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Abstract Background: Treatment of multiple myeloma (MM) has advanced dramatically over the past decade with median overall survival (OS) for newly diagnosed MM patients (pts) now exceeding 10 years. However, outcomes in pts with high-risk disease remain poor. While most pts with high-risk disease are identified through baseline cytogenetic risk stratification, presence of extramedullary disease, or R-ISS stage, there is also a population of pts who may or may not have these traditional high-risk factors who relapse earlier than expected and have similarly poor outcomes. These pts are often delineated as “functional high-risk,” but a standardized definition of this population in the era of modern myeloma therapy is lacking. Methods: Using the ASH Research Collaborative (ASH RC) Data Hub, we retrospectively searched 7,249 pts across 10 participating institutions for those with newly diagnosed multiple myeloma (NDMM) treated with a triplet or quadruplet induction regimen (with or without autologous stem cell transplant) who had response data available. We evaluated duration of response from first line therapy (PD1) on post-PD1 clinical outcomes such as: time to next treatment (TTNT), time to subsequent progression and/or death (PFS2), and OS. We also analyzed characteristics of those who did vs. did not progress at prespecified time points of interest (12-, 18-, 24-, and 36-months) from the start of induction. We utilized recursive partitioning analyses to iteratively explore all possible cutpoints and identify which one(s) provide the greatest differentiation in the risk of subsequent progression and/or death. Results: We identified 1,305 pts who met the eligibility criteria. Of these, 264 had PD1 data including 201 who had response follow-up (f/u) after PD1. Median f/u after PD1 in this population was 3.2 years, and median PFS2 f/u after PD1 (n=201) was 4.4 years. Months to PD1 was significantly associated with PFS2 (p=0.038). All prespecified time points examined showed inferior PFS2 compared to those with PD1 >36 mos, although each subgroup was not statistically significant, likely due to limited numbers in each group: PD1 <12 mos HR=1.44 (n=43; 95% CI: 0.90 – 2.30; p=0.126), PD1 12-24 mos HR=1.65 (n=30; 95% CI: 0.95 – 2.85; p=0.073), and PD1 24-36 mos HR=1.58 (n=52; 95% CI: 0.98 – 2.53; p=0.059). Similarly, timing of PD1 was also associated with OS post-PD1; pts with PD1 <12 mos had significantly lower OS compared to those with PD1 >36 mos (HR=2.35, 95% CI: 1.27 – 4.32; p=0.006). Our recursive partitioning analysis identified 30-months at the cutoff that produced the greatest differential in outcomes between those above vs. below the landmark. Approximately 23% of the pts analyzed had PD1 within 30 mos, and these pts had significantly lower OS compared to those with PD1 ≥30 mos (HR=1.99, 95% CI: 1.21 – 3.27; p=0.006). Similarly, pts who progressed within 30 mos of starting 1L therapy (n=98) had significantly shorter PFS2 (median=12.5 mos vs 22.8 mos; HR=1.55, 95% CI: 1.09 – 2.20; p=0.014) than those who did not (n=103). Those pts who progressed within 30 mos were more likely to have a higher R-ISS stage at diagnosis (p=0.048), but were not more likely to have high-risk cytogenetics (p=0.66) compared to pts with PD1 ≥30 mos. Interestingly, receiving SCT as part of 1L therapy was not significantly associated with improved PFS2 (HR=0.89, 95% CI: 0.62 – 1.29; 0.54) but was significantly associated with improved OS (HR=0.53, 95% CI: 0.31 – 0.90; 0.018) by univariate analysis. Conclusion: Our study is the first to use a large, multicenter, cohort to evaluate the definition of functional high-risk MM in the era of novel induction therapies. Our data suggest that pts who progress within 30 mos of induction on triplet or quadruplet regimens (irrespective of whether they underwent stem cell transplant) have poorer outcomes than those who progress at 30 mos or later, and that progression prior to 30 mos represents functional high-risk disease. These pts represent a clear unmet medical need, and additional focus should be taken to identify them early with the goal of designing trials to optimize their subsequent therapies.

Introduction Induction therapy (Tx) followed by ASCT and lenalidomide (LEN) maintenance is the standard of care in transplant-eligible pts with NDMM. Pts who have suboptimal response after front-line ASCT, however, have an increased risk of progression or death. Ide-cel demonstrated significantly improved median progression-free survival (PFS) and overall response rate (ORR) vs standard regimens in KarMMa-3 in pts with early-line triple-class-exposed relapsed and refractory multiple myeloma (MM). KarMMa-2 (NCT03601078) is a multicohort, phase 2, multicenter trial evaluating the efficacy and safety of ide-cel in several MM disease settings; cohort 2c includes pts with NDMM who achieved < very good partial response (VGPR) after front-line ASCT. An extended follow-up analysis (median follow-up of 39.4 months) of cohort 2c was presented at ASH 2023 (Dhodapkar M, et al. ASH 2023; P2101). Here, we report results with 55.4 months of median follow-up and additional data on pts who received LEN maintenance at the investigator's discretion. Methods Adult pts with NDMM who received ≥ 3 cycles of induction Tx (proteasome inhibitor, immunomodulatory agent, dexamethasone), and had < VGPR 70-110 days after ASCT (single/tandem) were eligible. Pts received a single ide-cel infusion (150-450 × 106 chimeric antigen receptor-positive T cells) and could receive maintenance Tx post-infusion at investigator discretion. The primary endpoint was complete response rate (CRR); secondary endpoints included ORR, duration of response (DOR), PFS, overall survival (OS), safety, cellular kinetics, and patient-reported outcomes on health-related quality of life (HRQoL). Minimal residual disease (MRD) negativity (sensitivity, 10-5), an exploratory endpoint, is also reported. Results At data cutoff (June 28, 2024), with a median follow-up of 55.4 (range, 51.4-59.5) months, all 31 pts treated with ide-cel were alive; 28 (90.3%) remained in follow-up and 3 (9.7%) discontinued due to disease progression or withdrawal by pt. Eight pts received LEN maintenance after ide-cel at the investigator's discretion. Among all 31 ide-cel-treated pts, CRR and ORR were 80.6% and 87.1%, respectively. Among the 8 pts who received LEN maintenance, CRR and ORR were 75.0% and 100.0%, respectively. Median DOR, PFS, and OS were not reached; 36-month event-free rates were 81.5%, 76.8%, and 100.0%, respectively. Of the 8 pts who received LEN maintenance after ide-cel infusion, none experienced disease progression. At 36 months, MRD negativity was confirmed in 12/18 (66.7%) evaluable pts; all 12 achieved ≥ CR. Since last data cutoff, 1 pt with MRD-negative MM changed to MRD-positive at 36 months, and 1 pt who had previously achieved MRD negativity was indeterminate at 36 months. Among the 6 evaluable pts who received LEN maintenance, 3/6 (50.0%) achieved MRD negativity at 36 months. There were no new safety signals nor reports of parkinsonism or Guillain-Barré syndrome since the previous data cutoff. In the 8 pts who received LEN maintenance, 6 (75.0%) experienced grade 3/4 AE, most commonly neutropenia, and 1 (12.5%) experienced grade 3/4 infection on or after LEN maintenance. All 8 pts were still receiving LEN at data cutoff. Robust CAR+ T cell expansion was observed in 31 pts, with higher expansion in pts with ≥ CR vs < CR. Following ide-cel treatment, HRQoL improved and disease symptoms were stable over time (assessed by European Organization for Research and Treatment of Cancer [EORTC] Quality of Life Core Questionnaire and EORTC Quality of Life Multiple Myeloma Module 20 Questionnaire). More than 75% of pts had improved or stable global health status/QoL by month 3. Conclusions With extended follow-up, ide-cel continued to demonstrate deep, durable responses in pts with < VGPR to front-line ASCT. No new safety signals were observed and no deaths were reported. Data in a small group of pts in cohort 2c showed that ide-cel followed by LEN maintenance extended disease control. Pts who received LEN maintenance did not experience disease progression regardless of MRD status. Stable or improved HRQoL was observed after ide-cel treatment. Overall, ide-cel continued to show a favorable clinical benefit-risk profile in NDMM after ASCT. The phase 3 KarMMa-9 study (NCT06045806), currently recruiting, will compare ide-cel followed by LEN maintenance vs LEN maintenance alone in pts with NDMM after suboptimal response to ASCT.

The use of 4-drug induction regimens for treatment naïve newly diagnosed multiple myeloma (NDMM) is associated with improved depth of response and progression-free survival (PFS). However, head-to-head trials of 4-drug combinations are lacking, and instead, these regimens are typically compared to 3-drug backbones; limiting the ability to discern whether any additional benefit (or toxicity) is simply additive or represents a synergy (or interaction). We conducted a meta-analysis of phase 2 and phase 3 clinical trials that randomized treatment naïve NDMM patients to either a 4-drug or 3-drug induction regimen. We included 11 trials which represented 6509 unique patients. PFS for all trials in the meta-analysis was 54 months with a 4-drug induction and 8.9 months with a 3-drug induction (HR: 0.49; 95% CI: 0.45; 0.54), but there was no benefit to using a 4-drug induction that did not include an anti-CD38 antibody (PFS 4-drug 8.1 months, PFS 3-drug 8.0 months; HR 0.95; 95% CI 0.86; 1.06). Adverse events were more frequent with the quadruplet regimens but were predominately mild. High-grade (≥3) adverse events (AEs) that were more common with 4-drug regimens were infections (RR: 1.34; 95% CI 1.17; 1.54) and thrombocytopenia (RR: 1.39; 95% CI 1.12; 1.74). This study suggests that 4-drug induction regimens which include an anti-CD38 antibody improve efficacy although with additional toxicity in NDMM patients.

Background: Although recently available immune-based approaches in multiple myeloma produce high response rates, curative outcomes have not yet been achieved, which highlights the need for new agents that further activate the immune system without increasing toxicity. ORIC-533 is a highly potent and selective, orally bioavailable, small molecule inhibitor of CD73, an enzyme expressed by various cell types, including immune cells and which catalyzes the generation of adenosine from adenosine monophosphate (AMP) (Allard 2019). Adenosine itself is highly immunosuppressive, diminishing activity of a range of immune cell types, including T, NK, and dendritic cells (Young 2014). By blocking adenosine production from AMP via inhibition of CD73, ORIC-533 has demonstrated the ability to restore and enhance immune function. Multiple myeloma (MM) patient bone marrow (BM) contains multiple cell types that also express CD73, including immune cells, cancer-associated fibroblasts, mesenchymal stem cells, and endothelial cells (Allard 2019). Moreover, elevated adenosine levels in the MM BM niche correlate with disease progression (Horenstein 2016; Yang 2020). We have previously observed that CD73-mediated adenosine activity suppresses the cytolytic antitumor immune function in the MM BM milieu (Ray 2022). ORIC-533 treatment reduced adenosine production, overcame this immune suppression with evidence of restored DC-mediated priming and activated cytotoxic CD8 + T-cell function, and significantly enhanced immune cell cytolytic behavior against RRMM tumor cells in autologous patient ex vivo experiments (Junttila 2022), making ORIC-533 a potential immunotherapy for patients with MM. Methods: ORIC-533-01 (NCT05227144) is an ongoing i3+3 Phase 1b dose escalation study in patients with RRMM. The primary objectives are to determine single agent safety/tolerability and pharmacokinetics (PK) of ORIC-533 to select the RP2D. Pharmacodynamic assessments include measurement of CD73 functional activity in serum and BM, as well as enumeration of immune cell subsets and activation state in blood and BM. Patients are also evaluated for clinical efficacy using IMWG criteria. Results: As of 23 June 2023, 17 patients received ORIC-533 once daily across 4 dose levels (400 mg to 1600 mg). All patients were triple-class refractory, 88% were penta-refractory, and 59% also received prior anti-BCMA/CD3 bispecific therapy or anti-BCMA CAR-T therapy. Overall, ORIC-533 was very well tolerated, with 5 patients experiencing a total of 10 treatment-related adverse events (TRAEs). Fatigue was the only G3 AE and the only TRAE seen in more than 1 patient (1 event each of G3 and G2); all other events seen in 1 patient each, were G1 or G2 in severity, and of no specific system organ class. No dose limiting toxicities, no Grade ≥4 TRAEs, and no treatment-related serious adverse events have been observed. PK showed increased exposure with dose with a plasma half-life of ~24 hrs. Soluble CD73 enzymatic activity was nearly completely inhibited by C1D15 in serum of all patients across all dose levels, while the largest relative suppression in surface CD73 activity in BM cells was observed at the highest dose level. There was also evidence for increases in the abundance of circulating NK and CD8+ T cells after 1 treatment cycle, with preliminary evidence of enhanced CD8+ T-cell activation at the highest dose levels tested in both peripheral blood and BM. In addition, early evidence of single agent clinical activity was observed. Conclusions: ORIC-533 has demonstrated an acceptable safety profile and preliminary evidence of immune activation in a heavily pretreated RRMM patient population. Enrollment is ongoing and updated data will be presented.

Background: Daratumumab-based combination with proteasome inhibitors (PI) and immunomodulatory drugs (IMiDs) have revolutionized the treatment outcomes of relapsed/refractory multiple myeloma (RRMM). Several daratumumab-based triplet combinations have been approved for the management of RRMM, but outcomes of daratumumab-based triplets in patients with high-risk cytogenetics RRMM (HC-RRMM) are not well described in the literature. This study aimed to evaluate the clinical outcome of daratumumab-based regimens in patients with HC-RRMM who are daratumumab-naïve. Methods: A retrospective review of 110 patients with HC-RRMM who received daratumumab-containing regimens from 6/2016 through 2/2023 at the University of Kansas Medical Center, in collaboration with the U.S Myeloma Innovations Research Collaborative (USMIRC) was conducted. Daratumumab was combined with bortezomib/dexamethasone (DVd), carfilzomib/dexamethasone (DKd), and pomalidomide/dexamethasone (DPd) for treatment of RRMM. HC-RRMM was defined as patients with one or more cytogenetic abnormalities (on FISH panel) including -17/del(17p), t(4;14), t(14;16), t(14;20), or amplification of 1q. Responses to therapy including overall response rate (ORR), complete response or better (≥CR), and very good partial response (VGPR) were evaluated using the International Myeloma Working Group (IMWG) criteria. Descriptive statistics were used to summarize patient and disease characteristics, along with efficacy outcomes. Progression-free survival (PFS) and overall survival (OS) were calculated by Kaplan-Meier methods. Results: Of the 110 patients with HC-RRMM, the median age was 66 years (range 43-84), 83 (75%) were Caucasian, 60 (54%) had IgG isotype, 41 (37%) had revised international staging system (R-ISS) stage III disease, 35 (32%) had extramedullary disease (EMD), 57 (52%) patients had one high-risk cytogenetic (HC) abnormality, 49 (44%) had 2 HC abnormalities, while four (4%) had 3 HC abnormalities. The median number of previous lines of therapy was 2 (1-4). Eighty-eight patients (80%) were lenalidomide refractory, while 53 patients (48%) had double refractory disease to both a PI and IMiD treatment. A total of 19 (17%), 27 (25%), and 64 (58%) patients received DVd, DKd, and DPd, respectively. The overall response rate (ORR) for the entire patient population was 73%, 45% had VGPR or better, and 35% achieved ≥CR. Median follow-up was 28 months (12.07-53.57), the median OS was 28 months (95% CI 23.2 - 43.9) and PFS was 8.03 months (95% CI 5.8 - 12.2) for all treatment groups. The median PFS for 1 HC vs ³ 2 HC was 14.8 vs 4.57 months (HR 1.72, 95%CI 1.17-2.72 p = 0.006), respectively. Similarly, median OS for 1 HC vs ³ 2 HC was 43.93 vs 22.47 months (HR 1.87, 95%CI 1.15-3.05, p = 0.011), respectively. (Figure 1). On multivariate analysis, R-ISS stage III, -17/del(17p), and the presence of more than one HC abnormality were associated with poor OS. The most common cause for treatment discontinuation was progressive disease, accounting for 73% of patients in the whole cohort. As for each daratumumab-based triplet, discontinuation due progression occurred in 78%, 48%, and 83% in DVd, DKd, and DPd, respectively. Other causes of discontinuation included > grade 3 neuropathy, which occurred in 16% of patients receiving DVd, and cardiac events, which occurred in 11% of patients receiving DKd. Treatment-related mortality occurred in 2% of patients in the whole cohort. At the time of evaluation, 14 patients remain on treatment. Conclusion: Advanced stage R-ISS disease at diagnosis, presence of -17/del(17p), and 2 or more HC abnormalities were associated with poor prognosis reflected by worse PFS and OS in HC-RRMM treated with daratumumab-based therapies. A larger cohort or a prospective study in patients with HC-RRMM is needed to compare long-term outcomes between daratumumab-based therapies (DKd, DVd, and DPd).

Abstract Introduction: Monoclonal antibodies (MoAbs) have shown activity in relapsed and refractory multiple myeloma (MM). Elotuzumab is an anti-SLAMF7 MoAb that synergizes with immunomodulatory agents (IMiDs) and is approved for treatment of relapsed and refractory MM. Anti-CD38 MoAbs (daratumumab, FDA approved and isatuximab, in clinical trials) have also entered the therapeutic MM landscape but little is known about how to best sequence MoAbs and the existence of cross resistance between different classes of MoAbs. We analyzed outcomes of patients who become refractory to elotuzumab with emphasis on the activity of anti-CD38 MoAbs in the post-elotuzumab setting. Methods: We identified patients from 14 academic institutions within the US who were refractory to elotuzumab administered alone or in combination with other agents. Patients were considered refractory to elotuzumab if treated with at least 4 weeks of therapy and had evidence of progressive disease (PD) within 60 days of last dose. Time zero (T0) was defined as point when myeloma became refractory to elotuzumab-containing regimen. Data were collected with an electronic platform and submitted to peer-based quality check for completeness and internal consistency. We analyzed demographics, characteristics of their myeloma and prior therapy including number and type of therapy prior to elotuzumab, duration of elotuzumab therapy, and outcome after becoming refractory to elotuzumab. We performed a multi-variable analysis of these factors to determine significant factors associated with survival post T0. We subsequently identified a subset of patients who received an anti-CD38 MoAb alone or in combination as next or subsequent line of therapy after T0 and report their outcomes. Results: Eighty-nine patients were evaluated. Eighty-six (96.6%) had progression on elotuzumab in combination with IMiD (N=82) or another agent (N=4). Median age at T0 was 65 years (range 28-90), 51% of patients were males, 65% were White, 54% had IgG subtype, 27% had ISS stage III at diagnosis, and 27% had high-risk cytogenetics. Patients received a median of 4 prior lines of treatment (range 1-11) prior to elotuzumab-containing regimen, including 66% who underwent autologous transplant. Sixty-one patients (69%) were refractory to at least one IMiD and one proteasome inhibitor (PI) at T0, classified as double-refractory. Twenty-one (23.6%) were quad-refractory, i.e. refractory to 2 IMiDs and 2 PIs. Twenty-eight patients (31.5%) were refractory to prior anti-CD38 MoAb. The median time from diagnosis to T0 was 59.5 months and duration of elotuzumab therapy was 3 months (range 0.4 - 59.5). Median OS from T0 for the entire cohort was 15.1 months (95% C.I. 9.3-20.9). In multivariate analysis, the only factors affecting OS after T0 were high-risk cytogenetics (HR 3.31, 95% C.I. 1.74-6.27, P<0.001) and refractoriness to prior anti-CD38 MoAb (HR 4.22, 95% C.I. 2.22-8.01, P<0.001). Interestingly, median OS for patients not refractory to anti-CD38 MoAb was 33.1 months vs. 4.5 months for anti-CD38 MoAb-refractory patients (Figure 1). After T0, 45 patients (50.6%) received an anti-CD38 MoAb in the next (N=28, 62.5%) or subsequent regimens (N=17, 37.8%). Median time between T0 and initiation of anti-CD38 MoAb was 1.2 months (0-15.0). Anti-CD38 MoAbs employed were: isatuximab in 4 (8.9%) and daratumumab in 41 patients (91.1%). In most cases, the anti-CD38 MoAb was administered in combination with IMiD (N=30, 66.6%) or bortezomib (N=7, 15.6%). The overall response rate (ORR) for anti-CD38 MoAb regimens was 35.6%; responses to specific combinations are outlined in the Table. Median PFS for elotuzumab-refractory patients receiving therapy with anti-CD38 MoAbs was 4.6 months (95% C.I. 1.6-7.6), while median OS was 15.3 months (95% C.I. 8.2-22.4) (Figure 2). Conclusion: Patient who become refractory to elotuzumab have guarded prognosis, particularly dismal among patients also refractory to anti-CD38 MoAb, defining a population with great need for novel therapies. Failure of elotuzumab-based therapy does not preclude disease control with anti-CD38 MoAb, particularly in combination. Disclosures Paul: Bristol Myer Squibb: Other: Stock and pension plan (past employee). Amarendra:Amgen: Consultancy; Celgene: Consultancy. Liedtke:Caelum: Membership on an entity's Board of Directors or advisory committees; celgene: Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Genentech/Roche: Research Funding; BlueBirdBio: Research Funding; Prothena: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen/Onyx: Consultancy, Honoraria, Research Funding; Gilead: Membership on an entity's Board of Directors or advisory committees, Research Funding. Hari:Kite Pharma: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding; Sanofi: Honoraria, Research Funding; Janssen: Honoraria; Bristol-Myers Squibb: Consultancy, Research Funding; Amgen Inc.: Research Funding; Spectrum: Consultancy, Research Funding. Vij:Karyopharm: Honoraria; Jazz Pharmaceuticals: Honoraria; Janssen: Honoraria; Amgen: Honoraria; Takeda: Honoraria, Research Funding; Bristol Myer Squibb: Honoraria, Research Funding; Celgene: Honoraria, Research Funding. Usmani:Abbvie, Amgen, Celgene, Genmab, Merck, MundiPharma, Janssen, Seattle Genetics: Consultancy; Amgen, BMS, Celgene, Janssen, Merck, Pharmacyclics,Sanofi, Seattle Genetics, Takeda: Research Funding. Kumar:Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; KITE: 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; AbbVie: Membership on an entity's Board of Directors or advisory committees, Research Funding. Costa:Karyopharm: Research Funding; Abbvie: Research Funding; Amgen: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; BMS: Research Funding; Sanofi: Honoraria; Janssen: Research Funding.

Abstract Introduction Development of anti-CD38 monoclonal antibody therapies (MoABs), including daratumumab and isatuximab, have drastically changed the therapeutic landscape for management of relapsed and/or refractory multiple myeloma (RRMM). However, there is a paucity of information regarding response to treatment after progression on CD38 MoABs. This collaborative research study (MAMMOTH: Monoclonal Antibodies in Multiple Myeloma: Outcomes after Therapy Failure) investigates the therapeutic choices and outcomes of subsequent treatments after refractoriness to MoABs. Methods Patients from 14 US academic institutions with diagnosis of MM and refractory to daratumumab or isatuximab, administered alone or in combination, were evaluated. Patients were considered refractory to a CD38 MoAB if treated with at least 4 weeks of therapy and had evidence of progressive disease (PD) while on therapy or within 60 days after last dose. Time of progression was defined as time zero (T0). Data was collected by electronic platform and submitted to peer-based quality check for completeness and internal consistency. Results Two hundred and seventy-five patients with MM refractory to CD38 MoAB were evaluated; 249 (90.5%) of those patients who received at least one subsequent line of therapy were included in this analysis. The median age at T0 was 65 years (range 27-90) and 54% were male. At the time of diagnosis, 28% had ISS stage III disease and 29% had high-risk cytogenetics. Patients were heavily pre-treated at T0 with a median of 5 lines of therapy (range 2-17); most (74%) underwent a prior autologous stem cell transplant. The majority of patients were refractory to other anti-myeloma therapies including lenalidomide (78%), pomalidomide (65%), bortezomib (69%) and carfilzomib (45%). In total, 97.1% of patients were exposed to at least one immunomodulatory agent (IMiD) and one proteasome inhibitor (PI), 79.3% were ≥ triple-refractory (refractory to 1 IMiD, 1 PI and 1 CD38 MoAB), while 25.3% were penta-refractory (refractory to 2 IMiDs, 2 PIs and 1 CD38 MoAB). Responses to the next subsequent line of therapy after progression with CD38 MoAB are shown (Table). The overall response rate (ORR; ≥PR) of first regimen post-T0 was 31% with a median progression free survival (PFS) of 3.4 mo and median overall survival (OS) of 9.3 mo. Carfilzomib-based therapy resulted in an ORR of 32% with median PFS 4.2 mo and overall OS of 10.9 mo. The addition of an IMiD to daratumumab yielded an ORR of 37% with median PFS 4.5 mo and OS 12.6 mo. The addition of a PI to daratumumab yielded no responses. Elotuzumab-based therapy had an ORR of 21% with median PFS 2.6 mo and OS 8.3 mo. By multivariate analysis, alkylator-based therapies were found to have the highest ORR (OR 3.1, 95% CI 1.8-5.7, p<0.001), however, resulted in median PFS of only 3 mo and OS of 7.7 mo. Multivariate analysis evaluating for PFS and adjusting for refractoriness to prior therapies found lower risk of progression with carfilzomib-based therapy (HR 0.60, 95% CI 0.42-0.85, p=0.004, Figure 1) and with the addition of an IMiD to daratumumab (HR 0.64, 95% CI 0.43-0.94, p=0.02). The benefit of IMiD + daratumumab was essentially restricted to patients previously refractory to CD38 MoAB as monotherapy. These patients demonstrated improved PFS with subsequent daratumumab-combination therapy at time of progression compared with patients refractory to CD38 MoAB combination therapy (log-rank p=0.02, Figure 2). No particular regimen was associated with improved OS compared with the rest. Conclusions After progression on CD38 MoAB, treatment with carfilzomib-based regimens and the addition of an IMiD to daratumumab-monotherapy resulted in a higher ORR and PFS compared with other regimens. Alkylating agents resulted in the highest ORR but relatively short PFS and OS. The use of elotuzumab and the addition of a PI to daratumumab monotherapy resulted in relatively low response rates. Overall, progression after CD38 MoAB therapy is associated with a poor prognosis and innovative therapeutic strategies are needed. Disclosures Malek: Celgene: Consultancy, Speakers Bureau; Amgen: Consultancy, Speakers Bureau; Takeda: Consultancy, Speakers Bureau; Janssen: Consultancy, Speakers Bureau; Sanofi: Consultancy, Speakers Bureau. Paul:Bristol Myer Squibb: Other: Stock and pension plan (past employee). Neppalli:Celgene: Consultancy; Amgen: Consultancy. Liedtke:Amgen/Onyx: Consultancy, Honoraria, Research Funding; Caelum: Membership on an entity's Board of Directors or advisory committees; Genentech/Roche: Research Funding; Gilead: 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; Prothena: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; celgene: Research Funding; BlueBirdBio: Research Funding; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Hari:Takeda: Consultancy, Honoraria, Research Funding; Spectrum: Consultancy, Research Funding; Kite Pharma: Consultancy, Honoraria; Sanofi: Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Amgen Inc.: Research Funding; Bristol-Myers Squibb: Consultancy, Research Funding; Janssen: Honoraria. Vij:Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Jazz Pharmaceuticals: 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, Research Funding; Jansson: Honoraria, Membership on an entity's Board of Directors or advisory committees; Karyopharma: 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; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Usmani:Amgen, BMS, Celgene, Janssen, Merck, Pharmacyclics,Sanofi, Seattle Genetics, Takeda: Research Funding; Abbvie, Amgen, Celgene, Genmab, Merck, MundiPharma, Janssen, Seattle Genetics: Consultancy. Costa:Amgen: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; BMS: Research Funding; Abbvie: Research Funding; Janssen: Research Funding; Karyopharm: Research Funding; Sanofi: Honoraria. Kumar:Roche: Research Funding; KITE: Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Membership on an entity's Board of Directors or advisory committees, Research Funding; Merck: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: 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; Oncopeptides: 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; Janssen: 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; KITE: Membership on an entity's Board of Directors or advisory committees, Research Funding.

Abstract Introduction Proteasome inhibitors (PIs) and immunomodulatory drugs (IMiDs) have significantly improved survival in patients with multiple myeloma (MM). Refractoriness to PIs and IMiDs results in poor outcomes with a median survival of about 13 months.Daratumumab and isatuximab are CD38-targeting monoclonal antibodies (MoABs) with remarkable activity in relapsed and refractory MM. The outcome of MM patients in US clinical practice who becomes refractory to CD38 MoABs is unknown. The aim of this multicenter, retrospective study is to investigate the natural history of patients with MM who become refractory to CD38 MoABs (Monoclonal Antibodies in Multiple Myeloma: Outcomes after Therapy Failure: MAMMOTH study). Methods We identified patients from 14 academic institutions in the US with diagnosis of active MM and refractory to daratumumab or isatuximab, administered alone or in combination. Patients were considered refractory to CD38 MoAB if treated with at least 4 weeks of therapy and had evidence of progressive disease (PD) while on therapy or within 60 days after last dose. The time when patients met the above criteria of progression was defined as time zero (T0). Data including patient-disease characteristics and all therapies administered before and after T0 were collected with an electronic platform and submitted to peer-based quality check for completeness and internal consistency. For survival outcome analysis, patients were classified into three groups: "Penta-refractory" (refractory to 1 CD38 MoAB + 2 PIs + 2 IMiDs), "Triple-refractory and quad-refractory" (refractory to 1 CD38 MoAB + 1 PI + 1/2 IMiDs, or 1 CD38 MoAB + 1/2 PIs + 1 IMiD), and "Not triple-refractory" (refractory to 1 CD38 MoAB, and not both of a PI and an IMiD). Responses were evaluated using the IMWG criteria. Results Two hundred and seventy-five patients were included in this study; median age at T0 was 65 years (range 27-90). Fifty five percent of patients were males, 52% had IgG isotype, 29% had ISS stage III disease, and 29% had high-risk cytogenetics at diagnosis. Median interval from diagnosis of MM to T0 was 50.1 months (mo) (range 2.5-230.1). Patients received a median of 4 lines of treatment (1-16) prior to the CD38 MoAB-containing regimen; 72% underwent prior autologous transplant. Daratumumab-refractory patients formed a majority (256, 93.1%) of this cohort. Most of the patients were refractory to lenalidomide (77%), pomalidomide (65%), bortezomib (68%), and carfilzomib (47%). Seventy patients (25.5%) were "penta-refractory" and 148 (53.8%) were "triple-refractory and quad-refractory". Median follow-up from T0 for survivors was 10.6 mo (range 1.9-42.3 mo). The median overall survival (OS) from T0 for the entire cohort was 8.6 mo (95% C.I. 7.2-9.9); OS for the groups based on refractoriness to prior treatments are shown (Figure). Two hundred and forty-nine patients (90.5%) received at least one line of treatment after T0, (median 2, range 1-10). The response rates and depth of responses to each line of treatment post T0 are shown (Table). Among patients who received ≥ 1 subsequent treatment, the median PFS and OS from T0 were 3.4 (95% C.I. 2.8-4.0) mo and 9.3 (95% C.I. 8.1-10.6) mo, respectively; the median OS for the 26 patients who received no further treatment was only 1.3 (95% C.I. 0.6-1.9) mo. By multivariate analysis, penta-refractoriness (HR 2.4, p <0.001), high-risk cytogenetics (HR 1.5, p=0.01), elevated LDH (HR 2, p<0.001) and creatinine > 2 (HR 2.4, p=0.01) at the onset of CD38-based therapy predicted inferior OS. Conclusions Patients with MM who become refractory to CD38 MoAB are increasingly less responsive to subsequent therapies. While survival is particularly dismal for penta-refractory patients, the outcomes are poor even for patients who are not refractory to other mainstream MM agents. The MAMMOTH study defines a MM population in urgent need for new therapies, and provides benchmark OS, PFS and response data for subsequent investigational therapies in this setting. Disclosures Malek: Celgene: Consultancy, Speakers Bureau; Janssen: Consultancy, Speakers Bureau; Takeda: Consultancy, Speakers Bureau; Sanofi: Consultancy, Speakers Bureau; Amgen: Consultancy, Speakers Bureau. Paul:Bristol Myer Squibb: Other: Stock and pension plan (past employee). Neppalli:Amgen: Consultancy; Celgene: Consultancy. Liedtke:Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Prothena: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen/Onyx: Consultancy, Honoraria, Research Funding; BlueBirdBio: Research Funding; celgene: Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead: Membership on an entity's Board of Directors or advisory committees, Research Funding; Genentech/Roche: Research Funding; Caelum: Membership on an entity's Board of Directors or advisory committees. Hari:Amgen Inc.: Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Spectrum: Consultancy, Research Funding; Kite Pharma: Consultancy, Honoraria; Bristol-Myers Squibb: Consultancy, Research Funding; Sanofi: Honoraria, Research Funding; Janssen: Honoraria; Takeda: Consultancy, Honoraria, Research Funding. Vij:Amgen: 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, Research Funding; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Karyopharma: Honoraria, Membership on an entity's Board of Directors or advisory committees; Jansson: Honoraria, Membership on an entity's Board of Directors or advisory committees; Jazz Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees. Usmani:Abbvie, Amgen, Celgene, Genmab, Merck, MundiPharma, Janssen, Seattle Genetics: Consultancy; Amgen, BMS, Celgene, Janssen, Merck, Pharmacyclics,Sanofi, Seattle Genetics, Takeda: Research Funding. Kumar:Celgene: 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; KITE: Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Membership on an entity's Board of Directors or advisory committees, Research Funding. Costa:Karyopharm: Research Funding; Sanofi: Honoraria; Janssen: Research Funding; BMS: Research Funding; Abbvie: Research Funding; Amgen: Honoraria, Research Funding; Celgene: Honoraria, Research Funding.