Targeted Delivery of BZLF1 to DEC205 Drives EBV-Protective Immunity in a Spontaneous Model of EBV-Driven Lymphoproliferative Disease

Epstein-Barr virus (EBV) is a human herpes virus that infects over 90% of the world's population and is linked to development of cancer. In immune-competent individuals, EBV infection is mitigated by a highly efficient virus-specific memory T-cell response. Risk of EBV-driven cancers increases with immune suppression (IS). EBV-seronegative recipients of solid organ transplants are at high risk of developing post-transplant lymphoproliferative disease (PTLD) due to iatrogenic IS. While reducing the level of IS may improve EBV-specific immunity and regression of PTLD, patients are at high risk for allograft rejection and need for immune-chemotherapy. Strategies to prevent PTLD in this vulnerable patient population represents an unmet need. We have previously shown that BZLF1-specific cytotoxic T-cell (CTL) expansion following reduced IS correlated with immune-mediated PTLD regression and improved patient survival. We have developed a vaccine to bolster EBV-specific immunity to the BZLF1 protein and show that co-culture of dendritic cells (DCs) loaded with a αDEC205-BZLF1 fusion protein with peripheral blood mononuclear cells (PMBCs) leads to expansion and increased cytotoxic activity of central-effector memory CTLs against EBV-transformed B-cells. Human-murine chimeric Hu-PBL-SCID mice were vaccinated with DCs loaded with αDEC205-BZLF1 or control to assess prevention of fatal human EBV lymphoproliferative disease. Despite a profoundly immunosuppressive environment, vaccination with αDEC205-BZLF1 stimulated clonal expansion of antigen-specific T-cells that produced abundant IFNγ and significantly prolonged survival. These results support preclinical and clinical development of vaccine approaches using BZLF1 as an immunogen to harness adaptive cellular responses and prevent PTLD in vulnerable patient populations.

Validation of protein arginine methyltransferase 5 (PRMT5) as a candidate therapeutic target in the spontaneous canine model of non-Hodgkin lymphoma

Non-Hodgkin lymphoma (NHL) is a heterogeneous group of blood cancers arising in lymphoid tissues that commonly effects both humans and dogs. Protein arginine methyltransferase 5 (PRMT5), an enzyme that catalyzes the symmetric di-methylation of arginine residues, is frequently overexpressed and dysregulated in both human solid and hematologic malignancies. In human lymphoma, PRMT5 is a known driver of malignant transformation and oncogenesis, however, the expression and role of PRMT5 in canine lymphoma has not been explored. To explore canine lymphoma as a useful comparison to human lymphoma while validating PRMT5 as a rational therapeutic target in both, we characterized expression patterns of PRMT5 in canine lymphoma tissue microarrays, primary lymphoid biopsies, and canine lymphoma-derived cell lines. The inhibition of PRMT5 led to growth suppression and induction of apoptosis, while selectively decreasing global marks of symmetric dimethylarginine (SDMA) and histone H4 arginine 3 symmetric dimethylation. We performed ATAC-sequencing and gene expression microarrays with pathway enrichment analysis to characterize genome-wide changes in chromatin accessibility and whole-transcriptome changes in canine lymphoma cells lines upon PRMT5 inhibition. This work validates PRMT5 as a promising therapeutic target for canine lymphoma and supports the continued use of the spontaneously occurring canine lymphoma model for the preclinical development of PRMT5 inhibitors for the treatment of human NHL.

Preclinical evaluation of the Hsp90 inhibitor SNX-5422 in ibrutinib resistant CLL

B-cell receptor (BCR) antagonists such as the BTK inhibitor ibrutinib have proven to effectively target chronic lymphocytic leukemia (CLL) tumor cells, leading to impressive response rates in these patients. However patients do still relapse on ibrutinib, and the progressive disease is often quite aggressive requiring immediate treatment. Several strategies are being pursued to treat patients who relapse on ibrutinib therapy. As the most common form of relapse is the development of a mutant form of BTK which limits ibrutinib binding, agents which lead to degradation of the BTK protein are a promising strategy. Our study explores the efficacy of the Hsp90 inhibitor, SNX-5422, in CLL. The SNX Hsp90 inhibitor was effective in primary CLL cells, as well as B-cell lines expressing either BTK wild type or C481 mutant BTK, which has been identified as the primary resistance mechanism to ibrutinib in CLL patients. Furthermore the combination of SNX-5422 and ibrutinib provided a remarkable in vivo survival benefit in the Eμ-TCL1 mouse model of CLL compared to the vehicle or single agent groups (51 day median survival in the vehicle and ibrutinib groups versus 100 day median survival in the combination). We report here preclinical data suggesting that the Hsp90 inhibitor SNX-5422, which has been pursued in clinical trials in both solid tumor and hematological malignancies, is a potential therapy for ibrutinib resistant CLL.

Targeting DNA Damage Repair Functions of Two Histone Deacetylases, HDAC8 and SIRT6, Sensitizes Acute Myeloid Leukemia to NAMPT Inhibition

PURPOSE

Nicotinamide phosphoribosyltransferase (NAMPT) inhibitors (NAMPTi) are currently in development, but may be limited as single-agent therapy due to compound-specific toxicity and cancer metabolic plasticity allowing resistance development. To potentially lower the doses of NAMPTis required for therapeutic benefit against acute myeloid leukemia (AML), we performed a genome-wide CRISPRi screen to identify rational disease-specific partners for a novel NAMPTi, KPT-9274.

EXPERIMENTAL DESIGN

Cell lines and primary cells were analyzed for cell viability, self-renewal, and responses at RNA and protein levels with loss-of-function approaches and pharmacologic treatments. In vivo efficacy of combination therapy was evaluated with a xenograft model.

RESULTS

We identified two histone deacetylases (HDAC), HDAC8 and SIRT6, whose knockout conferred synthetic lethality with KPT-9274 in AML. Furthermore, HDAC8-specific inhibitor, PCI-34051, or clinical class I HDAC inhibitor, AR-42, in combination with KPT-9274, synergistically decreased the survival of AML cells in a dose-dependent manner. AR-42/KPT-9274 cotreatment attenuated colony-forming potentials of patient cells while sparing healthy hematopoietic cells. Importantly, combined therapy demonstrated promising in vivo efficacy compared with KPT-9274 or AR-42 monotherapy. Mechanistically, genetic inhibition of SIRT6 potentiated the effect of KPT-9274 on PARP-1 suppression by abolishing mono-ADP ribosylation. AR-42/KPT-9274 cotreatment resulted in synergistic attenuation of homologous recombination and nonhomologous end joining pathways in cell lines and leukemia-initiating cells.

CONCLUSIONS

Our findings provide evidence that HDAC8 inhibition- or shSIRT6-induced DNA repair deficiencies are potently synergistic with NAMPT targeting, with minimal toxicity toward normal cells, providing a rationale for a novel-novel combination-based treatment for AML.

XPO1 expression worsens the prognosis of unfavorable DLBCL that can be effectively targeted by selinexor in the absence of mutant p53

The XPO1 inhibitor selinexor was recently approved in relapsed/refractory DLBCL patients but only demonstrated modest anti-DLBCL efficacy, prompting us to investigate the prognostic effect of XPO1 in DLBCL patients and the rational combination therapies in high-risk DLBCL. High XPO1 expression (XPO1high) showed significant adverse prognostic impact in 544 studied DLBCL patients, especially in those with BCL2 overexpression. Therapeutic study in 30 DLBCL cell lines with various molecular and genetic background found robust cytotoxicity of selinexor, especially in cells with BCL2-rearranged (BCL2-R+) DLBCL or high-grade B-cell lymphoma with MYC/BCL2 double-hit (HGBCL-DH). However, expression of mutant (Mut) p53 significantly reduced the cytotoxicity of selinexor in overall cell lines and the BCL2-R and HGBCL-DH subsets, consistent with the favorable impact of XPO1high observed in Mut-p53-expressing patients. The therapeutic effect of selinexor in HGBCL-DH cells was significantly enhanced when combined with a BET inhibitor INCB057643, overcoming the drug resistance in Mut-p53-expressing cells. Collectively, these data suggest that XPO1 worsens the survival of DLBCL patients with unfavorable prognostic factors such as BCL2 overexpression and double-hit, in line with the higher efficacy of selinexor demonstrated in BCL2-R+ DLBCL and HGBCL-DH cell lines. Expression of Mut-p53 confers resistance to selinexor treatment, which can be overcome by combined INCB057643 treatment in HGBCL-DH cells. This study provides insight into the XPO1 significance and selinexor efficacy in DLBCL, important for developing combination therapy for relapsed/refractory DLBCL and HGBCL-DH.

Transducin β-like protein 1 controls multiple oncogenic networks in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) is the most common non- Hodgkin lymphoma and is characterized by a remarkable heterogeneity with diverse variants that can be identified histologically and molecularly. Large-scale gene expression profiling studies have identified the germinal center B-cell (GCB-) and activated B-cell (ABC-) subtypes. Standard chemo-immunotherapy remains standard front-line therapy, curing approximately two thirds of patients. Patients with refractory disease or those who relapse after salvage treatment have an overall poor prognosis highlighting the need for novel therapeutic strategies. Transducin b-like protein 1 (TBL1) is an exchange adaptor protein encoded by the TBL1X gene and known to function as a master regulator of the Wnt signaling pathway by binding to β-CATENIN and promoting its downstream transcriptional program. Here, we show that, unlike normal B cells, DLBCL cells express abundant levels of TBL1 and its overexpression correlates with poor clinical outcome regardless of DLBCL molecular subtype. Genetic deletion of TBL1 and pharmacological approach using tegavivint, a first-in-class small molecule targeting TBL1 (Iterion Therapeutics), promotes DLBCL cell death in vitro and in vivo. Through an integrated genomic, biochemical, and pharmacologic analyses, we characterized a novel, β-CATENIN independent, post-transcriptional oncogenic function of TBL1 in DLBCL where TBL1 modulates the stability of key oncogenic proteins such as PLK1, MYC, and the autophagy regulatory protein BECLIN-1 through its interaction with a SKP1-CUL1-F-box (SCF) protein supercomplex. Collectively, our data provide the rationale for targeting TBL1 as a novel therapeutic strategy in DLBCL.

Evaluating the Therapeutic Potential of Zanubrutinib in the Treatment of Relapsed/Refractory Mantle Cell Lymphoma: Evidence to Date

Mantle cell lymphoma (MCL) is an uncommon B-cell non-Hodgkin lymphoma characterized by an aggressive clinical course in the majority of patients. Despite recent improvements in outcomes, MCL remains incurable and a major therapeutic challenge. BTK inhibitors are the preferred treatment option for patients with relapsed/refractory MCL, including those unfit for chemotherapy or those with chemoresistant disease. In addition to ibrutinib and acalabrutinib, the FDA recently approved zanubrutinib for the treatment of patients with relapsed/refractory MCL based on the results of two Phase 2 clinical trials showing overall response rates of 85–87% with complete responses in 30–77% of patients. Compared with ibrutinib, zanubrutinib is more selective for BTK and has less off-target inhibition, which is thought to limit certain toxicities although direct comparative data are still lacking. This review article summarizes data from clinical trials of currently FDA-approved BTK inhibitors in MCL with a focus on zanubrutinib.

Protein arginine methyltransferase 5 represses tumor suppressor miRNAs that down-regulate CYCLIN D1 and c-MYC expression in aggressive B-cell lymphoma

Protein arginine methyltransferase-5 (PRMT5) is overexpressed in aggressive B-cell non-Hodgkin's lymphomas, including mantle cell lymphoma and diffuse large B-cell lymphoma, and supports constitutive expression of CYCLIN D1 and c-MYC. Here, we combined ChIP analysis with next-generation sequencing to identify microRNA (miRNA) genes that are targeted by PRMT5 in aggressive lymphoma cell lines. We identified enrichment of histone 3 dimethylation at Arg-8 (H3(Me2)R8) in the promoter regions of miR33b, miR96, and miR503. PRMT5 knockdown de-repressed transcription of all three miRNAs, accompanied by loss of recruitment of epigenetic repressor complexes containing PRMT5 and either histone deacetylase 2 (HDAC2) or HDAC3, enhanced binding of co-activator complexes containing p300 or CREB-binding protein (CBP), and increased acetylation of specific histones, including H2BK12, H3K9, H3K14, and H4K8 at the miRNA promoters. Re-expression of individual miRNAs in B-cell lymphoma cells down-regulated expression of PRMT5, CYCLIN D1, and c-MYC, which are all predicted targets of these miRNAs, and reduced lymphoma cell survival. Luciferase reporter assays with WT and mutant 3'UTRs of CYCLIN D1 and c-MYC mRNAs revealed that binding sites for miR33b, miR96, and miR503 are critical for translational regulation of the transcripts of these two genes. Our findings link altered PRMT5 expression to transcriptional silencing of tumor-suppressing miRNAs in lymphoma cells and reinforce PRMT5's relevance for promoting lymphoma cell growth and survival.

Bruton tyrosine kinase inhibitors for the treatment of mantle cell lymphoma: review of current evidence and future directions

Mantle cell lymphoma (MCL) is a heterogeneous and uncommon non-Hodgkin lymphoma that affects predominantly older patients and often is associated with an aggressive clinical course. MCL relies upon B-cell receptor signaling through Bruton tyrosine kinase (BTK); therefore, the development of the BTK inhibitors ibrutinib and acalabrutinib represents a therapeutic breakthrough. In this review, we provide a summary of the efficacy and safety data from the landmark trials of single-agent ibrutinib and acalabrutinib that led to US Food and Drug Administration approval of these agents for patients with relapsed or refractory MCL. Toxicities of interest observed with ibrutinib include bleeding, atrial fibrillation, and increased risk for infection. The selectivity of acalabrutinib for BTK is greater than that of ibrutinib, which mitigates the risk for certain off-target toxicities, including atrial fibrillation; however, these toxicities, along with frequent headaches, still occur. Ongoing clinical trials are investigating both alternate BTK inhibitors and BTK inhibitors in combination with chemo-immunotherapy or other targeted agents in an effort to enhance the depth and duration of response. Trials to evaluate the use of these agents in the frontline setting are emerging and are likely to build upon the success of BTK inhibitors in patients with MCL.

Modulation of immune checkpoint molecule expression in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an aggressive B-cell malignancy for which novel therapeutics with improved efficacy are greatly needed. To provide support for clinical immune checkpoint blockade, we comprehensively evaluated the expression of therapeutically targetable immune checkpoint molecules on primary MCL cells. MCL cells showed constitutive expression of Programmed Death 1 (PD-1) and Programmed Death Ligand 1 (PD-L1), variable CD200, absent PD-L2, Lymphocyte Activation Gene 3 (LAG-3), and Cytotoxic T-cell Associated Protein 4 (CTLA-4). Effector cells from MCL patients expressed PD-1. Co-culture of MCL cells with T-cells induced PD-L1 surface expression, a phenomenon regulated by IFNγ and CD40:CD40L interaction. Induction of PD-L1 was attenuated by concurrent treatment with ibrutinib or duvelisib, suggesting BTK and PI3K are important mediators of PD-L1 expression. Overall, our data provide further insight into the expression of checkpoint molecules in MCL and support the use of PD-L1 blocking antibodies in MCL patients.

PLK1: a promising and previously unexplored target in double-hit lymphoma

Inhibitors that target specific kinases or oncoproteins have become popular additions to or replacements for cytotoxic chemotherapies to treat many different types of cancer. However, many tumors lack a discernable target kinase and an amplified oncoprotein and/or rely on several cooperating mechanisms for progression. Thus, combinations of targeted therapies are essential for treating many cancers to avoid the rapid emergence of resistance. In this issue of the JCI, Ren et al. use an elegant kinase activity-profiling method and identify activity of the oncogene polo-like kinase-1 (PLK1) as an important driver of double-hit lymphoma (DHL), an aggressive subgroup of B cell lymphoma characterized by chromosomal translocations involving c-MYC and BCL2 or BCL6. Moreover, PLK1 activity was associated with MYC expression and poor prognosis in DHL patients. PLK1 inhibition with volasertib, alone and in combination with the BCL-2 inhibitor venetoclax, was efficacious in multiple DHL models, including mice harboring DHL patient-derived xenografts. Together, these data support PLK1 as a promising prognostic marker and therapeutic target for DHL.

Pembrolizumab and its role in relapsed/refractory classical Hodgkin's lymphoma: evidence to date and clinical utility

Immune evasion is a critical mechanism of malignant cell survival, and relies in part on molecular signaling through the programmed cell death 1 (PD-1)/PD-1 ligand (PD-L1) axis that contributes to T cell exhaustion. Immune modulatory therapy with monoclonal antibodies against PD-1 designed to enhance antitumor immune response have shown promise in the treatment of advanced solid tumors and hematologic malignancies. Classical Hodgkin's lymphoma (cHL), a unique B cell malignancy characterized by an extensive but ineffective immune cell infiltrate surrounding a small number of tumor cells, has shown significant response to anti-PD-1 directed therapy. The anti-PD-1 monoclonal antibodies nivolumab and pembrolizumab have shown similarly remarkable activity in relapsed/refractory cHL and have been approved by the Food and Drug Administration for treatment of this disease. In this article we review the rationale of targeting the PD-1/PD-L1 axis in cHL and the pharmacology of pembrolizumab, and summarize the data on activity and safety profile of this agent in the treatment of relapsed/refractory cHL. We also discuss the potential benefits and pitfalls of using PD-1 blockade in the setting of allogeneic stem-cell transplantation, and summarize ongoing prospective trials of single-agent pembrolizumab and combination strategies as well as future directions.

BRD4 Profiling Identifies Critical Chronic Lymphocytic Leukemia Oncogenic Circuits and Reveals Sensitivity to PLX51107, a Novel Structurally Distinct BET Inhibitor

Bromodomain and extra-terminal (BET) family proteins are key regulators of gene expression in cancer. Herein, we utilize BRD4 profiling to identify critical pathways involved in pathogenesis of chronic lymphocytic leukemia (CLL). BRD4 is overexpressed in CLL and is enriched proximal to genes upregulated or de novo expressed in CLL with known functions in disease pathogenesis and progression. These genes, including key members of the B-cell receptor (BCR) signaling pathway, provide a rationale for this therapeutic approach to identify new targets in alternative types of cancer. Additionally, we describe PLX51107, a structurally distinct BET inhibitor with novel in vitro and in vivo pharmacologic properties that emulates or exceeds the efficacy of BCR signaling agents in preclinical models of CLL. Herein, the discovery of the involvement of BRD4 in the core CLL transcriptional program provides a compelling rationale for clinical investigation of PLX51107 as epigenetic therapy in CLL and application of BRD4 profiling in other cancers.Significance: To date, functional studies of BRD4 in CLL are lacking. Through integrated genomic, functional, and pharmacologic analyses, we uncover the existence of BRD4-regulated core CLL transcriptional programs and present preclinical proof-of-concept studies validating BET inhibition as an epigenetic approach to target BCR signaling in CLL. Cancer Discov; 8(4); 458-77. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 371.

Emerging treatment options for the management of Hodgkin's lymphoma: clinical utility of nivolumab

Classical Hodgkin's lymphoma (cHL) is a B-cell malignancy comprised of pathologic Reed Sternberg cells with a surrounding immune-tolerant inflammatory milieu. RS cells evade immune recognition in part through programmed death ligand 1 (PD-L1) overexpression, which is genetically programmed through copy number alterations, polysomy, and amplification of the 9p24.1 locus encoding PD-L1. By engaging with PD-1+ T-cells, PD-L1 delivers a potent immune suppressive signal promoting immunologic escape of the tumor cell. Enhancing antitumor immune response by targeting PD-1 with the monoclonal antibody nivolumab has proved to be effective in multiple solid tumors, but the highest response rates to date have been reported in patients with cHL, with over 65% of treated patients achieving an objective clinical response. In this review, we will summarize the published evidence regarding the activity of nivolumab in cHL as well as its current place in therapy. We will review the pharmacology, mechanism of action, and side effects of nivolumab as well as the emerging data indicating possible increased risk of graft versus host disease in patients treated with PD-1 inhibitors either pre- or post-allogeneic stem cell transplant. Given the remarkable single-agent activity and safety profile of PD-1 inhibitors in heavily pretreated patients with cHL, the possibility of employing nivolumab in combination with other active agents and earlier in therapy is a promising area of active investigation, and we will briefly summarize current clinical trials.

Pembrolizumab in classical Hodgkin's lymphoma

Pembrolizumab is a humanized monoclonal antibody directed against programmed cell death protein 1 (PD-1), a key immune-inhibitory molecule expressed on T cells and implicated in CD4+ T-cell exhaustion and tumor immune-escape mechanisms. Classical Hodgkin's lymphoma (cHL) is a unique B-cell malignancy in the sense that malignant Reed-Sternberg (RS) cells represent a small percentage of cells within an extensive immune cell infiltrate. PD-1 ligands are upregulated on RS cells as a consequence of both chromosome 9p24.1 amplification and Epstein-Barr virus infection and by interacting with PD-1 promote an immune-suppressive effect. By augmenting antitumor immune response, pembrolizumab and nivolumab, another monoclonal antibody against PD-1, have shown significant activity in patients with relapsed/refractory cHL as well as an acceptable toxicity profile with immune-related adverse events that are generally manageable. In this review, we explore the rationale for targeting PD-1 in cHL, review the clinical trial results supporting the use of checkpoint inhibitors in this disease, and present future directions for investigation in which this approach may be used.

De novo CD5+ diffuse large B-cell lymphoma: Adverse outcomes with and without stem cell transplantation in a large, multicenter, rituximab treated cohort

De novo CD5+ diffuse large B-cell lymphomas (DLBCL) are a distinct subgroup of DLBCL with poor prognosis. However the role of rituximab-containing therapy and salvage stem cell transplantation in this patients' population remain to be defined. We retrospectively reviewed clinical features and outcomes of 102 patients with de novo CD5+ DLBCL treated with rituximab-containing therapy at nine different institutions. By Hans' criteria, 64 patients had activated B-cell (ABC) subtype, 24 germinal center B-cell (GCB) subtype, and 14 were not evaluated. No patients had a myc translocation. Eighty-three patients were treated with rituximab, cyclophosphamide, doxorubicin, vincristine, prednisone (R-CHOP), 7 with rituximab, etoposide, cyclophosphamide, doxorubicin, vincristine, prednisone (R-EPOCH), and 6 with R-CHOP with methotrexate, 3 g/m(2) . The overall response rate to front-line therapy was 85%. The 3-year progression free survival (PFS) and overall survival (OS) for all patients were 40 and 65%, respectively. The 3-year PFS for ABC- and GCB-subtypes was 34 and 45%, respectively. The 3-year OS for ABC- and GCB-subtypes was 62 and 67%, respectively. The median time to second treatment failure was 3 months and 1 month for ABC- and GCB-subtypes, respectively. Twenty of 28 (71%) transplanted patients with autologous, allogeneic, or both, relapsed. This study confirms the poor prognosis of de novo CD5+ DLBCL in a large multi-center cohort despite initial rituximab-containing chemotherapy and suggests that stem cell transplantation fails to salvage the majority of these patients. Approaches to prevent recurrence and/or novel therapies for relapsed disease are needed for this subgroup of DLBCL patients.

Bruton's tyrosine kinase inhibitors in B-cell non-Hodgkin's lymphomas

The B-cell receptor pathway (BCR) is aberrantly activated in select B-cell malignancies. This knowledge has allowed for the development of inhibitors of different crucial steps of this pathway. Bruton's tyrosine kinase (BTK) is a key component of BCR signaling and functions as an important regulator of multiple cell functions including differentiation, proliferation, and survival in various B-cell malignancies. Ibrutinib is a potent, selective BTK inhibitor that has shown significant activity in specific subtypes of B-cell non-Hodgkin's lymphomas (NHLs). Given the high response rates, tolerability, and acceptable toxicities, ibrutinib was recently approved by the US Food and Drug Administration (FDA) for the treatment of patients with relapsed mantle cell lymphoma and chronic lymphocytic leukemia. It is also currently being evaluated in combination with chemotherapy and as frontline therapy in B-cell NHL. This review summarizes the preclinical and clinical development of ibrutinib in the treatment of B-cell NHL.

Genetic validation of the protein arginine methyltransferase PRMT5 as a candidate therapeutic target in glioblastoma

Glioblastoma is the most common and aggressive histologic subtype of brain cancer with poor outcomes and limited treatment options. Here, we report the selective overexpression of the protein arginine methyltransferase PRMT5 as a novel candidate theranostic target in this disease. PRMT5 silences the transcription of regulatory genes by catalyzing symmetric dimethylation of arginine residues on histone tails. PRMT5 overexpression in patient-derived primary tumors and cell lines correlated with cell line growth rate and inversely with overall patient survival. Genetic attenuation of PRMT5 led to cell-cycle arrest, apoptosis, and loss of cell migratory activity. Cell death was p53-independent but caspase-dependent and enhanced with temozolomide, a chemotherapeutic agent used as a present standard of care. Global gene profiling and chromatin immunoprecipitation identified the tumor suppressor ST7 as a key gene silenced by PRMT5. Diminished ST7 expression was associated with reduced patient survival. PRMT5 attenuation limited PRMT5 recruitment to the ST7 promoter, led to restored expression of ST7 and cell growth inhibition. Finally, PRMT5 attenuation enhanced glioblastoma cell survival in a mouse xenograft model of aggressive glioblastoma. Together, our findings defined PRMT5 as a candidate prognostic factor and therapeutic target in glioblastoma, offering a preclinical justification for targeting PRMT5-driven oncogenic pathways in this deadly disease.

Abstract 4637: Developing a first in class of drug to inhibit protein arginine methyltransferase 5 (PRMT5) enzyme dysregulation in glioblastoma multiforme

Glioblastoma Multiforme (GBM) is associated with a dismal prognosis despite intensive multimodal therapy, highlighting the need for novel therapeutic approaches. Chromatin remodeling complexes and associated co-repressors such as histone deacetylases (HDAC), DNA methyltransferases (DNMT) and protein arginine methyltransferase 5 (PRMT5) are involved in silencing tumor suppressor and regulatory gene expression and may contribute to glial cell transformation. PRMT5 silences the transcription of key regulatory genes by symmetric di-methylation (S2Me) of arginine (R) residues on histone proteins (H4R3 and H3R8) and works more efficiently when associated with other co-repressor enzymes. We have previously identified PRMT5 over-expression in GBM cell lines, primary GBM tumors, and GBM that spontaneously develop in a pre-clinical mouse model. The degree of PRMT5 over-expression inversely correlated with survival of GBM patients (r=-0.57, p=0.0001) and correlated with proliferation of GBM cell lines (r=0.81, p<0.0001), PRMT5 works concertedly with HDAC2, methyl-CpG binding domain protein 2 (MBD2) and DNMT3a to silence genes with anti-cancer and immune modulatory activities. PRMT5 silending in GBM cell lines leads to growth arrest, apoptosis and loss of HDAC2, DNMT3a and PRMT5 co-repressor complex recruitment at chemokine gene promoters. These findings provide evidence that PRMT5 is a central repressive element required for maintenance of target gene silencing. Thus, we explored methods to inhibit PRMT5 activity as a novel experimental therapeutic strategy for GBM. A rational design of small molecule compounds to inhibit PRMT5 activity led us to construct an in silico model of the human PRMT5 catalytic domain based on available homologous crystal structures. We screened a library of 10,000 compounds and eight small molecules were identified based on binding energy in the PRMT5 catalytic site. Enzyme inhibition assays showed that a lead compound (BLL1) was capable of selectively inhibiting PRMT5 and not PRMT1 or PRMT7 activity (p<0.0001). BLL1 interfered with maintenance of S2Me-H4R3 and S2Me-H3R8 in GBM cell lines by western blot and confocal microscopy. Dose titration experiments with BLL1 showed a dose-dependent response of inhibition of cellular proliferation in GBM cell lines. Combination treatment of GBM cells with subtoxic doses of BLL1, hypomethylating agent 5-azacitidine, and HDAC inhibitor TSA showed synergistic induction of cell death, loss of the epigenetic mark S2Me-H4R3, and de-repression of the immune modulating chemokine CXCL10. Preclinical in vivo studies have shown favorable toxicity and pharmacokinetic profiles for BLL1. We have successfully developed a first in class drug to selectively target dysregulated PRMT5 enzymatic activity in GBM. We are currently developing drugs with improved selectivity and potency.

Citation Format: Fengting Yan, Kate Gordon, Kiran Mahasenan, Mark Lustberg, Lapo Alinari, Christian T. Earl, Balveen Kaur, Chenglong Li, Robert A. Baiocchi. Developing a first in class of drug to inhibit protein arginine methyltransferase 5 (PRMT5) enzyme dysregulation in glioblastoma multiforme. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4637. doi:10.1158/1538-7445.AM2013-4637

Dual targeting of the cyclin/Rb/E2F and mitochondrial pathways in mantle cell lymphoma with the translation inhibitor silvestrol

PURPOSE

During cell-cycle progression, D-cyclins activate cyclin-dependent kinases (CDKs) 4/6 to inactivate Rb, permitting E2F1-mediated S-phase gene transcription. This critical pathway is typically deregulated in cancer, and novel inhibitory strategies would be effective in a variety of tumors. The protein synthesis inhibitor silvestrol has potent activity in B-cell leukemias via the mitochondrial pathway of apoptosis, and also reduces cyclin D1 expression in breast cancer and lymphoma cell lines. We hypothesized that this dual activity of silvestrol would make it especially effective in malignancies driven by aberrant cyclin D1 expression.

EXPERIMENTAL DESIGN

Mantle cell lymphoma (MCL), characterized by elevated cyclin D1, was used as a model to test this approach. The cyclin D/Rb/E2F1 pathway was investigated in vitro using MCL cell lines and primary tumor cells. Silvestrol was also evaluated in vivo using an aggressive model of MCL.

RESULTS

Silvestrol showed low nanomolar potency both in MCL cell lines and primary MCL tumor cells. D-cyclins were depleted with just 10 nmol/L silvestrol at 16 hours, with subsequent reductions of phosphorylated Rb, E2F1 protein, and E2F1 target transcription. As showed in other leukemias, silvestrol caused Mcl-1 depletion followed by mitochondrial depolarization and caspase-dependent apoptosis, effects not related to inhibition of CDK4/6. Silvestrol significantly (P < 0.0001) prolonged survival in a MCL xenograft model without detectable toxicity.

CONCLUSIONS

These data indicate that silvestrol effectively targets the cyclin/CDK/Rb pathway, and additionally induces cytotoxicity via intrinsic apoptosis. This dual activity may be an effective therapeutic strategy in MCL and other malignancies.

FTY720-induced blockage of autophagy enhances anticancer efficacy of milatuzumab in mantle cell lymphoma: is FTY720 the next autophagy-blocking agent in lymphoma treatment?

Inhibition of the autophagic pathway has recently revealed promising results in increasing pro-death activity of multiple cancer therapeutics. Here, we discuss our findings regarding the autophagy-blocking and anti-neoplastic effects of a synthetic sphingosine analog, FTY720, in mantle cell lymphoma (MCL). We also emphasize how FTY720 enhances the pro-death activity of the fully humanized monoclonal antibody milatuzumab by inhibiting the autophagy-lysosome dependent degradation of its therapeutic target, CD74. Our results provide justification for further evaluation of FTY720 and milatuzumab as a combination therapy for this aggressive B-cell malignancy.

Novel targeted therapies for mantle cell lymphoma

Mantle cell lymphoma (MCL) is an aggressive B-cell malignancy characterized by short median survival despite intensive therapies. The clinical behavior of MCL may be due to the complex pathophysiology of the disease which includes its genetic hallmark, the chromosomal translocation t(11;14) resulting in aberrant expression of cyclin D1, alteration in the DNA damage response, and constitutive activation of key anti-apoptotic pathways such as phosphatidyl-inositol 3-kinase (PI3K)/Akt and nuclear factor-kB (NF-kB). Collectively, these changes result in cell cycle dysregulation and give rise to profound genetic instability. Given this complex pathophysiology, the limited number of options for patients with relapsed/refractory MCL, and the difficulty in achieving long-lasting remissions with conventional approaches, it is essential to explore new treatment options targeting the numerous dysregulated pathways that are operable in MCL. We have recently reported that milatuzumab, a fully humanized anti-CD74 monoclonal antibody (mAb), in combination with anti-CD20 mAbs has significant preclinical and clinical activity in MCL. Here we discuss these results, provide additional insights into milatuzumab-mediated MCL cell death, and report preliminary data on the activity of other targeted biologic agents including PCI-32765 and CAL-101 currently undergoing evaluation at our institution and others.