Ibrutinib-resistant Mantle Cell Lymphoma Research Articles

MCIR1: A patient‐derived mantle cell lymphoma line for discovering new treatments for ibrutinib resistance

Despite the unprecedented success of ibrutinib in lymphoma therapy, the development of ibrutinib resistance due to acquired BTK or PLCγ2 mutations has become a new clinical problem. However, not all resistance is mediated by these mutations and these mechanisms are poorly understood due to a lack of study tools that truly recapitulate this clinical scenario. We established a novel patient-derived ibrutinib-resistant mantle cell lymphoma (MCL) line named MCIR1. Using immunological, molecular, and cytogenetic approaches, we comprehensively characterized MCIR1 and further demonstrated its utility in the study of resistance mechanisms and treatments to overcome this resistance. We show that MCIR1 is a bona fide ibrutinib-resistant MCL cell line with normal BTK-/PLCγ2 but ibrutinib-resistant ERK1/2 and AKT1 signaling. RNA-Seq analysis revealed a robust non-canonical NF-kB signaling that drives the ibrutinib resistance. We also demonstrate the potential utility of a MCIR1-based cell and mouse model for the discovery of new treatments to overcome BTK inhibitor resistance. We have established the first patient-derived ibrutinib-resistant MCL cell line MCIR1 that lacks BTK or PLCγ2 mutations but exhibits a hyperactive non-canonical NF-kB pathway. We further demonstrate its utility in the discovery and validation of new drugs to overcome this resistance.

Pharmacologic Targeting of Mcl-1 Induces Mitochondrial Dysfunction and Apoptosis in B-Cell Lymphoma Cells in a TP53- and BAX-Dependent Manner.

Bcl-2 has been effectively targeted in lymphoid malignancies. However, resistance is inevitable, and novel approaches to target mitochondrial apoptosis are necessary. AZD5991, a selective BH3-mimetic in clinical trials, inhibits Mcl-1 with high potency. We explored the preclinical activity of AZD5991 in diffuse large B-cell lymphoma (DLBCL) and ibrutinib-resistant mantle cell lymphoma (MCL) cell lines, MCL patient samples, and mice bearing DLBCL and MCL xenografts using flow cytometry, immunoblotting, and Seahorse respirometry assay. Cas9 gene editing and ex vivo functional drug screen assays helped identify mechanisms of resistance to Mcl-1 inhibition. Mcl-1 was expressed in DLBCL and MCL cell lines and primary tumors. Treatment with AZD5991 restricted growth of DLBCL cells independent of cell of origin and overcame ibrutinib resistance in MCL cells. Mcl-1 inhibition led to mitochondrial dysfunction as manifested by mitochondrial membrane depolarization, decreased mitochondrial mass, and induction of mitophagy. This was accompanied by impairment of oxidative phosphorylation. TP53 and BAX were essential for sensitivity to Mcl-1, and oxidative phosphorylation was implicated in resistance to Mcl-1 inhibition. Induction of prosurvival proteins (e.g., Bcl-xL) in stromal conditions that mimic the tumor microenvironment rendered protection of primary MCL cells from Mcl-1 inhibition, while BH3-mimetics targeting Bcl-2/xL sensitized lymphoid cells to AZD5991. Treatment with AZD5991 reduced tumor growth in murine lymphoma models and prolonged survival of MCL PDX mice. Selective targeting Mcl-1 is a promising therapeutic approach in lymphoid malignancies. TP53 apoptotic network and metabolic reprogramming underlie susceptibility to Mcl-1 inhibition.

Venetoclax As a Single Agent and in Combination with PI3K-mTOR1/2 Kinase Inhibitors Against Ibrutinib Sensitive and Resistant Mantle Cell Lymphoma

Mantle cell lymphoma (MCL) is an aggressive B-cell lymphoma and prognosis remains poor in MCL patients due to minimal residual disease (MRD), subsequent emergence of drug resistance and lymphoma progression. Ibrutinib is a bruton's tyrosine kinase (BTK) inhibitor that has high response rates in patients with MCL, however, over 30% of MCL patients showed partial or complete lack of response to ibrutinib and experienced disease progression within 12 months of treatment. Alarmingly, once patients relapse after ibrutinib treatment, the 1-year survival rate is only 22%. Our study demonstrated that the major hallmarks of ibrutinib resistant (IR) MCL are increased mitochondrial apoptosis priming, increased BCL-2 dependence, and sustained PI3K-AKT-mTOR pathway activation accompanied by enhanced stromal cell interaction. Sustained BCL-2 and PI3K-AKT-mTOR pathways cooperatively dictate interaction (adhesion) between MCL and stroma, promote MCL IR and proliferation. Disruption of both pathways establishes a novel ‘triple-hit’ or ‘multi-hit’ strategy: targeting the pathway (BCL-2) related to survival, targeting the pathway (PI3K-mTOR) related to cell proliferation, culminating in disruption of the lymphoma-stroma adhesion/interaction to sensitize and enhance cytotoxicity. BEZ235 and AZD8055 synergized with ABT-199 to suppress MCL viability and attenuated clonogenic growth in both ibrutinib sensitive and resistant MCL lines and primary cells. Moreover, we observed that compensatory upregulation of MCL1 following ABT-199 exposure could be abrogated by BEZ235 and AZD8055 - both in ibrutinib sensitive and IR cells. The combination of BEZ235 and AZD8055 with ABT-199 triggered a more dramatic reduction of MCL-1 protein expression, accompanied by an increase in apoptotic PARP cleavage, both in IR MCL cell lines and primary MCL. Together, these data suggest a novel strategy to overcome the dismal prognosis associated with IR. DisclosuresNo relevant conflicts of interest to declare.

Molecular Pathways Associated with Ibrutinib Resistance in Mantle Cell Lymphoma

Mantle cell lymphoma (MCL) is an aggressive and incurable subtype of B-cell lymphoma and constitutive activation of the B-cell receptor pathway is a hallmark of B-cell lymphomas. Bruton's tyrosine kinase (BTK) is a critical component of the B-cell receptor pathway, and ibrutinib, a first-in-class, once-daily, and oral covalent inhibitor of BTK, was developed to reduce/silence B-cell receptor pathway activity, leading to clinically remarkable anti-tumor activity. In our prior multiple-center phase II clinical trial, the overall response rate in relapsed/refractory MCL patients was 68%, with a median progression free survival (PFS) of 13.9 months, surpassing the effectiveness of other therapies. The follow-up data demonstrate the durability of responses and confirm the unprecedented single-agent activity of ibrutinib in relapsed/refractory MCL. Although ibrutinib is extremely efficacious in patients with relapsed/refractory MCL, ibrutinib-treated patients who experience disease progression die within 12 months.We used RNASeq to compare gene expression profiles between ibrutinib-sensitive and -resistant patient samples, and a molecular signature strongly associated with ibrutinib-resistant clinical responses was generated. As a new prognostic predictor for ibrutinib treatment, this molecular signature in combination with the MCL International Prognostic Index (MIPI) stratified the therapeutic outcome of MCL patients. In addition, gene set enrichment analysis (GSEA) revealed the marked upregulation of components of the c-Myc and mTOR signaling pathways in the ibrutinib-resistant patient samples, indicating that the activation of the pathways may mediate ibrutinib resistance. Furthermore, the gene expression of numerous members of metabolic pathways was significantly upregulated, including the OXPHOS pathway, the citric acid cycle, and pyruvate metabolism, suggesting that metabolic reprogramming may play a major role in ibrutinib resistance. To determine the effects of targeting these pathways, the PI3K/AKT/mTOR or OXPHOS signaling pathways were inhibited in both MCL cell lines and ibrutinib-resistant MCL patient-derived xenograft (PDX) models, greatly reducing MCL cell viability and tumor burden, respectively. Furthermore, synergistic combinations using agents targeting these two pathways are currently being investigated in both in vitro and in vivo models.In conclusion, we identified a molecular signature associated with ibrutinib resistance that may eventually be used as a prognostic indicator of ibrutinib response. In addition, these studies indicate that metabolic reprogramming mediated by the mTOR pathway may underlie ibrutinib resistance in MCL. DisclosuresWang:Celgene: Honoraria, Research Funding; Acerta Pharma: Consultancy, Research Funding; Juno Therapeutics: Research Funding; BeiGene: Research Funding; Dava Oncology: Honoraria; Pharmacyclics: Research Funding; Onyx: Research Funding; Kite Pharma: Research Funding; Asana Biosciences: Research Funding; Proteolix: Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.

Targeting the Paracaspase MALT1: A Potential Therapy to Overcome Ibrutinib Resistance in Relapsed/Refractory Mantle Cell Lymphoma Patients

Mantle cell lymphoma (MCL) is an aggressive B cell malignancy that is not yet curable. Ibrutinib was approved in 2013 for the treatment of relapsed/refractory MCL patients; however, ibrutinib resistance inevitably develops. Therefore, there is an urgent unmet need to overcome ibrutinib resistance and to study alternative treatment options.Constitutive NF-κB activation is the hallmark of MCL. Indeed, next generation sequencing analysis of 110 MCL patient samples revealed that genes belonging to the NF-κB signaling pathway had a high mutation rate (28.8%), indicating a significant contribution of NF-κB signaling to MCL malignancy. Mucosa-associated lymphoid tissue transformation protein (MALT1) plays a crucial role in NF-κB signaling. MALT1 is a unique paracaspase within the human genome, and the proteolytic activity of MALT1 has been found to be constitutively active in many MCL samples, suggesting that MALT1 may be a potential therapeutic target without significant off-target side effects. MI-2 is a specific inhibitor of MALT1 and its safety and efficacy are currently being examined in a clinical trial for the ABC subtype of diffuse large B cell lymphoma. In addition, animal models treated with MI-2 did not have any detectable physiological, histological or biochemical signs of toxicity. However, whether MALT1 activity contributes to ibrutinib resistance and whether targeting MALT1 can overcome ibrutinib resistance in relapsed/refractory MCL patients remain unclear.In this study, we found that both canonical and non-canonical NF-κB signaling is activated in ibrutinib-resistant MCL cells, which correlates with constitutive MALT1 activity. Interestingly, we found that MALT1 is highly mutated in four clusters including the death domain, TRAF6 binding site, Caspase like domain, and IKKγ binding site. Occurrence of L79P, K80R, E319D, L445P and N446S strongly correlated with ibrutinib resistance and disease progression. Treatment with MI-2 at nanomolar levels significantly reduced cell viability and induced apoptosis in several MCL cell lines. MI-2 treatment inhibited NF-κB activation, IL-6 production and downstream STAT3 activation. Combination of MI-2 and ibrutinib resulted in synergistic growth inhibition in both ibrutinib-resistant MCL cell lines and primary MCL cells. RPPA analysis showed that treatment with the MI-2 and ibrutinib combination resulted in marked changes in the cellular protein profiles, suggesting that targeting MALT1 catalytic activity in MCL is a promising therapy to overcome ibrutinib resistance in relapsed/refractory MCL patients. Furthermore, MI-2 treatment greatly inhibited the in vitro cell migration of the majority of examined MCL cell lines as well as cell adhesion to the extracellular matrix, including integrin and laminin; therefore, treatment with MI-2 may induce lymphocytosis (i.e., compartmental shift) in patients and mouse xenograft models.Altogether, our data suggest that targeting MALT1 is a very promising therapy for treating relapsed/refractory MCL patients. These studies support the development of preclinical and clinical studies to overcome ibrutinib resistance by targeting MALT1. [Display omitted] DisclosuresWang:Proteolix: Honoraria, Research Funding; Pharmacyclics: Research Funding; Onyx: Research Funding; Kite Pharma: Research Funding; Juno Therapeutics: Research Funding; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Dava Oncology: Honoraria; Celgene: Honoraria, Research Funding; BeiGene: Research Funding; Asana Biosciences: Research Funding; Acerta Pharma: Consultancy, Research Funding.

TAS4464, a Novel, Highly Potent NEDD8-Activating Enzyme Inhibitor Shows Antitumor Activity in Ibrutinib-Insensitive Mantle Cell Lymphoma Via Inactivation of the NF-κB Pathway

Background: Although ibrutinib, a Bruton tyrosine kinase inhibitor, is becoming widely used to treat patients with mantle cell lymphoma (MCL), about one-third of patients do not respond, leaving them with limited therapeutic options. Effective therapy is, therefore, urgently needed for ibrutinib-resistant MCL patients. The mechanisms of ibrutinib intrinsic and acquired resistance in MCL include activation of the alternative nuclear factor-κB (NF-κB) pathway, CARD11 mutation, and BTK C481S mutation. PLCG2, CXCR4, and MYD88 mutations are resistant mechanisms in other cancers. A characteristic common to these resistance mechanisms is the activation of NF-κB signaling mediated by two signaling cascades, the canonical and non-canonical pathways. TAS4464, a highly selective, and currently the most potent, inhibitor of NEDD8-activating enzyme (NAE), inhibits both the canonical and non-canonical pathways of NF-κB via NAE inhibition. Here, we report the antitumor activity of TAS4464 in ibrutinib-insensitive MCL preclinical models via modulation of both NF-κB pathways.Materials and methods: The biological activity of TAS4464 was evaluated in 7 different human MCL cell lines. Intracellular ATP levels were measured to assess in vitro cell growth. Effects on the cell cycle were analyzed by using fluorescence-activated cell sorting. The effects of TAS4464 on NEDD8 conjugation and cullin-RING ubiquitin ligase (CRL) substrate levels were evaluated by western blot analysis. Activated p65 and RelB (NF-κB subunits) were quantified by using commercial DNA-binding ELISA kits. The levels of NF-κB-targeted gene transcripts were assessed by quantitative real-time-PCR. Antitumor activities and the survival benefit of TAS4464 were evaluated in subcutaneous and systemic human MCL xenograft models, respectively.Results: TAS4464 in the nanomolar range inhibited cell growth and at 100 nmol/L induced apoptosis within 24 h in all tested MCL cell lines, whereas ibrutinib was less active (GI50 values >1 µmol/L) in 4 of the 7 cell lines tested. In ibrutinib-resistant cells, non-canonical NF-κB signaling (MAVER-1 and Z-138), or both non-canonical and B-cell antigen receptor (BCR)-independent canonical NF-κB signaling (GRANTA-519 and JVM-2), is reportedly activated. Ibrutinib suppressed p65 activity in BCR dependent-ibrutinib sensitive (REC-1) and moderately sensitive (JEKO-1 and Mino) cell lines but did not suppress p65 in the ibrutinib-insensitive cell lines, suggesting that ibrutinib inhibits only the BCR signal-driven canonical NF-κB pathway. In both ibrutinib-sensitive and -insensitive cell lines, TAS4464 inhibited NEDD8 conjugation to CRLs, with subsequent accumulation of CRL substrate proteins, including phosphorylated-IκBα (p-IκBα) and phosphorylated-p100 (p-p100), which are components of the canonical and non-canonical NF-κB pathways, respectively. This in turn suppressed p65 (canonical) and RelB (non-canonical) activity, and NF-κB-targeted gene expression. Intravenous administration of TAS4464 once or twice a week induced strong antitumor effects and led to tumor regression in subcutaneous human MCL Mino and Z-138 xenograft models; antitumor effects included inhibiting cullin neddylation, inducing caspase cleavage, and downregulating NF-κB-targeted gene transcription in the tumors. The antitumor activity of TAS4464 exceeded those of ibrutinib and bortezomib. Furthermore, in a human MCL GRANTA-519 systemic xenograft model, intravenous administration of TAS4464 once or twice a week doubled the survival times of mice, whereas ibrutinib did not prolong survival.Conclusions: TAS4464 inhibits cell growth and induces apoptosis in MCL cell lines through inactivation of both the canonical and non-canonical NF-κB pathways, in contrast to the limited effects of ibrutinib in BCR signal-driven cell lines. Our results suggest that inhibition of NF-κB pathways by modulating the amounts of p-IκBα and p-p100 is a potential treatment approach for MCL. TAS4464 exerts marked antitumor activity in subcutaneous human MCL xenograft models, in which neither bortezomib nor ibrutinib is effective, and it has a survival benefit in a human MCL systemic xenograft model. Thus, TAS4464 may be a valuable addition to current treatment options for refractory/relapsed MCL. DisclosuresMuraoka:Taiho Pharmaceutical Co., Ltd.: Employment. Yoshimura:Taiho Pharmaceutical Co., Ltd.: Employment. Tsuji:Taiho Pharmaceutical Co., Ltd.: Employment. Hashimoto:Taiho Pharmaceutical Co., Ltd.: Employment. Nakagawa:Taiho Pharmaceutical Co., Ltd.: Employment. Mizutani:Taiho Pharmaceutical Co., Ltd.: Employment. Ohkubo:Taiho Pharmaceutical Co., Ltd.: Employment. Matsuo:Taiho Pharmaceutical Co., Ltd.: Employment. Utsugi:Taiho Pharmaceutical Co., Ltd.: Employment. Iwasawa:Taiho Pharmaceutical Co., Ltd.: Employment.

SOX11 Inhibitors Are Cytotoxic in Mantle Cell Lymphoma.

Mantle cell lymphoma (MCL) is a fatal subtype of non-Hodgkin lymphoma. SOX11 transcription factor is overexpressed in the majority of nodal MCL. We have previously reported that B cell-specific overexpression of SOX11 promotes MCL pathogenesis via critically increasing BCR signaling in vivo. SOX11 is an attractive target for MCL therapy; however, no small-molecule inhibitor of SOX11 has been identified to date. Although transcription factors are generally considered undruggable, the ability of SOX11 to bind to the minor groove of DNA led us to hypothesize that there may exist cavities at the protein-DNA interface that are amenable to targeting by small molecules. Using a combination of in silico predictions and experimental validations, we report here the discovery of three structurally related compounds (SOX11i) that bind SOX11, perturb its interaction with DNA, and effect SOX11-specific anti-MCL cytotoxicity. We find mechanistic validation of on-target activity of these SOX11i in the inhibition of BCR signaling and the transcriptional modulation of SOX11 target genes, specifically, in SOX11-expressing MCL cells. One of the three SOX11i exhibits relatively superior in vitro activity and displays cytotoxic synergy with ibrutinib in SOX11-expressing MCL cells. Importantly, this SOX11i induces cytotoxicity specifically in SOX11-positive ibrutinib-resistant MCL patient samples and inhibits Bruton tyrosine kinase phosphorylation in a xenograft mouse model derived from one of these subjects. Taken together, our results provide a foundation for therapeutically targeting SOX11 in MCL by a novel class of small molecules.

CDKN1C‐mediated growth inhibition by an EZH1/2 dual inhibitor overcomes resistance of mantle cell lymphoma to ibrutinib

Mantle cell lymphoma (MCL) is a rare subtype of non‐Hodgkin's lymphoma, which is characterized by overexpression of cyclin D1. Although novel drugs, such as ibrutinib, show promising clinical outcomes, relapsed MCL often acquires drug resistance. Therefore, alternative approaches for refractory and relapsed MCL are needed. Here, we examined whether a novel inhibitor of enhancer of zeste homologs 1 and 2 (EZH1/2), OR‐S1 (a close analog of the clinical‐stage compound valemetostat), had an antitumor effect on MCL cells. In an ibrutinib‐resistant MCL patient–derived xenograft (PDX) mouse model, OR‐S1 treatment by oral administration significantly inhibited MCL tumor growth, whereas ibrutinib did not. In vitro growth assays showed that compared with an established EZH2‐specific inhibitor GSK126, OR‐S1 had a marked antitumor effect on MCL cell lines. Furthermore, comprehensive gene expression analysis was performed using OR‐S1–sensitive or insensitive MCL cell lines and showed that OR‐S1 treatment modulated B‐cell activation, differentiation, and cell cycle. In addition, we identified Cyclin Dependent Kinase Inhibitor 1C (CDKN1C, also known as p57, KIP2), which contributes to cell cycle arrest, as a direct target of EZH1/2 and showed that its expression influenced MCL cell proliferation. These results suggest that EZH1/2 may be a potential novel target for the treatment of aggressive ibrutinib‐resistant MCL via CDKN1C‐mediated cell cycle arrest.

Transcriptional programming drives Ibrutinib-resistance evolution in mantle cell lymphoma

SUMMARYIbrutinib, a bruton’s tyrosine kinase (BTK) inhibitor, provokes robust clinical responses in aggressive mantle cell lymphoma (MCL), yet many patients relapse with lethal Ibrutinib-resistant (IR) disease. Here, using genomic, chemical proteomic, and drug screen profiling, we report that enhancer remodeling-mediated transcriptional activation and adaptive signaling changes drive the aggressive phenotypes of IR. Accordingly, IR MCL cells are vulnerable to inhibitors of the transcriptional machinery and especially so to inhibitors of cyclin-dependent kinase 9 (CDK9), the catalytic subunit of the positive transcription elongation factor b (P-TEFb) of RNA polymerase II (RNAPII). Further, CDK9 inhibition disables reprogrammed signaling circuits and prevents the emergence of IR in MCL. Finally, and importantly, we find that a robust and facile ex vivo image-based functional drug screening platform can predict clinical therapeutic responses of IR MCL and identify vulnerabilities that can be targeted to disable the evolution of IR.

Outcomes of Acalabrutinib Failures in Relapsed Mantle Cell Lymphoma

Outcomes of Acalabrutinib Failures in Relapsed Mantle Cell Lymphoma

Oncogenic MALT1 Promotes Cell Survival and Mediates Ibrutinib Resistance and Ibrutinib-Venetoclax Resistance in Mantle Cell Lymphoma

Both as monotherapies and in combination, the Bruton's tyrosine kinase inhibitor ibrutinib and the BH3 mimetic BCL2 inhibitor venetoclax have proven to be efficacious and are now widely used treatment options for mantle cell lymphoma (MCL) patients. However, mono- and dual- resistance frequently develops, necessitating investigation into the mechanisms mediating resistance to these therapies. To investigate the mechanism of ibrutinib resistance, we generated two ibrutinib-resistant cells due to marked BTK knockdown via CRISPR/CAS9 from JeKo-1, which is ibrutinib-sensitive and venetoclax-resistant. To understand the mechanism of venetoclax resistance, we generated three venetoclax-resistant cell lines with acquired resistance via chronic exposure to increasing doses of venetoclax from ibrutinib/venetoclax double sensitive Mino and Rec-1 cells, and ibrutinib-resistant but venetoclax-sensitive Granta519 cells. All these paired cell lines with various resistance to ibrutinib and venetoclax were subject to whole transcriptome sequencing of these paired MCL cell lines. We discovered that mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) is significantly overexpressed in ibrutinib-resistant and ibrutinib-venetoclax dual-resistant MCL cells, especially in cells with BTK knockdown. This was further validated in primary MCL patient cells (n=24). Interestingly, MALT1 overexpression inversely correlates with CARD11 expression and enhances non-canonical NF-κB signaling, suggesting a switch from a highly-dependent BTK-CARD11 mechanism to an independent mechanism in both ibrutinib-resistant and ibrutinib-venetoclax dual-resistant MCL cells. Chromosomal translocations of MALT1 are the hallmarks of MALT lymphoma, which result in oncogenic fusion of MALT1 products. MALT1 is constitutively active in driving aggressive ABC-type DLBCL. This indicates MALT1 can be oncogenic when its activity is dysregulated. Therefore, we hypothesized that constitutive MALT1 activity may be responsible for the resistance to ibrutinib and venetoclax in MCL cells. To demonstrate this relationship, MALT1 ablation using genetic manipulation resulted in significant growth defects both in vitro and in vivo. Pharmaceutical approaches using MALT1 inhibitor MI-2 resulted in similar effects on cell survival and using cell viability assays. Whole transcriptome sequencing analysis revealed that MYC, NF-kB, ROS, cell cycle and mTOR signaling are the most significantly downregulated pathways upon MI-2 treatment. Intriguingly, MYC, NF-kB, PI3K-AKT-mTOR and mTOR signaling pathways were reported to be upregulated in ibrutinib-resistant MCL cells compared to sensitive MCL cells. To address this further, proteomics analysis by reverse phase protein array (RPPA) using more than 400 antibodies confirmed that MI-2 significantly downregulated AKT-mTOR signaling. NF-kB modulation, ROS production, AKT-mTOR, and metabolism changes were further confirmed through multiple biochemical approaches. In addition, MI-2 treatment resulted in a dramatic reduction of MALT1 expression, suggesting that MI-2 treatment affected both its scaffold and paracaspase activities. Furthermore, MI-2 treatment resulted in significant inhibition of in vivo tumor growth of ibrutinib-venetoclax dual-resistant MCL subcutaneous xenografts and tumor homing to the spleen and bone marrow in an ibrutinib-venetoclax dual-resistant MCL patient-derived xenograft (PDX) mouse model. In conclusion, we discovered that MALT1, an essential regulator of NF-κB signaling, is hyperactive in ibrutinib-resistant cells and ibrutinib-venetoclax dual-resistant MCL cells, which puts MALT1 forward as a potentially new therapeutic target in ibrutinib and venetoclax-resistant MCL tumors. Genetic depletion or pharmaceutical inhibition of MALT1 resulted in remarkable defects in cell survival and cell proliferation. The MALT1 inhibitor MI-2 proved its in vivo potency by its pro-apoptotic effect and its significant tumor growth inhibition. In conclusion, targeting a hyperactive MALT1 is a promising therapeutic strategy that could lead to clinical implementation of a new treatment strategy meant to overcome ibrutinib and ibrutinib-venetoclax dual resistance in MCL patients by reversing the NF-kB and ROS/mTOR- mediated resistance in these tumors. Disclosures Wang: Targeted Oncology: Honoraria; Loxo Oncology: Consultancy, Research Funding; Pulse Biosciences: Consultancy; Kite Pharma: Consultancy, Other: Travel, accommodation, expenses, Research Funding; Juno: Consultancy, Research Funding; BioInvent: Research Funding; VelosBio: Research Funding; Acerta Pharma: Research Funding; InnoCare: Consultancy; Oncternal: Consultancy, Research Funding; Nobel Insights: Consultancy; Guidepoint Global: Consultancy; Dava Oncology: Honoraria; Verastem: Research Funding; Molecular Templates: Research Funding; OncLive: Honoraria; Beijing Medical Award Foundation: Honoraria; Lu Daopei Medical Group: Honoraria; MoreHealth: Consultancy; Celgene: Consultancy, Other: Travel, accommodation, expenses, Research Funding; AstraZeneca: Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding; Pharmacyclics: Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding; Janssen: Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding; OMI: Honoraria, Other: Travel, accommodation, expenses.

Abemaciclib-Based Combinational Therapies to Overcome Therapeutic Resistance in Mantle Cell Lymphoma

Introduction The past decades witnessed dramatic improvement of overall survival rate of mantle cell lymphoma (MCL) patients by constant efforts in developing novel therapeutic strategies that include ibrutinib and venetoclax. Nevertheless, resistance is still a major challenge in refractory/relapsed MCL patients. Chromosomal translocation t(11:14)(q13:q32) of the cyclin D1 (CCND1) gene is the hallmark of MCL, which leads to overexpression of cyclin D1. This overexpression promotes aberrant cell cycle progression by activating CDK4/6. Abemaciclib is a selective CDK4/6 inhibitor used as a clinical treatment of breast cancer and has been shown to be effective in preclinical human MCL xenograft models. It has also been used in a phase II clinical trial as a single agent among refractory/relapsed MCL patients with an objective response rate of 35.7%. In this preclinical study, we aim to evaluate the benefit of a combinational therapeutic strategy using abemaciclib with other molecular targeting agents among MCL patients with therapeutic resistance. Methods Cytotoxic efficacy of abemaciclib as a single agent and in combination with other drugs on different MCL cell lines and primary lymphoma cells from MCL patients with or without resistance was used as a key criterion for screening beneficial therapeutic strategies. Cell apoptosis and cell cycle arrest assays were conducted to further evaluate those effective combinations. Western blot was performed to investigate the mechanism of action of the combinations. Finally, the efficacy of abemaciclib alone or in combination were assessed in ibrutinib-resistant or venetoclax-resistant MCL PDX models in vivo. Results Our preliminary data showed that all MCL cell lines involved in this study were highly sensitive to abemaciclib treatment with IC50 values ranging from 50 nM to 1 µM. Further investigation of abemaciclib cytotoxicity on ibrutinib and/or venetoclax resistant MCL cell lines showed effective inhibition with a higher IC50 values ranging from 5 µM to 10 µM. More importantly, abemaciclib had potent efficacy on cells from primary MCL patients as well as from patients with acquired ibrutinib resistance. Our recent findings revealed that the addition of PI3K inhibitor TGR-1202 significantly enhanced cytotoxicity of abemaciclib in both sensitive and resistant MCL cell lines. Abemaciclib significantly inhibited phosphorylation of Rb1, the active form of the protein, in 4 different MCL cell lines. The active Rb1 maintains the cell in the G1 phase, preventing progression through the cell cycle and acting as a growth suppressor. The result suggests that CDK4/6 inhibition with abemaciclib disrupts CDK4/6 suppressive activity towards pRb-E2F and induce cell cycle arrest in the MCL cells. Interestingly, abemaciclib somehow interrupted phosphorylation of Chk1, which is continuously phosphorylated and hence activated in the MCL cell lines. Inhibiting activation of Chk1 by abemaciclib may induce cell death via unmonitored and accumulated DNA damage. The efficacy of abemaciclib in combination with Bcl-2 or BTK inhibitors in MCL cell lines and isolated cells from MCL patients are ongoing. These data suggest that abemaciclib in combination with other therapeutic drugs could be beneficial in targeting therapeutic resistant MCL cells. Conclusions Abemaciclib showed impressive therapeutic potency on both MCL cell lines and isolated primary cells from MCL patients, which is likely due to the predominant contribution of cyclin D1-CDK4/6 pathway to malignancy. Other agents, such as PI3K inhibitors, can sensitize abemaciclib in therapeutic resistant MCL cells. Thus, an abemaciclib based multi-drug combinational strategy may be a promising therapy for refractory/relapsed MCL patients in the near future. Disclosures Wang: Beijing Medical Award Foundation: Honoraria; Lu Daopei Medical Group: Honoraria; Kite Pharma: Consultancy, Other: Travel, accommodation, expenses, Research Funding; Pulse Biosciences: Consultancy; Loxo Oncology: Consultancy, Research Funding; Targeted Oncology: Honoraria; OMI: Honoraria, Other: Travel, accommodation, expenses; Nobel Insights: Consultancy; Guidepoint Global: Consultancy; Dava Oncology: Honoraria; Verastem: Research Funding; Molecular Templates: Research Funding; OncLive: Honoraria; Celgene: Consultancy, Other: Travel, accommodation, expenses, Research Funding; AstraZeneca: Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding; Oncternal: Consultancy, Research Funding; Juno: Consultancy, Research Funding; BioInvent: Research Funding; VelosBio: Research Funding; Acerta Pharma: Research Funding; InnoCare: Consultancy; MoreHealth: Consultancy; Pharmacyclics: Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding; Janssen: Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding.

Targeting Glutamine Metabolism Overcomes Resistance to Targeted Therapies in Refractory Mantle Cell Lymphoma

Background: Mantle cell lymphoma (MCL) is an incurable B cell non-Hodgkin's lymphoma characterized by high refractory occurrence following drug treatment. Despite the encouraging initial MCL tumor response to ibrutinib (IBN), relapse occurs only after few months of treatment due to multiple resistance mechanisms. Thus, the novel therapeutic strategies targeting resistant mechanisms are crucial. Our group has recently shown that among the highly proliferative MCL population, a subpopulation of IBN-R cells exhibits increased OXPHOS activity that is fueled by increased glutaminolysis and rely more on mitochondrial respiration for their grow and survival. The aim of this work was to uncover potential targets responsible for the upregulation of OXPHOS pathway in the refractory/relapsed (R/R) MCL by using multiple biochemical and biological strategies. We focused the present study on glutaminase (GLS), the enzyme that converts glutamine to glutamate, a precursor of α-ketoglutarate (α-KG) that links glutamate to the TCA cycle. Incorporation of α-KG into the TCA cycle is a major anaplerotic step in proliferating cells and is critical for the maintenance of TCA cycle function. To further demonstrate the reliance of OXPHOS on glutamine anaplerosis, we have further tested the combinatory effects of targeting GLS and OXPHOS using their respective inhibitors, CB-839 and IACS-010759, on tumor killing activity in R/R MCL. Methods:Primary MCL cells from patient leukapheresis or whole blood specimens, as well as established MCL cell lines were used as experimental models of MCL. Metabolomic profiling was used to determine intracellular metabolite fluxes and levels. Cell Titer Glo assay was used to measure cell proliferation/viability after treatment with inhibitors. Annexin V and propidium iodide were used to measure cell apoptosis and cell cycle arrestviaflow cytometry analysis. Magnetic microbeads-based B-cell isolation method were used for the purification of malignant B cells from patient samples. Western blot analysis was used to evaluate protein level expression. Patient-derived Xenograft (PDX) mouse model created from patients with MCL was used to evaluate the in vivo anti-tumor activity and potential clinical value of GLS and OXPHOS inhibitors. Results:Our recent metabolomic profiling studies have demonstrated that glutaminolysis and OXPHOS are upregulated in IBN-R MCL, manifested by increased glutamine uptake in the ibrutinib-resistant MCL cell lines (p=0.03).Inhibition of glutamine metabolism with the allosteric GLS1-selective inhibitor BPTES resulted in inhibition of cell viability (0.2381uM-9.98uM), indicating that MCL cells are dependent on glutamine metabolism for their proliferation. To corroborate with the above finding, we also presented evidence that GLS1 is highly increased in IBN-R and CART-R MCL patient samples and cell lines confirmed by immunoblotting. Inhibiting of GLS would lead to significant reduction in OXPHOS, mitochondria membrane potential and ATP production, as either single drug or in combination with other targeting agents. To identify a clinical actionable GLS inhibitor for the treatment of MCL, we chose a GLS1 specific inhibitor CB-839 (Selleckchem), currently under several phase II and III clinical trials investigation on solid tumors. Inhibiting GLS1 with CB-839 leads to the decreased cell viability in MCL (0.5626nM-308.4nM). Of note, the treatment with CB-839 to MCL cell lines induces S phase reduction in both Jeko-1 (17.23%) and Z-138 (14.01%), as well as induces significant apoptosis (p=0.013 and p=0.002 in Jeko-1 and Z-138 cells). GLS inhibition will be further explored in the context of mitochondria defect or hypoxia, where OXPHOS maybe deficient. Importantly, while CB-839 is continuing its validation in several solid tumor models, this is the first study providing data on its efficacy in preclinical models of MCL. Conclusion:In conclusion, we report that glutaminolysis and OXPHOS are upregulated in IBN-R MCL that could be partially due to high expression of GLS1. Our preliminary results revealed that the new GLS inhibitor, GCB-839, may present a clinical potential for a new indication and warrants more in-depth investigation. Deciphering the mechanisms involved in MCL metabolic heterogeneity and adaptability during drug resistance development would be crucial to identify key actors enabling MCL cells to escape from therapy. Disclosures Wang: Acerta Pharma:Research Funding;Molecular Templates:Research Funding;InnoCare:Consultancy;Oncternal:Consultancy, Research Funding;Celgene:Consultancy, Other: Travel, accommodation, expenses, Research Funding;Targeted Oncology:Honoraria;MoreHealth:Consultancy;Kite Pharma:Consultancy, Other: Travel, accommodation, expenses, Research Funding;Lu Daopei Medical Group:Honoraria;OMI:Honoraria, Other: Travel, accommodation, expenses;Verastem:Research Funding;Nobel Insights:Consultancy;BioInvent:Research Funding;Guidepoint Global:Consultancy;AstraZeneca:Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding;Pharmacyclics:Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding;Janssen:Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding;Juno:Consultancy, Research Funding;Dava Oncology:Honoraria;Loxo Oncology:Consultancy, Research Funding;Pulse Biosciences:Consultancy;OncLive:Honoraria;Beijing Medical Award Foundation:Honoraria;VelosBio:Research Funding.

Targeting ROR1 Using the Antibody Drug Conjugate Vls-101 in Aggressive Mantle Cell Lymphoma

ROR1 is a transmembrane receptor with tightly controlled expression during embryonic development. While it is expressed on multiple tumor types it is not expressed in normal adult tissues. ROR1-positive B cell non-Hodgkin's lymphomas (B-NHL) include mantle cell lymphoma (MCL), and diffuse large B cell lymphoma. Given its unique expression pattern, ROR1 represents a tumor-specific therapeutic target with little or no normal tissue toxicity. VLS-101, utilizes the UC-961 anti-ROR1 antibody which is conjugated to monomethyl auristatin E (MMAE) via a cleavable linker. VLS-101/ROR1 complex induce internalization facilitates the release of a cytotoxic agent resulting in killing of ROR1-expressing cancer cells. With the rise of novel targeted therapies, resistance has been a major challenge among MCL patients. MCL is one of the most therapeutically resistant and aggressive forms of NHL-B, making it a challenging cancer to treat. The advancement of ibrutinib, a Bruton's tyrosine kinase (BTK) inhibitor, has been a beneficial change; however, ibrutinib-resistant MCL remains an urgent unmet clinical need. Furthermore, MCLs are either intrinsically resistant to ibrutinib, as well as to venetoclax (Bcl-2 inhibitor) and CAR T-cell therapy, or they may acquire resistance after a short response to these therapies. While many studies have looked into overcoming each of these different types of resistance, no studies have been conducted to overcome a multiple-resistance phenotype simultaneously. Therefore, identifying effective therapies that target these resistance mechanisms is crucial. In this study, we assessed the efficacy of VLS-101 in preclinical models of MCL with intrinsic or aquired resistance to ibrutinib, venetoclax, and CD19 CAR T therapies. By using MCL cell lines, primary patient samples and our established MCL patient-derived xenografts, we explored the expression of ROR1 in MCL and investigated the effects of in vitro and in vivo effects ROR1 targeting using VLS-101. To assess total and cell surface ROR1 expression we used flow cytometry (FACS), western blot analysis, and reverse-transcriptase-polymerase chain reaction (RT-PCR). Cell viability and apoptosis assays were performed to evaluate the in vitro efficacy of VLS-101 in MCL cell lines and primary patient MCL samples. To assess the apoptosis and cell cycle arrest in cells treated with VLS-101, we used annexin V/propidium iodide-staining followed by flow cytometry analyses. In vivo efficacy was tested using various PDX models with various multiple resistances. All 5 cell lines and all 9 PDX models of MCL tested showed high ROR1 mRNA and protein expression levels, whereas 3 out of 4 primary human samples expressed cell surface ROR1. VLS-101 treatment showed anti-MCL activity at the concentrations of 0.3-9 μg/ml in most of cell models tested, which correlated with a significant G2/M cell cycle arrest. Furthermore, VLS-101 induced a time- and dose-dependent apoptosis, as shown by increases in annexin V/propidium iodide-staining. VLS-101 treatment of ibrutinib-venetoclax dual-resistant ROR1+ PDX model resulted in significant regressions of the tumor bearing spleens and livers when compared to vehicle controls (p=0.0001 and p=0.002 respectively). In the ibrutinib-CD19 CAR T dual-resistant ROR1+ model, vehicle-treated animals showed s.c. tumor masses with a mean volume of 1440 mm3, whereas tumor masses were barely palpable (mean = 160 mm3) or had completely regressed in animals treated with VLS-101 at 1.0 mg/kg or 2.0 mg/kg, respectively. VLS-101 showed no adverse effects as monitored by animal clinical observations and weekly body weight measurements. Collectively, these results indicate that ROR1 expression on MCL cells can be utilized for selective targeting. These preclinical data document that the MMAE-containing ADC, VLS-101, can cause cell cycle arrest and induce apoptosis in vitro and safely induce tumor regressions in highly resistant in vivo PDX models of MCL derived from patient tumors. Importantly, our study revealed that even the heavyly pretreated tumors in the clinical setting may express targetable levels of ROR1 and that targeting ROR1 using ADC is a promising approach for the treatment of MCL. Building on these types of results, a Phase 1 clinical trial on VLS-101 (NCT03833180) is ongoing in patients with lymphoid cancers. Disclosures Jessen: VelosBio: Current Employment, Current equity holder in private company; eFFECTOR: Current equity holder in private company. Lannutti:VelosBio: Current Employment, Current equity holder in private company; Gilead Sciences: Current equity holder in publicly-traded company. Wang:Dava Oncology: Honoraria; AstraZeneca: Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding; MoreHealth: Consultancy; BioInvent: Research Funding; VelosBio: Research Funding; Acerta Pharma: Research Funding; Lu Daopei Medical Group: Honoraria; Molecular Templates: Research Funding; Oncternal: Consultancy, Research Funding; OMI: Honoraria, Other: Travel, accommodation, expenses; Janssen: Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding; Pharmacyclics: Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding; Celgene: Consultancy, Other: Travel, accommodation, expenses, Research Funding; InnoCare: Consultancy; Pulse Biosciences: Consultancy; Juno: Consultancy, Research Funding; Beijing Medical Award Foundation: Honoraria; Nobel Insights: Consultancy; Verastem: Research Funding; Kite Pharma: Consultancy, Other: Travel, accommodation, expenses, Research Funding; OncLive: Honoraria; Guidepoint Global: Consultancy; Loxo Oncology: Consultancy, Research Funding; Targeted Oncology: Honoraria.

Abstract 652: Exploiting PRMT5, a metabolic and epigenetic regulator, as a novel therapeutic target in mantle cell lymphoma

Abstract Distinct genetic and epigenetic alterations as well as metabolic reprogramming are among the hallmarks of mantle cell lymphoma (MCL), an incurable cancer. Deletion of MTAP, encoding 5-Methylthioadenosine phosphorylase (MTAP), results in the accumulation of metabolite methylthioadenosine (MTA), a selective inhibitor of the catalytic activity of protein arginine methyltransferase 5 (PRMT5). MTAP loss therefore presents a passenger vulnerability to PRMT5 inhibition. PRMT5 is a type II protein arginine methyltransferase that is involved in cellular processes via the regulation of the active status of transcription by symmetric dimethylation of arginine in histones, and the post-translational modification by symmetric dimethylation of arginine in nuclear and cytoplasmic oncoproteins and tumor suppressors. We recently reported that CDKN2A and MTAP were commonly co-deleted in ibrutinib-resistant MCL tumors. Consistently, PRMT5 was found to be selectively over expressed in ibrutinib-resistant MCL cells. Metabolomics analysis revealed that both the substrate (S-Adenosylmethionine) and the product (S-adenosylhomocysteine) of the PRMT5 enzyme are elevated in ibrutinib-resistant MCL, suggesting an increase in PRMT5 enzymatic activity. Importantly, our clinical data analysis indicates that the expression levels of PRMT5 in MCL patients are inversely correlated with patient responses to ibrutinib treatment. Moreover, pharmacological blockade with the PRMT5 specific inhibitor, GSK3226595, significantly inhibited tumor growth in ibrutinib-resistant MCL PDX models, demonstrating that PRMT5 is an actionable target in MCL. Mechanistically, PRMTs play an important role in mRNA splicing, among which MDM4 mRNA was identified as one of the key mRNAs that senses defects in the spliceosomal machinery during PRMT5 deficiency. Alternative splicing of MDM4 mRNA reduces the expression of Mdm2 and Mdm4 E3 ligases, leading to the upregulation of p53. Selective overexpression of PRMT5 in MCL may drive repressive epigenetic marks on chromatin to silence regulatory and tumor suppressor genes. Inhibition of PRMT5 reverses PRMT5-catalyzed epigenetic marks, restores regulatory pathways and induces cell death in ibrutinib-resistant primary MCL. Taken together, sustained PRMT5 expression and activity presents a vulnerability in ibrutinib-resistant MCL where MTAP is frequently deleted. As metabolic reprogramming towards OXPHOS is one of the hallmarks of ibrutinib-resistant MCL, combinatory inhibition of OXPHOS and PRMT5 may represent a rational therapeutic strategy that ultimately eradicates MCL. Citation Format: Yixin Yao, Lingzhi Li, Liang Zhang, Shaojun Zhang, Hui Guo, Hui Zhang, Maria Badillo, Zhihong Chen, Kimberly Hartig, Preetesh Jain, Krystle Nomie, Linghua Wang, Michael Wang. Exploiting PRMT5, a metabolic and epigenetic regulator, as a novel therapeutic target in mantle cell lymphoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 652.

Abstract 650: MCIR1: A patient-derived ibrutinib-resistant mantle cell lymphoma line for the study of ibrutinib resistance and drug discovery

Abstract Constitutive activation of B-cell receptor (BCR) signaling is a major driving mechanism for the proliferation and survival of various B-cell lineage non-Hodgkin lymphomas (NHL). Blocking BCR signaling using the first-in-class Bruton's tyrosine kinase (BTK) inhibitor, ibrutinib, has proved effective and is currently FDA approved for the treatment of several B-cell lymphomas. Unfortunately, significant subsets of patients possess either primary or acquired resistance to ibrutinib, often resulting in worse prognosis and poorer responses to subsequent therapies. Therefore, overcoming ibrutinib resistance is an urgent clinical need requiring new tools to both delineate the molecular mechanisms of resistance and develop viable therapeutics. To that end, we developed a mantle cell lymphoma (MCL) cell line, MCIR1, from a patient with clinically acquired ibrutinib resistance. MCIR1 is a bona fide MCL cell line featuring CD19+/CD5+/CD10-/CD23-/t(11;14)+/TP53+/-; and lacking the previously described mutations in BTK (C481S) and phospholipase C-gamma-2 (R665W, L845F). Importantly, MCIR1 cells possess hallmarks of ibrutinib resistance, including the insensitivity of ERK1/2 phosphorylation to ibrutinib in vitro, and the irresponsiveness of MCIR1 xenograft tumors to ibrutinib treatment in mice as compared with ibrutinib-sensitive Jeko-1 tumors. To delineate the molecular mechanism(s) of ibrutinib resistance, we identified by RNA-Seq analysis that MCIR1 cells possess robust activity of the non-canonical NFkB pathway and elevated expression of NFkB target genes, suggesting that this pathway likely drives their resistance to ibrutinib. Finally, we demonstrated the utility of MCIR1 as a tool for testing new drugs to combat ibrutinib resistance in vitro by showing its sensitivity to the anti-apoptotic B-cell lymphoma-2 (Bcl-2) protein inhibitor, venetoclax. To our knowledge, MCIR1 is the first patient-derived ibrutinib-resistant cell line to be established. Hence, MCIR1 is especially useful because uniquely possesses the relevant clinical and biological features of patients with ibrutinib-resistant MCL. We propose MCIR1 as a tool to further the understanding of molecular mechanisms conferring ibrutinib resistance by providing a clinically relevant system with the capacity for molecular and genetic manipulations. Further, these features make MCIR1 an ideal model to screen drugs for combating ibrutinib resistance through both in vivo and in vitro approaches. Citation Format: Kevin E. Nowakowski, Jithma P. Abeykoon, Mary J. Stenson, Michael M. Timm, Curtis A. Hanson, Daniel L. Van Dyke, Anne J. Novak, Xiaosheng Wu, Thomas E. Witzig. MCIR1: A patient-derived ibrutinib-resistant mantle cell lymphoma line for the study of ibrutinib resistance and drug discovery [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 650.

Abstract 2205: TG-1701, a novel irreversible Bruton's kinase (BTK) inhibitor, cooperates with ublituximab-driven ADCC and ADCP in in vitro and in vivo models of ibrutinib-resistant mantle cell lymphoma

Abstract Mantle cell lymphoma (MCL) is a rare subtype of aggressive B-cell non-Hodgkin lymphoma (NHL) that remains incurable with standard therapy. The first-in-class Bruton's tyrosine kinase (BTK) inhibitor ibrutinib has proven to be an effective agent for patients with relapsed/refractory MCL, although development of a cysteine to serine mutation at the BTK catalytic site (BTKC481S) or over-activation of the NF-kB pathway can impair MCL response to most BTK inhibitors (BTKis). TG-1701 is a novel irreversible inhibitor highly specific to BTK, with improved selectivity when compared to ibrutinib, currently being evaluated in a phase 1 clinical trial in non-Hodgkin lymphoma (NHL) and chronic lymphocytic leukemia (CLL) patients. Here we show that, when compared to irreversible and reversible BTKis including ibrutinib, acalabrutinib, and ARQ-531, TG-1701 showed similar cytotoxic activity either in ibrutinib-sensitive or in ibrutinib-resistant MCL cells. We then evaluated the activity of TG-1701 and ibrutinib on anti-CD20 antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis (ADCP), as ibrutinib has previously been show to block rituximab-mediated ADCC due to off-target inhibition of ITK (Kohrt et al., Blood 2014). In sharp contrast with ibrutinib, which partially antagonized rituximab activity, we found that TG-1701 did not impair the anti-CD20-mediated ADCC and ADCP in a multicellular co-culture system that associated MCL to effector cells. In another set of cell-based assays, we showed that the treatment with TG-1701 led to a strong decrease in IL-10 expression, together with a significant increase in IL-2, IL-6 and IL1RN gene transcription, thus associating TG-1701 exposure with the promotion of an immune response within the tumor microenvironment (TME). Accordingly, TG-1701 cooperated with the novel anti-CD20 antibody ublituximab and the novel PI3Kδ-CK1ϵ dual inhibitor umbralisib in reducing the tumor growth in both ibrutinib-sensitive (REC1) and ibrutinib-resistant (UPN-1res) mouse models of MCL. In accordance with in vitro data, the antitumor activity of the drug combination was associated with increased production of IL-2, IL-6 and TNFα, and with an increased infiltration of mouse macrophages and NK cells. Altogether, these data warrant further clinical investigation of TG-1701-mediated engagement of TME-related anti-tumor effect, and clinical evaluation of the triple combination of TG-1701, ublituximab, and umbralisib, is ongoing. Citation Format: Marcelo L. Ribeiro, Diana Reyes-Garau, Meritxell Vinyoles, Marc Armengol, Hari Miskin, Francesc Bosch, Pablo Menendez, Emmanuel Normant, Gael Roue. TG-1701, a novel irreversible Bruton's kinase (BTK) inhibitor, cooperates with ublituximab-driven ADCC and ADCP in in vitro and in vivo models of ibrutinib-resistant mantle cell lymphoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2205.

Correlation of PI3K upregulation with NOTCH2 mutations in ibrutinib-resistant mantle cell lymphoma.

e20065 Background: PI3K dysregulation has been linked to ibrutinib resistance in mantle cell lymphoma (MCL); however, the investigation of PI3K signaling as an important ibrutinib resistance mechanism has been rarely assessed in clinical samples. Therefore, we sought to more fully characterize PI3K dysregulation in ibrutinib-sensitive and -resistant MCL clinical samples at the genomic and transcriptomic levels. Methods: Whole exome sequencing (WES; n = 41) and RNA-seq (n = 93) were performed on fresh peripheral blood, apheresis, or biopsy patient primary samples. All analysis was performed using the BostonGene automated pipeline. Results: To evaluate ibrutinib resistance in MCL, we compared the activity of 11 cellular pathways calculated by PROGENy between ibrutinib-sensitive and -resistant MCL. The PI3K pathway was the most differentially expressed between the two groups (q < 0.005; > 1 median absolute deviation in ibrutinib-resistant MCL). Increased PI3K pathway expression in the ibrutinib-resistant MCL tumors strongly correlated with hyperproliferation (r = 0.49; p = 6e-07). Both hyperproliferation and enriched PI3K expression associated with more frequent NOTCH2 somatic mutations (~22% ibrutinib-resistant MCL with high PI3K expression (p = 0.004) and hyperproliferation (p = 0.003)) that generate a premature stop codon in the PEST domain, likely resulting in hyperactive NOTCH2. Frequent TP53 mutations were identified in ibrutinib-resistant MCL tumors (58%; 3-fold greater vs ibrutinib-sensitive tumors; p = 0.02) based on our cohort and the Agarwal et al., 2019, Nature Medicine cohort. PI3K signaling is a known p53 activator, which may lead to a compensatory tumor suppressive mechanism, indicating that PI3K activation may induce mutational pressure on TP53 to promote MCL survival. VH gene usage variability of the B-cell receptor (BCR), the inducer of PI3K signaling, was not different between ibrutinib-sensitive and -resistant MCL, suggesting that diverse BCR expression does not underlie enriched ibrutinib-resistant MCL PI3K expression. Conclusions: PI3K inhibitors have resulted in underwhelming MCL clinical outcomes; yet, the advent of next-generation PI3K inhibitors such as copanlisib with potentially greater efficacy and less toxicity warrants the continued investigation of PI3K signaling in ibrutinib-resistant MCL. We identified a strong correlation between gain-of-function NOTCH2 mutations and enriched PI3K signaling, suggesting that dual inhibition of NOTCH and PI3K may overcome ibrutinib resistance in MCL.

Multi-omics analysis of mantle cell lymphoma reveals an immune-cold tumor microenvironment associated with ibrutinib resistance.

8055 Background: Mantle cell lymphoma (MCL) is an aggressive and incurable B-cell lymphoma. Although the role of the MCL tumor microenvironment (TME) in survival and therapeutic response has been studied, greater knowledge regarding the tumor-immune interaction is needed to develop MCL immunotherapeutic strategies. Methods: Whole exome sequencing (WES; n = 41) and RNA-seq (n = 93) were performed on fresh peripheral blood, apheresis, or biopsy MCL patient primary samples. Joint WES and RNA-seq mutation calling, expression analysis were performed by BostonGene. Results: Both tumor and TME molecular signatures were characterized based on ibrutinib response. Concurrent analysis of MCL biopsy samples with an additional previously published cohort (n = 122; Scott et al., JCO, 2017) identified 4 MCL microenvironment signatures (Nomie et al., Blood, 2019) in which the ibrutinib-resistant MCL samples primarily belonged to the “stroma-enriched” subtype (29%; 6/8 resistant, non-immune with increased stromal signature and tumor-promoting cytokines), whereas most of the ibrutinib-sensitive samples were assigned to the “immune-hot” subtype (53%, 9/9 sensitive; anti-tumor infiltration, high immune and checkpoint molecule expression with low stromal expression, Chi-square test p-value = 0.001). NOTCH1 gain-of-function mutations (25%, 3/12 resistant) in the PEST domain were found exclusively in the ibrutinib-resistant cohort associated with the microenvironment-depleted subtype. Frequent recurring inactivating mutations in the epigenetic modifier KMT2D (30%) were identified in MCL cells associated with the ‘immune-suppressed” subtype (p < 0.05). Loss-of-function mutations in epigenetic modifiers have been tied to immune evasion. PD-L1 was significantly downregulated in the ibrutinib-resistant MCL tumors (p = 0.03), indicating that targeting the PD-L1 and PD-1 immune checkpoint axis may not be beneficial. Conclusions: Ibrutinib sensitivity and resistance were defined by immune-hot and immune-cold TME portraits, respectively, suggesting that the TME has a prominent role in mediating ibrutinib response. Ibrutinib has been suggested to activate the immune TME through its off target inhibition of interleukin 2–inducible T-cell kinase (ITK). The immune activation by ibrutinib suggests that anti-tumor activity of ibrutinib may be better harnessed by combining ibrutinib with immunotherapy.

Genetically Defined Metabolic Targets Overcome Ibrutinib Resistance in Mantle Cell Lymphoma

Background: Mantle cell lymphoma (MCL), which accounts for around 7% of non-Hodgkin lymphomas, is currently incurable. MCL's key survival pathway is the B-cell receptor pathway, and Bruton's tyrosine kinase (BTK), the critical component of this pathway, is fundamentally activated in MCL. Ibrutinib, an agent that inhibits BTK, produced a 68% clinical response rate in patients with relapsed/refractory MCL, leading to its FDA approval. However, if MCL patients experience relapse following ibrutinib therapy or do not respond to ibrutinib, the 1-year survival rate is only 22%. Therefore, determining the mechanisms underlying ibrutinib resistance and identifying alternative therapeutic options are urgent unmet needs. Our group and others have shown that the BTK mutation associated with ibrutinib resistance is rare in MCL, and targeting alternative survival pathways to overcome ibrutinib resistance has not shown promising results. Methods: To explore novel biomarkers or therapeutic targets, we performed whole exome and transcriptome sequencing and RNA-seq of ibrutinib-treated clinical specimens. Patient primary cells were isolated from MCL patients treated with ibrutinib either prior to treatment or at treatment discontinuation. Whole exome sequencing (WES) was performed to determine the mutational landscape of ibrutinib resistance. RNA-seq was employed to compare the gene expression profiles between ibrutinib-sensitive and -resistant patient samples. Gene set enrichment analysis was utilized to identify dysregulated molecular pathways associated with the resistant phenotype. The RNA-seq data were then validated with western blots. Functional studies targeting this molecular pathway were conducted, and in vivo efficacy was investigated in the ibrutinib-resistant MCL patient-derived xenograft (PDX) mouse model. Results: Co-deletion of CDKN2A and MTAP was frequently detected in ibrutinib-resistant tumors but not in ibrutinib-sensitive tumors. We also found that metabolic reprogramming towards oxidative phosphorylation (OXPHOS) potentially drives ibrutinib resistance. IACS-010759, a small molecule developed by MD Anderson Cancer Center, inhibits OXPHOS and overcomes ibrutinib resistance in vitro and in PDX models. Nevertheless, the potential drivers of this observed metabolic reprogramming towards OXPHOS remain unclear. MTAP encodes 5-methylthioadenosine phosphorylase, a critical enzyme in the methionine salvage pathway, and is frequently lost in cancer cells, resulting in accumulation of metabolite methylthioadenosine and overexpression of protein arginine methyltransferase 5 (PRMT5). Thus, PRMT5 is a potential druggable therapeutic target to overcome ibrutinib resistance in MCL. We found that PRMT5 is highly expressed in ibrutinib-resistant MCL but not ibrutinib-sensitive MCL (7 ibrutinib-resistant vs. 15 ibrutinib-sensitive patient samples, p = 0.0051), especially in MTAP-deleted MCL specimens. Furthermore, patients with high PRMT5 levels had poor clinical outcomes on ibrutinib treatment. PRMT5 regulates tumor survival pathways by the methylation of histones H3R8, H4R3, and post-translational methylation of non-histone proteins, including p53, E2F1, BCL6, p65, and others. A novel PRMT5 inhibitor, GSK3226595, significantly inhibited tumor growth in an ibrutinib-resistant MCL PDX model (n = 5; vehicle vs. GSK3226595, p = 0.000048) and had no significant mouse body weight change (GSK3226595 vs. vehicle, p = 0.08). Conclusion: This study may pave the way to develop a PRMT5/OXPHOS-based therapeutic strategy to overcome ibrutinib resistance in MCL. Disclosures Wang: Juno Therapeutics: Research Funding; Aviara: Research Funding; Dava Oncology: Honoraria; Celgene: Honoraria, Research Funding; Loxo Oncology: Research Funding; VelosBio: Research Funding; Janssen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Pharmacyclics: Honoraria, Research Funding; AstraZeneca: Consultancy, Honoraria, Research Funding, Speakers Bureau; MoreHealth: Consultancy, Equity Ownership; Acerta Pharma: Consultancy, Research Funding; Kite Pharma: Consultancy, Research Funding; Guidepoint Global: Consultancy; BioInvent: Consultancy, Research Funding.