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Introduction Acute myeloid leukemia (AML) is a very heterogeneous hematological malignancy characterized by the accumulation of myeloid blasts. Treatment options for unfit AML patients greater than 65 year of age are still limited and outcomes are dismal. The current standard of care for older AML patients are hypomethylating agents (HMA) or low dose Ara-C. However, even though inducing hematological remissions in up to 30 percent of the patients, responses are not durable and survival of these patients is only marginally prolonged. The poor durability is due to insufficient action on leukemia stem cells (LSC) which drive and maintain the disease and are resistance to therapy (Craddock et al, 2013 and DiNardo et al, 2019). We described how aberrant CD70/CD27 signaling drives stemness of AML LSCs and identified CD70 as potential new target for the treatment of AML patients (Riether et al, 2017). In a recent Phase 1 clinical trial, treatment of older and unfit AML patients with the ADCC-enhanced humanized monoclonal anti CD70 antibody (mAb) cusatuzumab in combination with HMA demonstrated promising clinical activity and a favorable tolerability profile. The BCL-2 antagonist, venetoclax, targets and eliminates LSCs by suppression of oxidative phosphorylation and demonstrated very promising activity in older AML patients in clinical phase I and II studies in combination with standard of care (Pollyea et al, 2018). However, even with novel agents such as venetoclax, there are still patients that become refractory or relapse. We hypothesized that combining venetoclax and cusatuzumab with distinct but complementary mechanisms of action could successfully eliminate LSCs. Experimental design To test this hypothesis, we performed a drug-combination study according to the Chou-Talalay method (Chou 2010) in CD70-expressing AML cell lines such as MOLM-13, MV4-11, and NOMO-1 cells in vitro. In addition, we tested the effect of the cusatuzumab/venetoclax and the cusatuzumab/venetoclax/HMA combination on colony formation and re-plating capacity of primary CD34+CD38- LSCs from newly diagnosed AML patients. Results We first treated MOLM-13, MV4-11, and NOMO-1 AML cells with vehicle, cusatuzumab alone or in combination with venetoclax or decitabine in a constant ratio in the presence of CFSE-labeled NK cells (ratio 1:1). AML cell numbers were assessed 72 hours later. Cusatuzumab in combination with venetoclax or decitabine and NK cells synergistically eliminated CD70-expressing AML cells in a broad dose range (Figure 1). To assess the effect of the cusatuzumab/venetoclax combination on primary human AML LSCs, we treated CD34+ CD38- LSCs with cusatuzumab or venetoclax monotherapy or in combination in the presence of NK cells and assessed colony formation. Cusatuzumab/venetoclax co-treatment was more efficacious than each monotherapy alone and strongly reduced LSCs and leukemia progenitors (Figure 2). To analyze the effect of the cusatuzumab/venetoclax treatment on LSC function in a more stringent way, we performed serial re-plating experiments in vitro. The impaired colony formation after combination treatment observed after the first plating was maintained during subsequent the re-plating, even though cusatuzumab and venetoclax were not present in the re-plating, indicating an effective reduction of LSCs. Mechanistically, we could show that treatment with venetoclax results in up-regulation of CD70 on LSCs, suggesting that venetoclax renders LSCs more susceptible to cytolytic killing with cusatuzumab. In older AML patients, cusatuzumab and venetoclax each have demonstrated promising clinical activity in combination with HMA. We determined whether addition of HMA to the cusatuzumab/venetoclax co-treatment could more effectively eliminate LSCs. The triplet combination cusatuzumab/venetoclax/decitabine did not further reduce growth of AML cell lines in vitro (Figure 1) nor colony and re-plating capacity of human LSCs compared to the cusatuzumab/venetoclax co-treatment. Conclusions Overall, these results indicate that a combination of cusatuzumab with venetoclax eliminates LSCs synergistically and more efficiently than as a monotherapy. Tolerability and efficacy will be tested in mouse models using both primary AML and cell line xenografts. The results suggest that targeting LSCs by combining venetoclax and cusatuzumab treatments is a promising novel treatment strategy in AML. Disclosures Johnson: Janssen R&D: Employment. Drenberg:Janssen R&D: Employment. Syed:Janssen R&D: Employment. Moshir:Argenx: Employment, Equity Ownership. Hultberg:Argenx: Employment. Leupin:Argenx: Employment, Equity Ownership, Patents & Royalties. De Haard:Argenx: Employment, Equity Ownership, Patents & Royalties.

Introduction Acute myeloid leukemia (AML) is a heterogenous hematological malignancy driven by leukemia stem cells (LSCs) (Lapidot et al, 1994). LSCs resistant against conventional chemotherapy represent the major cause of relapse. Elderly or unfit AML patients not eligible for intensive chemotherapy are treated in a palliative setting with hypomethylating agents (HMA) or low dose Ara-C, but responses are modest and not durable. The reason for the low efficacy of HMA treatment is their insufficient action on the disease initiating- and -maintaining LSC population (Craddock et al, 2013). We recently demonstrated that CD34+ AML cells (progenitors and LSC) consistently express the tumor necrosis factor family ligand CD70 as well as its receptor CD27 and that cell-autonomous CD70/CD27-signaling propagates the disease (Riether et al. 2017). The aim of the current study was to evaluate whether resistance to HMA treatment can be overcome by combining HMA with an anti-CD70 monoclonal antibody treatment. Experimental design The effect of HMA treatment on the expression of CD70 on primary human CD34+CD38- AML LSCs was determined in vitro cultures and in patients treated with HMA in vivo. The therapeutic potential of targeting CD70-expressing LSCs in presence and absence of HMA was assessed using the anti-CD70 ADCC-optimized monoclonal antibody (mAb), cusatuzumab, and an effector-dead anti-CD70 mAb in colony formation and re-plating assays as well as patient-derived xenograft models (Silence et al, 2014). The clinical relevance of the findings was determined in a clinical phase 1 trial in previously untreated elderly AML patients with a single dose of cusatuzumab monotherapy followed by a combination therapy with the HMA azacitidine (AZA, NCT03030612). Four different dose levels of cusatuzumab (1, 3, 10 and 20 mg/kg Q2W) were studied; AZA was administered at 75 mg/m² for 7 days every 28 days. Results We found that resistance of AML LSCs to HMA treatment is mediated by the up-regulation of the CD70. The up-regulation of CD70 triggered cell-autonomous CD70/CD27 signaling on AML LSCs. Based on these findings we hypothesized that the upregulation of CD70 by HMA may render LSCs more susceptible to CD70-targeting interventions. Targeting CD70-expressing LSCs by a blocking anti-CD70 mAb and the anti-CD70 mAb cusatuzumab, which blocks CD70/CD27-signaling and additionally mediates ADCC and CDC, eradicated LSCs in colony and re-plating assays in vitro and in xenotransplantation experiments in vivo. HMA in combination with blocking αCD70 mAb synergistically reduced LSC numbers in vivo and this was even more efficient when ADCC-enhanced αCD70 mAb cusatuzumab was added in the presence of NK cells. In order to test the hypothesis that targeting CD70 in combination with HMA eliminates LSCs in AML patients, we initiated a phase 1 dose-escalation trial in previously untreated elderly AML patients with a single dose of cusatuzumab monotherapy followed by a combination therapy with azacitidine. No dose-limiting toxicities (DLT) were observed in the dose-escalation phase 1 trial and responses were observed across the dose levels (1-20 mg/kg). A single dose of cusatuzumab reduced bone marrow blasts in just two weeks in all patients on average by 32%. Cusatuzumab monotherapy significantly reduced LSC numbers and frequencies in all patients analyzed in the bone marrow as assessed in limiting dilution colony assays. Single cell sequencing analysis revealed that cusatuzumab induced gene signatures related to myeloid differentiation and apoptosis in LSCs. In combination with azacitidine, cusatuzumab induced CR/CRi in 10 out of 12 patients. Responses were observed at all dose levels of cusatuzumab and median time to response was 3.3 months. Conclusions Blocking CD70/CD27-signaling and targeting CD70-expressing LSCs by the ADCC-optimized mAb, cusatuzumab, eliminated LSCs in vitro and in xenotransplantation experiments. In a phase 1 study promising activity of cusatuzumab in combination with HMA was observed in AML patients, in which translational data indicate that cusatuzumab selectively eliminates CD70-expressing LSCs. Disclosures Van Rompaey: argenx: Employment, Equity Ownership, Patents & Royalties. Moshir:Argenx: Employment, Equity Ownership. Delahaye:argenx: Employment, Equity Ownership. Gandini:Argenx: Employment, Equity Ownership. Erzeel:argenx: Employment, Equity Ownership. Hultberg:Argenx: Employment. Fung:argenx: Consultancy, Equity Ownership. De Haard:Argenx: Employment, Equity Ownership, Patents & Royalties. Leupin:Argenx: Employment, Equity Ownership, Patents & Royalties.

Introduction Cutaneous T-cell lymphoma (CTCL), a heterogeneous group of rare non-Hodgkin's lymphomas, is characterized by infiltration of malignant clonally expanded T lymphocytes in the skin and is very challenging to treat at advanced stages. Around 65% of the CTCL cases are mycosis fungoides (MF), which usually starts as indolent disease with accumulation of neoplastic resident memory T lymphocytes in the skin, evolving from patch to plaque and later to tumor stage with poor prognosis. Sézary syndrome (SS) involves erythroderma in the skin, malignant circulating clone, and lymphadenopathy. The CTCL pathogenesis is incompletely understood but suggests a complex interplay between neoplastic T cells and the tumor microenvironment. Glycoengineered ARGX-110 potently blocks CD70/CD27 signaling, which is thought to inhibit evasion of tumor immune surveillance as well as tumor cell proliferation and survival. The ARGX-110 effector functions, i.e., complement-dependent cytotoxicity (CDC), antibody-dependent cellular phagocytosis (ADCP), and enhanced antibody-dependent cellular cytotoxicity (ADCC), efficiently kill CD70-expressing tumor cells. With only limited expression in normal tissues and strong expression on tumor cells, CD70 is a very attractive target for antibody-based therapy in cancer. The role of CD70 and treatment with ARGX-110 was further investigated as part of a Phase 1/2 study in advanced CTCL patients. Methods As part of a non-randomized, open-label, multicenter Phase 1/2 study (NCT01813539), a total of 27 patients with CD70-expressing advanced CTCL of different subtypes and stages were included for determination of the optimal dose, to evaluate exploratory efficacy and safety, and to characterize pharmacokinetics and biomarkers of drug activity (immunohistochemistry [IHC] staining for drug- and disease-related markers, profiling of malignant and non-malignant immune cells by flow cytometry, quantitative polymerase chain reaction [qPCR] and Nanostring). ARGX-110 was administered intravenously at 1 (N=11) or 5 mg/kg (N=16) every three weeks (Q3W). Adverse events were graded according to NCI-CTCAE v. 4.03. Results ARGX-110 was safe and well tolerated at both 1 and 5 mg/kg in heavily treated CTCL patients with a median age of 67 years (range: 25-84 years). At cut-off (26th June 2018), a total of 110 treatment-emergent adverse events (TEAE) were reported in 27 patients of which 12 were grade 3-5; 40 events in 18 patients were considered drug-related by the investigator. The most common TEAE was pyrexia (7 events in 5 patients), and infusion-related reactions (IRRs) was the most common drug-related TEAE. The best response was 1 complete response (CR), 5 partial response (PR), and 7 stable disease (SD) in 26/27 patients evaluable for response, meaning overall response (ORR) in 23% of the patients and disease stabilization in 27% of the patients. The mean duration on the study was around 4.8 months. Two patients were still on the study at cut-off: 1 patient (subcutaneous panniculitis-like T-cell lymphoma, 24 months) with CR and 1 SS patient with PR (6.5 months). Pharmacokinetics (PK) revealed a mean half-life of 8-12 days, in line with previously published data. The 5 mg/kg dose was chosen as the recommended phase 2 dose (RP2D) based on PK and immunogenicity data. IHC staining for drug- and disease-specific markers showed results in line with clinical reduction of modified Severity Weighted Assessment Tool (mSWAT) and regression from plaque to patch stages. Molecular and cellular profiling indicated a complex interplay between the tumor and the microenvironment, in which CD70/CD27 plays a role. Conclusions ARGX-110 was safe and well tolerated at both dose levels (1 and 5 mg/kg) in advanced CD70-expressing CTCL patients. Clinical anti-tumor activity in patients with various types of CTCL was observed after treatment with ARGX-110, indicating ARGX-110 as a safe and promising treatment option for advanced CTCL. Disclosures Bagot: Kyowa Kirin: Membership on an entity's Board of Directors or advisory committees; Actelion: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Innate Pharma: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Maerevoet:Karyopharm: Membership on an entity's Board of Directors or advisory committees; takeda: Membership on an entity's Board of Directors or advisory committees, Other: travel grant; roche: Other: travel grant; abbvie: Membership on an entity's Board of Directors or advisory committees, Other: travel grant. Zinzani:Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Bayer: Membership on an entity's Board of Directors or advisory committees; PFIZER: Honoraria, Membership on an entity's Board of Directors or advisory committees; PFIZER: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bayer: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; MSD: Honoraria, Speakers Bureau; Janssen: Honoraria, Speakers Bureau; Celltrion: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Astra Zeneca: Speakers Bureau; TG Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees; Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Merck: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Merck: Honoraria, Membership on an entity's Board of Directors or advisory committees; Verastem: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; TG Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees; SERVIER: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Morschhauser:Janssen: Other: Scientific Lectures; Epizyme: Consultancy; Roche: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees; Gilead: Consultancy, Membership on an entity's Board of Directors or advisory committees. Ribrag:Incyte Corporation: Consultancy; Amgen: Research Funding; Infinity: Consultancy, Honoraria; Gilead: Consultancy, Honoraria; epizyme: Consultancy, Honoraria; NanoString Technologies: Consultancy, Honoraria; Roche: Honoraria, Other: travel; Servier: Consultancy, Honoraria; pharmamar: Other: travel; argenX: Research Funding; BMS: Consultancy, Honoraria, Other: travel; MSD: Honoraria. Dalle:Kyowa Hakko Kirin Pharmaceutical: Research Funding. Cayuela:Cepheid: Other: financial sponsor to attend John Goldman Conference 2017. Hultberg:argenx: Employment. Gandini:argenx: Employment. Moshir:argenx: Employment. Delahaye:argenx: Employment. Zabrocki:argenx: Employment. Silence:argenx: Consultancy. Van Rompaey:argenx: Employment. De Haard:argenx: Employment. Leupin:argenx: Employment.

Outcomes in elderly patients with acute myeloid leukemia (AML) are still adverse, as the majority does not qualify for intensive therapy or allogenic stem cell transplantation (ASCT). DNA hypomethylating agents (HMAs) induce remissions and prolong survival in a fraction of these patients. However, overall prognosis remains dismal and all patients progress due to therapy-resistant leukemia stem cells (LSCs). We recently demonstrated that HMAs upregulate the expression of CD70 on primary human AML LSCs, potentially contributing to HMA resistance and that blocking the cell-autonomous CD70/CD27 signaling inhibits proliferation and myeloid differentiation of LSCs and contributes to HMA resistance. Consequently, combining HMA treatment with a blocking αCD70 monoclonal antibody potently reduced colony formation of AML LSCs in vitro and effectively eliminated human AML LCSs in xenograft experiments. Based on these results, we initiated an open-label, non-controlled, non-randomized Phase 1/2 trial combining the HMA azacitidine (AZA) with ARGX-110, a human monoclonal antibody targeting CD70, in newly diagnosed AML patients unfit for intensive chemotherapy (ARGX-110-1601, NCT03030612). Trial design ARGX-110 with optimized antibody-dependent cell-mediated cytotoxicity (ADCC), is administered intravenously at 1, 3, 10 or 20 mg/kg once every two weeks (Q2W), in combination with a standard dose of AZA (75 mg/m² subcutaneously for 7 days every 28 days). A 2-week lead-in of ARGX-110 enables studying the effect of αCD70 antibody monotherapy. Primary objectives in the Phase 1 include determining the maximum tolerated dose of ARGX-110 in combination with AZA and the recommended phase 2 dose (RP2D). Secondary objectives comprise safety, pharmacokinetics and anti-leukemic activity of ARGX-110 alone or in combination with AZA (including overall response rate (ORR), frequencies of LSCs and minimal/measurable residual disease (MRD)). Results The trial is ongoing and as of 16th July 2018, 12 newly diagnosed AML patients were treated in the dose-escalation part of the trial, the results of which will be further updated in December 2018. The median age over the dose cohorts was 75 years (range 64-84) and included intermediate (N=6) and adverse (N=6) ELN risk groups. WHO subtypes were AML with recurrent genetic abnormalities (N=2), MDS-related changes (N=6), AML-NOS (N=3) and therapy-related myeloid neoplasms (N=1). No dose-limiting toxicity was observed and the combination of ARGX-110 with a standard dose of AZA was well tolerated. Hematological toxicities, in line with expected AZA toxicities, were the most frequently reported adverse events (AEs). At the cut-off, the ORR (complete remission [CR] + incomplete recovery [CRi] + partial remission [PR] + morphologic leukemia-free status [MLFS]) was 92% (11/12 patients); among the 11 evaluable subjects, the best response was CR/CRi for 9 patients (82%), 1 patient (9%) reached MLFS, and 1 (9%) PR, with 7 patients still on the trial (median duration on the trial at cut-off was 6.9 months [range: 2-14.4 months]). Significantly, 1 patient reached CR and was discontinued after 5 months to undergo ASCT. Five evaluable patients (45%) achieved MRD negativity by flow cytometry. Molecular MRD negativity was achieved in 3/8 patients with available results, two correlating with flow-MRD negativity. The mean time to response for the first 9 patients was 2.8 months and 3.4 months to reach best response. The 3 patients in the 20 mg/kg cohort did not all reach response at the time of cut-off. Based on pharmacokinetics, safety, and pharmacodynamic data, the RP2D of 10 mg/kg was established. Translational studies assessing LSC frequencies in bone marrow aspirates by limiting dilution colony assays and xenograft experiments revealed that ARGX-110 monotherapy and in combination with AZA significantly reduced LSC frequencies in AML patients. Conclusions ARGX-110 in combination with AZA is well tolerated and leads to a higher ORR compared to historical data for AZA alone. Clinical activity of the combination is observed in different AML subtypes and risk categories. Preliminary data indicate that ARGX-110 alone, and in combination with AZA, efficiently eliminates LSCs. These observations warrant further accelerated investigation of ARGX-110 in combination with AZA in AML patients unfit for intensive therapy. Disclosures Ochsenbein: argenx: Research Funding. Riether:argenx: Research Funding. Bacher:argenx: Research Funding. Höpner:argenx: Research Funding. Hinterbrandner:argenx: Research Funding. Van Rompaey:argenx: Employment. Moshir:argenx: Employment. Delahaye:argenx: Employment. Gandini:argenx: Employment. Erzeel:argenx: Employment. Hultberg:argenx: Employment. Fung:argenx: Consultancy. De Haard:argenx: Employment. Leupin:argenx: Employment.