Overcoming Acquired Epigenetic Resistance to BTK Inhibitors

An inducible Cd79b mutation confers ibrutinib sensitivity in mouse models of Myd88-driven diffuse large B-cell lymphoma

ABSTRACT

Diffuse large B-cell lymphoma (DLBCL) is the most common aggressive lymphoma and constitutes a highly heterogenous disease. Recent comprehensive genomic profiling revealed the identity of numerous molecularly defined DLBCL subtypes, including a cluster which is characterized by recurrent aberrations in MYD88, CD79B, and BCL2, as well as various lesions promoting a block in plasma cell differentiation, including PRDM1, TBL1XR1, and SPIB. Here, we generated a series of autochthonous mouse models to mimic this DLBCL cluster and specifically focused on the impact of Cd79b mutations in this setting. We show that canonical Cd79b immunoreceptor tyrosine-based activation motif (ITAM) mutations do not accelerate Myd88- and BCL2-driven lymphomagenesis. Cd79b-mutant murine DLBCL were enriched for IgM surface expression, reminiscent of their human counterparts. Moreover, Cd79b-mutant lymphomas displayed a robust formation of cytoplasmic signaling complexes involving MYD88, CD79B, MALT1, and BTK. These complexes were disrupted upon pharmacological BTK inhibition. The BTK inhibitor-mediated disruption of these signaling complexes translated into a selective ibrutinib sensitivity of lymphomas harboring combined Cd79b and Myd88 mutations. Altogether, this in-depth cross-species comparison provides a framework for the development of molecularly targeted therapeutic intervention strategies in DLBCL.

ERBB4-Mediated Signaling Is a Mediator of Resistance to PI3K and BTK Inhibitors in B-cell Lymphoid Neoplasms

BTK and PI3K inhibitors are among the drugs approved for the treatment of patients with lymphoid neoplasms. Although active, their ability to lead to long-lasting complete remission is rather limited, especially in the lymphoma setting. This indicates that tumor cells often develop resistance to the drugs. We started from a marginal zone lymphoma cell line, Karpas-1718, kept under prolonged exposure to the PI3Kδ inhibitor idelalisib until acquisition of resistance, or with no drug. Cells underwent transcriptome, miRNA and methylation profiling, whole-exome sequencing, and pharmacologic screening, which led to the identification of the overexpression of ERBB4 and its ligands HBEGF and NRG2 in the resistant cells. Cellular and genetic experiments demonstrated the involvement of this axis in blocking the antitumor activity of various BTK/PI3K inhibitors, currently used in the clinical setting. Addition of recombinant HBEGF induced resistance to BTK/PI3K inhibitors in parental cells and in additional lymphoma models. Combination with the ERBB inhibitor lapatinib was beneficial in resistant cells and in other lymphoma models already expressing the identified resistance factors. An epigenetic reprogramming sustained the expression of the resistance-related factors, and pretreatment with demethylating agents or EZH2 inhibitors overcame the resistance. Resistance factors were also shown to be expressed in clinical specimens. In conclusion, we showed that the overexpression of ERBB4 and its ligands represents a novel mechanism of resistance for lymphoma cells to bypass the antitumor activity of BTK and PI3K inhibitors and that targeted pharmacologic interventions can restore sensitivity to the small molecules.

Response to Bruton's tyrosine kinase inhibitors in aggressive lymphomas linked to chronic selective autophagy

Diffuse large B cell lymphoma (DLBCL) is an aggressive, profoundly heterogeneous cancer, presenting a challenge for precision medicine. Bruton's tyrosine kinase (BTK) inhibitors block B cell receptor (BCR) signaling and are particularly effective in certain molecular subtypes of DLBCL that rely on chronic active BCR signaling to promote oncogenic NF-κB. The MCD genetic subtype, which often acquires mutations in the BCR subunit, CD79B, and in the innate immune adapter, MYD88L265P, typically resists chemotherapy but responds exceptionally to BTK inhibitors. However, the underlying mechanisms of response to BTK inhibitors are poorly understood. Herein, we find a non-canonical form of chronic selective autophagy in MCD DLBCL that targets ubiquitinated MYD88L265P for degradation in a TBK1-dependent manner. MCD tumors acquire genetic and epigenetic alterations that attenuate this autophagic tumor suppressive pathway. In contrast, BTK inhibitors promote autophagic degradation of MYD88L265P, thus explaining their exceptional clinical benefit in MCD DLBCL.

Kinase-impaired BTK mutations are susceptible to clinical-stage BTK and IKZF1/3 degrader NX-2127

Increasing use of covalent and noncovalent inhibitors of Bruton's tyrosine kinase (BTK) has elucidated a series of acquired drug-resistant BTK mutations in patients with B cell malignancies. Here we identify inhibitor resistance mutations in BTK with distinct enzymatic activities, including some that impair BTK enzymatic activity while imparting novel protein-protein interactions that sustain B cell receptor (BCR) signaling. Furthermore, we describe a clinical-stage BTK and IKZF1/3 degrader, NX-2127, that can bind and proteasomally degrade each mutant BTK proteoform, resulting in potent blockade of BCR signaling. Treatment of chronic lymphocytic leukemia with NX-2127 achieves >80% degradation of BTK in patients and demonstrates proof-of-concept therapeutic benefit. These data reveal an oncogenic scaffold function of mutant BTK that confers resistance across clinically approved BTK inhibitors but is overcome by BTK degradation in patients.

Degradation trumps mutations in cancer

Redirecting targeted proteins for degradation can overcome acquired drug resistance.

Diffuse large B-cell lymphoma involving the central nervous system: biologic rationale for targeted therapy

Diffuse large B-cell lymphoma (DLBCL) is an aggressive B-cell lymphoma curable even in advanced stages. DLBCL involving the central nervous system (CNS) is more difficult to cure and fewer treatment options exist. Primary CNS lymphoma (PCNSL) refers to aggressive lymphomas confined to the CNS, and are almost always DLBCL. Standard approaches for PCNSL use high-dose methotrexate-based combinations as induction therapy and younger patients often receive dose-intensive consolidation. However, dose-intensive therapies are not suitable for all patients, and older patients have fewer effective treatment options. Patients with relapsed or chemotherapy-refractory disease have a very poor prognosis. Secondary CNS lymphoma (SCNSL) describes aggressive lymphomas involving the CNS at initial presentation or relapses within the CNS after treatment for systemic DLBCL. Isolated CNS relapse is often managed as PCNSL, but patients with synchronous involvement of DLBCL in both the periphery and the CNS pose a unique clinical challenge. Insights into the molecular circuitry of DLBCL have identified distinct genetic subtypes including cases with a predilection for CNS invasion. PCNSL and subsets of SCNSL are characterized by chronically activated B-cell receptor and NFκB signaling along with genetic evidence of immune evasion which may be exploited therapeutically. Improved mechanistic understanding of targetable pathways underpinning CNS lymphomas has led to numerous clinical trials testing targeted agent combinations and immunotherapy approaches with promising early results. Biologically rational strategies may further improve the cure rate of CNS lymphomas, either by overcoming intrinsic or acquired treatment resistance and/or by being broadly applicable to patients of all ages.

Bruton's Tyrosine Kinase Inhibitors with Distinct Binding Modes Reveal Differential Functional Impact on B-Cell Receptor Signaling

Small molecule inhibitors of Bruton's tyrosine kinase (BTK) have been approved for the treatment of multiple B-cell malignancies and are being evaluated for autoimmune and inflammatory diseases. Various BTK inhibitors (BTKi) have distinct potencies, selectivity profiles, and binding modes within the ATP-binding site. On the basis of the latter feature, BTKis can be classified into those that occupy the back-pocket, H3 pocket, and the hinge region only. Hypothesizing that differing binding modes may have differential impact on the B-cell receptor (BCR) signaling pathway, we evaluated the activities of multiple BTKis in B-cell lymphoma models in vitro and in vivo. We demonstrated that, although all three types of BTKis potently inhibited BTK-Y223 autophosphorylation and phospholipase C gamma 2 (PLCγ2)-Y1217 transphosphorylation, hinge-only binders were defective in inhibiting BTK-mediated calcium mobilization upon BCR activation. In addition, PLCγ2 activation was effectively blocked by back-pocket and H3 pocket binders but not by hinge-only binders. Further investigation using TMD8 cells deficient in Rac family small GTPase 2 (RAC2) revealed that RAC2 functioned as a bypass mechanism, allowing for residual BCR signaling and PLCγ2 activation when BTK kinase activity was fully inhibited by the hinge-only binders. These data reveal a kinase activity-independent function of BTK, involving RAC2 in transducing BCR signaling events, and provide mechanistic rationale for the selection of clinical candidates for B-cell lymphoma indications.

Resisting the Resistance: Navigating BTK Mutations in Chronic Lymphocytic Leukemia (CLL)

Bruton's tyrosine kinase (BTK) plays a key role in the B-cell receptor (BCR) signaling pathway and confers anti-apoptotic and proliferative properties to malignant B-cells in chronic lymphocytic leukemia (CLL). Small molecule BTK inhibitors were designed to bind BTK's active site and block downstream signaling. These drugs have now been used in the treatment of thousands of patients with CLL, the most common form of leukemia in the western hemisphere. However, adverse effects of early generations of BTK inhibitors and resistance to treatment have led to the development of newer, more selective and non-covalent BTK inhibitors. As the use of these newer generation BTK inhibitors has increased, novel BTK resistance mutations have come to light. This review aims to discuss previously known and novel BTK mutations, their mechanisms of resistance, and their relationship with patient treatment. Also discussed here are future studies that are needed to investigate the underlying cause allowing these mutations to occur and how they incite resistance. New treatments on the horizon that attempt to maneuver around these resistance mutations can be met with new resistance mutations, creating an unmet need for patients with CLL. Novel therapies and combinations that address all forms of resistance are discussed.

Leukemic presentation and progressive genomic alterations of MCD/C5 diffuse large B-cell lymphoma (DLBCL)

Diffuse large B-cell lymphoma (DLBCL) is a heterogenous group of lymphoid malignancies. Based on gene expression profiling, it has been subdivided into germinal center (GC)-derived and activated B-cell (ABC) types. Advances in molecular methodologies have further refined the subclassification of DLBCL, based on recurrent genetic abnormalities. Here, we describe a distinct case of DLBCL that presented in leukemic form. DNA sequencing targeting 275 genes revealed pathogenically relevant mutations of CD79B, MyD88, TP53, TBL1XR1, and PIM1 genes, indicating that this lymphoma would be best classified as MCD/C5 DLBCL, an ABC subtype. Despite an initial good clinical response to BTK inhibitor ibrutinib, anti-CD20 antibody rituxan, alkylating agent bendamustine, and hematopoietic stem-cell transplant, the lymphoma relapsed, accompanied by morphologic and molecular evidence of disease progression. Specifically, the recurrent tumor developed loss of TP53 heterozygosity (LOH) and additional chromosomal changes central to ABC DLBCL pathogenesis, such as PRDM1 loss. Acquired resistance to ibrutinib and rituxan was indicated by the emergence of BTK and FOXO1 mutations, respectively, as well as apparent activation of alternative cell-activation pathways, through copy-number alterations (CNAs), detected by high-resolution chromosomal microarrays. In vitro, studies of relapsed lymphoma cells confirmed resistance to standard BTK inhibitors but sensitivity to vecabrutinib, a noncovalent inhibitor active against both wild-type as well as mutated BTK. In summary, we provide in-depth molecular characterization of a de novo leukemic DLBCL and discuss mechanisms that may have contributed to the lymphoma establishment, progression, and development of drug resistance.

EGR1-mediated metabolic reprogramming to oxidative phosphorylation contributes to ibrutinib resistance in B-cell lymphoma

The use of Bruton tyrosine kinase inhibitors, such as ibrutinib, to block B-cell receptor signaling has achieved a remarkable clinical response in several B-cell malignancies, including mantle cell lymphoma (MCL) and diffuse large B-cell lymphoma (DLBCL). Acquired drug resistance, however, is significant and affects the long-term survival of these patients. Here, we demonstrate that the transcription factor early growth response gene 1 (EGR1) is involved in ibrutinib resistance. We found that EGR1 expression is elevated in ibrutinib-resistant activated B-cell-like subtype DLBCL and MCL cells and can be further upregulated upon ibrutinib treatment. Genetic and pharmacological analyses revealed that overexpressed EGR1 mediates ibrutinib resistance. Mechanistically, TCF4 and EGR1 self-regulation induce EGR1 overexpression that mediates metabolic reprogramming to oxidative phosphorylation (OXPHOS) through the transcriptional activation of PDP1, a phosphatase that dephosphorylates and activates the E1 component of the large pyruvate dehydrogenase complex. Therefore, EGR1-mediated PDP1 activation increases intracellular adenosine triphosphate production, leading to sufficient energy to enhance the proliferation and survival of ibrutinib-resistant lymphoma cells. Finally, we demonstrate that targeting OXPHOS with metformin or IM156, a newly developed OXPHOS inhibitor, inhibits the growth of ibrutinib-resistant lymphoma cells both in vitro and in a patient-derived xenograft mouse model. These findings suggest that targeting EGR1-mediated metabolic reprogramming to OXPHOS with metformin or IM156 provides a potential therapeutic strategy to overcome ibrutinib resistance in relapsed/refractory DLBCL or MCL.

CARD11 gain of function upregulates BCL2A1 expression and promotes resistance to targeted therapies combination in B-cell lymphoma

A strategy combining targeted therapies is effective in B-cell lymphomas (BCL), such as mantle cell lymphoma (MCL), but acquired resistances remain a recurrent issue. In this study, we performed integrative longitudinal genomic and single-cell RNA-sequencing analyses of patients with MCL who were treated with targeted therapies against CD20, BCL2, and Bruton tyrosine kinase (OAsIs trial). We revealed the emergence of subclones with a selective advantage against OAsIs combination in vivo and showed that resistant cells were characterized by B-cell receptor (BCR)-independent overexpression of NF-κB1 target genes, especially owing to CARD11 mutations. Functional studies demonstrated that CARD11 gain of function not only resulted in BCR independence but also directly increased the transcription of the antiapoptotic BCL2A1, leading to resistance against venetoclax and OAsIs combination. Based on the transcriptional profile of OAsIs-resistant subclones, we designed a 16-gene resistance signature that was also predictive for patients with MCL who were treated with conventional chemotherapy, underlying a common escape mechanism. Among druggable strategies to inhibit CARD11-dependent NF-κB1 transduction, we evaluated the selective inhibition of its essential partner MALT1. We demonstrated that MALT1 protease inhibition led to a reduction in the expression of genes involved in OAsIs resistance, including BCL2A1. Consequently, MALT1 inhibition induced synergistic cell death in combination with BCL2 inhibition, irrespective of CARD11 mutational status, both in vitro and in vivo. Taken together, our study identified mechanisms of resistance to targeted therapies and provided a novel strategy to overcome resistance in aggressive BCL. The OAsIs trial was registered at www.clinicaltrials.gov #NCT02558816.

miR-28-based combination therapy impairs aggressive B cell lymphoma growth by rewiring DNA replication

Diffuse large B cell lymphoma (DLBCL) is the most common aggressive B cell lymphoma and accounts for nearly 40% of cases of B cell non-Hodgkin lymphoma. DLBCL is generally treated with R-CHOP chemotherapy, but many patients do not respond or relapse after treatment. Here, we analyzed the therapeutic potential of the tumor suppressor microRNA-28 (miR-28) for DLBCL, alone and in combination with the Bruton's tyrosine kinase inhibitor ibrutinib. Combination therapy with miR-28 plus ibrutinib potentiated the anti-tumor effects of monotherapy with either agent by inducing a specific transcriptional cell-cycle arrest program that impairs DNA replication. The molecular actions of miR-28 and ibrutinib synergistically impair DNA replication by simultaneous inhibition of origin activation and fork progression. Moreover, we found that downregulation of the miR-28-plus-ibrutinib gene signature correlates with better survival of ABC-DLBCL patients. These results provide evidence for the effectiveness of a new miRNA-based ibrutinib combination therapy for DLBCL and unveil the miR-28-plus-ibrutinib gene signature as a new predictor of outcome in ABC-DLBCL patients.

Phase I Study of Acalabrutinib Plus Danvatirsen (AZD9150) in Relapsed/Refractory Diffuse Large B-Cell Lymphoma Including Circulating Tumor DNA Biomarker Assessment

PURPOSE

Novel targeted and immunotherapies have improved outcomes in relapsed/refractory (R/R) diffuse large B-cell lymphoma (DLBCL), but toxicities limit widespread use. The selective Bruton tyrosine kinase (BTK) inhibitor acalabrutinib has activity in patients with R/R DLBCL but durable responses are uncommon. STAT3 inhibition has demonstrated clinical activity in DLBCL.

PATIENTS AND METHODS

Final results of the phase I study of acalabrutinib plus STAT3 inhibitor (danvatirsen; AZD9150) in patients with R/R DLBCL are reported. Danvatirsen 200 mg intravenous infusion [Days 1, 3, 5 (Cycle 1); weekly infusions starting Day 8, Cycle 1] was administered in combination with oral acalabrutinib 100 mg twice daily until progressive disease (PD) or unacceptable toxicity. Primary endpoints were safety and tolerability. Secondary endpoints included efficacy, pharmacokinetics, and immunogenicity.

RESULTS

Seventeen patients received combination treatment. One dose-limiting toxicity (Grade 3 liver transaminase) occurred in 1 patient. The most common reason for treatment discontinuation was PD (65%). In evaluable patients (n = 17), objective response rate was 24%; median duration of response was 1.9 months. All responders with available DLBCL cell-of-origin data were either activated B-cell or nongerminal center B-cell like subtype. Genetic subtype did not correlate with response. Baseline and longitudinal plasma cell-free DNA (cfDNA) concentrations were mostly higher in nonresponding patients. cfDNA changes were generally concordant with imaging. Pretreatment circulating B-cell levels were higher in responders versus nonresponders.

CONCLUSIONS

Targeting both STAT3 and BTK in combination is safe and tolerable but efficacy is limited in R/R DLBCL. Results support evaluation of circulating tumor DNA as a biomarker for clinical response.

Pirtobrutinib in Covalent Bruton Tyrosine Kinase Inhibitor Pretreated Mantle-Cell Lymphoma

PURPOSE

Pirtobrutinib is a highly selective, noncovalent (reversible) Bruton tyrosine kinase inhibitor (BTKi). We report the safety and efficacy of pirtobrutinib in patients with covalent Bruton tyrosine kinase inhibitor (cBTKi) pretreated mantle-cell lymphoma (MCL), a population with poor prognosis.

METHODS

Patients with cBTKi pretreated relapsed/refractory (R/R) MCL received pirtobrutinib monotherapy in a multicenter phase I/II trial (BRUIN; ClinicalTrials.gov identifier: NCT03740529). Efficacy was assessed in the first 90 consecutively enrolled patients who met criteria for inclusion in the primary efficacy cohort. The primary end point was overall response rate (ORR). Secondary end points included duration of response (DOR) and safety.

RESULTS

The median patient age was 70 years (range, 46-87), the median prior lines of therapy was 3 (range, 1-8), 82.2% had discontinued a prior cBTKi because of disease progression, and 77.8% had intermediate- or high-risk simplified MCL International Prognostic Index score. The ORR was 57.8% (95% CI, 46.9 to 68.1), including 20.0% complete responses (n = 18). At a median follow-up of 12 months, the median DOR was 21.6 months (95% CI, 7.5 to not reached). The 6- and 12-month estimated DOR rates were 73.6% and 57.1%, respectively. In the MCL safety cohort (n = 164), the most common treatment-emergent adverse events (TEAEs) were fatigue (29.9%), diarrhea (21.3%), and dyspnea (16.5%). Grade ≥3 TEAEs of hemorrhage (3.7%) and atrial fibrillation/flutter (1.2%) were less common. Only 3% of patients discontinued pirtobrutinib because of a treatment-related adverse event.

CONCLUSION

Pirtobrutinib is a first-in-class novel noncovalent (reversible) BTKi and the first BTKi of any kind to demonstrate durable efficacy after prior cBTKi therapy in heavily pretreated R/R MCL. Pirtobrutinib was well tolerated with low rates of treatment discontinuation because of toxicity.

Targeting N-linked Glycosylation for the Therapy of Aggressive Lymphomas

Diffuse large B-cell lymphoma (DLBCL) can be subdivided into the activated B-cell (ABC) and germinal center B cell-like (GCB) subtypes. Self-antigen engagement of B-cell receptors (BCR) in ABC tumors induces their clustering, thereby initiating chronic active signaling and activation of NF-κB and PI3 kinase. Constitutive BCR signaling is essential in some GCB tumors but primarily activates PI3 kinase. We devised genome-wide CRISPR-Cas9 screens to identify regulators of IRF4, a direct transcriptional target of NF-κB and an indicator of proximal BCR signaling in ABC DLBCL. Unexpectedly, inactivation of N-linked protein glycosylation by the oligosaccharyltransferase-B (OST-B) complex reduced IRF4 expression. OST-B inhibition of BCR glycosylation reduced BCR clustering and internalization while promoting its association with CD22, which attenuated PI3 kinase and NF-κB activation. By directly interfering with proximal BCR signaling, OST-B inactivation killed models of ABC and GCB DLBCL, supporting the development of selective OST-B inhibitors for the treatment of these aggressive cancers.

SIGNIFICANCE

DLBCL depends on constitutive BCR activation and signaling. There are currently no therapeutics that target the BCR directly and attenuate its pathologic signaling. Here, we unraveled a therapeutically exploitable, OST-B-dependent glycosylation pathway that drives BCR organization and proximal BCR signaling. This article is highlighted in the In This Issue feature, p. 1749.

The preclinical discovery and development of orelabrutinib as a novel treatment option for B-cell lymphoid malignancies

INTRODUCTION

Bruton's tyrosine kinase (BTK) inhibitors have recently been approved for clinical use against several B-cell indolent lymphoid malignancies, both as single agents or in combination. One second-generation BTK inhibitor that is being developed for the treatment of B-cell hematological malignancies, as well as for autoimmune disorders, is orelabrutinib.

AREAS COVERED

This paper reviews recent developments in the use of orelabrutinib against B-cell indolent lymphoid malignancies such as chronic lymphocytic leukemia, mantle cell lymphoma, diffuse large B-cell lymphoma, Waldenstrom macroglobulinemia and central nervous system lymphoma. Google Scholar and PubMed were initially searched for articles, and the corpus of articles was broadened by reviewing the references of the identified papers. All were in English. The corpus comprised papers from 2016 to April 2023. In addition, a manual search was performed of conference proceedings from the last five years of The American Society of Hematology, American Society of Clinical Oncology and the European Hematology Association.

EXPERT OPINION

Orelabrutinib is an active drug in indolent and aggressive B-cell lymphoid malignancies. It demonstrates high selectivity, good efficacy and an excellent safety profile. Nevertheless, further clinical trials are required to optimize its use. In addition, several other highly selective BTK inhibitors are being examined in early-phase studies.

Synergistic disruption of BTK and BCL-2 causes apoptosis while inducing ferroptosis in double-hit lymphoma

Double-hit lymphoma (DHL) is an aggressive subset of Diffuse Large B-cell Lymphoma (DLBCL) with poor outcomes and without satisfying treatment options. BTK inhibitor monotherapy is ineffective to suppress aggressive lymphoma. Hence, combination with other potential agents is warranted. Here, we demonstrated the second generation of BTK inhibitor, zanubrutinib, and a BCL-2 inhibitor, navitoclax, worked in synergistic manner to suppress DHL. Comprehensive in silico approach by interrogating single-cell to bulk-level profiling was employed along with in vitro and in vivo validation in DHL cell lines. Ablation of BTK enhanced sensitivity to navitoclax and suppressed proliferation of DHL cells. Combination of second generation of BTK inhibitor with navitoclax synergistically suppressed DLBCL cells with higher synergy score in DHL subset. The drug combination triggered apoptosis and ferroptosis, with the latter being characterized by reactive oxygen species (ROS) accumulation, extensive lipid peroxidation, and depletion of reduced glutathione. Moreover, ablation of BTK sensitized DHL cells to ferroptosis. Mechanistically, disruption of BTK and BCL-2 triggered ferroptosis by downregulating NRF2 and HMOX1, while deactivating GPX4. Combination of zanubrutinib and navitoclax effectively suppressed tumor growth in vivo. Our data suggest that zanubrutinib and navitoclax synergistically suppressed DHL by inducing apoptosis and ferroptosis.

ERBB4-mediated signaling is a mediator of resistance to BTK and PI3K inhibitors in B cell lymphoid neoplasms

BTK and PI3K inhibitors are among the drugs approved for the treatment of patients with lymphoid neoplasms. Although active, their ability to lead as single agents to long-lasting complete remission is rather limited especially in the lymphoma setting. This indicates that tumor cells often develop resistance to the drugs. Here, we show that the overexpression of ERBB4 and its ligands represents a modality for B cell neoplastic cells to bypass the anti-tumor activity of BTK and PI3K inhibitors and that targeted pharmacological interventions can restore sensitivity to the small molecules. We started from a marginal zone lymphoma (MZL) cell line, Karpas-1718, kept under prolonged exposure to the PI3Kδ inhibitor idelalisib until acquisition of resistance, or with no drug. Cells underwent transcriptome, miRNA and methylation profiling, whole exome sequencing, and pharmacological screening which led to the identification of the overexpression of ERBB4 and its ligands HBEGF and NRG2 in the resistant cells. Cellular and genetic experiments demonstrated the involvement of this axis in blocking the anti-tumor activity of various BTK and PI3K inhibitors, currently used in the clinical setting. Addition of recombinant HBEGF induced resistance to BTK and PI3K inhibitors in parental cells but also in additional lymphoma models. Combination with the ERBB inhibitor lapatinib was beneficial in resistant cells and in other lymphoma models already expressing the identified resistance factors. Multi-omics analysis underlined that an epigenetic reprogramming affected the expression of the resistance-related factors, and pretreatment with demethylating agents or EZH2 inhibitors overcame the resistance. Resistance factors were shown to be expressed in clinical samples, further extending the findings of the study. In conclusions, we identified a novel ERBB4-driven mechanism of resistance to BTK and PI3K inhibitors and treatments that appear to overcome it.

Key points

A mechanism of secondary resistance to the PI3Kδ and BTK inhibitors in B cell neoplasms driven by secreted factors.Resistance can be reverted by targeting ERBB signaling.

Resistance to Bruton tyrosine kinase inhibition in chronic lymphocytic leukaemia and non-Hodgkin lymphoma

Bruton tyrosine kinase inhibitors (BTKi) have transformed the therapeutic landscape of chronic lymphocytic leukaemia (CLL) and non-Hodgkin lymphoma. However, primary and acquired resistance to BTKi can be seen due to a variety of mechanisms including tumour intrinsic and extrinsic mechanisms such as gene mutations, activation of bypass signalling pathways and tumour microenvironment. Herein, we provide an updated review of the key clinical data of BTKi treatment in CLL, mantle cell lymphoma, and diffuse large B-cell lymphoma (DLBCL). We incorporate the most recent findings regarding mechanisms of resistance to covalent and non-covalent inhibitors, including ibrutinib, acalabrutinib, zanubrutinib and pirtobrutinib. We also cover the clinical sensitivity of certain molecular subtypes of DLBCL to an ibrutinib-containing regimen. Lastly, we summarise ongoing clinical investigations aimed at overcoming resistance via use of BTKi-containing combined therapies or the novel non-covalent BTKi. The review article targets an audience of clinical practitioners, clinical investigators and translational researchers.

Bruton's tyrosine kinase inhibitors in the treatment of primary central nervous system lymphoma: A mini-review

Primary central nervous system lymphoma (PCNSL) is a highly aggressive brain tumor with poor prognosis if no treatment. The activation of the NF-κB (nuclear factor kappa-B) is the oncogenic hallmark of PCNSL, and it was driven by B cell receptor (BCR) and Toll-like receptor (TLR) signaling pathways. The emergence of Bruton's tyrosine kinase inhibitors (BTKis) has brought the dawn of life to patients with PCNSL. This review summarizes the management of PCNSL with BTKis and potential molecular mechanisms of BTKi in the treatment of PCNSL. And the review will focus on the clinical applications of BTKi in the treatment of PCNSL including the efficacy and adverse events, the clinical trials currently being carried out, the underlying mechanisms of resistance to BTKi and possible solutions to drug resistance.

The Landscape of Nucleic-Acid-Based Aptamers for Treatment of Hematologic Malignancies: Challenges and Future Directions

Hematologic malignancies, including leukemia, lymphoma, myeloproliferative disorder and plasma cell neoplasia, are genetically heterogeneous and characterized by an uncontrolled proliferation of their corresponding cell lineages in the bone marrow, peripheral blood, tissues or plasma. Although there are many types of therapeutic drugs (e.g., TKIs, chemotherapy drugs) available for treatment of different malignancies, the relapse, drug resistance and severe side effects due to the lack of selectivity seriously limit their clinical application. Currently, although antibody–drug conjugates have been well established as able to target and deliver highly potent chemotherapy agents into cancer cells for the reduction of damage to healthy cells and have achieved success in leukemia treatment, they still also have shortcomings such as high cost, high immunogenicity and low stability. Aptamers are ssDNA or RNA oligonucleotides that can also precisely deliver therapeutic agents into cancer cells through specifically recognizing the membrane protein on cancer cells, which is similar to the capabilities of monoclonal antibodies. Aptamers exhibit higher binding affinity, lower immunogenicity and higher thermal stability than antibodies. Therefore, in this review we comprehensively describe recent advances in the development of aptamer–drug conjugates (ApDCs) with cytotoxic payload through chemical linkers or direct incorporation, as well as further introduce the latest promising aptamers-based therapeutic strategies such as aptamer–T cell therapy and aptamer–PROTAC, clarifying their bright application, development direction and challenges in the treatment of hematologic malignancies.

Epigenetic regulation of B cells and its role in autoimmune pathogenesis

B cells play a pivotal role in the pathogenesis of autoimmune diseases. Although previous studies have shown many genetic polymorphisms associated with B-cell activation in patients with various autoimmune disorders, progress in epigenetic research has revealed new mechanisms leading to B-cell hyperactivation. Epigenetic mechanisms, including those involving histone modifications, DNA methylation, and noncoding RNAs, regulate B-cell responses, and their dysregulation can contribute to the pathogenesis of autoimmune diseases. Patients with autoimmune diseases show epigenetic alterations that lead to the initiation and perpetuation of autoimmune inflammation. Moreover, many clinical and animal model studies have shown the promising potential of epigenetic therapies for patients. In this review, we present an up-to-date overview of epigenetic mechanisms with a focus on their roles in regulating functional B-cell subsets. Furthermore, we discuss epigenetic dysregulation in B cells and highlight its contribution to the development of autoimmune diseases. Based on clinical and preclinical evidence, we discuss novel epigenetic biomarkers and therapies for patients with autoimmune disorders.

NF-κB Mutations in Germinal Center B-Cell Lymphomas: Relation to NF-κB Function in Normal B Cells

Most B cell lymphomas arise from the oncogenic transformation of B cells that have undergone the germinal center (GC) reaction of the T cell-dependent immune response, where high-affinity memory B cells and plasma cells are generated. The high proliferation of GC B cells coupled with occasional errors in the DNA-modifying processes of somatic hypermutation and class switch recombination put the cell at a risk to obtain transforming genetic aberrations, which may activate proto-oncogenes or inactivate tumour suppressor genes. Several subtypes of GC lymphomas harbor genetic mutations leading to constitutive, aberrant activation of the nuclear factor-κB (NF-κB) signaling pathway. In normal B cells, NF-κB has crucial biological roles in development and physiology. GC lymphomas highjack these activities to promote tumour-cell growth and survival. It has become increasingly clear that the separate canonical and non-canonical routes of the NF-κB pathway and the five downstream NF-κB transcription factors have distinct functions in the successive stages of GC B-cell development. These findings may have direct implications for understanding how aberrant NF-κB activation promotes the genesis of various GC lymphomas corresponding to the developmentally distinct GC B-cell subsets. The knowledge arising from these studies may be explored for the development of precision medicine approaches aimed at more effective treatments of the corresponding tumours with specific NF-κB inhibitors, thus reducing systemic toxicity. We here provide an overview on the patterns of genetic NF-κB mutations encountered in the various GC lymphomas and discuss the consequences of aberrant NF-κB activation in those malignancies as related to the biology of NF-κB in their putative normal cellular counterparts.

BTK-independent regulation of calcium signalling downstream of the B-cell receptor in malignant B-cells

BTK inhibitors (BTKi) have dramatically improved outcomes for patients with chronic lymphocytic leukaemia (CLL) and some forms of B-cell lymphoma. However, new strategies are needed to enhance responses. Here we have performed a detailed analysis of the effects of BTKi on B-cell receptor (BCR)-induced signalling using primary malignant cells from CLL patients and B-lymphoma cell lines. Although BTK is considered as a key activator of PLCγ2, BTKi (ibrutinib and acalabrutinib) failed to fully inhibit calcium responses in CLL samples with strong BCR signalling capacity. This BTKi-resistant calcium signalling was sufficient to engage downstream calcium-dependent transcription and suppress CLL cell apoptosis and was entirely independent of BTK and not just its kinase activity as similar results were obtained using a BTK-degrading PROTAC. BTK-independent calcium signalling was also observed in two B-lymphoma cell lines where BTKi had little effect on the initial phase of the calcium response but did accelerate the subsequent decline in intracellular calcium. In contrast to BTKi, calcium responses were completely blocked by inhibition of SYK in CLL and lymphoma cells. Engagement of BTK-independent calcium responses was associated with BTK-independent phosphorylation of PLCγ2 on Y753 and Y759 in both CLL and lymphoma cells. Moreover, in CLL samples, inhibition of RAC, which can mediate BTK-independent activation of PLCγ2, cooperated with ibrutinib to suppress calcium responses. BTK-independent calcium signalling may limit the effectiveness of BTKi to suppress BCR signalling responses and our results suggest inhibition of SYK or dual inhibition of BTK and RAC as alternative strategies to strengthen pathway blockade.

Epigenetic Rewiring of BCR Signaling as a Novel Mechanism of Ibrutinib Resistance in ABC-DLBCL

In this issue of Blood Cancer Discovery, Schaffer and colleagues uncover a novel epigenetic mechanism of resistance to the Bruton tyrosine kinase inhibitor ibrutinib in activated B-cell-like diffuse large B-cell lymphoma (ABC-DLBCL), whereby tumor cells rewire the B-cell receptor (BCR)-driven NF-κB signaling cascade through the small GTPase RAC2. This circuit can be efficiently targeted by RAC1/2 small-molecule inhibitors, suggesting a promising new therapeutic approach to overcome acquired ibrutinib resistance in ABC-DLBCL and possibly other B-cell malignancies relying on active BCR signaling.See related article by Shaffer et al., p. 630.