Bergsagel, P. Leif

DISCUSSION AND CONCLUSION Chimeric antigen receptor (CAR)-T cells directed against B-cell maturation antigen (BCMA), have yielded impressive results in clinical trials for multiply relapsed/refractory multiple myeloma (MM). However, progression-free survival is short, demonstrating a need for improvements. While numerous advances have been made to improve this treatment modality, these have not always translated to superior clinical outcomes. A major limitation is that novel therapeutics are often not tested in a relevant in vivo model. These novel strategies have been largely tested only in immunodeficient models of MM. To better recapitulate the host-tumour-effector cell interactions, particularly in the TME, we sought to develop a system to study CAR-T cell activity in the Vk*MYC model of MM. Here, we report on the first known successful model of CAR-T cell activity in an immunocompetent model of MM. We sought to identify a BCMA CAR construct that is capable of binding to both human and mouse BCMA. A novel nanobody recognizing BCMA was developed by immunizing llamas with human BCMA and the resulting single-domain antibodies (sdAbs) were subsequently isolated and cloned. These sdAbs were then screened for binding to mouse BCMA. A binder capable of recognizing both human and mouse BCMA was identified and cloned into our murine CAR construct, with CD28 and CD3z signaling domains, herein referred to as sdBCMA2.CD28.CD3z. Murine CAR-T cells were then generated by transducing activated healthy murine splenocytes with a retroviral vector carrying the sdBCMA2.CD28.CD3z construct. These CAR-T cells were then characterized in vitro using standard cytotoxicity, proliferation, and cytokine release assays. To assess activity in an immunocompetent model, female C57BL/6J mice were injected intravenously with Vk12598 Vk*MYC cells. CAR-T cells were later administered intravenously. Mice were bled weekly to quantify M-protein disease burden by serum protein electrophoresis and for phenotypic characterization of circulating cells. When co-cultured with BCMA+ human MM cell lines and with Vk*MYC cell lines, murine sdBCMA2.CD28.CD3z CAR-T cells effectively lysed target cells, proliferated, and released inflammatory cytokines. To assess the in vivo activity of these CAR-T cells, we injected intravenously female C57BL/6J mice with Vk12598 cells, and then administered sdBCMA2.CD28.CD3z CAR-T cells intravenously. While there was some evidence of survival improvement, the median overall survival (mOS) of 35 days compared to 28 days, for CAR-T vs. Mock T-cells, respectively, the difference was non-significant, and the survival benefit was not durable. To address this, we incorporated lymphodepletion by total body irradiation (TBI) followed by CAR-T cell administration the next day. By incorporating lymphodepletion prior to CAR-T cell administration we were able to significantly reduce M-protein surrogates of disease burden, as well as achieve durable remissions in the TBI + CAR-T arm with an mOS not yet reached after experimental day 91, compared to an mOS of 42 days in the TBI + Mock T-cell arm. The addition of TBI also significantly improved the engraftment of CAR-T cells, as assessed by flow cytometry. The TBI + CAR-T arm also had significantly increased IL-6, IL-1b, IL-2, and TNF-a in the serum, compared to the TBI + Mock arm, at 7 days post administration of CAR-T cells. The durable remission achieved using the combination of our novel sdBCMA2.CD28.CD3z CAR-T cells with lymphodepletion is a pivotal advancement in modelling MM CAR-T cell activity in this clinically relevant model system. Our approach represents the first time that CAR-T cells were successfully used to treat the Vk12598 model of MM that faithfully recapitulates relapsed MM. The incorporation of lymphodepletion significantly improves CAR-T cell engraftment, in line with clinical CAR-T cell therapy paradigms. The cytokine profile observed is consistent with the cytokine elevations typically observed after CAR-T cell administration in patients. As our construct is capable of binding to both human and mouse BCMA, we are therefore able to fully characterize our lead assets in a relevant model system. Our strategy will enable further research to dissect host-tumour-effector cell interactions, to better inform the design and characterization of novel CAR-T cell therapies in MM.

Abstract Immunomodulatory drugs (IMiD) are a backbone therapy for multiple myeloma (MM). Despite their efficacy, most patients develop resistance, and the mechanisms are not fully defined. Here, we show that IMiD responses are directed by IMiD-dependent degradation of IKZF1 and IKZF3 that bind to enhancers necessary to sustain the expression of MYC and other myeloma oncogenes. IMiD treatment universally depleted chromatin-bound IKZF1, but eviction of P300 and BRD4 coactivators only occurred in IMiD-sensitive cells. IKZF1-bound enhancers overlapped other transcription factor binding motifs, including ETV4. Chromatin immunoprecipitation sequencing showed that ETV4 bound to the same enhancers as IKZF1, and ETV4 CRISPR/Cas9-mediated ablation resulted in sensitization of IMiD-resistant MM. ETV4 expression is associated with IMiD resistance in cell lines, poor prognosis in patients, and is upregulated at relapse. These data indicate that ETV4 alleviates IKZF1 and IKZF3 dependency in MM by maintaining oncogenic enhancer activity and identify transcriptional plasticity as a previously unrecognized mechanism of IMiD resistance. Significance: We show that IKZF1-bound enhancers are critical for IMiD efficacy and that the factor ETV4 can bind the same enhancers and substitute for IKZF1 and mediate IMiD resistance by maintaining MYC and other oncogenes. These data implicate transcription factor redundancy as a previously unrecognized mode of IMiD resistance in MM. See related article by Welsh, Barwick, et al., p. 34. See related commentary by Yun and Cleveland, p. 5. This article is featured in Selected Articles from This Issue, p. 4

Abstract The approach to patients with high-risk smoldering multiple myeloma (SMM) varies among clinicians; while some advocate early intervention, others reserve treatment at progression to multiple myeloma (MM). We aimed to describe the myeloma-defining events (MDEs) and clinical presentations leading to MM diagnosis among SMM patients seen at our institution. We included 406 patients diagnosed with SMM between 2013-2022, seen at Mayo Clinic, Rochester, MN. The 2018 Mayo 20/2/20 criteria were used for risk stratification. Median follow-up was 3.9 years. Among high-risk patients who did not receive treatment in the SMM phase (n=71), 51 progressed by last follow-up; the MDEs included: bone lesions(37%), anemia(35%), hypercalcemia(8%), and renal failure(6%); 24% met MM criteria based on marrow plasmacytosis (≥60%) and/or free light chain ratio (>100); 45% had clinically significant MDEs (hypercalcemia, renal insufficiency, and/or bone lesions). MM diagnosis was made based on surveillance labs/imaging(45%), testing obtained due to provider suspicion for progression(14%), bone pain(20%), and hospitalization/ED presentations due to MM complications/symptoms(4%). The presentation was undocumented in 14%. A high proportion (45%) of patients with high-risk SMM on active surveillance develop end-organ damage at progression. About a quarter of patients who progress to MM are not diagnosed based on routine interval surveillance testing.

Abstract Multiple myeloma (MM) is a malignancy that is often driven by MYC and that is sustained by IRF4, which are upregulated by super-enhancers. IKZF1 and IKZF3 bind to super-enhancers and can be degraded using immunomodulatory imide drugs (IMiD). Successful IMiD responses downregulate MYC and IRF4; however, this fails in IMiD-resistant cells. MYC and IRF4 downregulation can also be achieved in IMiD-resistant tumors using inhibitors of BET and EP300 transcriptional coactivator proteins; however, in vivo these drugs have a narrow therapeutic window. By combining IMiDs with EP300 inhibition, we demonstrate greater downregulation of MYC and IRF4, synergistic killing of myeloma in vitro and in vivo, and an increased therapeutic window. Interestingly, this potent combination failed where MYC and IRF4 expression was maintained by high levels of the AP-1 factor BATF. Our results identify an effective drug combination and a previously unrecognized mechanism of IMiD resistance. Significance: These results highlight the dependence of MM on IKZF1-bound super-enhancers, which can be effectively targeted by a potent therapeutic combination pairing IMiD-mediated degradation of IKZF1 and IKZF3 with EP300 inhibition. They also identify AP-1 factors as an unrecognized mechanism of IMiD resistance in MM. See related article by Neri, Barwick, et al., p. 56. See related commentary by Yun and Cleveland, p. 5. This article is featured in Selected Articles from This Issue, p. 4

Abstract Most patients with solitary bone plasmacytomas (SBP) progress to multiple myeloma (MM) after definitive radiation therapy as their primary treatment. Whether the presence of high-risk (HR) cytogenetic abnormalities by fluorescence in situ hybridization (FISH) in the clonal plasma cells, obtained either directly from the diagnostic SBP tissue or the corresponding bone marrow examination at the time of diagnosis, is associated with a shorter time to progression (TTP) to MM is unknown. This study evaluated all patients diagnosed with SBP at the Mayo Clinic from January 2012 to July 2022. The presence of del(17p), t(14;16), t(4;14), or +1q (gain or amplification) by FISH in clonal plasma cells was defined as HR. A total of 114 patients were included in this cohort, and baseline FISH was available for 55 patients (48%), of which 22 were classified as HR (40%). The median TTP to MM for patients with SBP and HR FISH was 8 months (95% confidence interval [CI], 6.3-26) compared with 42 months (95% CI, 25-not reached [NR]) in patients with SBP without HR FISH (P < .001). In a multivariate analysis, only HR FISH was a significant predictor for shorter TTP to MM, independent of minimal marrow involvement and an abnormal serum free light chain ratio at diagnosis. Deletion (17p) and gain 1q abnormalities were the most common FISH abnormalities responsible for the short TTP to MM. Thus, assessing for HR FISH abnormalities in clonal plasma cells derived from either the diagnostic SBP tissue or the staging bone marrow examination of patients with newly diagnosed SBP is feasible and prognostic for a shorter TTP to MM.

Abstract Multiple myeloma (MM) makes up approximately 10% of all hematologic malignancies, with increasing incidence worldwide. Though treatment options for newly diagnosed patients have evolved in recent years, they are not curative, and relapse is common with 2,500-3,000 relapsed or refractory (R/R) MM US patients seeking novel treatments, annually. Cereblon (CRBN)-binding Ikaros and Aiolos (I/A) protein degraders (i.e., lenalidomide (LEN) and pomalidomide (POM)) have had great success in treating MM. However, due to the unstable chiral nature of the CRBN-binding moiety, common to these molecules, many exist as racemic mixtures (1:1 mixture of interconverting (R)- and (S)-enantiomers). A growing body of evidence suggests that the (S)- enantiomer is the active anticancer species, while the (R)-enantiomer is inactive and potentially results in undesired activity. The ability to stabilize the active (S)- enantiomer represents a potential strategy to improve therapeutic outcomes. We are developing SP-3164, the deuterium-stabilized (S)-enantiomer of avadomide (AVA), an I/A-degrading molecular glue which has been studied in over 400 patients, demonstrating safety and activity in hematologic diseases. Our in vitro data demonstrate that SP-3164 more potently and rapidly degrades the cancer-promoting transcription factors, I/A, compared to LEN, POM, and AVA, while the stabilized (R)-enantiomer, SP-3165, neither binds CRBN nor degrades I/A. In preliminary in vivo studies, SP-3164 elicited significant tumor growth inhibition (TGI). Here, we investigate the activity of SP-3164 in MM, compared to established treatment regimens. SP-3164’s potential to degrade Aiolos in MM.1S cells was compared to SP-3165, AVA, and POM. SP-3164 rapidly and robustly degraded Aiolos in a time- and dose-dependent manner, more potently than AVA and POM, while SP-3165 demonstrated minimal Aiolos degradation. To investigate the effect of SP-3164 on cell survival, we treated NCI-H929 cells with SP-3164 (0.1 µM, 1 µM, and 10 µM) and SP-3165 (1 µM) and assessed apoptosis by flow cytometry. SP-3164 induced dose-dependent early and late apoptosis after 72 hours, while SP-3165 showed no induction of apoptosis. SP-3164’s in vivo activity was assessed and compared to SP-3165, LEN, and POM in an NCI-H929 mouse xenograft model. SP-3164 showed significant TGI as a single agent while SP-3165 seemed to support tumor growth. LEN and POM demonstrated minimal TGI. When combined with dexamethasone (DEX), SP-3164 had improved TGI compared to the combination of POM+DEX. These preclinical data demonstrate that SP-3164 is a potent CRBN-binding molecular glue with the ability to rapidly degrade cancer-promoting transcription factors and induce apoptosis in vitro. In vivo, SP-3164 has superior single agent and combination activity compared to approved molecular glues. These data support the continued development as a novel therapy in MM. Citation Format: Aundrietta D. Duncan, Daniela Y. Santiesteban, Nadeem Q. Mirza, Steve Horrigan, Sheila DeWitt, Vincent Jacques, P. Leif Bergsagel. SP-3164, a novel molecular glue degrader with activity in preclinical models of multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6253.

8024 Background: Minimal residual disease (MRD) is prognostic for survival in multiple myeloma (MM). MRD is usually assessed in the bone marrow (BM) by flow cytometry or NGS. MRD by FDG PET/CT provides a global representation of the tumor burden, including response assessment of extramedullary disease. We examined MRD status in MM pts using both BM and PET/CT at Mayo Clinic. Methods: Medical records were reviewed retrospectively for MM pts who received CAR-T between 4/2018 and 12/2022. All PET/CT scans were assessed by radiologists. BM MRD was assessed by flow cytometry with a sensitivity of 10-5. Progression-free survival (PFS) and overall survival (OS) were analyzed by Kaplan-Meier method. Results: Among the 157 CAR-T pts, including 89 pts who received FDA-approved CAR-T, median age was 63 years, 59% (92/157) were males, 42% (66/157) had high risk cytogenetics with median of 5 prior lines of therapy, 36% (57/157) had plasmacytoma. Incidence of CRS was 83%, 5% grade ≥ 3 CRS. Incidence of ICANS was 19%, 4% grade ≥ 3 ICANS. CR/sCR rate was 37%. One hundred and thirty-seven pts (87%) had evaluable BM at month (mo) 1, 85% (117/137) were BM MRDneg at mo 1. Baseline demographics were comparable between the two groups except age, % BM plasma cells and use of bridging therapy. Among the MRDneg pts, CR/sCR rate was 44% (51/117) and 95% (111/117) had sFLC below normal. At median follow-up of 13.8 months, median PFS among pts with BM MRDneg at 1 mo was 12 mo (95% CI: 11, 30) vs 3 mo (95%CI: 2, 7) for BM MRDpos (p<0.001). Median OS among pts with BM MRDneg at 1 mo was 34 mo (95% CI: 24, NR) vs 22 mo (95%CI: 7, NA) for BM MRDpos (p<0.01). At month 1, 112 pts had both BM and PET/CT assessments available, 64/112(57%) were both BM MRDneg/PET MRDneg, 38/112 (34%) were MRDneg for either BM or PET and 10/112 (9%) were positive for both BM and PET (MRDpos/PET MRDpos). Baseline demographics except age were comparable between the 2 groups (Table). Rate of conversion from MRDpos to MRDneg was low. Rate of sustained BM and PET MRDneg at mo 12 was 44% (8/18). The median PFS and OS for BM MRDneg/PET MRDneg was significantly longer as compared to others (Table). Conclusions: Achieving FLC below normal, BM MRDneg and PET MRDneg at 1 month, regardless of IMWG response at that time, is prognostic for both PFS and OS in MM pts receiving CAR-T. Failure to achieve any of these confers poor prognosis. [Table: see text]