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Introduction: Daratumumab (DARA) is a human, CD38-targeting, IgG1κ monoclonal antibody approved as monotherapy in relapsed/refractory multiple myeloma (RRMM) and in combination with standard of care for RRMM and NDMM. The addition of DARA to standard-of-care regimens in phase 3 studies has consistently improved progression-free survival (PFS) and led to deep and durable responses, including higher rates of minimal residual disease (MRD) negativity compared with standard of care. In the primary analysis of the phase 3 MAIA study (median follow-up, 28.0 mo), D-Rd vs Rd significantly improved PFS and MRD-negativity rates in transplant-ineligible NDMM (Facon T, N Engl J Med 2019). With longer follow-up (36.4 mo), D-Rd maintained a PFS benefit and deeper and more durable responses vs Rd alone (Bahlis N, Blood 2019. 134[Suppl 1]:1875). Here, we report updated efficacy and safety findings from MAIA after approximately 4 years of follow-up. Methods: Patients with NDMM ineligible for high-dose chemotherapy and autologous stem cell transplantation due to age ≥65 years or comorbidities were randomly assigned (1:1) to receive Rd ± DARA. Stratification factors included International Staging System stage (ISS [I vs II vs III]), region (North America vs other), and age ( Results: A total of 737 patients were randomized (D-Rd, n = 368; Rd, n = 369). Patient baseline characteristics were well balanced between the two treatment arms. Median (range) age was 73 (45-90) years, with 44% of patients ≥75 years of age. 27%, 43%, and 29% of all patients were ISS stage I, II, and III, respectively. Among 642 patients evaluable for FISH/karyotyping analysis, 86% had standard and 14% had high cytogenetic risk. After a median follow-up of 47.9 months, 176 (48%) and 273 (75%) patients discontinued study treatment in the D-Rd vs Rd groups, respectively, with 85 (23%) and 113 (31%) patients discontinuing treatment due to progressive disease. PFS remained improved for D-Rd vs Rd (median, not reached [NR] vs 34 mo; HR, 0.54; 95% CI, 0.43-0.67; P Grade 3/4 treatment-emergent adverse events (TEAEs; D-Rd/Rd) occurring in ≥10% of patients were neutropenia (53%/37%), pneumonia (18%/11%), anemia (16%/21%), lymphopenia (16%/11%), hypokalemia (12%/10%), leukopenia (11%/6%), and cataract (11%/10%); grade 3/4 infection rates were 40%/29%. The most common serious TEAE was pneumonia (17%/11%). 11% of patients in the D-Rd arm and 22% in the Rd arm discontinued treatment due to an adverse event. The complete updated data set will be presented at the meeting with additional efficacy endpoints, including MRD-negativity rate. Conclusion : After 48 months follow up, the addition of DARA to Rd continues to demonstrate a superior PFS benefit. More patients continued to have deeper and more durable responses with D-Rd vs Rd alone. No new safety concerns were observed with longer follow-up. These results continue to support the use of D-Rd in the first line of treatment for patients with transplant-ineligible NDMM. Disclosures Kumar: Sanofi: Research Funding; Carsgen: Other, Research Funding; Cellectar: Other; Celgene/BMS: Other: Research funding for clinical trials to the institution, Consulting/Advisory Board participation with no personal payments; Genentech/Roche: Other: Research funding for clinical trials to the institution, Consulting/Advisory Board participation with no personal payments; Takeda: Other: Research funding for clinical trials to the institution, Consulting/Advisory Board participation with no personal payments; Janssen Oncology: Other: Research funding for clinical trials to the institution, Consulting/Advisory Board participation with no personal payments; Novartis: Research Funding; Oncopeptides: Consultancy, Other: Independent Review Committee; IRC member; Karyopharm: Consultancy; Merck: Consultancy, Research Funding; Kite Pharma: Consultancy, Research Funding; AbbVie: Other: Research funding for clinical trials to the institution, Consulting/Advisory Board participation with no personal payments; Tenebio: Other, Research Funding; BMS: Consultancy, Research Funding; MedImmune: Research Funding; Genecentrix: Consultancy; Dr. Reddy's Laboratories: Honoraria; Amgen: Consultancy, Other: Research funding for clinical trials to the institution, Consulting/Advisory Board participation with no personal payments, Research Funding; Adaptive Biotechnologies: Consultancy. Facon:Celgene, Janssen, Takeda, Amgen, Roche, Karyopharm, Oncopeptides, BMS, Sanofi: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Usmani:Abbvie: Consultancy; Janssen: Consultancy, Honoraria, Other: Speaking Fees, Research Funding; Takeda: Consultancy, Honoraria, Other: Speaking Fees, Research Funding; Sanofi: Consultancy, Honoraria, Research Funding; Celgene: Other; GSK: Consultancy, Research Funding; Merck: Consultancy, Research Funding; BMS, Celgene: Consultancy, Honoraria, Other: Speaking Fees, Research Funding; Amgen: Consultancy, Honoraria, Other: Speaking Fees, Research Funding; SkylineDX: Consultancy, Research Funding; Seattle Genetics: Consultancy, Research Funding; Pharmacyclics: Research Funding; Incyte: Research Funding; Array Biopharma: Research Funding. Plesner:Janssen: Consultancy. Orlowski:Founder of Asylia Therapeutics, Inc., with associated patents and an equity interest, though this technology does not bear on the current submission.: Current equity holder in private company, Patents & Royalties; STATinMED Research: Consultancy; Laboratory research funding from BioTheryX, and clinical research funding from CARsgen Therapeutics, Celgene, Exelixis, Janssen Biotech, Sanofi-Aventis, Takeda Pharmaceuticals North America, Inc.: Research Funding; Amgen, Inc., AstraZeneca, BMS, Celgene, EcoR1 Capital LLC, Forma Therapeutics, Genzyme, GSK Biologicals, Ionis Pharmaceuticals, Inc., Janssen Biotech, Juno Therapeutics, Kite Pharma, Legend Biotech USA, Molecular Partners, Regeneron Pharmaceuticals, Inc.,: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi-Aventis, Servier, Takeda Pharmaceuticals North America, Inc.: Honoraria, Membership on an entity's Board of Directors or advisory committees. Touzeau:Takeda: Consultancy, Honoraria, Other: Travel, Accommodations, Expenses; Abbvie: Consultancy, Honoraria, Other: Travel, Accommodations, Expenses, Research Funding; Amgen: Consultancy, Honoraria, Other: Travel, Accommodations, Expenses; GlaxoSmithKline: Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Other: Travel, Accommodations, Expenses; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi: Honoraria, Research Funding. Bahlis:BMS/Celgene and Janssen: Consultancy, Honoraria, Other: Travel, Accomodations, Research Funding; Genentech: Consultancy, Honoraria; GSK: Consultancy, Honoraria; Sanofi: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Karyopharm Therapeutics: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria. Goldschmidt:Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Grants and/or provision of Investigational Medicinal Product:, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Johns Hopkins University: Other: Grants and/or provision of Investigational Medicinal Product; Incyte: Research Funding; University Hospital Heidelberg, Internal Medicine V and National Center for Tumor Diseases (NCT), Heidelberg, Germany: Current Employment; GlaxoSmithKline (GSK): Honoraria; Adaptive Biotechnology: Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Grants and/or provision of Investigational Medicinal Product, Research Funding; Mundipharma GmbH: Research Funding; Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other, Research Funding; Novartis: Honoraria, Research Funding; Chugai: Honoraria, Other: Grants and/or provision of Investigational Medicinal Product:, Research Funding; Dietmar-Hopp-Foundation: Other: Grants and/or provision of Investigational Medicinal Product:; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Grants and/or provision of Investigational Medicinal Product:, Research Funding; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Grants and/or provision of Investigational Medicinal Product:, Research Funding; Molecular Partners: Research Funding; Merck Sharp and Dohme (MSD): Research Funding. O'Dwyer:Celgene: Other: TRAVEL, ACCOMMODATIONS, EXPENSES (paid by any for-profit health care company); BMS: Research Funding; Carrick Therapeutics: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologies: Consultancy, Other: TRAVEL, ACCOMMODATIONS, EXPENSES (paid by any for-profit health care company); Janssen: Consultancy, Other: TRAVEL, ACCOMMODATIONS, EXPENSES (paid by any for-profit health care company); AbbVie: Consultancy; ONK Therapeutics: Consultancy, Current Employment, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding. Venner:Celgene, Amgen: Research Funding; Janssen, BMS/Celgene, Sanofi, Takeda, Amgen: Honoraria. Weisel:Adaptive: Consultancy, Honoraria; Bristol-Myers Squibb: Consultancy, Honoraria; Sanofi: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Roche: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Karyopharm: Consultancy, Honoraria; GlaxoSmithKline: Honoraria; Takeda: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria. Hulin:Celgene/Bristol-Myers Squibb, Janssen, GlaxoSmithKline, and Takeda: Honoraria. Karlin:GlaxoSmithKline: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support, personal fees; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support, personal fees; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support, personal fees; Celgene: Other: Personal fees; Sanofi: Honoraria; AbbVie: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support; Celgene/Bristol-Myers Squibb: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support. Preis:Janssen: Other: for factor XI inhibitor. Broyl:Janssen, Celgene, Takeda, Amgen: Honoraria. Hansson:Amgen, Celgene, Takeda, Janssen Cilag: Consultancy. Krevvata:Janssen: Current Employment. Wang:Janssen: Current Employment. Van Rampelbergh:Janssen: Current Employment. Ukropec:Janssen: Current Employment, Current equity holder in publicly-traded company. Uhlar:Janssen: Current Employment, Current equity holder in publicly-traded company. Kobos:Janssen: Current Employment, Current equity holder in publicly-traded company. Perrot:Amgen, BMS/Celgene, Janssen, Sanofi, Takeda: Consultancy, Honoraria, Research Funding.

Introduction The introduction of new treatment regimens has significantly increased the progression free survival (PFS) of newly diagnosed multiple myeloma (MM) patients. However, even with these novel treatments, for some the disease remains refractory, highlighting the need to identify the pathobiology of high-risk MM. In MM patients, high levels of circulating tumor cells (CTCs) is associated with an inferior prognosis independent of high-risk cytogenetics (Chakraborty et al., 2016), suggesting that CTC numbers are a relevant reflection of tumor cell biology. We hypothesized that high levels of CTCs in MM patients are either the result of a transcriptionally distinct tumor clone with enhanced migration capacities, or driven by transcriptional differences present in the bone marrow (BM) tumor cells. To test these hypotheses, we 1) compared MM cells from paired blood and BM samples, and 2) compared BM tumor cells of patients with high and low CTC levels, using single cell RNA-sequencing. Results We isolated plasma cell (PCs) from viably frozen mononuclear cells of paired peripheral blood (PB) and BM aspirates from five newly diagnosed MM patients (0.5%-8% CTCs) to determine the presence of a distinct CTC subclone. We generated single cell transcriptomes from 44,779 CTCs and 35,697 BM PCs. In the total 9 clusters common to BM PCs and CTCs were identified upon single cell data integration, but no cluster specific for either source was detected. Only 25 genes were significantly differential expressed between CTCs and BM PCs. The absence of transcriptional clusters unique to either CTCs or BM PCs, and the transcriptional similarity between these two anatomical sites makes it highly unlikely that CTC levels are driven by the presence of a transcriptionally-primed migratory clone. We next set out to identify possible transcriptional differences in BM PCs from eight patients with high (2-22%) versus thirteen patients with low (0.004%-0.08%) percentages of CTCs. Recurrent high-risk mutations were present in both groups. Single cell transcriptomes were generated from 74,830 BM PCs. Single cell data integration across all patients led to the identification of 8 distinct PC clusters, one of which was characterized by enhanced proliferation as defined by STMN1 and MKI67 transcription. Interestingly, this proliferative cluster was increased in patients with a high percentage of CTCs. Furthermore, cell cycle analyses based on canonical G2M and S phase markers revealed that actively cycling PCs were more frequent in the BM of patients with a high percentage of CTCs (64% versus 30%, p In order to substantiate this, we isolated BM immune cells from the same 21 patients and generated a library of 301,045 single immune cell transcriptomes. This library contained all major immune cell subsets, including CD4 + and CD8 + T cells, NK cells, B cells and monocytes. Comparative analyses of these cell populations in patients with either high or low levels of CTC are ongoing. Conclusion Through single cell transcriptomic analyses, we demonstrate that CTCs and BM PCs are transcriptionally similar. Importantly, we identify increased BM PC proliferation as a significant difference between patients with high and low levels of CTCs, implicating an increased tumor proliferation as one of the potential mechanisms driving CTC levels and MM disease pathobiology. The relation of the BM immune micro-environment to this altered proliferative state is currently under investigation. Disclosures van der Velden: Janssen: Other: Service Level Agreement; BD Biosciences: Other: Service Level Agreement; Navigate: Other: Service Level Agreement; Agilent: Research Funding; EuroFlow: Other: Service Level Agreement, Patents & Royalties: for network, not personally. Sonneveld: SkylineDx: Honoraria, Research Funding; Karyopharm: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Celgene/BMS: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding. Broyl: Sanofi: Honoraria; Janssen Pharmaceuticals: Honoraria; Celgene: Honoraria; Bristol-Meyer Squibb: Honoraria; Amgen: Honoraria.

Introduction: The 2-part phase 3 CASSIOPEIA study (NCT02541383) investigated the combination of DARA with VTd (D-VTd) in transplant-eligible NDMM pts. D-VTd induction/consolidation (ind/cons) led to increased rates of MRD negativity and prolonged progression-free survival (PFS) compared with VTd (Moreau P, et al. Lancet. 2019;394(10192):29-38). In Part 2, DARA as post-autologous stem cell therapy (ASCT) maintenance significantly improved PFS in pts who received VTd ind/cons. Most common (≥2.5%) grade 3/4 adverse events included pneumonia (DARA: 2.5%; observation [OBS]: 1.4%), lymphopenia (3.6%; 1.8%), and hypertension (3.0%; 1.6%; Moreau P, et al. J Clin Oncol. 2021;39(no. 15_suppl):8004). Here, we present results from a detailed analysis of MRD negativity. Methods: Eligible pts were 18-65 years of age, had NDMM and were ASCT eligible. Pts were randomized 1:1 to 4 (28-day) cycles of pre-ASCT induction and 2 (28-day) cycles of post-ASCT consolidation with D-VTd or VTd (bortezomib, 1.3 mg/m 2 SC on Days 1, 4, 8, 11; thalidomide, 100 mg PO daily; dexamethasone, 20-40 mg IV/PO; ± DARA, 16 mg/kg IV weekly (QW) in Cycles 1-2, Q2W in Cycles 3-6). Pts who completed consolidation and achieved partial response or better were re-randomized 1:1 to maintenance DARA at reduced intensity (DARA, 16 mg/kg Q8W) for a maximum of 2 years, or OBS. Samples were collected for MRD analysis at predefined timepoints from all pts regardless of response to ind/cons and in pts with very good partial response or better during maintenance. The primary MRD assessment methodology was multiparametric flow cytometry during ind/cons and next-generation sequencing during maintenance, each at the 10 -5 threshold. MRD negativity is reported here for pts who achieved complete response or better. Results: 1,085 pts were randomized to ind/cons (D-VTd, n=543 or VTd, n=542) and 886 pts were re-randomized for post-ASCT maintenance (DARA, n=442 or OBS, n=444). The rate of MRD negativity was higher with D-VTd than with VTd following induction (9.2% vs 5.4%; odds ratio [OR], 1.79; P=0.0150) and consolidation (33.7% vs 19.9%; OR, 2.06; P During maintenance, the rate of MRD negativity significantly favored DARA over OBS (58.6% vs 47.1%; OR, 1.80; P=0.0001). In pts who received D-VTd ind/cons, the MRD-negativity rates with DARA and OBS were 64.2% and 57.6% respectively (OR, 1.43; P=0.1037). In contrast, pts who had received VTd ind/cons showed significantly higher MRD-negativity rates during DARA maintenance vs OBS (52.6% vs 35.8%; OR, 2.26; P=0.0002). The rates of sustained MRD negativity in the D-VTd group were not significantly different with DARA vs OBS (1yr sustained: 48.5% vs 41.0%; OR, 1.41; P=0.0885; 2yr sustained: 28.8% vs 21.8%; OR, 1.47; P=0.0789). In the VTd group, the 1-year sustained MRD-negativity rate was significantly higher with DARA vs OBS (35.7% vs 21.4%; OR, 2.22; P=0.0006) but no difference was observed in the 2-year sustained MRD rate (11.3% vs 13.0%; OR, 0.83; P=0.5481). Conclusion: In CASSIOPEIA, the highest and most durable rates of MRD negativity were achieved after D-VTd ind/ASCT/cons and DARA maintenance. Reduced intensity (Q8W) DARA maintenance did not significantly improve MRD negativity compared to OBS in patients treated with D-VTd. In patients treated with VTd, DARA maintenance did improve MRD negativity, but this effect was not long lasting. Longer follow-up is required to assess the potential long-term benefits of sustained MRD negativity for DARA vs OBS after D-VTd. Figure 1 Figure 1. Disclosures Sonneveld: Karyopharm: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; SkylineDx: Honoraria, Research Funding; Celgene/BMS: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding. Moreau: Amgen: Honoraria; Janssen: Honoraria; Celgene BMS: Honoraria; Sanofi: Honoraria; Abbvie: Honoraria; Oncopeptides: Honoraria. van der Velden: Janssen: Other: Service Level Agreement; BD Biosciences: Other: Service Level Agreement; Navigate: Other: Service Level Agreement; Agilent: Research Funding; EuroFlow: Other: Service Level Agreement, Patents & Royalties: for network, not personally. Hulin: abbvie: Honoraria; Sanofi: Honoraria; Celgene/BMS: Honoraria; Janssen: Honoraria; Takeda: Honoraria. Arnulf: Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celene/BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees. Mohty: Sanofi: Honoraria, Research Funding; Pfizer: Honoraria; Novartis: Honoraria; Takeda: Honoraria; Jazz: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Gilead: Honoraria; Celgene: Honoraria, Research Funding; Bristol Myers Squibb: Honoraria; Astellas: Honoraria; Amgen: Honoraria; Adaptive Biotechnologies: Honoraria. Karlin: Janssen: Honoraria, Other: member of advisory board, travel support; Abbvie: Honoraria; Amgen: Honoraria, Other: travel support and advisory board ; Sanofi: Honoraria; Takeda: Honoraria, Other: member of advisory board; Celgene-BMS: Honoraria, Other: member of advisory board; GSK: Honoraria, Other: member of advisory board; oncopeptide: Honoraria. Macro: Sanofi: Honoraria; GSK: Honoraria; Takeda: Honoraria, Other: Travel accomodation, Research Funding; Janssen: Honoraria, Other: Travel accomodation, Research Funding; Celgen/BMS: Honoraria. Perrot: GSK: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Abbvie: Honoraria; Sanofi: Honoraria, Research Funding; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Levin: Roche, Janssen, Abbvie: Other: Travel Expenses, Ad-Board. Delforge: Amgen, Celgene, Janssen, Sanofi: Honoraria, Research Funding. Zweegman: Oncopeptides: Membership on an entity's Board of Directors or advisory committees; Sanofi: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; 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. Van De Donk: BMS/Celgene: Consultancy, Honoraria; Janssen: Consultancy, Research Funding; Amgen: Consultancy, Research Funding; Cellectis: Research Funding; Takeda: Consultancy; Roche: Consultancy; Novartis /bayer/servier: Consultancy. Krevvata: Janssen: Current Employment. Rigat: Janssen: Current Employment, Current equity holder in publicly-traded company. Yang: Janssen: Current Employment. Vanquickelberghe: Janssen: Current Employment. de Boer: Janssen: Current Employment. Kampfenkel: Janssen: Current Employment. Vermeulen: Janssen: Current Employment, Current equity holder in publicly-traded company. Broyl: Celgene: Honoraria; Janssen Pharmaceuticals: Honoraria; Sanofi: Honoraria; Bristol-Meyer Squibb: Honoraria; Amgen: Honoraria. OffLabel Disclosure: The specific regimen combination is not yet approved in the maintenance setting.

Introduction and background Treatment of newly diagnosed Multiple Myeloma (MM) with Bortezomib, Imid and Dexamethasone followed by high-dose therapy and Lenalidomide maintenance has become standard with an estimated progression-free survival (PFS) of 5 years. In the EMN02 collaborative trial VCD induction (Bortezomib, Cyclophosphamide, Dexamethasone) followed by HDM/ASCT or VMP (Bortezomib, Melphalan, Prednisone), followed by VRD consolidation and Lenalidomide maintenance until progression resulted in a median PFS of 57.5 months from start of maintenance at a median follow-up of 73 months (Cavo et al, Lancet Haematol 2020; Sonneveld et al, J Clin Oncol 2021). Patients with progressive disease during Lenalidomide maintenance are defined as double refractory and have limited options for treatment. The present Phase 2 trial was designed to evaluate a salvage treatment of next generation proteasome inhibition and IMId, i.e., Carfilzomib, continuous Pomalidomide and Dexamethasone (KPd) for patients who developed a first progression after treatment in EMN02 The primary endpoint was progression-free survival (PFS). This trial is registered as NTR5349 and EudraCT 2013-003265-34. Methods Patients were eligible if they had PD according to IMWG criteria during treatment in EMN02. Treatment consisted of 8 cycles of KPd, i.e. Carfilzomib (20/36mg/m 2, days 1,2,8,9,15,16) with Pomalidomide (4 mg days 1-21) and Dexamethasone (20mg days 1,2,8,9,15,16). In patients who had not previously received HDM/ASCT, HDM(200 mg/m 2) was administered followed by autologous transplantation with stem cells previously harvested.. Patients who achieved stable disease or better were randomized to receive Pomalidomide 4mg (21/28 days) with or without Dexamethasone 40mg (days 1, 8, 15, 22) in 28 days cycles until progression. Results At the time of the final analysis 112 patients were registered of whom 1 was ineligible. 59% had received prior HDM/ASCT and 41% VMP. Prior best responses in the EMN02 trial were 39% ³CR , 81% ≥VGPR, 96% ≥PR. The median duration of maintenance in EMN02 had been 33 months and median PFS from start of maintenance 59 months. At inclusion adverse risk factors were available in 92/111 patients, ie R-ISS II or III 62%, del17p 14%, t(14;16) 1%, t(4;14) 20%, amp1q 40%. One hundred (90%) of patients had progressed during or within 6 months after discontinuation of Lenalidomide maintenance. 86 (77%) patients completed 8 cycles of KPd, of whom 69(62%) without dose reduction and received continuous Pomalidomide with Dexamethasone (38%) or without (40%). The median time to discontinuation of Pomalidomide w/o Dexamethasone was 17 months (18 vs 15 months, n.s.). In addition, thirty-three of 42 (79%) eligible patients received their first HDM plus ASCT. Time to first response was 2 months. Best response on protocol was 37% ≥CR, 75% ≥VGPR, 92% ≥PR, respectively. At a median follow-up of 40 months (range 9-66 months) median PFS from registration was 26 months. PFS from randomization was 27 months (Pom/Dex) and 18 months (Pom) respectively (HR 0.68, 95%CI 0.41-1.13, p=0.14). 70 (63%) of patients are alive and in follow-up. With Cox regression analysis predefined risk factors including high-risk cytogenetics (HR 1.36, 95%CI 0.80-2.41), prior HDM/ASCT (HR 1.25, 95%CI 0.78-2.01) and duration of prior maintenance >36 months (HR 3.56, 95%CI 1.42-8.96) were not significant. Median overall survival (OS) was 67 months. KPd-emerging grade 3 and 4 adverse events included hematologic (41%), cardiovascular (6%), respiratory (3%), infections (17%) and neuropathy (2%). There were 6 fatal SAEs not related to progression (1 patient cardiovascular). Discussion This Phase 2 trial demonstrates that KPd followed by Pomalidomide until progression is an effective and safe triple drug regimen in second-line for patients who are refractory to Lenalidomide. The benefit is observed across subgroups of risk factors. A 92% overall response and 26 months PFS is clinically relevant in this population and compares favourably to Pom/Vd. Using this regimen HDM/ASCT could be performed in the majority of patients. Acknowledgements This trial was conducted as an investigator sponsored trial by HOVON and the European Myeloma Network EMN and supported by a grant from the Dutch Cancer Foundation and by independent grants and drug supply from Amgen and BMS/Celgene. Disclosures Sonneveld: Karyopharm: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; SkylineDx: Honoraria, Research Funding; Celgene/BMS: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding. Zweegman: Sanofi: Membership on an entity's Board of Directors or advisory committees; Takeda: 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; Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding. Cavo: Novartis: Honoraria; Adaptive Biotechnologies: Consultancy, Honoraria; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; GlaxoSmithKline: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: TRAVEL, ACCOMMODATIONS, EXPENSES, Speakers Bureau; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel Accommodations, Speakers Bureau; Bristol-Myers Squib: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Broyl: Sanofi: Honoraria; Janssen Pharmaceuticals: Honoraria; Celgene: Honoraria; Bristol-Meyer Squibb: Honoraria; Amgen: Honoraria. Corradini: Amgen; Takeda; AbbVie: Consultancy, Honoraria, Other: Travel and accommodations; KiowaKirin; Incyte; Daiichi Sankyo; Janssen; F. Hoffman-La Roche; Kite; Servier: Consultancy; AbbVie, ADC Theraputics, Amgen, Celgene, Daiichi Sankyo, Gilead/Kite, GSK, Incyte, Janssen, KyowaKirin, Nerviano Medical Science, Novartis, Roche, Sanofi, Takeda: Honoraria; AbbVie, ADC Theraputics, Amgen, Celgene, Daiichi Sankyo, Gilead/Kite, GSK, Incyte, Janssen, KyowaKirin, Nerviano Medical Science, Novartis, Roche, Sanofi, Takeda: Consultancy; Novartis; Gilead; Celgene: Consultancy, Other: Travel and accommodations; BMS: Other: Travel and accommodation; Sanofi: Consultancy, Honoraria; Incyte: Consultancy; Novartis, Janssen, Celgene, BMS, Takeda, Gilead/Kite, Amgen, AbbVie: Other: travel and accomodations. Minnema: BMS: Consultancy; Janssen: Consultancy; Cilag: Consultancy; Celgene: Other: Travel expenses; Alnylam: Consultancy; Kite/Gilead: Consultancy. Boccadoro: Janssen and GSK: Membership on an entity's Board of Directors or advisory committees; Sanofi, Celgene, Amgen, Janssen, Novartis, Bristol-Myers Squibb, and AbbVie: Honoraria; Sanofi, Celgene, Amgen, Janssen, Novartis, Bristol-Myers Squibb, and Mundipharma: Research Funding.

Introduction: The introduction of new treatments over the past 2 decades has improved survival outcomes in patients with multiple myeloma (MM), yet MM remains incurable. Despite therapeutic advances, most patients with MM eventually relapse and/or become refractory to treatment, and cycle through standard drug classes of proteasome inhibitors (PIs), immunomodulatory drugs (IMiDs), and anti-CD38 monoclonal antibodies (mAbs), becoming triple-class exposed. These triple-class exposed patients represent a population with an urgent unmet clinical need and are a major focus for the development of new therapies. There are currently no prospective data outside of clinical trials on standard-of-care (SOC) in MM patients who progress after these treatment regimens. LocoMMotion (NCT04035226) is the first prospective study to assess the efficacy and safety of real-life SOC treatments in triple-class exposed patients with relapsed/refractory MM (RRMM). Initial analysis at a median follow-up of 3.7 months (range: 0.0-12.7) showed an overall response rate (ORR) of 20.1% (95% CI: 15.0-26.0), with only 5% of patients achieving very good partial response or better. Here, we report updated results from LocoMMotion with a longer median duration of follow-up (7.8 months), along with progression-free survival (PFS) and overall survival (OS) data. Methods: LocoMMotion is a non-interventional study of 246 patients across 10 countries (the US and 9 in Europe) and 75 sites. Patients (aged ≥18 years) with a documented diagnosis of MM were included if they received ≥3 prior lines of therapy or were double-refractory to a PI and IMiD; had measurable disease at screening; received at least a PI, an IMiD, and anti-CD38 mAb with documented progressive disease since their last line of therapy; and had an Eastern Cooperative Oncology Group performance status (ECOG PS) score of 0 or 1. Patient-level data and the most common SOC treatments were collected and analyzed during the study period (August 2019 through October 2020). Real-life SOC treatments were defined as those used in local clinical practice. Responses were assessed per International Myeloma Working Group response criteria. A review committee assessed the ORR (primary objective) of real-life current SOC as well as disease progression for the determination of PFS. Secondary objectives included evaluation of additional efficacy endpoints, including OS, and safety. Results: As of the data cut-off date of March 9, 2021, 246 patients were included with a median follow-up of 7.8 months (range: 0.1-16.9); 23 (9.3%) patients were from the US and 223 (90.7%) were from Europe. Median age was 68 years (range: 41-89), 133 (54.1%) were male, 180 (73.8%) had a baseline ECOG PS score of 1, and median time since initial MM diagnosis was 6.3 years (range: Conclusions: Updated results of this first, prospective study of real-life SOC treatment in triple-class exposed patients with RRMM confirm rapid disease progression after application of salvage therapy and poor outcomes with currently available treatments. New treatment approaches with novel therapies are needed to improve outcomes in this patient population. Disclosures Moreau: Oncopeptides: Honoraria; Amgen: Honoraria; Abbvie: Honoraria; Janssen: Honoraria; Sanofi: Honoraria; Celgene BMS: Honoraria. Weisel: Roche: Honoraria; Oncopeptides: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Karyopharm: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologies: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; GSK: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Sanofi: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Abbvie: Consultancy; Novartis: Honoraria; Pfizer: Honoraria. Goldschmidt: Incyte: Research Funding; BMS: Consultancy, Honoraria, Other: Grants and/or Provision of Investigational Medicinal Product, Research Funding; Celgene: Consultancy, Honoraria, Other: Grants and/or Provision of Investigational Medicinal Product, Research Funding; Chugai: Honoraria, Other: Grants and/or Provision of Investigational Medicinal Product, Research Funding; GSK: Honoraria; Amgen: Consultancy, Honoraria, Other: Grants and/or Provision of Investigational Medicinal Product, Research Funding; Janssen: Consultancy, Honoraria, Other: Grants and/or Provision of Investigational Medicinal Product, Research Funding; Johns Hopkins University: Other: Grant; Molecular Partners: Research Funding; MSD: Research Funding; Mundipharma: Research Funding; Novartis: Honoraria, Research Funding; Dietmar-Hopp-Foundation: Other: Grant; Sanofi: Consultancy, Honoraria, Other: Grants and/or Provision of Investigational Medicinal Product, Research Funding; Takeda: Consultancy, Research Funding; Adaptive Biotechnology: Consultancy. Delforge: Amgen, Celgene, Janssen, Sanofi: Honoraria, Research Funding. Mohty: Novartis: Honoraria; Astellas: Honoraria; Gilead: Honoraria; Bristol Myers Squibb: Honoraria; Pfizer: Honoraria; Adaptive Biotechnologies: Honoraria; Sanofi: Honoraria, Research Funding; Jazz: Honoraria, Research Funding; Takeda: Honoraria; Celgene: Honoraria, Research Funding; Amgen: Honoraria; Janssen: Honoraria, Research Funding. Cavo: Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Honoraria; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel Accommodations, Speakers Bureau; Adaptive Biotechnologies: Consultancy, Honoraria; GlaxoSmithKline: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: TRAVEL, ACCOMMODATIONS, EXPENSES, Speakers Bureau; Sanofi: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Bristol-Myers Squib: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Vij: BMS: Research Funding; Takeda: Honoraria, Research Funding; Sanofi: Honoraria, Research Funding; BMS: Honoraria; GSK: Honoraria; Oncopeptides: Honoraria; Karyopharm: Honoraria; CareDx: Honoraria; Legend: Honoraria; Biegene: Honoraria; Adaptive: Honoraria; Harpoon: Honoraria. Dytfeld: Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen Cilag: Consultancy, Current Employment, Honoraria, Membership on an entity's Board of Directors or advisory committees. Angelucci: Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene BSM: Honoraria, Other: DMC; Blue Bird Bio: Honoraria, Membership on an entity's Board of Directors or advisory committees; Menarini-Stemline: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: steering commitee, Speakers Bureau; Vertex Pharmaceuticals: Honoraria, Other: DMC; Crispr therapeutics: Honoraria, Other: DMC; Glaxo: 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. Perrot: GSK: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Abbvie: Honoraria; Sanofi: Honoraria, Research Funding; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Benjamin: Servier: Honoraria, Research Funding; Allogene: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; BMS: Honoraria, Research Funding; Gileai: Honoraria, Research Funding. Van de Donk: BMS/Celgene: Consultancy, Honoraria; Janssen: Consultancy, Research Funding; Amgen: Consultancy, Research Funding; Cellectis: Research Funding; Takeda: Consultancy; Roche: Consultancy; Novartis /bayer/servier: Consultancy. Ocio: MSD: Honoraria; Amgen: Consultancy, Honoraria; Bristol-Myers Squibb/Celgene: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Speakers Bureau; Takeda: Consultancy, Honoraria, Speakers Bureau; Sanofi: Consultancy, Honoraria; Karyopharm: Consultancy; Oncopeptides: Consultancy, Honoraria; Pfizer: Consultancy; Secura-Bio: Consultancy. Scheid: Roche: Consultancy; Abbvie: Honoraria; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees. Gay: Oncopeptides: Membership on an entity's Board of Directors or advisory committees; Bluebird Bio: Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologies: Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; GlaxoSmithKline: Honoraria, Membership on an entity's Board of Directors or advisory committees; AbbVie: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Honoraria; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi: Consultancy, Honoraria. Rodriguez-Otero: Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Clínica Universidad de Navarra: Current Employment; Celgene-BMS: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; GSK: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy; Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees; Kite: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria; Regeneron: Honoraria. Broyl: Celgene: Honoraria; Janssen Pharmaceuticals: Honoraria; Sanofi: Honoraria; Bristol-Meyer Squibb: Honoraria; Amgen: Honoraria. Potamianou: Janssen: Current Employment, Current holder of stock options in a privately-held company. Sakabedoyan: Janssen: Current Employment. Semerjian: Janssen: Current Employment. Keim: Janssen Research & Development: Current Employment. Strulev: Janssen Pharmaceutica NV: Current Employment. Schecter: Janssen: Current Employment, Current holder of stock options in a privately-held company. Vogel: Janssen Global Services, LLC: Current Employment, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company, Divested equity in a private or publicly-traded company in the past 24 months. Wapenaar: Janssen: Current Employment; Johnson & Johnson: Current holder of individual stocks in a privately-held company. Nesheiwat: Legend Biotech USA: Current Employment. San-Miguel: AbbVie, Amgen, Bristol-Myers Squibb, Celgene, GlaxoSmithKline, Janssen, Karyopharm, Merck Sharpe & Dohme, Novartis, Regeneron, Roche, Sanofi, SecuraBio, and Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees. Sonneveld: SkylineDx: Honoraria, Research Funding; Karyopharm: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Celgene/BMS: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding. Einsele: Janssen, Celgene/BMS, Amgen, GSK, Sanofi: Consultancy, Honoraria, Research Funding. Mateos: Oncopeptides: Honoraria; GSK: Honoraria; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; Regeneron: Honoraria, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologies: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bluebird bio: Honoraria; Celgene - Bristol Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; AbbVie: Honoraria; Oncopeptides: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sea-Gen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi: 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; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Background. Bispecific antibodies that target tumors by engaging innate-like T cell subsets with inherent antitumor activity, such as Vγ9Vδ2-T and type 1 natural killer T (NKT) cells, may combine high therapeutic efficacy with limited off-tumor toxicity. Type 1 NKT cells respond to self and foreign (glyco)lipid antigens presented in the context of the MHC class I like molecule CD1d which is expressed on various malignancies. Vγ9Vδ2-T cells respond to intracellular accumulation of phosphoantigens in cancer cells by sensing conformational alterations in the butyrophilin (BTN) 2A1-3A1 complex. CD1d is expressed by the majority of patients with CLL and MM, while expression in AML is most pronounced on (myelo)monocytic subtypes. Methods. LAVA-051 is a 27kD humanized bispecific single domain antibody (bsVHH) that directly engages CD1d and the Vδ2-TCR chain of Vγ9Vδ2-T cells. The anti-CD1d VHH specifically stabilizes the interaction between CD1d and the type 1 NKT cell TCR and thereby triggers strong activation of type 1 NKT cells (Nature Cancer 2020;1:1054-1065). Vγ9Vδ2-T and type 1 NKT effector cell activation, proliferation, cytokine production and target cell lysis were assessed in in vitro, ex vivo, and in vivo studies. Due to lack of cross reactivity of LAVA-051 with non-human primate (NHP) CD1d and Vγ9Vδ2-T cells, a cross-reactive surrogate bispecific engager was generated to assess tolerability, pharmacokinetic (PK) and pharmacodynamic (PD) parameters. Results. The CD1d-Vδ2 bsVHH LAVA-051 triggers activation of both Vγ9Vδ2-T and type 1 NKT cells (EC 50 4 pM for Vγ9Vδ2-T and 366 pM for type 1 NKT; induction of > 80% degranulation in 4h assays) and mediates potent killing of CD1d expressing tumor cells by engagement of Vγ9Vδ2-T and/or type 1 NKT cells (EC 50 1 pM for Vγ9Vδ2-T and 216 pM for type 1 NKT; > 85% target cell lysis in 16h assays at a low 1:2 E:T ratio). Further, LAVA-051 triggered pro-inflammatory cytokine production, proliferation of Vγ9Vδ2-T and type 1 NKT cells, and exerted substantial antitumor activity against patient AML, CLL and MM cells that express CD1d and improved survival in in vivo T-ALL, AML and MM mouse models. Multiple dose studies in NHP (7 daily doses up to 1 mg/kg iv) showed clear Vγ9Vδ2-T cell engagement and some cytokine release after the first administration, but no clinical, laboratory, or histopathological toxicity. Reflecting the low molecular size of this bispecific engager, PK studies revealed a short plasma half-life which was however compensated for by prolonged (up to 5 days) binding of the engager to peripheral blood Vγ9Vδ2-T cells allowing intermittent dosing. Conclusions. In this study, we demonstrate that the CD1d-Vδ2 bsVHH LAVA-051 triggers activation of both type 1 NKT and Vγ9Vδ2-T cells, which translates directly into antitumor activity. Based on the expression of CD1d in CLL, MM, and AML, the strong preclinical activity of LAVA-051 against CD1d-expresssing tumors, and the favorable tolerability profile of the surrogate engager in NHP, LAVA-051 is currently evaluated in a first-in-human clinical Phase 1/2a study in patients with CD1d-expressing CLL, MM, or AML refractory to prior therapy (NCT04887259). Disclosures Lameris: Lava Therapeutics: Honoraria, Patents & Royalties, Research Funding. Ruben: Lava Therapeutics: Current Employment, Honoraria, Research Funding. Weerdt: LAVA Therapeutics: Research Funding. Roovers: LAVA Therapeutics: Current Employment, Current equity holder in publicly-traded company. van de Donk: 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; Adaptive Biotechnologies: 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; Takeda: 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; Novartis: 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; Servier: Membership on an entity's Board of Directors or advisory committees; Cellectis: Research Funding. Broyl: Amgen: Honoraria; Bristol-Meyer Squibb: Honoraria; Celgene: Honoraria; Janssen Pharmaceuticals: Honoraria; Sanofi: Honoraria. Kater: Abbvie: Honoraria, Other: Ad Board, Research Funding; Janssen, AstraZeneca: Other: Ad Board, steering committee, Research Funding; Genmab, LAVA: Other: Ad Board, Steering Committee; BMS, Roche/Genentech: Other: Ad Board, , Research Funding. Riedl: LAVA THerapeutics: Current Employment, Current equity holder in publicly-traded company; Genmab BV: Current equity holder in publicly-traded company. Iglesias: LAVA therapeutics: Current Employment. Winograd: LAVA therapeutics: Current Employment, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Celgene: Ended employment in the past 24 months; BMS: Current equity holder in publicly-traded company. Adang: LAVA therapeutics: Current Employment, Current equity holder in publicly-traded company, Honoraria, Membership on an entity's Board of Directors or advisory committees. de Gruijl: LAVA therapeutics: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties; DCPrime: Membership on an entity's Board of Directors or advisory committees; Macrophage Pharma: Membership on an entity's Board of Directors or advisory committees, Research Funding; Partner Tx: Membership on an entity's Board of Directors or advisory committees; Idera Pharmaceuticals: Research Funding; ORCA Therapeutics: Patents & Royalties. Parren: Lava Therapeutics: Current Employment, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties; Sparring Bioconsult BV: Membership on an entity's Board of Directors or advisory committees; Genmab: Patents & Royalties; Roche: Consultancy. Vliet: Lava Therapeutics: Current Employment, Current equity holder in publicly-traded company, Honoraria, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding; Glycostem: Research Funding.

Background: Multiple myeloma (MM) disease progression is influenced by signals from the bone marrow (BM) microenvironment. Recently, we showed that the MM BM is characterized by inflammatory mesenchymal stromal cells (iMSCs) that transcribe MM survival factors and are predicted to recruit proliferating myeloma cells via CCL2-CCR2 interactions (de Jong et al. Nat Immunol. 2021). iMSCs also transcribed high levels of chemokines that can bind to CXCR1 and 2. Myeloid cells are known to express CXCR1/2, and have been implicated in both pro- and anti-tumor responses in various malignancies. Therefore, we hypothesized that iMSCs attract and influence myeloid populations in the MM BM. Results: Using flow cytometry, we verified expression of CXCR1/2 on myeloid cell populations in the BM of 5 newly diagnosed MM (NDMM) patients. CD15 + neutrophils were the most dominant population expressing these receptors, as 22.4% (± 9.8%) of cells expressed CXCR2 alone, and 72.6% (± 8.0%) expressed both CXCR1 and CXCR2. CD14 + monocytes only expressed CXCR2 (86.9% ± 15.8%). Importantly, less than 1% of myeloma cells expressed these receptors (n = 17 NDMM). As these findings suggested neutrophils and monocytes as potential targets of iMSC-mediated chemotaxis, we set out to identify MM-associated alterations in this population by performing single cell RNA sequencing of the full neutrophilic and monocytic lineages (n = 5 NDMM and 2 controls). In line with our flow cytometric data, CXCR1 transcripts were absent in monocytes, while CXCR2 was transcribed by classical monocytes of both myeloma patients and controls. Interestingly, CXCR1 and CXCR2 transcription was increased in mature neutrophils of MM patients compared to controls. Additionally, both mature classical monocytes as well as mature neutrophils of MM patients had an activated transcriptome as defined by increased transcription of C3AR1, SLPI, and IL6R, the plasma cell supportive factor TNFSF13B (encoding BAFF), and the inflammatory cytokines IL1B and IL18. Transcription of IL1B and IL18 can be regulated by pattern-recognition receptors (PRRs) binding damage-associated molecular patterns (DAMPs) resulting from e.g. matrix breakdown. Transcription of PRRs as TLR1, 2 and 4 was increased in mature neutrophils and classical monocytes of MM patients compared to controls. Secretion of IL-1β and IL-18 relies on the cleavage of pro-forms of these cytokines by the inflammasome, a multiprotein complex that is assembled in response to alarmins. Transcription of inflammasome components PYCARD, NLRP3 and CASP1 was increased in mature neutrophils and classical monocytes of patients with MM. Additionally, protein levels of both IL-18 and IL-1β are increased in BM plasma from MM patients, implicating activated neutrophils and monocytes as a potential sources of these cytokines. Conclusion: In MM, mature neutrophils and classical monocytes are activated and might interact with iMSCs via CXCR1 and/or 2. Moreover, these myeloid cells are inflammasome-primed and are likely to be sources of the increased IL-1β levels in the MM BM. Therefore, myeloid cells and iMSCs may form a feed-forward loop in which myeloid cells contribute to a pro-MM environment by maintaining iMSC and by directly providing BAFF to tumor cells. Disclosures Broyl: Celgene/BMS: Consultancy, Honoraria; Sanofi: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Janssen: Consultancy, Honoraria. Sonneveld: Janssen: Consultancy, Honoraria, Research Funding; Karyopharm: Consultancy, Honoraria, Research Funding; SkylineDx: Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Celgene/BMS: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding.

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.