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HexaBody®-CD38 (GEN3014) is a next-generation CD38-specific IgG1 molecule with a hexamerization-enhancing mutation that leads to highly efficient induction of complement-dependent cytotoxicity (CDC) upon binding to CD38 positive tumor cells. HexaBody-CD38 is designed to induce strong anti-tumor activity in patients with CD38-expressing hematological malignancies through CDC and other Fc-mediated effector functions. Previously we have shown that HexaBody-CD38 induces highly potent CDC-mediated cell death in multiple myeloma (MM), acute myeloid leukemia (AML), and B-cell non-hodgkin lymphoma (B-NHL) cells, including tumor cells in bone marrow samples from MM patients. In addition, HexaBody-CD38 demonstrated efficient ADCC and ADCP and induction of direct cell kill after Fc-crosslinking. Moreover HexaBody-CD38 induced strong inhibition of CD38 cyclase activity, which may contribute to reduction of immune suppression in the tumor microenvironment. In this study, we show that in a panel of 23 MM, AML and B-NHL cell lines HexaBody-CD38 induced CDC, with EC50 values on average 7-fold lower than those of the CD38-targeting mAb daratumumab, which is part of the standard of care for MM. Cytotoxic activity of HexaBody-CD38 showed a positive correlation with CD38 expression levels. HexaBody-CD38 was especially more potent than daratumumab in cell lines with lower CD38 expression. Sensitivity to HexaBody-CD38-induced CDC was correlated with lower expression levels of complement regulatory proteins, including CD46, CD55 and CD59. To exclude that the enhanced potency of HexaBody-CD38 resulted in lysis of healthy cell subsets known to express low levels of CD38, CDC in healthy donor leukocytes was assessed in vitro. HexaBody-CD38 (20 μg/mL) did not induce lysis of monocytes, B cells, T cells, granulocytes or erythrocytes, and only minor cytotoxicity of NK cells (median 13% lysis, range 9.5-55.1%). NK-cell lysis by daratumumab, assessed in parallel, was in the same range (median 8%, range 4.5-21.2%). Thus, HexaBody-CD38 shows superior potency in CD38-positive tumor cells while sparing healthy donor leukocytes and erythrocytes in vitro. The anti-tumor activity of HexaBody-CD38 in vivo was evaluated in patient-derived AML and B-NHL xenograft (PDX) models in nude mice. In a screen using 9 B-NHL PDX models in a 1-3 mice-per-treatment-group design, administration of two weekly doses of 5 mg/kg HexaBody-CD38 induced a strong anti-tumor response (relative tumor growth ≤10%, i.e. tumor stasis or tumor regression) in 2 models. Two models were classified as non-responder (relative tumor growth >70%) and 5 models could not be classified as either responder or non-responder (intermediates, relative tumor growth between 10 and 70%). A dose-response experiment in one of the responding models (Ly12638, 9 mice per group), confirmed potent anti-tumor activity of HexaBody-CD38, inducing complete tumor regression at a dose of 10 mg/kg (Figure 1A). In a screen using 5 AML PDX models in a 1-3 mouse-per-treatment-group design, HexaBody-CD38 (5 mg/kg) induced a strong anti-tumor response in one model. One model was classified as non-responder and the remaining 3 models could not be classified as either responder or non-responder (intermediates). A dose-response experiment in one of the intermediate models (AML11810), demonstrated statistically significant and dose-dependent anti-tumor activity of HexaBody-CD38 in an in vivo study using a classical 9 mice per treatment group design (Figure 1B). In summary, HexaBody-CD38 induced potent CDC in MM, B-NHL and AML cell lines, with superior potency compared to benchmark antibody daratumumab. HexaBody-CD38 was able to induce CDC in tumor cell lines with lower levels of CD38 expression, while sparing healthy leukocyte subsets. HexaBody-CD38 showed potent anti-tumor activity in vivo both in B-NHL and AML PDX models in nude mice. In these models, the contribution of mouse complement to the anti-tumor activity is limited and therapeutic activity is thought to be driven mainly through FcγR-mediated effector functions. This indicates that besides highly potent CDC, FcɣR-mediated effector mechanisms contribute to the preclinical activity of HexaBody-CD38 in hematological tumor models. These preclinical data support clinical investigation of HexaBody-CD38 in CD38 positive hematologic malignancies, including MM, AML, and B cell lymphomas. Figure Disclosures Hiemstra: Genmab: Current Employment, Current equity holder in publicly-traded company. Janmaat:Genmab: Current Employment, Current equity holder in publicly-traded company. Boross:Genmab: Current Employment, Current equity holder in publicly-traded company. van den Brakel:Genmab: Current Employment, Current equity holder in publicly-traded company. ten Hagen:Genmab: Current Employment, Current equity holder in publicly-traded company. van Dooremalen:Genmab: Current Employment, Current equity holder in publicly-traded company. Bosgra:Genmab: Current Employment, Current equity holder in publicly-traded company. De Goeij:Genmab: Current Employment, Current equity holder in publicly-traded company. Andringa:Genmab: Current Employment, Current equity holder in publicly-traded company. Kil:Genmab: Current Employment, Current equity holder in publicly-traded company. Sasser:Genmab: Current Employment, Current equity holder in publicly-traded company. Ahmadi:Genmab: Current Employment, Current equity holder in publicly-traded company. Satijn:Genmab: Current Employment, Current equity holder in publicly-traded company. Breij:Genmab: Current Employment, Current equity holder in publicly-traded company.

The CD38 molecule, with its high expression on Multiple Myeloma (MM), is considered a suitable target for antibody therapy of MM. We developed daratumumab (DARA), a human CD38 monoclonal antibody (mAb) with direct and Fc-mediated cell killing activity. DARA induces killing of tumor cells, mainly via complement-dependent cytotoxicity (CDC), antibody-dependent cell-mediated cytotoxicity (ADCC) (de Weers M. J Immunol 2011), and antibody-dependent cellular phagocytosis (ADCP) by macrophages (both murine and human). In addition, DARA induces apoptosis upon secondary cross-linking and modulates CD38 enzymatic function. DARA is currently in phase I, II and III clinical evaluation in patients with MM. Besides DARA, several other anti-CD38 mAb are in development; SAR650894 (SAR; clone 38SB19; Sanofi-Aventis) for MM and other CD38+ hematological malignancies, MOR03087 (MOR; Morphosys) for relapsed/refractory MM and Ab79 (Millenium/Takeda) which is in preclinical development. Similar mechanisms of action (MoA) are described for these mAb; nevertheless direct comparison studies would be critical for differentiation among these antibodies. . In this study, the efficacy of these anti-CD38 mAb was directly compared to DARA with respect to binding, apoptosis, CD38 ectoenzyme activity, and the induction of ADCC, ADCP and CDC. Surrogate antibodies of MOR, SAR and Ab79 were generated on the basis of protein sequences, as published in their corresponding patents families, and were attached to the backbone of DARA. Binding to CD38 expressing Daudi tumor cells was assessed by flow cytomery. All CD38 antibodies showed similar EC50 (~0.1 µg/mL) and maximal binding, except MOR which showed a lower apparent affinity (~0.3 µg/mL). Previously, CD38 amino acid residues Q272 and S274 were reported as critical for DARA binding. ELISA analyses using CD38 point mutants revealed MOR, SAR and Ab79 not to be affected by mutation of these residues. All CD38 mAb were equally potent in inducing ADCC of Daudi cells (40-60% lysis, 0.02 µg/mL), in classic Cr51-release ADCC assay using human PBMC effector cells (E:T ratio 100:1). Important differences were observed with respect to induction of CDC. SAR was unable to induce CDC in Daudi cells at concentrations up to 30 µg/mL, while DARA induced more than 80% lysis at concentrations above 1 µg/mL. Ab79 and MOR induced CDC, yet maximum lysis was 50% and 20%, respectively. Evaluation of Annexin V/propidium iodide (AnnV/PI) staining and activation of caspase-3 showed that only SAR induced AnnV/PI+ in Ramos cells (~40%) after 48 h exposure without Fc crosslinking, but did not activate caspase-3. In the presence of Fc crosslinking antibodies, all anti-CD38 mAb induced AnnV/PI+, caspase-3 mediated apoptosis. In enzyme activity assays using purified CD38 protein, SAR inhibited generation of cGDPR (indicative of the combined CD38 cyclase activity generating fluorescent cGDPR and hydrolase activity converting cGDPR into GDPR)more effectively than DARA (~70% vs. ~20% inhibition at 30 µg/mL). Ab79 had a modest effect on CD38 enzyme activity (~10% inhibition). MOR did not affect CD38 enzyme activity at all. The capacity to induce ADCP was only tested for DARA, MOR and Ab79 using mouse macrophages (mφ) as effector cells and Daudi target cells. mφ, isolated and matured from bone marrow cells, and calcein-AM labelled target cells (E:T ratio 1:1) were cultured in the presence of 0.0006-5 µg/mL antibody for 4 h. Non-phagocytosed target cells and mφ were collected and ADCP was evaluated by flow cytometry. All CD38 mAb induced mφ-mediated phagocytosis, as observed by a concentration dependent increase in the number of double positive mφ and killing of target cells. Ab79 was as effective as DARA (EC50 ~0.01 µg/mL) in ADCP induction, whereas MOR was less effective (EC50 0.04 µg/mL). In summary, DARA and surrogate mAb of MOR, SAR and Ab79 showed similar binding to cells and induced similar amounts of ADCC. Differences between these mAb involved the ability to directly induce apoptosis, to inhibit the enzymatic activity of CD38 and to induce ADCP. The most striking difference was observed for the ability to induce CDC, the MoA which is currently believed the most important mechanism of MM cell killing in the clinic. DARA efficiently induced high levels of CDC at low concentrations, whereas the other CD38 mAb were unable or less capable to induce CDC. Disclosures Lammerts van Bueren: Genmab: Employment, warrents Other. Jakobs:Genmab: Employment, warrents Other. Kaldenhoven:Genmab: Employment, warrents Other. Roza:Genmab: Employment, warrents Other. Hiddingh:Genmab: Employment. Meesters:Genmab: Employment, warrents Other. Voorhorst:Genmab: Employment, warrents Other. Gresnigt:Genmab: Employment, warrents Other. Wiegman:Genmab: Employment, warrents Other. Ortiz Buijsse:Genmab: Employment, warrents Other. Andringa:Genmab: Employment, warrents Other. Overdijk:Genmab: Employment, warrents Other. Doshi:Janssen R&D: Employment. Sasser:Janssen R&D: Employment. de Weers:Genmab: Employment, warrents Other. Parren:Genmab: Employment, inventor on patents regarding daratumumab Patents & Royalties, stock and warrents Other.