessed per RECIST v1.1 and iRECIST. Study enrollment began in Oct 2017. Clinical trial information: NCT03288545.

Immunoconjugates for Osteosarcoma Therapy: Preclinical Experiences and Future Perspectives.

Osteosarcoma (OS) is an aggressive osteoid-producing tumor of mesenchymal origin, which represents the most common primary bone malignancy. It is characterized by a complex and frequently uncertain etiology. The current standard care for high-grade OS treatment is neoadjuvant chemotherapy, followed by surgery and post-operative chemotherapy. In order to ameliorate survival rates of patients, new therapeutic approaches have been evaluated, mainly immunotherapy with antibody-drug conjugates or immunoconjugates. These molecules consist of a carrier (frequently an antibody) joined by a linker to a toxic moiety (drug, radionuclide, or toxin). Although several clinical trials with immunoconjugates have been conducted, mainly in hematological tumors, their potential as therapeutic agents is relatively under-explored in many types of cancer. In this review, we report the immunoconjugates directed against OS surface antigens, considering the in vitro and in vivo studies. To date, several attempts have been made in preclinical settings, reporting encouraging results and demonstrating the validity of the idea. The clinical experience with glembatumumab vedotin may provide new insights into the real efficacy of antibody-drug conjugates for OS therapy, possibly giving more information about patient selection. Moreover, new opportunities could arise from the ongoing clinical trials in OS patients with unconjugated antibodies that could represent future candidates as carrier moieties of immunoconjugates.

T cell engaging bispecific antibody (T-BsAb): From technology to therapeutics.

Harnessing the power of the human immune system to treat cancer is the essence of immunotherapy. Monoclonal antibodies engage the innate immune system to destroy targeted cells. For the last 30years, antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity have been the main mechanisms of anti-tumor action of unconjugated antibody drugs. Efforts to exploit the potentials of other immune cells, in particular T cells, culminated in the recent approval of two T cell engaging bispecific antibody (T-BsAb) drugs, thereby stimulating new efforts to accelerate similar platforms through preclinical and clinical trials. In this review, we have compiled the worldwide effort in exploring T cell engaging bispecific antibodies. Our special emphasis is on the lessons learned, with the hope to derive insights in this fast evolving field with tremendous clinical potential.

“External” antibodies as the simplest tool for sensitive immunochromatographic tests

The scheme of immunochromatographic competitive analysis with "external" specific antibodies is offered for the first time. The test strip includes all the necessary components for analysis (membranes, protein antigen conjugates, colored nanoparticles), except specific free unconjugated antibodies that are transferred to the buffer used for sample dilution. Thus, the concentrations of the antibodies and markers can be chosen independently, and dilution of the sample with buffer (a routine procedure for any immunochromatographic test) ensures pre-incubation of the antibodies with the sample. Both factors increase the sensitivity of the test. The proposed scheme has been proven to be effective for the determination of deoxynivalenol and T-2 toxin; detection limits of 500pg/mL and 50pg/mL, respectively, were achieved. This is an order of magnitude lower than the limits of detection of traditional tests using the same reagents. The analysis was conducted in water-organic extracts (20% methanol); the duration of the analysis is 10min (it's the same time as for a traditional test). The proposed approach is universal, and it can be used to detect a variety of compounds.

Abstract 4061: Computational exploration of mechanistic determinants of antibody drug-conjugate pharmacokinetics using quantitative systems pharmacology modeling strategies

Abstract The pharmacokinetics of antibody drug conjugate (ADC) therapeutics typically show a discrepancy between the PK of total antibody (conjugated and unconjugated antibody) and that of conjugated antibody, carrying one or more payload molecules. This discrepancy is often attributed to deconjugation (Kamath, 2014), however recent evidence suggests that the underlying mechanisms may be more complex. This work employs a computational quantitative systems pharmacology (QSP), or mechabistic PK/PD approach to understand the impact of drug antibody ratio (DAR) and the resulting changes in molecular properties on overall PK and relative payload disposition as observed in preclinical and clinical studies. Our work establishes the benefit of using computational models to design novel ADCs and to optimize the discovery and development of existing ADCs. Citation Format: A. Katharina Wilkins, Andrew Matteson, Lore Gruenbaum, Jennifer Park, John M. Burke, Joshua Apgar. Computational exploration of mechanistic determinants of antibody drug-conjugate pharmacokinetics using quantitative systems pharmacology modeling strategies [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 4061. doi:10.1158/1538-7445.AM2017-4061

Abstract 2861: Dose-dependent tissue distribution and tumor targeting of Notch3-ADC using fluorescence molecular tomography imaging

Abstract Background: NOTCH3, a cell surface receptor involved in cell-cell communications, is over-expressed or amplified in certain human tumors. NOTCH3 is known to regulate proliferation, differentiation and survival of cancer cells or cancer stem cells and thus an important therapeutic target. NOTCH3 antibody drug conjugate (ADC) is comprised of humanized anti-NOTCH3 antibody conjugated to an auristatin based cytotoxic payload. NOTCH3-ADC has shown promising results in pre-clinical tumor models. In this study we evaluated the kinetics, dose-dependent tissue distribution, tumor accumulation and targeting specificity of the NOTCH3-ADC in OVCAR3 xenograft model using fluorescence molecular tomography (FMT) imaging. The NOTCH3-ADC is mouse cross-reactive thus providing an accurate assessment of biodistribution. Methods: NOTCH3-ADC was conjugated to the near-infrared dye, AlexaFluor680 (AF680). The in vitro cellular binding was evaluated by cell-based ELISA. In vivo biodistribution was evaluated using OVCAR3 subcutaneous xenograft model. NOTCH3-ADC-AF680 (1mg/kg; 3mg/kg and 10mg/kg) was injected when the tumors were ~300mm3 and imaged 5 min, 24, 48, 96, 168 and 240 h post-injection. Ex vivo FMT imaging, pharmacokinetic analysis and immunohistochemistry (IHC) was performed at 48 and 240 h after whole-body perfusion. An in vivo receptor competition FMT study was performed by injecting excess of unconjugated anti-NOTCH3 antibody (Ab) or a non-targeted control Ab. Results: In vitro cell binding studies showed that conjugation of AF680 to NOTCH3-ADC did not change its binding ability. A dose-dependent tumor regression was also observed after a single injection of NOTCH3-ADC-AF680. In vivo FMT imaging showed dose-dependent whole-body clearance kinetics of NOTCH3-ADC. Dose-dependent accumulation in the tumors was observed with peak accumulation at 24-48 h post-injection and a slow decline at later time-points. A maximum accumulation of ~10 %ID/g was observed which was independent of the dose of NOTCH3-ADC-AF680. Ex vivo FMT quantitation of tumor was consistent with the IHC for antibody and LC/MS analysis of released payload. Pharmacological competition with excess unlabeled control Ab did not block tumor accumulation of NOTCH3-ADC-AF680, whereas excess unlabeled NOTCH3-Ab blocked ~47% of NOTCH3-ADC-AF680 accumulation. There was no significant specific accumulation of NOTCH3-ADC in other organs as observed by FMT imaging or IHC. Conclusions: These imaging studies provided understanding of the kinetics, tumor accumulation and biodistribution of NOTCH3-ADC. Further this work showcase the utility of non-invasive FMT imaging in better understanding of pharmacology and behavior of biologic drugs. Citation Format: Anand Giddabasappa, Parul Gupta, Mauricio Leal, Jonathan Golas, Fengping Li, Bing Yang, Antonio Esparza, Christopher Winkelmann, Kenneth Geles. Dose-dependent tissue distribution and tumor targeting of Notch3-ADC using fluorescence molecular tomography imaging [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 2861. doi:10.1158/1538-7445.AM2017-2861

Therapeutic Use of Brentuximab Vedotin in CD30+ Hematologic Malignancies.

The CD30 antigen is strongly expressed on neoplastic cells in classical Hodgkin lymphoma (HL), anaplastic large cell lymphoma (ALCL) and other hematologic malignancies (such as DLBCL and cutaneous TCL), while is almost undetectable on healthy tissues, representing an ideal immunotherapeutic target. Since unconjugated anti-CD30 antibody (SGN-30) demonstrated limited clinical activity, researchers' effort aimed to create an antibody-drug conjugate (ADC), leading to discovery of SGN-35 (brentuximab vedotin), in which an anti-CD30 antibody is linked to the antimitotic agent monomethyl auristatin E (MMAE). In the first phase I study in CD30+ hematologic malignancies (the majority of patients with HL), the maximum tolerated dose was fixed respectively at 1.8mg/Kg every 3 weeks, overall response rate (ORR) and complete response (CR) rate were 38% and 24%. In 2 subsequent phase II studies, amazing results were reported, that permitted accelerated FDA approval for relapsed/refractory patients and led to the development of many clinical trials including BV as first-line HL and ALCL treatment. Moreover, as CD30 antigen may be expressed by other malignancies, the potential therapeutic application is increasing, including at least diffuse large B-cell lymphoma, T-cell lymphomas other than ALCL and cutaneous lymphoproliferative disorders. BV is administrated as outpatient regimen and is usually well tolerated; sensorial peripheral neuropathy represents the most common toxic effect, although it is dose-dependent and at least partially reversible in most cases, after dose reduction and/or treatment ending.

Abstract MIP-071: TARGETING A CHEMORESISTANT OVARIAN CANCER CELL POPULATION VIA THE CARBOHYDRATE ANTIGEN SIALYL TN

Abstract Key words: Sialyl Tn, drug resistance, antibody-drug conjugates, cancer stem cells OBJECTIVES: A successful therapeutic strategy for ovarian cancer will require direct targeting of inherently chemoresistant tumor cells which are comprised in part of cancer stem cells (CSCs) that survive current cytotoxic treatment regimes and drive tumor resurgence. The sialyl-Tn (STn) antigen is a carbohydrate moiety present on tumor cells but rarely seen in normal adult tissue. Importantly, STn has been shown to be present on CSCs in pancreatic, colon, and gastric malignancies. Our objective was to assess the expression of STn and the known CSC marker CD133 in human ovarian cancer (OvCa) cell lines and primary serous carcinomas, and evaluate the ability of STn+ and STn- cells to both grow in an anchorage independent manner and survive standard-of-care cytotoxic therapy. Furthermore, we sought to assess the effect of murine and humanized α -STn antibody-drug conjugates (ADCs) on OvCa cells in vitro and tumor viability in vivo. METHODS: STn and CD133 expression in established OvCa cell lines was analyzed by flow cytometry. STn-CD133-, STn+CD133-, STn-CD133+ and STn+CD133+ cells were purified from OVCAR3 and OVCAR4 by FACS, plated in soft agar, and incubated for 21 days. Colony forming efficiency of each sub-population was calculated. Unsorted cells were treated in vitro with either murine α -STn-monomethyl auristatin E (MMAE) ADC or vehicle control and cell viability was assessed by MTT assay. Subsequently, cells were treated in vitro with α -STn-MMAE, paclitaxel and carboplatin, or appropriate controls, and the profile of cells surviving 72 hours post-treatment was determined by flow cytometric analysis. Finally, OVCAR3-derived mouse xenografts were treated with murine and humanized α-STn-MMAE, unconjugated mAbs alone, and vehicle control. Mice were assessed regularly for tumor growth and cytotoxic effects. RESULTS: In the OvCa cell lines OV90, OVCAR3 and OVCAR4, when grown in traditional 2D culture, STn+ cells comprised 98.4%, 40.0%, and 26.4% of the total cell population, respectively. In each of these cell lines, we readily detected STn+CD133+ sub-populations suggesting that STn is expressed on CD133+ ovarian CSCs. Colony formation assays analyzing FACS-purified STn-CD133-, STn+CD133-, STn-CD133+ and STn+ CD133+ sub-populations suggest that STn expression correlates with anchorage independent growth, a characteristic of cell stemness. Paclitaxel and carboplatin treatment in vitro significantly increased the proportion of STn+ and CD133+ cells, demonstrating the chemoresistant characteristics of these cells. Treatment with the murine α-STn-MMAE ADCs reduced the viability of OvCa cell lines in vitro in a dose-dependent manner. Treatment with murine and humanized α -STn- MMAE antibodies in vivo reduced tumor volumes, whereas vehicle treatment did not impede tumor growth. Interestingly, the unconjugated antibody also had a modest negative impact on tumor volume. CONCLUSION: A novel, highly specific STn antibody identifies the STn antigen in OvCa cell lines and patient samples. STn+ and CD133+ cells demonstrate stem-like characteristics such as anchorage-independent growth and chemoresistance. STn ADCs decreased cell viability in vitro and reduced tumor volumes in vivo, suggesting that specific therapeutic targeting of STn in ovarian tumors may be an effective clinical strategy to eliminate quiescent CSCs. Citation Format: B.R. Rueda, K. Starbuck, D. Eavarone, J. Prendergast, J. Stein, R. Foster, J. Behrens. TARGETING A CHEMORESISTANT OVARIAN CANCER CELL POPULATION VIA THE CARBOHYDRATE ANTIGEN SIALYL TN [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr MIP-071.