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ADAM metallopeptidase domain 9 (ADAM9) is a member of the ADAM family of multifunctional, multidomain type 1 transmembrane proteins. ADAM9 is overexpressed in many cancers, including non–small cell lung, pancreatic, gastric, breast, ovarian, and colorectal cancer, but exhibits limited expression in normal tissues. A target-unbiased discovery platform based on intact tumor and progenitor cell immunizations, followed by an IHC screen, led to the identification of anti-ADAM9 antibodies with selective tumor-versus-normal tissue binding. Subsequent analysis revealed anti-ADAM9 antibodies were efficiently internalized and processed by tumor cells making ADAM9 an attractive target for antibody–drug conjugate (ADC) development. Here, we describe the preclinical evaluation of IMGC936, a novel ADC targeted against ADAM9. IMGC936 is comprised of a high-affinity humanized antibody site-specifically conjugated to DM21-C, a next-generation linker-payload that combines a maytansinoid microtubule-disrupting payload with a stable tripeptide linker, at a drug antibody ratio of approximately 2.0. In addition, the YTE mutation (M252Y/S254T/T256E) was introduced into the CH2 domain of the antibody Fc to maximize in vivo plasma half-life and exposure. IMGC936 exhibited cytotoxicity toward ADAM9-positive human tumor cell lines, as well as bystander killing, potent antitumor activity in human cell line-derived xenograft and patient-derived xenograft tumor models, and an acceptable safety profile in cynomolgus monkeys with favorable pharmacokinetic properties. Our preclinical data provide a strong scientific rationale for the further development of IMGC936 as a therapeutic candidate for the treatment of ADAM9-positive cancers. A first-in-human study of IMGC936 in patients with advanced solid tumors has been initiated (NCT04622774).

5000 Hs700T parental cells only in the presence of 6.7 nM MGC018

5000 Hs700T B7-H3 KO/RFP cells alone in the presence of 6.7 nM MGC018

Supplementary Data

Co-culture of 5000 Hs700T parental cells and 5000 Hs700T B7-H3 KO/RFP cells in the presence of 6.7 nM MGC018

B7-H3, also referred to as CD276, is a member of the B7 family of immune regulatory proteins. B7-H3 is overexpressed on many solid cancers, including prostate cancer, renal cell carcinoma, melanoma, squamous cell carcinoma of the head and neck, non–small cell lung cancer, and breast cancer. Overexpression of B7-H3 is associated with disease severity, risk of recurrence and reduced survival. In this article, we report the preclinical development of MGC018, an antibody–drug conjugate targeted against B7-H3. MGC018 is comprised of the cleavable linker-duocarmycin payload, valine-citrulline-seco duocarmycin hydroxybenzamide azaindole (vc-seco-DUBA), conjugated to an anti-B7-H3 humanized IgG1/kappa mAb through reduced interchain disulfides, with an average drug-to-antibody ratio of approximately 2.7. MGC018 exhibited cytotoxicity toward B7-H3–positive human tumor cell lines, and exhibited bystander killing of target-negative tumor cells when cocultured with B7-H3–positive tumor cells. MGC018 displayed potent antitumor activity in preclinical tumor models of breast, ovarian, and lung cancer, as well as melanoma. In addition, antitumor activity was observed toward patient-derived xenograft models of breast, prostate, and head and neck cancer displaying heterogeneous expression of B7-H3. Importantly, MGC018 exhibited a favorable pharmacokinetic and safety profile in cynomolgus monkeys following repeat-dose administration. The antitumor activity observed preclinically with MGC018, together with the positive safety profile, provides evidence of a potentially favorable therapeutic index and supports the continued development of MGC018 for the treatment of solid cancers.

We have developed MGD007 (anti-glycoprotein A33 x anti-CD3), a DART protein designed to redirect T cells to target gpA33 expressing colon cancer. The gpA33 target was selected on the basis of an antibody-based screen to identify cancer antigens universally expressed in both primary and metastatic colorectal cancer specimens, including putative cancer stem cell populations. MGD007 displays the anticipated-bispecific binding properties and mediates potent lysis of gpA33-positive cancer cell lines, including models of colorectal cancer stem cells, through recruitment of T cells. Xenograft studies showed tumor growth inhibition at doses as low as 4 μg/kg. Both CD8 and CD4 T cells mediated lysis of gpA33-expressing tumor cells, with activity accompanied by increases in granzyme and perforin. Notably, suppressive T-cell populations could also be leveraged to mediate lysis of gpA33-expressing tumor cells. Concomitant with CTL activity, both T-cell activation and expansion are observed in a gpA33-dependent manner. No cytokine activation was observed with human PBMC alone, consistent with the absence of gpA33 expression on peripheral blood cell populations. Following prolonged exposure to MGD007 and gpA33 positive tumor cells, T cells express PD-1 and LAG-3 and acquire a memory phenotype but retain ability to support potent cell killing. In cynomolgus monkeys, 4 weekly doses of 100 μg/kg were well tolerated, with prolonged PK consistent with that of an Fc-containing molecule. Taken together, MGD007 displays potent activity against colorectal cancer cells consistent with a mechanism of action endowed in its design and support further investigation of MGD007 as a potential novel therapeutic treatment for colorectal cancer. Mol Cancer Ther; 17(8); 1761–72. ©2018 AACR.

Supplementary Table 1: Sixteen-locus STR analysis of tumor tissue and banked CSLC lines Supplementary Table 2: CSLC derivation and characteristics Supplementary Table 3: Immunohistochemical Analysis of Colorectal Cancer Tumor Specimens Supplementary Table 4: Equilibrium Dissociation Constants (KD) for Binding of MGD007 to Human CD3 or gpA33 Supplementary Table 5: MGD007 Pharmacokinetic Parameters Derived from Two-compartment Analysis in Cynomolgus Monkeys Supplementary Figure 1: In vitro CSLC differentiation in MatrigelTM 3D cultures Supplementary Figure 2: RECA47 mAb binding restricted to intestinal epithelium Supplementary Figure 3: MGD007 Flow Cytometry Analyses on Tumor Cell Lines Supplementary Figure 4: Expanded Treg population exhibits suppressive T-cell properties Supplementary Figure 5: Cytokine Levels in Monkeys Following Treatment with MGD007

Supplementary Data from Preclinical Evaluation of IMGC936, a Next-Generation Maytansinoid-based Antibody–drug Conjugate Targeting ADAM9-expressing Tumors

Supplementary Materials and Methods

PDF file, 81KB, Kinetic and Equilibrium binding constants, determined by surface plasmon resonance, for binding of murine BRCA84D, RES240 and MGA271 to human and cynomolgus monkey B7-H3.

MGD011 mediated concentration-dependent cytotoxicity against CD19+ HBL-2 and Jeko-1 lymphoma cell lines

Supplemental figure legends for supplemental figures S1, S2, S3, S4, S5, S6

PDF file, 75KB, Expression level of B7-H3 on ATCC cancer cell lines as determined by FLOW cytometry with anti-B7-H3 mAb BRCA84D. Solid curves represent isotype control staining; open curves represent staining with FI.

Supplementary Figures

PDF file, 78KB, Measurement of the absolute level of circulating B cells (CD20+; Panel A), T cells (CD3+; Panel B) and NK cells (CD159a+)Panel C) in blood samples collected from cynomolgus monkeys at the indicated days prior to administration (Day -2), or following intravenous administration of PBS vehicle on Day 1 (all animals received PBS vehicle on Day 1) or MGA271 on Day 8 at the indicated dose. The panels on the left for each group represent all animals (N=6) in each dosing group, while the panels on the right represent only the recovery group animals (N=2) in each dosing group.

PDF file, 89KB, Measurement of IL-5 (Panel A), IL-6 (Panel B) and TNF-B (Panel C) cytokine levels in serum samples collected from cynomolgus monkeys prior to infusion (Pre) or at the indicated time points following intravenous administration of PBS vehicle (Day 1) or MGA271 at the indicated dose (Day 8). The top two panels of each cytokine data set represent all animals (N=6) in each dosing group, while the bottom three panels represent only the recovery group animals (N=2) in each dosing group. Sera from the samples were analyzed for cytokine levels using the Becton Dickinson Cytometric Bead Array Non-Human Primate Th1/Th2 Cytokine kit.

Purpose: CD19, a B-cell lineage-specific marker, is highly represented in B-cell malignancies and an attractive target for therapeutic interventions. MGD011 is a CD19 x CD3 DART bispecific protein designed to redirect T lymphocytes to eliminate CD19-expressing cells. MGD011 has been engineered with a modified human Fc domain for improved pharmacokinetic (PK) properties and designed to cross-react with the corresponding antigens in cynomolgus monkeys. Here, we report on the preclinical activity, safety and PK properties of MGD011.Experimental Design: The activity of MGD011 was evaluated in several in vitro and in vivo models. PK, safety and pharmacodynamic activity was also assessed in dose-escalation and repeat-dose studies of MGD011 administered once weekly in cynomolgus monkeys.Results: MGD011 mediated killing of human B-cell lymphoma lines by human or cynomolgus monkey PBMCs as well as autologous B-cell depletion in PBMCs from both species. MGD011-mediated killing was accompanied by target-dependent T-cell activation and expansion, cytokine release and upregulation of perforin and granzyme B. MGD011 demonstrated antitumor activity against localized and disseminated lymphoma xenografts reconstituted with human PBMCs. In cynomolgus monkeys, MGD011 displayed a terminal half-life of 6.7 days; once weekly intravenous infusion of MGD011 at doses up to 100 μg/kg, the highest dose tested, was well tolerated and resulted in dose-dependent, durable decreases in circulating B cells accompanied by profound reductions of B lymphocytes in lymphoid organs.Conclusions: The preclinical activity, safety and PK profile support clinical investigation of MGD011 as a therapeutic candidate for the treatment of B-cell malignancies. Clin Cancer Res; 23(6); 1506–18. ©2016 AACR.

Supplemental Table 1: MGD011 repeat-dose and dose-escalation study design in non-human primates; Supplemental Table 2: PK properties of MGD011 derived from non-human primate studies

PDF file, 204KB, Panel A: Representative reactivity of anti-B7-H3 mAb BRCA84D with frozen tissue sections of prostate, breast, colon, lung, gastric and renal cancer. Panel B: Reactivity of anti-B7-H3 mAb BRCA69D on formalin-fixed paraffin embedded normal human spleen and lymph node tissues.

Purpose:Patients with relapsed pediatric solid tumors and CNS malignancies have few therapeutic options and frequently die of their disease. Chimeric antigen receptor (CAR) T cells have shown tremendous success in treating relapsed pediatric acute lymphoblastic leukemia, but this has not yet translated to treating solid tumors. This is partially due to a paucity of differentially expressed cell surface molecules on solid tumors that can be safely targeted. Here, we present B7-H3 (CD276) as a putative target for CAR T-cell therapy of pediatric solid tumors, including those arising in the central nervous system.Experimental Design:We developed a novel B7-H3 CAR whose binder is derived from a mAb that has been shown to preferentially bind tumor tissues and has been safely used in humans in early-phase clinical trials. We tested B7-H3 CAR T cells in a variety of pediatric cancer models.Results:B7-H3 CAR T cells mediate significant antitumor activity in vivo, causing regression of established solid tumors in xenograft models including osteosarcoma, medulloblastoma, and Ewing sarcoma. We demonstrate that B7-H3 CAR T-cell efficacy is largely dependent upon high surface target antigen density on tumor tissues and that activity is greatly diminished against target cells that express low levels of antigen, thus providing a possible therapeutic window despite low-level normal tissue expression of B7-H3.Conclusions:B7-H3 CAR T cells could represent an exciting therapeutic option for patients with certain lethal relapsed or refractory pediatric malignancies, and should be tested in carefully designed clinical trials.

PDF file, 78KB.

Purpose: The goal of this research was to harness a monoclonal antibody (mAb) discovery platform to identify cell-surface antigens highly expressed on cancer and develop, through Fc optimization, potent mAb therapies toward these tumor-specific antigens.Experimental Design: Fifty independent mAbs targeting the cell-surface immunoregulatory B7-H3 protein were obtained through independent intact cell-based immunizations using human tissue progenitor cells, cancer cell lines, or cell lines displaying cancer stem cell properties. Binding studies revealed this natively reactive B7-H3 mAb panel to bind a range of independent B7-H3 epitopes. Immunohistochemical analyses showed that a subset displayed strong reactivity to a broad range of human cancers while exhibiting limited binding to normal human tissues. A B7-H3 mAb displaying exquisite tumor/normal differential binding was selected for humanization and incorporation of an Fc domain modified to enhance effector-mediated antitumor function via increased affinity for the activating receptor CD16A and decreased binding to the inhibitory receptor CD32B.Results: MGA271, the resulting engineered anti–B7-H3 mAb, mediates potent antibody-dependent cellular cytotoxicity against a broad range of tumor cell types. Furthermore, in human CD16A-bearing transgenic mice, MGA271 exhibited potent antitumor activity in B7-H3–expressing xenograft models of renal cell and bladder carcinoma. Toxicology studies carried out in cynomolgus monkeys revealed no significant test article-related safety findings.Conclusions: This data supports evaluation of MGA271 clinical utility in B7-H3–expressing cancer, while validating a combination of a nontarget biased approach of intact cell immunizations and immunohistochemistry to identify novel cancer antigens with Fc-based mAb engineering to enable potent antitumor activity. Clin Cancer Res; 18(14); 3834–45. ©2012 AACR.

PDF file, 59KB, Expanded normal tissue IHC analysis of B7-H3 on frozen tissues with MGA271 at optimal staining concentration (4 ug/ml) and at 5x the optimal concentration (20 ug/ml). The optimal staining concentration was determined by titration on the CaKi2 and Hs700T positive control cell lines to determine the lowest MGA271 concentration that yielded the greatest specific staining. (c) = cytoplasmic; (m) = membrane; (c,m) = cytoplasmic plus membrane.

Monoclonal antibodies (mAb) are widely used in the treatment of non-Hodgkin's lymphoma and autoimmune diseases. Although the mechanism of action in vivo is not always known, the therapeutic activity of several approved mAbs depends on the binding of the Fcγ regions to low-affinity Fcγ receptors (FcγR) expressed on effector cells. We did functional genetic screens to identify IgG1 Fc domains with improved binding to the low-affinity activating Fc receptor CD16A (FcγRIIIA) and reduced binding to the low-affinity inhibitory Fc receptor, CD32B (FcγRIIB). Identification of new amino acid residues important for FcγR binding guided the construction of an Fc domain that showed a dramatically enhanced CD16A binding and greater than a 100-fold improvement in antibody-dependent cell-mediated cytotoxicity. In a xenograft murine model of B-cell malignancy, the greatest enhancement of an Fc-optimized anti-human B-cell mAb was accounted for by improved binding to FcγRIV, a unique mouse activating FcγR that is expressed by monocytes and macrophages but not natural killer (NK) cells, consistent with experimental and clinical data suggesting that mononuclear phagocytes, effector cells expressing both activating and inhibitory FcγR, are critical mediators of B-cell depletion in vivo. By using mice transgenic for human CD16A, enhanced survival was observed due to expression of CD16A-158phe on monocytes and macrophages as well as on NK cells in these mice. The design of new generations of improved antibodies for immunotherapy should aim at Fc optimization to increase the engagement of activating FcγR present on the surface of tumor-infiltrating effector cell populations. [Cancer Res 2007;67(18):8882–90]

Supplementary Methods from Fc Optimization of Therapeutic Antibodies Enhances Their Ability to Kill Tumor Cells In vitro and Controls Tumor Expansion In vivo via Low-Affinity Activating Fcγ Receptors

Acute myeloid leukemia (AML) relapse is one of the most common and significant adverse events following allogeneic hematopoietic cell transplantation (HCT). Downregulation of major histocompatibility class II (MHC-II) surface expression on AML blasts may represent a mechanism of escape from the graft-versus-malignancy effect and facilitate relapse. We hypothesized that T-cell immunotherapies targeting AML antigens would upregulate MHC-II surface expression via localized release of interferon gamma (IFN-γ), a protein known to upregulate MHC-II expression via JAK-STAT signaling. We demonstrate that flotetuzumab (FLZ), a CD123 × CD3 bispecific DART molecule, and chimeric antigen receptor expressing T cells targeting CD123, CD33, or CD371 upregulate MHC-II surface expression in vitro on a THP-1 AML cell line with intermediate MHC-II expression and 4 primary AML samples from patients relapsing after HCT with low MHC-II expression. We additionally show that FLZ upregulates MHC-II expression in a patient-derived xenograft model and in patients with relapsed or refractory AML who were treated with FLZ in a clinical trial. Finally, we report that FLZ-induced MHC-II upregulation is mediated by IFN-γ. In conclusion, we provide evidence that T-cell immunotherapies targeting relapsed AML can kill AML via both MHC-independent mechanisms and by an MHC-dependent mechanism through local release of IFN-γ and subsequent upregulation of MHC-II expression.

Advanced Breast Cancer; Efficacy; Margetuximab Cáncer de mama avanzado; Eficacia; Margetuximab Càncer de mama avançat; Eficàcia; Margetuximab Importance ERRB2 (formerly HER2)–positive advanced breast cancer (ABC) remains typically incurable with optimal treatment undefined in later lines of therapy. The chimeric antibody margetuximab shares ERBB2 specificity with trastuzumab but incorporates an engineered Fc region to increase immune activation. Objective To compare the clinical efficacy of margetuximab vs trastuzumab, each with chemotherapy, in patients with pretreated ERBB2-positive ABC. Design, Setting, and Participants The SOPHIA phase 3 randomized open-label trial of margetuximab plus chemotherapy vs trastuzumab plus chemotherapy enrolled 536 patients from August 26, 2015, to October 10, 2018, at 166 sites in 17 countries. Eligible patients had disease progression on 2 or more prior anti-ERBB2 therapies and 1 to 3 lines of therapy for metastatic disease. Data were analyzed from February 2019 to October 2019. Interventions Investigators selected chemotherapy before 1:1 randomization to margetuximab, 15 mg/kg, or trastuzumab, 6 mg/kg (loading dose, 8 mg/kg), each in 3-week cycles. Stratification factors were metastatic sites (≤2, >2), lines of therapy (≤2, >2), and chemotherapy choice. Main Outcomes and Measures Sequential primary end points were progression-free survival (PFS) by central blinded analysis and overall survival (OS). All α was allocated to PFS, followed by OS. Secondary end points were investigator-assessed PFS and objective response rate by central blinded analysis. Results A total of 536 patients were randomized to receive margetuximab (n = 266) or trastuzumab (n = 270). The median age was 56 (27-86) years; 266 (100%) women were in the margetuximab group, while 267 (98.9%) women were in the trastuzumab group. Groups were balanced. All but 1 patient had received prior pertuzumab, and 489 (91.2%) had received prior ado-trastuzumab emtansine. Margetuximab improved primary PFS over trastuzumab with 24% relative risk reduction (hazard ratio [HR], 0.76; 95% CI, 0.59-0.98; P = .03; median, 5.8 [95% CI, 5.5-7.0] months vs 4.9 [95% CI, 4.2-5.6] months; October 10, 2018). After the second planned interim analysis of 270 deaths, median OS was 21.6 months with margetuximab vs 19.8 months with trastuzumab (HR, 0.89; 95% CI, 0.69-1.13; P = .33; September 10, 2019), and investigator-assessed PFS showed 29% relative risk reduction favoring margetuximab (HR, 0.71; 95% CI, 0.58-0.86; P

BackgroundAvailability of checkpoint inhibitors has created a paradigm shift in the management of patients with solid tumors. Despite this, most patients do not respond to immunotherapy, and there is considerable interest in developing combination therapies to improve response rates and outcomes. B7-H3 (CD276) is a member of the B7 family of cell surface molecules and provides an alternative immune checkpoint molecule to therapeutically target alone or in combination with programmed cell death-1 (PD-1)–targeted therapies. Enoblituzumab, an investigational anti-B7-H3 humanized monoclonal antibody, incorporates an immunoglobulin G1 fragment crystallizable (Fc) domain that enhances Fcγ receptor-mediated antibody-dependent cellular cytotoxicity. Coordinated engagement of innate and adaptive immunity by targeting distinct members of the B7 family (B7-H3 and PD-1) is hypothesized to provide greater antitumor activity than either agent alone.MethodsIn this phase I/II study, patients received intravenous enoblituzumab (3–15 mg/kg) weekly plus intravenous pembrolizumab (2 mg/kg) every 3 weeks during dose-escalation and cohort expansion. Expansion cohorts included non–small cell lung cancer (NSCLC; checkpoint inhibitor [CPI]–naïve and post-CPI, programmed death-ligand 1 [PD-L1] ResultsOverall, 133 patients were enrolled and received ≥1 dose of study treatment. The maximum tolerated dose of enoblituzumab with pembrolizumab at 2 mg/kg was not reached. Intravenous enoblituzumab (15 mg/kg) every 3 weeks plus pembrolizumab (2 mg/kg) every 3 weeks was recommended for phase II evaluation. Treatment-related adverse events occurred in 116 patients (87.2%) and were grade ≥3 in 28.6%. One treatment-related death occurred (pneumonitis). Objective responses occurred in 6 of 18 (33.3% [95% CI 13.3 to 59.0]) patients with CPI-naïve HNSCC and in 5 of 14 (35.7% [95% CI 12.8 to 64.9]) patients with CPI-naïve NSCLC.ConclusionsCheckpoint targeting with enoblituzumab and pembrolizumab demonstrated acceptable safety and antitumor activity in patients with CPI-naïve HNSCC and NSCLC.Trial registration numberNCT02475213.

Introduction: MGC018 is an ADC composed of a humanized anti-B7-H3 IgG1 monoclonal antibody conjugated via a cleavable linker to vc-seco-duocarmycin-hydroxybenzamide azaindole (DUBA), a synthetic DNA alkylating agent whose cytotoxic activity is cell-cycle independent and retained in multidrug-resistant cells. MGC018 has shown preliminary clinical activity in B7-H3-expressing tumors, including castration-resistant metastatic prostate cancer. Owing to its expression in several childhood tumors, B7-H3 is a potential target for pediatric cancers, including neuroblastoma (NB). Methods: B7-H3 expression was evaluated by flow-cytometry (FCM) in a panel of human NB cell lines. Cytotoxicity was evaluated in monolayer and in multicellular tumor spheroid (MCTS) models by MTS assay and Cell Titer Glo 3D cell viability assay, respectively. Apoptotic cell death was investigated by the Annexin V staining. Pseudometastatic, orthotopic, and resected mouse NB models were developed via tumor cells tail vein injection, implantation in the adrenal gland, or implantation followed by surgical resection of the primary tumor mass, respectively, to mimic disease conditions related to circulating tumor cells and metastases, primary tumor growth, and minimal residual disease. Results: All cell lines expressed cell surface B7-H3 in a unimodal fashion, ranging between 200 and 400 in mean ratio fluorescence intensity over unstained cells. MGC018 was cytotoxic in a dose- and time-dependent manner against all NB cell lines (IC50 range 5.1-53.9 ng/mL) and NB MCTS (IC50 range 17.8-364 ng/mL). MGC018 was inactive against a murine NB cell line (NX-S2) that did not express human B7-H3; however, NX-S2 cells were killed in the presence of MGC018 when co-cultured with human B7-H3-expressing cells, demonstrating by-stander activity. In the pseudometastatic model and in five orthotopic NB mouse models, weekly iv treatments with 1 mg/kg MGC018 for 3 weeks resulted in delayed tumor growth and increased survival rates compared to animals treated with an irrelevant (anti-CD20) duocarmycin-ADC or an anti-GD2 antibody (dinutuximab beta). A 4-week course of treatment further ameliorated MGC018 antitumor effect in both the orthotopic and resected NB models and increased the survival of NB-bearing mice co-treated with TOpotecan-TEMozolomide (TOTEM), the standard-of-care therapy for relapsed disease. In the orthotopic model, tumor relapse was temporarily or completely arrested by 2 or 3 courses of MGC018, respectively. MGC018 treatment was not associated with body weight loss, hematological toxicity, or clinical chemistry abnormalities. Conclusion: MGC018 exerts relevant antitumor activity in pre-clinical NB models and may represent a potential candidate for future NB clinical translation. Citation Format: Chiara Brignole, Veronica Bensa, Enzo Calarco, Elena Giusto, Eleonora Ciampi, Patrizia Perri, Maria Valeria Corrias, Simonetta Astigiano, Michele Cilli, Deryk Loo, Ezio Bonvini, Fabio Pastorino, Mirco Ponzoni. Anti-tumor activity of vobramitamab duocarmazine (MGC018), an investigational duocarmycin-based anti-B7-H3 antibody-drug conjugate (ADC), in preclinical neuroblastoma models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2981.

Children with acute myeloid leukemia (AML) have a poor prognosis despite the intensification of chemotherapy. Future efforts to improve outcomes should focus on more precise targeting of leukemia cells. CD123, or IL3RA, is expressed on the surface of nearly all pediatric AML samples and is a high-priority target for immunotherapy. The efficacy of an investigational dual-affinity retargeting antibody (DART) molecule (CD123 × CD3; MGD006 or flotetuzumab) was assessed in two distinct patient-derived xenograft (PDX) models of pediatric AML. MGD006 simultaneously binds to CD123 on target cells and CD3 on effector T cells, thereby activating T cells and redirecting them to induce cytotoxicity in target cells. The concurrent treatment of cytarabine and MGD006 was performed to determine the effect of cytarabine on T-cell counts and MGD006 activity. Treatment with MGD006 along with an allogeneic human T-cell infusion to act as effector cells induced durable responses in both PDX models, with CD123 positivity. This effect was sustained in mice treated with a combination of MGD006 and cytarabine in the presence of T cells. MGD006 enhanced T-cell proliferation and decreased the burden of AML blasts in the peripheral blood with or without cytarabine treatment. These data demonstrate the efficacy of MGD006 in prolonging survival in pediatric AML PDX models in the presence of effector T cells and show that the inclusion of cytarabine in the treatment regimen does not interfere with MGD006 activity.

Introduction. Approximately 40% of patients (pts) with newly diagnosed AML either fail to achieve complete remission with intensive induction therapy or experience disease recurrence after a short remission (CR1 6 months), the probability of response for PIF/ER pts is particularly poor (~12%) with median expected overall survival of ~3.5 month and no approved therapy for this specific population. We have recently shown that increased immune infiltration of the tumor microenvironment (TME) is associated with induction failure and poor prognosis; conversely, an infiltrated TME predisposes for immunotherapy response1. We provide an update of the first-in-human study of flotetuzumab (FLZ), an investigational CD123 x CD3 bispecific DART® molecule currently in clinical development for PIF/ER AML pts. Methods. In this phase of the study, PIF is defined as being refractory to induction with: ≥1 high-intensity cytarabine-based chemotherapy (CTx) cycles, or ≥2 but ≤4 Bcl-2 inhibitor-based combinations, or gemtuzumab ozogamicin only. ER is defined as relapse following CR1 < 6 months. Pts who receive up to one prior salvage attempt are included. Pts whose AML recurred following HSCT are excluded. The recommended Phase 2 dose (RP2D) of FLZ is 500 ng/kg/day administered as a continuous infusion in 28-day cycles following a step-up ('priming') lead-in dose during Cycle 1 Week 1. Disease status is assessed by modified IWG criteria. Duration of response is measured from initial response to relapse or death. Results. As of July 1, 2020, 38 PIF/ER (as defined above) AML patients have been treated at the RP2D (median age 63yrs [range 28-81]; 31.6% [12] pts female). Most pts (63.2%, 24/38) were PIF and the large majority (94.7%, 36/38) had non-favorable risk by ELN 2017 criteria (25 pts adverse, 11 pts intermediate); 34.2% (13/38) had secondary AML. For ER pts, median duration of CR1 was 2.9 months (range: 0.7-4.0 months). Cytokine release syndrome (CRS) was the most frequently reported treatment related adverse event (TRAE), with all pts experiencing mild-to-moderate (grade ≤ 2) CRS. No grade ≥ 3 CRS events have been reported in this cohort. Most CRS events (51.5%) occurred in the first week of treatment during step-up dosing. The incidence of CRS progressively decreased during dosing at RP2D (34.8% in week 2, 4.5% in week 3, and 6.1% in week 4), allowing outpatient treatment in most cases. Neurologic AEs have been infrequent, with the most prominent event being grade 1 or grade 2 headache in 23.7% (9/38) treated at the RP2D. Two pts experienced grade 3 confusion of short duration (1-2 days) that was fully reversible. Over half (57.9%) of pts had evidence of antileukemic activity (reduction in blast count) with a median decrease of 92.7% in BM blasts (Fig. 1). The overall complete response rate (CRR, Conclusion: FLZ demonstrated encouraging activity in pts with PIF/ER AML, a population with poor prognosis and high unmet medical need, with 42.1% achieving CRR and over half of those receiving a stem cell transplant. Treatment is tolerable with a minimum 8 day inpatient treatment. The study is currently enrolling patients [NCT02152956] 1 Vadakekolathu J, Minden MD, Hood T, Church SE, Reeder S, Altmann H et al. Immune landscapes predict chemotherapy resistance and immunotherapy response in acute myeloid leukemia. Sci Trans Med 2020. Disclosures Aldoss: abbvie: Consultancy, Research Funding; agios: Honoraria; kite: Consultancy; autolus limited: Consultancy; JAZZ: Honoraria, Speakers Bureau; Amgen: Consultancy; Agios: Consultancy. Uy:Genentech: Consultancy; Agios: Consultancy; Pfizer: Consultancy; Jazz Pharmaceuticals: Consultancy; Daiichi Sankyo: Consultancy; Astellas Pharma: Honoraria. Emadi:Amgen: Membership on an entity's Board of Directors or advisory committees; NewLink Genetics: Research Funding; Jazz Pharmaceuticals: Research Funding; Genentech: Membership on an entity's Board of Directors or advisory committees; KinaRx: Other: co-founder and scientific advisor; Servier: Membership on an entity's Board of Directors or advisory committees. Walter:Aptevo Therapeutics: Research Funding. Foster:Daiichi Sankyo: Consultancy; Bellicum Pharmaceuticals: Research Funding; Macrogenics: Consultancy, Research Funding. Arellano:Hanmi: Research Funding; Gilead Sciences, Inc: Consultancy, Membership on an entity's Board of Directors or advisory committees; Cephalon Oncology: Research Funding. Wieduwilt:Amgen: Research Funding; Macrogeneics: Research Funding; Leadiant: Research Funding; Merck: Research Funding; Shire: Research Funding; Daiichi Sankyo: Membership on an entity's Board of Directors or advisory committees. Michaelis:Jazz Pharmaceuticals: Research Funding. Stiff:Kite, a Gilead Company: Research Funding; Gamida Cell: Research Funding; Atara: Research Funding; Unum: Research Funding; Delta-Fly: Research Funding; Macrogenics: Research Funding; Amgen: Research Funding. Advani:Novartis: Consultancy, Other: advisory board; Pfizer: Honoraria, Research Funding; Takeda: Research Funding; OBI: Research Funding; Kite: Other: Advisory board/ honoraria; Amgen: Consultancy, Other: steering committee/ honoraria, Research Funding; Seattle Genetics: Other: Advisory board/ honoraria, Research Funding; Immunogen: Research Funding; Glycomimetics: Consultancy, Other: Steering committee/ honoraria, Research Funding; Macrogenics: Research Funding; Abbvie: Research Funding. Wermke:MacroGenics: Honoraria. Erba:AbbVie, Daiichi Sankyo, Forma, ImmunoGen, Jazz Pharmaceuticals, MacroGenics, Novartis, PTC: Research Funding; Glycomimetics: Other: member of Scientific Steering Committee; Celgene: Other: chair of the Scientific Steering Committee; Covance (AbbVie): Other: chair of the Independent Review Committee; AbbVie, Agios, Celgene, Incyte, Jazz Pharmaceuticals, and Novartis: Speakers Bureau; AbbVie, Agios, Amgen, Astellas, Celgene, Daiichi Sankyo, Glycomimetics, ImmunoGen, Incyte, Jazz Pharmaceuticals, MacroGenics, Novartis, and Pfizer: Consultancy. Topp:Amgen, Boehringer Ingelheim, KITE, Regeneron, Roche: Research Funding; Amgen, KITE, Novartis, Regeneron, Roche: Consultancy. Ravandi:Abbvie: Consultancy, Honoraria, Research Funding; Jazz Pharmaceuticals: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Xencor: Consultancy, Honoraria, Research Funding; BMS: Consultancy, Honoraria, Research Funding; Astellas: Consultancy, Honoraria, Research Funding; Macrogenics: Research Funding; Celgene: Consultancy, Honoraria; AstraZeneca: Consultancy, Honoraria; Orsenix: Consultancy, Honoraria, Research Funding. Muth:MacroGenics, Inc.: Current Employment, Current equity holder in publicly-traded company. Collins:IQVIA: Other: I have worked as a contractor for IQVIA in the past, within the past 24 months.; MacroGenics: Current equity holder in publicly-traded company, Other: I currently work as a contractor for MacroGenics. Guo:Macrogenics: Current Employment. Tran:MacroGenics: Current Employment. Kaminker:MacroGenics, Inc.: Current Employment, Current equity holder in publicly-traded company. Patel:MacroGenics: Current Employment. Bakkacha:MacroGenics: Current Employment. Jacobs:MacroGenics: Current Employment. Seiler:MacroGenics: Current Employment. Rutella:Kura Oncology: Research Funding; MacroGenics Inc.: Research Funding; NanoString Technologies Inc.: Research Funding. Bonvini:MacroGenics, Inc.: Current Employment, Current equity holder in publicly-traded company. Davidson-Moncada:Macrogenics: Current Employment. DiPersio:Magenta Therapeutics: Membership on an entity's Board of Directors or advisory committees.

Introduction: CRS is a potentially life-threatening toxicity observed following T cell-redirecting therapies. CRS is associated with elevated cytokines, including IL6, IFNγ, TNFα, IL2 and GM-CSF. Glucocorticosteroids (GC) and the IL6 receptor blocking antibody tocilizumab (TCZ) can reduce CRS severity; however, CRS may still occur and limit the therapeutic window of novel immunotherapeutic agents. Disruption of cytokine signaling via Janus kinase (JAK) pathway interference may represent a complementary approach to blocking CRS. Ruxolitinib (RUX), an oral JAK1/2 inhibitor approved for the treatment of myelofibrosis and polycythemia vera, interferes with signaling of several cytokines, including IFNγ and IL6, via blockade of the JAK/STAT pathway. We hypothesized that RUX may reduce the frequency and severity of CRS in R/R AML patients (pts) undergoing treatment with flotetuzumab (FLZ), an investigational CD123 x CD3 bispecific DART® molecule. Methods: Relapse/refractory (including primary induction failure, early relapse and late relapse) AML pts were included in this study. RUX pts were treated at a single site, Washington University, St. Louis, MO. RUX was dosed at 10 mg or 20mg BID days -1 through 14. Comparator (non-RUX) pts (n=23) were treated at other clinical sites. FLZ was administered at 500 ng/kg/day continuously in 28-day cycles following multi-step lead-in dosing in week 1 of cycle 1. CRS was graded per Lee criteria1. Results: As of July 1st, 2020, 10 R/R AML pts, median age 65 (range 40-82) years, have been enrolled and treated in the RUX cohort (6 at 10mg, 4 at 20 mg of RUX). All pts had non-favorable risk by ELN 2017 criteria (8 adverse and 2 intermediate); 1 (10.0%) pt had secondary AML; pt characteristics in the RUX and non-RUX cohorts were balanced, except for median baseline BM blasts which was higher in non-RUX pts: 15% (range 5-72) vs (40% (range 7-84), RUX and non-RUX pts respectively. Cytokine analysis showed statistically significant (p Conclusion: Prophylactic RUX produced a clear difference in cytokine profiles but no discernable improvement in clinical CRS or response rates in FLZ treated patients. A larger study may be required to determine the prophylactic role of RUX in CRS. References: 1. Lee DW, Gardner R, Porter DL, Louis CU, Ahmed N, Jensen M et al. Current concepts in the diagnosis and management of cytokine release syndrome. Blood 2014; 124(2): 188-195. doi: 10.1182/blood-2014-05-552729 Disclosures Uy: Pfizer: Consultancy; Agios: Consultancy; Genentech: Consultancy; Jazz Pharmaceuticals: Consultancy; Daiichi Sankyo: Consultancy; Astellas Pharma: Honoraria. Aldoss:abbvie: Consultancy, Research Funding; kite: Consultancy; agios: Honoraria; autolus limited: Consultancy; JAZZ: Honoraria, Speakers Bureau; Amgen: Consultancy; Agios: Consultancy. Arellano:Cephalon Oncology: Research Funding; Gilead Sciences, Inc: Consultancy, Membership on an entity's Board of Directors or advisory committees; Hanmi: Research Funding. Foster:Daiichi Sankyo: Consultancy; Bellicum Pharmaceuticals: Research Funding; Macrogenics: Consultancy, Research Funding. Ravandi:Celgene: Consultancy, Honoraria; Xencor: Consultancy, Honoraria, Research Funding; Macrogenics: Research Funding; Abbvie: Consultancy, Honoraria, Research Funding; Astellas: Consultancy, Honoraria, Research Funding; BMS: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Orsenix: Consultancy, Honoraria, Research Funding; AstraZeneca: Consultancy, Honoraria; Jazz Pharmaceuticals: Consultancy, Honoraria, Research Funding. Advani:Takeda: Research Funding; Glycomimetics: Consultancy, Other: Steering committee/ honoraria, Research Funding; Macrogenics: Research Funding; Abbvie: Research Funding; Immunogen: Research Funding; Seattle Genetics: Other: Advisory board/ honoraria, Research Funding; Amgen: Consultancy, Other: steering committee/ honoraria, Research Funding; Kite: Other: Advisory board/ honoraria; Pfizer: Honoraria, Research Funding; Novartis: Consultancy, Other: advisory board; OBI: Research Funding. Wieduwilt:Macrogeneics: Research Funding; Amgen: Research Funding; Leadiant: Research Funding; Merck: Research Funding; Shire: Research Funding; Daiichi Sankyo: Membership on an entity's Board of Directors or advisory committees. Emadi:Genentech: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; NewLink Genetics: Research Funding; Jazz Pharmaceuticals: Research Funding; KinaRx: Other: co-founder and scientific advisor; Servier: Membership on an entity's Board of Directors or advisory committees. Michaelis:Jazz Pharmaceuticals: Research Funding. Stiff:Macrogenics: Research Funding; Kite, a Gilead Company: Research Funding; Delta-Fly: Research Funding; Unum: Research Funding; Atara: Research Funding; Gamida Cell: Research Funding; Amgen: Research Funding. Wermke:MacroGenics: Honoraria. Topp:Amgen, Boehringer Ingelheim, KITE, Regeneron, Roche: Research Funding; Amgen, KITE, Novartis, Regeneron, Roche: Consultancy. Muth:MacroGenics, Inc.: Current Employment, Current equity holder in publicly-traded company. Collins:MacroGenics: Current equity holder in publicly-traded company, Other: I currently work as a contractor for MacroGenics; IQVIA: Other: I have worked as a contractor for IQVIA in the past, within the past 24 months.. Guo:Macrogenics: Current Employment. Tran:MacroGenics: Current Employment. Kaminker:MacroGenics, Inc.: Current Employment, Current equity holder in publicly-traded company. Patel:MacroGenics: Current Employment. Bakkacha:MacroGenics: Current Employment. Jacobs:MacroGenics: Current Employment. Seiler:MacroGenics: Current Employment. Rutella:MacroGenics Inc.: Research Funding; Kura Oncology: Research Funding; NanoString Technologies Inc.: Research Funding. Walter:Aptevo Therapeutics: Research Funding. Bonvini:MacroGenics, Inc.: Current Employment, Current equity holder in publicly-traded company. Davidson-Moncada:Macrogenics: Current Employment. DiPersio:Magenta Therapeutics: Membership on an entity's Board of Directors or advisory committees.

Purpose: Patients with relapsed pediatric solid tumors and CNS malignancies have few therapeutic options and frequently die of their disease. Chimeric antigen receptor (CAR) T cells have shown tremendous success in treating relapsed pediatric acute lymphoblastic leukemia, but this has not yet translated to treating solid tumors. This is partially due to a paucity of differentially expressed cell surface molecules on solid tumors that can be safely targeted. Here, we present B7-H3 (CD276) as a putative target for CAR T-cell therapy of pediatric solid tumors, including those arising in the central nervous system. Experimental Design: We developed a novel B7-H3 CAR whose binder is derived from a mAb that has been shown to preferentially bind tumor tissues and has been safely used in humans in early-phase clinical trials. We tested B7-H3 CAR T cells in a variety of pediatric cancer models. Results: B7-H3 CAR T cells mediate significant antitumor activity in vivo, causing regression of established solid tumors in xenograft models including osteosarcoma, medulloblastoma, and Ewing sarcoma. We demonstrate that B7-H3 CAR T-cell efficacy is largely dependent upon high surface target antigen density on tumor tissues and that activity is greatly diminished against target cells that express low levels of antigen, thus providing a possible therapeutic window despite low-level normal tissue expression of B7-H3. Conclusions: B7-H3 CAR T cells could represent an exciting therapeutic option for patients with certain lethal relapsed or refractory pediatric malignancies, and should be tested in carefully designed clinical trials.

Introduction: Prostate cancer is the second most common cancer among men worldwide. In 2021, it is estimated that 248,530 men in the United States will be diagnosed with prostate cancer, and 34,130 will die from the disease. Although current treatments have success initially, development of resistance commonly leads to recurrence of an incurable castrate-resistant form of the disease. Thus, significant need for novel therapies to improve the outcome of castrate-resistant prostate cancer remains. B7-H3 (CD276), a member of the B7 family of immunomodulatory molecules, is overexpressed in primary and metastatic prostate cancer, and correlates with disease severity and poor clinical outcome. MGC018, a duocarmycin-based B7-H3 antibody-drug conjugate, is currently being evaluated in clinical studies. Here, MGC018 was explored preclinically to assess the potential for targeting B7-H3 in prostate cancer. Methods: Immunohistochemistry studies were performed to define the expression of B7-H3 in prostate cancer tissue microarrays (TMA). In vivo efficacy studies were conducted with human prostate cancer cell line-derived xenograft (CDX) models to explore the antitumor activity of MGC018 as a single agent and in combination with Poly (ADP-ribose) polymerase (PARP) and androgen receptor (AR) inhibitors. Based on the results in the CDX studies, in vivo efficacy studies were extended to a panel of metastatic prostate cancer patient-derived xenograft (PDX) models, which exhibit heterogenous expression of B7-H3 and more closely mimic the biological characteristics of patient tumors. Results: Staining of prostate tumor TMAs revealed high expression of B7-H3 in primary and metastatic prostate cancer. Of the prostate samples evaluated, 95% (38/40) of the tumor samples were positive for B7-H3 (H-score ≥ 20): 65% had H-scores greater than 200, while 20% and 10% had H-scores between 101-200 and 1-100, respectively. MGC018 demonstrated in vitro cytotoxicity toward B7-H3-positive human prostate cancer cell lines. The in vitro cytotoxicity translated to potent antitumor activity in vivo toward prostate cancer CDX models, and the antitumor activity of MGC018 was enhanced when combined with inhibitors of PARP or AR. In PDX models of metastatic prostate cancer, MGC018 was active as a single agent toward heterogeneous B7-H3-expressing tumors, and combining MGC018 with inhibitors of PARP or AR led to a greater response in some models. Conclusion: B7-H3 is frequently overexpressed in prostate cancer. MGC018 demonstrated potent antitumor activity in vivo toward CDX and PDX models of prostate cancer, and enhanced antitumor activity when combined with inhibitors of PARP or AR. These results support prostate cancer as an indication that may be responsive to ADC-based treatments directed toward B7-H3. MGC018 is being investigated in metastatic prostate cancer in a Phase 1/2 clinical study. Citation Format: Juniper A. Scribner, Francine Z. Chen, Anushka De Costa, Ying Li, Michael Chiechi, Thomas Son, Jeff Hooley, Jonathan Li, Scott Koenig, Chet Bohac, Ezio Bonvini, Paul A. Moore, Deryk Loo. Targeting B7-H3 in prostate cancer: Preclinical proof of concept with MGC018, an investigational anti-B7-H3 antibody-drug conjugate [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 330.

Pre-clinical murine models are critical for translating drug candidates from the bench to the bedside. There is interest in better understanding how anti-human CD3 therapy works based on recent longitudinal studies of short-term administration. Although several models have been created in this pursuit, each have their own advantages and disadvantages in Type-1 diabetes. In this study, we report a murine genetic knock-in model which expresses both a murine and a humanized-CD3ε-exon, rendering it sensitive to manipulation with anti-human CD3. These huCD3εHET mice are viable and display no gross abnormalities. Specifically, thymocyte development and T cell peripheral homeostasis is unaffected. We tested immune functionality of these mice by immunizing them with T cell-dependent antigens and no differences in antibody titers compared to wild type mice were recorded. Finally, we performed a graft-vs-host disease model that is driven by effector T cell responses and observed a wasting disease upon transfer of huCD3εHET T cells. Our results show a viable humanized CD3 murine model that develops normally, is functionally engaged by anti-human CD3 and can instruct on pre-clinical tests of anti-human CD3 antibodies.

Background Interleukin-10 (IL-10) is a multifunctional cytokine that can mediate immune suppression or activation depending on the immunological context. Mouse studies have demonstrated that blockade of IL-10 enhances immune response against tumors and chronic viral infections;1 2 intriguingly, high concentrations of long-acting, pegylated IL-10 have also shown anti-tumor activity.3 Here we investigated IL-10 and IL-10 receptor-alpha (IL-10RA) expression profiles in normal and tumor tissues as well as the immunological effects of modulating the IL-10 pathway via antibody-mediated blockade of IL-10RA. Methods IL-10 and IL-10RA mRNA are expressed by several tumors, including renal, lung, breast, and colon cancers. Fluorescent in-situ hybridization revealed that the majority of IL-10RA was expressed by CD3-negative tumor-infiltrating cells, localized in close proximity to T cells in the tumor microenvironment (TME). Immunohistochemistry studies confirmed expression of IL-10RA in the TME, while no expression was detected in healthy tissues. Furthermore, dissociated tumor cells produced biologically active levels of IL-10 in culture. Results Monoclonal antibodies (mAbs) against IL-10RA prevented IL-10 signaling and enhanced release of IL-12 by monocyte-derived dendritic cells activated with suboptimal LPS concentrations. The effect of IL-10RA blockade was greater than that observed with IL-10 neutralizing mAbs. In mixed lymphocyte reactions and superantigen-driven T-cell activation, IL-10RA blockade enhanced IL-2 secretion by T lymphocytes. Consistent with earlier observations in mouse models,4 the effect of IL-10RA blockade was nonredundant with blockade of the PD-1/PD-L1 axis, resulting in enhanced IL-2 and interferon-gamma secretion by T cells when both pathways were inhibited. Blockade of IL-10RA during CD3-redirected in vitro killing of tumor cells by PBMC induced IL-12 release as well as upregulation of CD86 and HLA-DR by CD3-negative cells. In in vitro dissociated tumor cells, IL-10RA blockade induced release of IL-2, interferon-gamma and other proinflammatory cytokines; additional PD-1/PD-L1 axis blockade further enhanced cytokine release. Conclusions In summary, antibody-mediated IL-10RA blockade can potentiate immune activation in the dissociated tumor cells and may be a valuable addition to cancer immunotherapies, including redirected T-cell killing and checkpoint blockade. References Vicari A, et al. J. Exp. Med 2002;196:541–549. Ejrnaes M, et al. J Exp Med 2006;203:2461–72. Emmerich J, et al. Cancer Res 2012. 72:3570–3581. Brooks D, et al. Proc Natl Acad Sci U S A 2008;105:20428–33.

B7-H3, also referred to as CD276, is a member of the B7 family of immune regulatory proteins. B7-H3 is overexpressed on many solid cancers, including prostate cancer, renal cell carcinoma, melanoma, squamous cell carcinoma of the head and neck, non–small cell lung cancer, and breast cancer. Overexpression of B7-H3 is associated with disease severity, risk of recurrence and reduced survival. In this article, we report the preclinical development of MGC018, an antibody–drug conjugate targeted against B7-H3. MGC018 is comprised of the cleavable linker-duocarmycin payload, valine-citrulline-seco duocarmycin hydroxybenzamide azaindole (vc-seco-DUBA), conjugated to an anti-B7-H3 humanized IgG1/kappa mAb through reduced interchain disulfides, with an average drug-to-antibody ratio of approximately 2.7. MGC018 exhibited cytotoxicity toward B7-H3–positive human tumor cell lines, and exhibited bystander killing of target-negative tumor cells when cocultured with B7-H3–positive tumor cells. MGC018 displayed potent antitumor activity in preclinical tumor models of breast, ovarian, and lung cancer, as well as melanoma. In addition, antitumor activity was observed toward patient-derived xenograft models of breast, prostate, and head and neck cancer displaying heterogeneous expression of B7-H3. Importantly, MGC018 exhibited a favorable pharmacokinetic and safety profile in cynomolgus monkeys following repeat-dose administration. The antitumor activity observed preclinically with MGC018, together with the positive safety profile, provides evidence of a potentially favorable therapeutic index and supports the continued development of MGC018 for the treatment of solid cancers.

We have developed MGD007 (anti-glycoprotein A33 x anti-CD3), a DART protein designed to redirect T cells to target gpA33 expressing colon cancer. The gpA33 target was selected on the basis of an antibody-based screen to identify cancer antigens universally expressed in both primary and metastatic colorectal cancer specimens, including putative cancer stem cell populations. MGD007 displays the anticipated-bispecific binding properties and mediates potent lysis of gpA33-positive cancer cell lines, including models of colorectal cancer stem cells, through recruitment of T cells. Xenograft studies showed tumor growth inhibition at doses as low as 4 μg/kg. Both CD8 and CD4 T cells mediated lysis of gpA33-expressing tumor cells, with activity accompanied by increases in granzyme and perforin. Notably, suppressive T-cell populations could also be leveraged to mediate lysis of gpA33-expressing tumor cells. Concomitant with CTL activity, both T-cell activation and expansion are observed in a gpA33-dependent manner. No cytokine activation was observed with human PBMC alone, consistent with the absence of gpA33 expression on peripheral blood cell populations. Following prolonged exposure to MGD007 and gpA33 positive tumor cells, T cells express PD-1 and LAG-3 and acquire a memory phenotype but retain ability to support potent cell killing. In cynomolgus monkeys, 4 weekly doses of 100 μg/kg were well tolerated, with prolonged PK consistent with that of an Fc-containing molecule. Taken together, MGD007 displays potent activity against colorectal cancer cells consistent with a mechanism of action endowed in its design and support further investigation of MGD007 as a potential novel therapeutic treatment for colorectal cancer. Mol Cancer Ther; 17(8); 1761–72. ©2018 AACR.

Introduction: Notwithstanding recent progress, acute myeloid leukemia (AML) remains an incurable disease, particularly in patients (pts) with relapsing/refractory disorder or ineligible for intensive induction therapy (unfit pts). Redirected T-cell mediated killing via CD3-engaging bispecific molecules may offer an alternate therapeutic opportunity for aggressive disease or unfit pts. Flotetuzumab, a continuous infusion CD123 x CD3 DART molecule, has shown preliminary single-agent activity in pts refractory to induction therapy (Uy et al., Blood. 2021, 137:751). CD123, the IL-3 receptor alpha chain, is expressed by both leukemic blasts and leukemic stem cells and is a suitable therapeutic target in AML (Testa et al., Cancers (Basel). 2019, 11:1358). MGD024 is an Fc-bearing CD123 x CD3 DART molecule designed for prolonged circulating half-life and intermittent delivery. MGD024 was also designed with a CD3-binding arm with reduced affinity to diminish the propensity for cytokine release compared to flotetuzumab. The potentially improved tolerability and dosing convenience of MGD024 may provide a framework for introducing T-cell immunotherapy in early-stage AML or unfit pts. To explore whether MGD024 could complement AML standard of care (SOC), we investigated combination therapy in mouse models. Materials and Methods: The DART molecules, flotetuzumab and MGD024, shared identical CD123 (humanized 7G3) and CD3 (humanized XR32) Fv arms, save for a mutation in the anti-CD3 arm of MGD024 that decreases its affinity for the CD3-epsilon chain. While flotetuzumab has no Fc domain, MGD024 includes an ala-ala-mutated human IgG1 Fc that extends its circulating half-life via the neonatal Fc receptor-mediated salvage pathway together with impairing binding to Fc-gamma receptors and complement. An IgG1-ala-ala Fc-bearing version of flotetuzumab (RES234M1.1) was also engineered to allow delivery at identical time intervals as MGD024 and avoid continuous infusion in experimental animals. MHC class I-null, NOD/SCID/IL2R-gamma-null mice were reconstituted with human PBMC (8x10 6 cells/mouse, retro orbital). Two human AML cell lines expressing low or high levels of CD123 (KG1a < MOLM-13) were implanted SC at 2.5 x 10 6 (KG1a) or 5 x 10 6 (MOLM-13) cells/mouse. Treatments (IV, IP or PO by gavage, as indicated) were initiated when tumor volumes reached ~150 mm 3, with volumes recorded weekly or twice weekly thereafter. Results and Conclusions: Consistent with its decreased affinity for CD3, MGD024 demonstrated reduced in vitro potency in killing CD123-positive target cells compared to flotetuzumab or RES234M1.1, but proportionally greater reduction in cytokine release. MGD024, however, achieved maximal cytolytic activity as flotetuzumab or RES234M1.1, albeit at increased concentrations. Similarly, MGD024 showed reduced potency in vivo against CD123-positive tumors compared to RES234M1.1; nevertheless, tumor growth reduction of the same magnitude as that observed with RES234M1.1 was attained at higher doses of MGD024 (0.5-1 mg/kg IV 2QW MGD024 vs. 0.05-0.1 mg/kg IV 2QW RES234M1.1, depending on the model). Reduced cytokine release was also observed with MGD024 compared to RES234M1.1 in vivo. To explore MGD024 suitability for combination therapy, sub-active doses of cytarabine (CYT, 10 mg/kg IV 2QW or 7.5-10 mg/kg IP QD), venetoclax (VEN, range 10-80 mg/kg PO QD), or azacitidine (AZA, 2 mg/kg PO QD) were co-administered with suboptimal regimens of MGD024 (range 0.005-0.1 mg/kg IV 2QW, depending on the model). Complete or near complete tumor elimination was observed with the combination of suboptimal MGD024 and CYT or VEN. In contrast, AZA, at the dose tested, did not contribute to the antitumor effect of MGD024. CYT, VEN or AZA did not inhibit a fully active dose of MGD024, confirming no detrimental impact of the SOC agents at the doses employed on the effector cell population engaged by the DART molecule. All treatments were well tolerated, as indicated by body weigh profiles across treatment groups. These data support clinical exploration of the combination of MGD024 with SOC in patients with AML. An investigational new drug (IND) application of MGD024 in pts with selected relapsed or refractory hematologic malignancies is planned. Disclosures Alderson: MacroGenics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Huang: MacroGenics: Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company, Ended employment in the past 24 months. Zhang: MacroGenics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Li: MacroGenics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Kaufman: MacroGenics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Diedrich: MacroGenics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Moore: MacroGenics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Bonvini: MacroGenics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company.

Approximately 50% of acute myeloid leukemia (AML) patients do not respond to induction therapy (primary induction failure [PIF]) or relapse after