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Objectives The therapeutic monoclonal antibody (t-mAb) daratumumab, used to treat multiple myeloma (MM) patients, interferes with routine, electrophoretic based M-protein diagnostics. Electrophoretic response assessment becomes increasingly difficult when multiple t-mAbs are combined for use in a single patient. This is the first study to address the analytical challenges of M-protein monitoring when multiple t-mAbs are combined. Methods In this proof-of-principle study we evaluate two different methods to monitor M-protein responses in three MM patients, who receive both daratumumab and nivolumab. The double hydrashift assay aims to resolve t-mAb interference on immunofixation. The MS-MRD (mass spectrometry minimal residual disease) assay measures clonotypic peptides to quantitate both M-protein and t-mAb concentrations. Results After exposure to daratumumab and nivolumab, both t-mAbs become visible on immunofixation electrophoresis (IFE) as two IgG-kappa bands that migrate close to each other at the cathodal end of the γ-region. In case the M-protein co-migrates with these t-mAbs, the observed interference was completely abolished with the double IFE hydrashift assay. In all three patients the MS-MRD assay was also able to distinguish the M-protein from the t-mAbs. Additional advantage of the MS-MRD assay is that this multiplex assay is more sensitive and allows quantitative M-protein-, daratumumab- and nivolumab-monitoring. Conclusions Daratumumab and nivolumab interfere with electrophoretic M-protein diagnostics. However, the M-protein can be distinguished from both t-mAbs by use of a double hydrashift assay. The MS-MRD assay provides an alternative method that allows sensitive and simultaneous quantitative monitoring of both the M-protein and t-mAbs.

In the 2010s, immunotherapy revolutionized the treatment landscape of multiple myeloma. CD38-targeting antibodies were initially applied as monotherapy in end-stage patients, but are now also approved by EMA/FDA in combination with standards-of-care in newly diagnosed disease or in patients with early relapse. The approved SLAMF7-targeting antibody can also be successfully combined with lenalidomide or pomalidomide in relapsed/refractory myeloma. Although this has resulted in improved clinical outcomes, there remains a high unmet need in patients who become refractory to immunomodulatory drugs, proteasome inhibitors and CD38-targeting antibodies. Several new antibody formats, such as antibody–drug conjugates (e.g., belantamab mafodotin, which was approved in 2020 and targets BCMA) and T cell redirecting bispecific antibodies (e.g., teclistamab, talquetamab, cevostamab, AMG-420, and CC-93269) are active in these triple-class refractory patients. Based on their promising efficacy, it is expected that these new antibody formats will also be combined with other agents in earlier disease settings.

B-cell maturation antigen (BCMA)-targeting bispecific antibodies and bispecific T-cell engagers (BiTEs) redirect T-cells to BCMA-expressing multiple myeloma (MM) cells. These MM cells are subsequently eliminated via various mechanisms of action including the release of granzymes and perforins. Several phase 1, dose-escalation studies show pronounced activity of BCMA-targeting bispecific antibodies, including teclistamab, AMG420 and CC-93269, in heavily pretreated MM patients. Cytokine release syndrome is the most common adverse event, which can be adequately managed with tocilizumab or steroids. Several clinical trials are currently evaluating combination therapy with a BCMA-specific bispecific antibody, based on preclinical findings showing that immunomodulatory drugs or CD38-targeting antibodies enhance the activity of bispecific antibodies. In addition, bispecific antibodies, targeting other MM cell surface antigens (i. e. GPRC5D, CD38 and FcRH5), are also evaluated in early phase clinical trials. Such bispecific antibodies, targeting other antigens, may be given to patients with low baseline BCMA expression, disease with substantial heterogeneity in BCMA expression, following prior BCMA-targeted therapy, or combined with BCMA bispecific antibodies to prevent development of antigen escape.

PURPOSE OF REVIEW: Immunotherapy is transforming treatment of multiple myeloma patients in all stages of their disease. This review will discuss recent developments in immunotherapy in multiple myeloma with a focus on antibodies, antibody-drug conjugates, and T-cell-redirection strategies. RECENT FINDINGS: CD38-targeting antibodies have single agent activity in multiple myeloma, and especially when combined with other drugs, are improving the clinical outcome of patients with newly diagnosed or relapsed/refractory multiple myeloma. Also the SLAMF7-targeting antibody, elotuzumab, improves the survival of relapsed/refractory multiple myeloma patients, when it is combined with either lenalidomide or pomalidomide. Several novel immunotherapies, such as chimeric antigen receptor T cells, antibody-drug conjugates, and bispecific antibodies, are active in patients who developed resistance to all currently available antimultiple myeloma drugs, including immunomodulatory drugs, proteasome inhibitors, and CD38 antibodies. These new immunotherapeutic agents frequently target B-cell maturation antigen, which is highly and uniformly expressed on multiple myeloma cells. However, other targets, such as GPRC5D, are also being investigated. SUMMARY: Immunotherapy is incorporated into first-line and relapse regimens, and is improving the survival of both newly diagnosed and relapsed/refractory multiple myeloma patients.

Multiple myeloma (MM) patients eventually develop multi-drug-resistant disease with poor survival. Hence, the development of novel treatment strategies is of great importance. Recently, different classes of immunotherapeutic agents have shown great promise in heavily pre-treated MM, including T cell-redirecting bispecific antibodies (BsAbs). These BsAbs simultaneously interact with CD3 on effector T cells and a tumor-associated antigen on MM cells, resulting in redirection of T cells to MM cells. This leads to the formation of an immunologic synapse, the release of granzymes/perforins, and subsequent tumor cell lysis. Several ongoing phase 1 studies show substantial activity and a favorable toxicity profile with BCMA-, GPRC5D-, or FcRH5-targeting BsAbs in heavily pre-treated MM patients. Resistance mechanisms against BsAbs include tumor-related features, T cell characteristics, and impact of components of the immunosuppressive tumor microenvironment. Various clinical trials are currently evaluating combination therapy with a BsAb and another agent, such as a CD38-targeting antibody or an immunomodulatory drug (e.g., pomalidomide), to further improve response depth and duration. Additionally, the combination of two BsAbs, simultaneously targeting two different antigens to prevent antigen escape, is being explored in clinical studies. The evaluation of BsAbs in earlier lines of therapy, including newly diagnosed MM, is warranted, based on the efficacy of BsAbs in advanced MM.

Simple Summary The CD38-targeting antibody daratumumab has marked activity in multiple myeloma through direct anti-tumor effects and immunomodulatory activity. However, eventually most patients will develop daratumumab-refractory disease. We hypothesized that daratumumab-resistance could be reversed by the addition of an inhibitor of the PD-1/PD-L1 signaling pathway, resulting in improved T- and NK-cell mediated anti-tumor immune responses. We therefore performed a phase 2 study to investigate the efficacy and safety of adding the PD-L1 checkpoint inhibitor durvalumab to daratumumab at the time of daratumumab failure. The toxicity profile of the daratumumab/durvalumab combination was acceptable, but none of the 18 enrolled patients achieved a clinical response. Immunomonitoring of bone marrow samples at baseline and during treatment showed a reduction of regulatory T-cell numbers and a decrease in the proportion of T-cells expressing LAG3 and CD8+ T-cells expressing TIM-3, whereas tumor cell characteristics were not affected. These results indicate that co-targeting PD-L1 at the time of daratumumab failure is insufficient to reverse daratumumab-resistance. Abstract Daratumumab is active both as a single agent and in combination with other agents in multiple myeloma (MM) patients. However, the majority of patients will develop daratumumab-refractory disease, which carries a poor prognosis. Since daratumumab also has immunomodulatory effects, addition of the PD-L1 blocking antibody durvalumab at the time of progression may reverse daratumumab-resistance. The efficacy and safety of daratumumab and durvalumab in daratumumab-refractory relapsed/refractory MM patients was evaluated in this prospective, single-arm phase 2 study (NCT03000452). None of the 18 enrolled patients achieved PR or better. The frequency of serious adverse events was 38.9%, with one patient experiencing an immune related adverse event (grade 2 hyperthyroidism). No infusion-related reactions were observed. Analysis of tumor- and immune cell characteristics was performed on bone marrow samples obtained at baseline and during treatment. Daratumumab combined with durvalumab reduced the frequency of regulatory T-cells and decreased the proportion of T-cells expressing LAG3 and CD8+ T-cells expressing TIM-3, without altering T- and NK-cell frequencies. Durvalumab did not affect tumor cell characteristics associated with daratumumab resistance. In conclusion, the addition of durvalumab to daratumumab following development of daratumumab-resistance was associated with an acceptable toxicity profile, but was not effective. This indicates that inhibition of the PD-1/PD-L1 signaling pathway at the time of daratumumab-resistance is insufficient to reverse daratumumab-resistance.

Cell surface expression levels of GPRC5D, an orphan G protein–coupled receptor, are significantly higher on multiple myeloma (MM) cells, compared with normal plasma cells or other immune cells, which renders it a promising target for immunotherapeutic strategies. The novel GPRC5D-targeting T-cell redirecting bispecific antibody, talquetamab, effectively kills GPRC5D+ MM cell lines in the presence of T cells from both healthy donors or heavily pretreated MM patients. In addition, talquetamab has potent anti-MM activity in bone marrow (BM) samples from 45 patients, including those with high-risk cytogenetic aberrations. There was no difference in talquetamab-mediated killing of MM cells from newly diagnosed, daratumumab-naïve relapsed/refractory (median of 3 prior therapies), and daratumumab-refractory (median of 6 prior therapies) MM patients. Tumor cell lysis was accompanied by T-cell activation and degranulation, as well as production of pro-inflammatory cytokines. High levels of GPRC5D and high effector:target ratio were associated with improved talquetamab-mediated lysis of MM cells, whereas an increased proportion of T cells expressing PD-1 or HLA-DR, and elevated regulatory T-cell (Treg) counts were associated with suboptimal killing. In cell line experiments, addition of Tregs to effector cells decreased MM cell lysis. Direct contact with bone marrow stromal cells also impaired the efficacy of talquetamab. Combination therapy with daratumumab or pomalidomide enhanced talquetamab-mediated lysis of primary MM cells in an additive fashion. In conclusion, we show that the GPRC5D-targeting T-cell redirecting bispecific antibody talquetamab is a promising novel antimyeloma agent. These results provide the preclinical rationale for ongoing studies with talquetamab in relapsed/refractory MM.

Introduction: Daratumumab (DARA)-based regimens are effective and well tolerated in both newly diagnosed and relapsed/refractory multiple myeloma (RRMM) patients. However, the prognosis for patients who have become refractory to proteasome inhibitors (PIs), immunomodulatory drugs (IMiDs) and DARA is poor. A better understanding of determinants of response and mechanisms of resistance to DARA may lead to new rationally designed treatment strategies. We therefore characterized the effect of DARA on the immune system in MM patients treated with DARA monotherapy in part A of the DARA-ATRA study (ClinicalTrials.gov NCT02751255). Methods: In part A of this prospective multicenter phase 2 trial, 63 DARA-naïve patients were treated with DARA monotherapy (16 mg/kg; approved schedule). Median number of prior lines of treatment was 4 (range 2-11), all patients were previously exposed to lenalidomide and a PI; 89% was refractory to an IMiD, 71% to a PI and 67% to both IMiD and PI. Bone marrow (BM) aspirates obtained at baseline (BL) and at progression (PD; primary refractory, or acquired resistance after prior response) were subjected to deep immune profiling using 28 surface- and intracellular proteins measured by flow cytometry (BL n=51, PD n=47). In a subset of these patients (BL n=39, PD n=33) mass cytometry (CyTOF) was also performed profiling 39 proteins. In addition, peripheral blood (PB) samples obtained at BL (n=44) and PD (n=37) were analyzed using CyTOF. Computational analyses of flow data were performed using UMAP and FlowSOM; CyTOF data were analyzed using SPADE and Freeviz. Statistical analyses included Wilcoxon rank-sum and signed-rank tests, Generalized Linear Mixed Models and ANOVA. Results: A partial response or better was achieved in 41% of patients treated with DARA monotherapy. We compared immune profiles of responding and non-responding (primary-refractory) patients. At BL, the percentages of MM-, T-, B- and NK cells in BM were similar between both groups. However, NK cells of non-responding patients had a lower proportion of CD16 + (P=0.029), and a higher proportion of TIM-3 + (P=0.010) and HLA-DR + (P=0.043) NK cells, suggesting an exhausted phenotype. Non-responders also had a higher proportion of TIM-3 + CD4 + (P=0.022) and TIM-3 + CD8 + T-cells (P=0.004), and a higher proportion of TIM-3 + regulatory T-cells (Tregs) (P=0.042). A higher proportion of TIM-3 + -NK cells, -T-cells, or -Tregs in BM was also associated with poor progression-free and overall survival (PFS; OS). In addition, clinical characteristics associated with poor PFS such as LDH (P=0.016), extramedullary disease (P=0.001) and (R-)ISS stage III (P=0.015) were associated with an increased number of phenotypically exhausted NK- and T-cells. Similar to prior studies, DARA treatment resulted in reduced levels of CD38 on all immune cell subsets and a marked decrease in Tregs, regulatory B cells (Bregs) and NK cells in both BM and PB. Upon acquired resistance, remaining NK cells displayed higher proportions of TIM-3 + (P=0.022), HLA-DR + (P=0.0007) and LAG-3 + (P=0.011), and lower proportions of CD16 + (P=0.002) (Figure 1), compatible with an exhausted phenotype. Furthermore, disease progression was associated with an increase in CD4 + and CD8 + terminally differentiated effector memory T-cells and a decrease in CD4 + and CD8 + central memory T-cells in BM. There was no significant change in the proportion of T-cells expressing immune checkpoint molecules. CyTOF analysis of BM and PB samples confirmed flow cytometric findings. Furthermore, data-driven analysis identified immune profiles specific to progression, including a significant decrease in the fraction of Granzyme B + NK cells, and increase in the fraction of Granzyme B + Bregs at the time of progression. Based on NK cell depletion and -exhaustion observed at PD, we hypothesized that NK cell repletion may restore DARA sensitivity. Indeed, in ex vivo experiments we show that DARA-resistance can be overcome by addition of healthy donor derived NK cells to MM cells obtained from DARA-refractory patients. Conclusion: Here we show that an increased proportion of NK cells with an exhausted phenotype is associated with primary and acquired DARA-resistance, which is in line with the important role of NK cells in DARA-mediated tumor cell elimination ex vivo. Future DARA-based treatment strategies may benefit from reinvigorating NK cells and restoring their cytotoxic capacities. Figure 1 Figure 1. Disclosures Zweegman: Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding; BMS: Membership on an entity's Board of Directors or advisory committees; Oncopeptides: Membership on an entity's Board of Directors or advisory committees; Sanofi: Membership on an entity's Board of Directors or advisory committees. Minnema: Celgene: Other: Travel expenses; Alnylam: Consultancy; Cilag: Consultancy; Janssen: Consultancy; Kite/Gilead: Consultancy; BMS: Consultancy. Broyl: Celgene/BMS: Consultancy, Honoraria; Sanofi: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Janssen: Consultancy, Honoraria. Levin: Roche, Janssen, Abbvie: Other: Travel Expenses, Ad-Board. Kersten: Kite/Gilead: Consultancy, Honoraria, Research Funding; Novartis: Consultancy, Honoraria; Roche: Honoraria; Takeda: Consultancy; BMS/Celgene: Consultancy; Miltenyi Biotech: Consultancy. Krevvata: Janssen: Current Employment. Casneuf: Janssen: Current Employment. Abraham: Janssen: Current Employment. Verona: Janssen: Current Employment. Smets: Janssen: Current Employment. Vanhoof: Janssen: Current Employment. Cortes-Selva: Janssen: Current Employment. van Steenbergen: Biolizard working for Janssen: Current Employment. Vieyra: Janssen: Current Employment. Sonneveld: Karyopharm: Consultancy, Honoraria, Research Funding; SkylineDx: Honoraria, Research Funding; Celgene/BMS: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding. Mutis: Janssen: Honoraria; Genmab: Research Funding; Takeda: Research Funding; Novartis: Research Funding; ONK Therapeutics: Research Funding. van de Donk: Adaptive Biotechnologies: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees; Janssen Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Cellectis: Research Funding; Servier: Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; Bayer: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees, Research Funding.

Introduction: New immunotherapies directed against CD38, SLAMF7 or BCMA have significantly improved the outcome of multiple myeloma (MM) patients. However, most patients eventually relapse, underscoring the need for additional immunotherapeutic targets. We have previously shown that expression levels of GPRC5D, an orphan G protein-coupled receptor, are significantly higher on MM cells, compared to normal plasma cells or other immune cells. We also showed that the novel GPRC5DxCD3 bispecific antibody (BsAb) JNJ-7564, has promising anti-MM activity in patient-derived BM samples (Verkleij et al., EHA 2019). To elucidate which factors contribute to the observed heterogeneity in ex vivo response, we analyzed the impact of tumor and patient characteristics on efficacy of JNJ-7564. We further investigated whether tumor-intrinsic factors may be determinants of response by also testing in these assays JNJ-7957, a BCMA-targeting BsAb that differs from JNJ-7564 only in the tumor-antigen-binding domain. Methods: Bone marrow (BM) samples obtained from 13 newly diagnosed (ND), 17 daratumumab-naive relapsed/refractory (DARA-naive RR; median of 3 prior therapies) and 15 daratumumab-refractory (DARA-R, median of 6 prior therapies) MM patients were analyzed for tumor- and immune cell composition, and subsequently incubated with JNJ-7564 (0.00128-4.0 µg/mL) or JNJ-7957 (0.8 µg/mL). After 48 hours, MM cell lysis was assessed by flow cytometry. Luciferase-transduced MM cell lines were incubated with JNJ-7564 (0.032-4.0 µg/mL) in the presence of healthy peripheral blood mononuclear cells (PBMCs), purified CD4+CD25- T-cells or regulatory T-cells (Tregs). After 48 hours, MM cell lysis was assessed by bioluminescence assay. Results: We found no difference in JNJ-7564 efficacy with respect to disease stage (NDMM vs DARA-naive RRMM vs DARA-R MM, P=0.48). Importantly, the presence of high-risk cytogenetic abnormalities [del(17p), t(4;14) and t(14;16)] did not impair JNJ-7564 efficacy. The level of target expression was an important determinant of response, as evidenced by superior MM cell lysis in samples with higher than median GPRC5D expression, when compared to lower GPRC5D expression (Fig. 1A). Inferior MM cell lysis was observed in older patients (>67 years), in samples with low T-cell counts or low effector:target (E:T) ratios, and in those with a high frequency of PD-1+ T-cells, HLA-DR+ activated T-cells, or Tregs. These determinants of response also affected JNJ-7564-mediated T-cell activation and degranulation. To further analyze the impact of Tregs, we performed additional cell line experiments. Purified Tregs impaired T-cell proliferation, and were significantly less potent to kill MM cells when redirected by JNJ-7564, compared to CD4+CD25- T-cells (Fig. 1B). This was accompanied by reduced secretion of IFN-γ, TNF-α, IL-2 and granzyme B. To evaluate the impact of BM stromal cells (BMSCs) on JNJ-7564 activity, MM cell lines were co-incubated with PBMCs and patient-derived BMSCs. Direct cell-cell contact hampered MM cell lysis, while indirect contact (transwell) did not affect JNJ-7564 activity. Direct contact also decreased secretion of TNF-α and IL-2, and reduced GPRC5D expression on MM cells, contributing to BMSC-mediated resistance to JNJ-7564. Finally, we simultaneously evaluated the single agent activity of both JNJ-7564 and JNJ-7957 (0.8 µg/mL, dose whereby a plateau in MM cell lysis was observed with both BsAbs) in 40 BM samples. MM cell lysis induced by both agents was strongly correlated (Fig. 1C). In 6 samples, both agents exhibited poor activity (80% lysis). Comparison of characteristics between these groups showed that a low E:T ratio (Fig. 1D) and high frequency of Tregs (Fig. 1E) significantly impaired efficacy of both BsAbs, suggesting patient-specific factors can determine response to T-cell redirectors targeting different antigens. Conclusion: We show that tumor-related factors, such as GPRC5D expression, as well as differences in the composition of the BM microenvironment, including E:T ratio, frequency of PD-1+ or HLA-DR+ T-cells or immune-suppressing Tregs or BMSCs, contribute to the variability in response to JNJ-7564. Our data indicate that strategies aiming at optimizing E:T ratio (e.g. induction therapy) or Treg depletion, may improve response to T-cell redirecting antibodies in MM. Disclosures Wong: Jhonson & Jhonson: Current Employment. Zweegman:Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees; Sanofi: Membership on an entity's Board of Directors or advisory committees. Verona:Johnson & Johnson: Current Employment, Current equity holder in publicly-traded company. Adams:Johnson & Johnson: Ended employment in the past 24 months. Mutis:Janssen Pharmaceuticals: Research Funding; Genmab: Research Funding; Takeda: Research Funding; Onkimmune: Research Funding; Gadeta: Research Funding. van de Donk:Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: 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; Ferrer: Membership on an entity's Board of Directors or advisory committees.

Introduction: Daratumumab (DARA) has potent single agent activity in relapsed/refractory multiple myeloma (RRMM). The effectivity of DARA is partially dependent on the expression of its target, CD38, on tumor cells. Upregulation of CD38 on MM cells by all-trans retinoic acid (ATRA) improved the ex vivo efficacy of DARA, even in DARA-resistant MM cells. We therefore evaluated the efficacy and safety of DARA combined with ATRA in RRMM patients in the DARA/ATRA study (NCT02751255). Methods: In part A of this prospective, multicenter phase 1/2 trial, DARA was administered according to the approved schedule (16 mg/kg; cycles 1-2: weekly; cycles 3-6: biweekly; cycles ≥7: monthly). In part B, ATRA was given twice daily during 3 days prior to each DARA infusion. DARA dosing was resumed according to the aforementioned schedule. Inclusion criteria were ≥2 prior lines of treatment, WHO performance score 0-2, adequate bone marrow reserve, kidney and hepatic function. Patients were enrolled in part B, after treatment in part A, if they developed disease progression (PD), or if they had insufficient response defined as < minimal response (MR) after cycle 2, or < partial response (PR) after cycle 3, and if they still fulfilled the other inclusion criteria. All patients gave written informed consent. In phase 1 of part B, we determined the maximum tolerated dose (MTD) of ATRA (dose levels: 15, 30 and 45mg/m2) and the recommended dose level (RDL) for phase 2. We here report overall response rate (ORR), progression free survival (PFS(A)), overall survival (OS) and adverse events (AEs) for all patients in part A. For patients treated at the RDL in part B, we report ORR, PFS(A), PFS for part A + B combined (PFS(A+B)) and AEs. Results: Between July 2016 and October 2017, 63 patients were enrolled in part A, baseline characteristics are described in Table 1. At data cut-off (July 1st, 2019), 6 patients were still on treatment in part A. The median duration of follow-up of surviving patients at data cut-off was 26 months (range 18.2 - 39.4 months). The ORR in part A was 41% (22% PR, 14% very good PR [VGPR] and 5% (stringent) complete remission; Figure 1). The median PFS(A) and OS from start of DARA for all enrolled patients were 7.0 months (95% CI 5.3-8.8) and 28.2 months (95% CI 20.4-36.0), respectively. Eight patients did not meet eligibility criteria for part B, and 1 patient refused further treatment in part B. In part B phase 1, 14 patients were treated with ATRA at the 3 dose levels. The MTD was not reached and the RDL for phase 2 was defined as 45mg/m2. At data cut-off, 34 patients were enrolled in part B phase 2, of whom 2 patients were still on treatment. Thirty-two patients stopped treatment due to PD (n=28) or withdrawal of consent (n=4). Next, we focused on all 42 patients who were treated at the RDL in part B (8 patients in phase 1, and 34 patients in phase 2). The ORR in part B was 5% (VGPR n=2). However, 57% achieved MR or stable disease (SD; Figure 1). During part A, median PFS(A) in these patients was 6.7 months (95% CI 2.7-10.6). When stratified by ≥PR vs SD/MR vs PD/not evaluable (NE), median PFS(A) was 9.9 months (95% CI 3.1-16.7) vs 3.2 months (95% CI 2.5-3.9) vs 1.2 months (95% CI 0.8-1.5), respectively. Addition of ATRA and re-intensification of DARA in DARA-refractory patients resulted in a median PFS(A+B) of 7.9 months (95% CI 5.6-10.1 months). When stratified by ≥PR vs SD/MR vs PD/NE in part A, median PFS(A+B) was 17.7 months (95% CI 9.6-25.7) vs 5.4 months (95% CI 3.8-7.0) vs 2.5 months (95% CI 2.1-2.9), respectively. The presence of extramedullary plasmacytomas (P=0.004 and P=0.012) and WHO ≥ 1 (P=0.002 and P=0.005) were associated with lower PFS(A) and PFS(A+B). Importantly, combination of ATRA with DARA did not increase the rate of AEs (Table 2). The incidence of any grade infusion related reactions (IRR) was 34.9%. There were no complications with red blood cell transfusions. Conclusion: Here, we report for the first time on the efficacy of DARA re-intensification, combined with ATRA in DARA-refractory patients. Overall, this approach resulted in a modest prolongation of PFS. Patients with primary DARA-refractory disease did not benefit from the addition of ATRA. However, in patients with ≥PR on DARA monotherapy prior to progression, addition of ATRA and re-intensification of DARA was of marked clinical benefit with an additional 7.8 months of disease control. The combination of DARA and ATRA was well tolerated and safe. Disclosures Minnema: Celgene Corporation: Honoraria, Research Funding; Amgen: Honoraria; Gilead: Honoraria; Servier: Honoraria; Jansen Cilag: Honoraria. Levin:Takeda: Membership on an entity's Board of Directors or advisory committees, Other: Educational grant ; Amgen: Membership on an entity's Board of Directors or advisory committees, Other: Educational grant ; Janssen: Membership on an entity's Board of Directors or advisory committees, Other: Educational Grant; Roche: Membership on an entity's Board of Directors or advisory committees, Other: Educational Grant; Abbvie: Membership on an entity's Board of Directors or advisory committees, Other: Educational Grant. Broyl:Celgene, amgen, Janssen,Takeda: Honoraria. Bos:Celgene: Research Funding; Kiadis Pharma: Other: Shareholder (of Cytosen, acquired by Kiadis). Kersten:Miltenyi: Honoraria; Mundipharma: Honoraria, Research Funding; Kite Pharma: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding; Gilead: Honoraria; Amgen: Honoraria, Research Funding; Takeda Oncology: Research Funding; Celgene: Honoraria, Research Funding; Roche: Honoraria, Research Funding; Novartis: Honoraria. Mutis:Janssen Research and Development: Research Funding; Celgene: Research Funding; Onkimmune: Research Funding; Genmab: Research Funding. Zweegman:Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding. Sonneveld:SkylineDx: Research Funding; Takeda: Honoraria, Research Funding; Karyopharm: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; BMS: Honoraria; Amgen: Honoraria, Research Funding. Van De Donk:Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees, Research Funding; Roche: Membership on an entity's Board of Directors or advisory committees; Janssen Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Servier: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Bayer: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; AMGEN: Membership on an entity's Board of Directors or advisory committees, Research Funding. OffLabel Disclosure: ATRA for treatment of MM

Autoantibody-producing plasma cells are frequently resistant to conventional immunosuppressive treatments and B-cell depletion therapy. As a result of this resistance, autoreactive plasma cells survive conventional therapy, resulting in persistent autoantibody production and inflammation. CD38 is highly and uniformly expressed on normal and malignant plasma cells. Daratumumab is the first in class CD38-targeting monoclonal antibody approved for the treatment of multiple myeloma (MM). To evaluate the potential activity of daratumumab in antibody-mediated autoimmune disorders by targeting autoantibody-producing plasma cells, we evaluated serum levels of autoantibodies in MM patients during daratumumab treatment. We found that 6 out of 41 (15%) had detectable autoantibodies before initiation of daratumumab therapy, and that these autoantibodies rapidly disappeared in 5 out of 6 patients during daratumumab treatment. Our data provide support for the evaluation of daratumumab in patients with autoantibody-dependent autoimmune disorders., Highlights • Autoantibody-producing plasma cells express CD38. • The CD38-targeting antibody daratumumab reduces autoantibody levels. • Our data provide support for the evaluation of daratumumab in antibody-mediated autoimmune disease.