Bruton tyrosine kinase inhibitor ibrutinib (PCI-32765)

Bruton tyrosine kinase inhibitor ONO/GS-4059: from bench to bedside

The Bruton tyrosine kinase (BTK) inhibitor, ibrutinib, has been approved for the treatment of chronic lymphocytic leukemia, mantle cell lymphoma, and Waldenstroms macroglobulinemia. Acquired resistance to ibrutinib due to BTK C481S mutation has been reported. Mutations in PLC?2 can also mediate resistance to ibrutinib. Untoward effects due to off-target effects are also disadvantages of ibrutinib. More selective and potent BTK inhibitors (ACP-196, ONO/GS-4059, BGB-3111, CC-292) are being investigated. This review summarized the preclinical research and clinical data of ONO/GS-4059.

Hair and Nail Changes During Long-term Therapy With Ibrutinib for Chronic Lymphocytic Leukemia

IMPORTANCE

Ibrutinib, a Bruton tyrosine kinase inhibitor, is a new targeted agent approved by the US Food and Drug Administration for the treatment of chronic lymphocytic leukemia (CLL), mantle cell lymphoma, and Waldenström macroglobulinemia. Ibrutinib is overall well tolerated but long-term treatment is required until disease progression or intolerable toxic effects occur. Little is known regarding its cutaneous adverse effects.

OBJECTIVE

To describe the hair and nail manifestations associated with the long-term use of ibrutinib for the treatment of CLL.

DESIGN, SETTING, AND PARTICIPANTS

Prospective study of 66 patients with CLL enrolled in a single-arm phase 2 clinical trial of ibrutinib for CLL between March 2014 and October 2015 at the National Institutes of Health.

MAIN OUTCOMES AND MEASURES

The primary outcome, nail and hair changes associated with ibrutinib therapy, was assessed by an 11-question survey. In addition, the severity of nail changes was determined from a 0 to 3 rating scale for both onychoschizia and onychorrhexis.

RESULTS

Among 66 patients (43 men and 23 women with ages ranging from 55 to 85 years), 44 (67%) reported brittle fingernails at a median of 6.5 (95% CI, 6-12) months after starting ibrutinib therapy. Fifteen patients (23%) developed brittle toenails after a median of 9 (95% CI, 6-15) months of ibrutinib therapy. Textural hair changes were reported in 17 patients (26%), at a median of 9 (95% CI, 6-12) months of ibrutinib treatment.

CONCLUSIONS AND RELEVANCE

Hair and nail abnormalities are commonly associated with ibrutinib and appear several months after initiating therapy. Ibrutinib inhibits Bruton tyrosine kinase by covalently binding to cysteine 481. Whether ibrutinib affects the hair and nails by binding and altering cysteine-rich proteins of hair and nails or by means of another mechanism remains unknown.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT01500733.

The Bruton Tyrosine Kinase (BTK) Inhibitor Acalabrutinib Demonstrates Potent On-Target Effects and Efficacy in Two Mouse Models of Chronic Lymphocytic Leukemia

Purpose: Acalabrutinib (ACP-196) is a novel, potent, and highly selective Bruton tyrosine kinase (BTK) inhibitor, which binds covalently to Cys481 in the ATP-binding pocket of BTK. We sought to evaluate the antitumor effects of acalabrutinib treatment in two established mouse models of chronic lymphocytic leukemia (CLL).Experimental Design: Two distinct mouse models were used, the TCL1 adoptive transfer model where leukemic cells from Eμ-TCL1 transgenic mice are transplanted into C57BL/6 mice, and the human NSG primary CLL xenograft model. Mice received either vehicle or acalabrutinib formulated into the drinking water.Results: Utilizing biochemical assays, we demonstrate that acalabrutinib is a highly selective BTK inhibitor as compared with ibrutinib. In the human CLL NSG xenograft model, treatment with acalabrutinib demonstrated on-target effects, including decreased phosphorylation of PLCγ2, ERK, and significant inhibition of CLL cell proliferation. Furthermore, tumor burden in the spleen of the mice treated with acalabrutinib was significantly decreased compared with vehicle-treated mice. Similarly, in the TCL1 adoptive transfer model, decreased phosphorylation of BTK, PLCγ2, and S6 was observed. Most notably, treatment with acalabrutinib resulted in a significant increase in survival compared with mice receiving vehicle.Conclusions: Treatment with acalabrutinib potently inhibits BTK in vivo, leading to on-target decreases in the activation of key signaling molecules (including BTK, PLCγ2, S6, and ERK). In two complementary mouse models of CLL, acalabrutinib significantly reduced tumor burden and increased survival compared with vehicle treatment. Overall, acalabrutinib showed increased BTK selectivity compared with ibrutinib while demonstrating significant antitumor efficacy in vivo on par with ibrutinib. Clin Cancer Res; 23(11); 2831-41. ©2016 AACR.

BTK suppresses myeloma cellular senescence through activating AKT/P27/Rb signaling

We previously explored the role of BTK in maintaining multiple myeloma stem cells (MMSCs) self-renewal and drug-resistance. Here we investigated the elevation of BTK suppressing MM cellular senescence, a state of irreversible cellular growth arrest. We firstly discovered that an increased expression of BTK in MM samples compared to normal controls by immunohistochemistry (IHC), and significant chromosomal gain in primary samples. In addition, BTK high-expressing MM patients are associated with poor outcome in both Total Therapy 2 (TT2) and TT3 cohorts. Knockdown BTK expression by shRNA induced MM cellular senescence using β-galactosidase (SA-b-gal) staining, cell growth arrest by cell cycle staining and decreased clonogenicity while forcing BTK expression in MM cells abrogated these characteristics. We also validated this feature in mouse embryonic fibroblast cells (MEFs), which showed that elevated BTK expression was resistant to MEF senescence after serial cultivation in vitro. Further mechanism study revealed that BTK activated AKT signaling leading to down-regulation of P27 expression and hindered RB activity while AKT inhibitor, LY294002, overcame BTK-overexpression induced cellular senescence resistance. Eventually we demonstrated that BTK inhibitor, CGI-1746, induced MM cellular senescence, colony reduction and tumorigenecity inhibition in vivo. Summarily, we designate a novel mechanism of BTK in mediating MM growth, and BTK inhibitor is of great potential in vivo and in vitro suggesting BTK is a promising therapeutic target for MM.

Leukemia cell proliferation and death in chronic lymphocytic leukemia patients on therapy with the BTK inhibitor ibrutinib

BACKGROUND. Ibrutinib is an effective targeted therapy for patients with chronic lymphocytic leukemia (CLL) that inhibits Bruton's tyrosine kinase (BTK), a kinase involved in B cell receptor signaling. METHODS. We used stable isotopic labeling with deuterated water (²H₂O) to measure directly the effects of ibrutinib on leukemia cell proliferation and death in 30 patients with CLL. RESULTS. The measured average CLL cell proliferation ("birth") rate before ibrutinib therapy was 0.39% of the clone per day (range 0.17%-1.04%); this decreased to 0.05% per day (range 0%-0.36%) with treatment. Death rates of blood CLL cells increased from 0.18% per day (average, range 0%-0.7%) prior to treatment to 1.5% per day (range 0%-3.0%) during ibrutinib therapy, and they were even higher in tissue compartments. CONCLUSIONS. This study provides the first direct in vivo measurements to our knowledge of ibrutinib's antileukemia actions, demonstrating profound and immediate inhibition of CLL cell proliferation and promotion of high rates of CLL cell death. TRIAL REGISTRATION. This trial was registered at clinicaltrials.gov (NCT01752426). FUNDING. This study was supported by a Cancer Center Support Grant (National Cancer Institute grant P30 CA016672), an NIH grant (CA081554) from the National Cancer Institute, MD Anderson's Moon Shots Program in CLL, and Pharmacyclics, an AbbVie company.

Ibrutinib Therapy Increases T Cell Repertoire Diversity in Patients with Chronic Lymphocytic Leukemia

The Bruton's tyrosine kinase inhibitor ibrutinib is a highly effective, new targeted therapy for chronic lymphocytic leukemia (CLL) that thwarts leukemia cell survival, growth, and tissue homing. The effects of ibrutinib treatment on the T cell compartment, which is clonally expanded and thought to support the growth of malignant B cells in CLL, are not fully characterized. Using next-generation sequencing technology, we characterized the diversity of TCRβ-chains in peripheral blood T cells from 15 CLL patients before and after 1 y of ibrutinib therapy. We noted elevated CD4⁺ and CD8⁺ T cell numbers and a restricted TCRβ repertoire in all pretreatment samples. After 1 y of ibrutinib therapy, elevated peripheral blood T cell numbers and T cell-related cytokine levels had normalized, and T cell repertoire diversity increased significantly. Dominant TCRβ clones in pretreatment samples declined or became undetectable, and the number of productive unique clones increased significantly during ibrutinib therapy, with the emergence of large numbers of low-frequency TCRβ clones. Importantly, broader TCR repertoire diversity was associated with clinical efficacy and lower rates of infections during ibrutinib therapy. These data demonstrate that ibrutinib therapy increases diversification of the T cell compartment in CLL patients, which contributes to cellular immune reconstitution.

Extended Treatment with Single-Agent Ibrutinib at the 420 mg Dose Leads to Durable Responses in Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma

Purpose: Ibrutinib, a first-in-class, once-daily, oral inhibitor of Bruton tyrosine kinase, promotes apoptosis, and inhibits B-cell proliferation, adhesion, and migration. Ibrutinib has demonstrated single-agent efficacy and acceptable tolerability at doses of 420 and 840 mg in patients with chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) who were treatment-naïve (TN) or had relapsed/refractory (R/R) CLL after ≥1 prior therapy in a phase Ib/II study (PCYC-1102). Subsequently, the ibrutinib 420 mg dose was approved in CLL.Experimental Design: We report data with 44 months of follow-up on 94 patients with TN and R/R CLL/SLL receiving ibrutinib 420 mg once-daily in PCYC-1102 and the long-term extension study PCYC-1103.Results: Ninety-four CLL/SLL patients (27 TN, 67 R/R) were treated with ibrutinib (420 mg/day). Patients with R/R disease had received a median of four prior therapies (range, 1-12). Responses were rapid and durable and median duration of response was not reached. Best overall response was 91% [85% TN (complete response, CR 26%) and 94% R/R (9% CR)]. Median progression-free survival (PFS) was not reached in either group. The 30-month PFS rate was 96% and 76% for TN and R/R patients, respectively. Ibrutinib was well tolerated with extended follow-up; rates of grade ≥3 cytopenias and fatigue, as well as discontinuations due to toxicities decreased over time.Conclusions: Single-agent ibrutinib at 420 mg once-daily resulted in durable responses and was well tolerated with up to 44 months follow-up in patients with TN and R/R CLL/SLL. Currently, 66% of patients continue on ibrutinib. Clin Cancer Res; 23(5); 1149-55. ©2017 AACR.

Recommendations for Clinical Trial Development in Follicular Lymphoma

Follicular lymphoma (FL) is the second most common lymphoid malignancy, representing 20% to 25% of all cases of non-Hodgkin's lymphoma (NHL), and the most common of the indolent NHLs. FL is considered incurable in the majority of patients with the current standard therapeutic approaches, although outcomes have improved in the last few decades with our current therapies, with a median overall survival that now exceeds 18 years. While the majority of patients with FL have improved outcomes with our current therapeutic approaches, there are patients with high-risk disease features that have inferior outcomes to these therapies. There is an urgent need to integrate novel therapeutic agents into the treatment regimens for these patients to improve outcomes with continued evaluation of biomarkers indicative of prognosis and effects of these regimens on quality of life.

Combination of Targeted Drugs to Control Chronic Lymphocytic Leukemia: Harnessing the Power of New Monoclonal Antibodies in Combination With Ibrutinib

The landscape of treatment for chronic lymphocytic leukemia is rapidly changing at present. Considerable improvement has been achieved with the introduction of the anti-CD20 antibodies, and chemoimmunotherapy has now become an established standard for patients without the high-risk features del(17p)/TP53 mutation. Also, the outcome of patients with these adverse genetic aberrations was dramatically improved with the introduction of the kinase inhibitors ibrutinib and idelalisib. Different combinations of these and additional novel agents are currently evaluated in clinical trials. The combination of the Bruton tyrosine kinase inhibitor ibrutinib with an anti-CD20 antibody is an attractive option, because both drugs act synergistically: ibrutinib redistributes the CLL cells from their homing organs to the peripheral blood, and obinutuzumab eliminates the leukemic cells in the blood with particular efficiency. Adding the Bcl-2 antagonist venetoclax could further intensify the treatment of CLL. This combination might hold the potential to achieve a deep remission with an eradication of residual CLL cells and thus lead to long-term remissions of CLL.

Pharmacokinetic and pharmacodynamic evaluation of ibrutinib for the treatment of chronic lymphocytic leukemia: rationale for lower doses

INTRODUCTION

Ibrutinib, a first-in-class covalent inhibitor of Bruton's tyrosine kinase (BTK), is approved in many countries for the treatment of relapsed/refractory chronic lymphocytic leukemia (CLL) and for previously untreated disease with a 17p deletion and, most recently, as a frontline therapy for CLL. In controlled trials in CLL, ibrutinib produced high response rates and improved survival in both the frontline and relapsed settings. While ibrutinib controls CLL with impressive efficacy, it only infrequently induces complete remissions, particularly of relapsed CLL, and does not eradicate minimal residual disease. Finally, ibrutinib is extremely expensive, has off-target toxicities, and requires indefinite therapy. Areas covered: In this article, we provide an overview of the CLL therapeutic landscape and discuss the pharmacokinetic and pharmacodynamic aspects of ibrutinib. Major clinical trials of ibrutinib in CLL are summarized, and its safety profile explored. Expert opinion: Ibrutinib represents a transformative advance in CLL management and has validated BTK as a therapeutic target in this disease, but has some limitations, leading to the emergence of other BTK inhibitors and mechanism-based combination strategies. Given complete BTK occupancy at lower doses of ibrutinib and declining levels of BTK on ibrutinib therapy, lower doses of ibrutinib in CLL are being explored.

Ibrutinib Inhibits ERBB Receptor Tyrosine Kinases and HER2-Amplified Breast Cancer Cell Growth

Ibrutinib is a potent, small-molecule Bruton tyrosine kinase (BTK) inhibitor developed for the treatment of B-cell malignancies. Ibrutinib covalently binds to Cys481 in the ATP-binding domain of BTK. This cysteine residue is conserved among 9 other tyrosine kinases, including HER2 and EGFR, which can be targeted. Screening large panels of cell lines demonstrated that ibrutinib was growth inhibitory against some solid tumor cells, including those inhibited by other HER2/EGFR inhibitors. Among sensitive cell lines, breast cancer lines with HER2 overexpression were most potently inhibited by ibrutinib (<100 nmol/L); in addition, the IC₅₀s were lower than that of lapatinib and dacomitinib. Inhibition of cell growth by ibrutinib coincided with downregulation of phosphorylation on HER2 and EGFR and their downstream targets, AKT and ERK. Irreversible inhibition of HER2 and EGFR in breast cancer cells was established after 30-minute incubation above 100 nmol/L or following 2-hour incubation at lower concentrations. Furthermore, ibrutinib inhibited recombinant HER2 and EGFR activity that was resistant to dialysis and rapid dilution, suggesting an irreversible interaction. The dual activity toward TEC family (BTK and ITK) and ERBB family kinases was unique to ibrutinib, as ERBB inhibitors do not inhibit or covalently bind BTK or ITK. Xenograft studies with HER2⁺ MDA-MB-453 and BT-474 cells in mice in conjunction with determination of pharmacokinetics demonstrated significant exposure-dependent inhibition of growth and key signaling molecules at levels that are clinically achievable. Ibrutinib's unique dual spectrum of activity against both TEC family and ERBB kinases suggests broader applications of ibrutinib in oncology. Mol Cancer Ther; 15(12); 2835-44. ©2016 AACR.

Ibrutinib in Waldenström macroglobulinemia: latest evidence and clinical experience

Ibrutinib is an oral Bruton's tyrosine kinase (BTK) inhibitor, which has recently gained approval by the United States (US) Food and Drug Administration (FDA) and the European Medicines Agency (EMA) for the treatment of patients with symptomatic Waldenström macroglobulinemia (WM). Herein, we review the role of BTK in the pathophysiology of WM, and present the results of the preclinical and clinical studies that led to the initial investigation and later approval of ibrutinib in WM. We also discuss aspects associated with ibrutinib therapy in WM patients, especially focusing on genomic profiling and the impact on response to ibrutinib, and the management of adverse events.

Substitution scanning identifies a novel, catalytically active ibrutinib-resistant BTK cysteine 481 to threonine (C481T) variant

Irreversible Bruton tyrosine kinase (BTK) inhibitors, ibrutinib and acalabrutinib have demonstrated remarkable clinical responses in multiple B-cell malignancies. Acquired resistance has been identified in a sub-population of patients in which mutations affecting BTK predominantly substitute cysteine 481 in the kinase domain for catalytically active serine, thereby ablating covalent binding of inhibitors. Activating substitutions in the BTK substrate phospholipase Cγ2 (PLCγ2) instead confers resistance independent of BTK. Herein, we generated all six possible amino acid substitutions due to single nucleotide alterations for the cysteine 481 codon, in addition to threonine, requiring two nucleotide substitutions, and performed functional analysis. Replacement by arginine, phenylalanine, tryptophan or tyrosine completely inactivated the catalytic activity, whereas substitution with glycine caused severe impairment. BTK with threonine replacement was catalytically active, similar to substitution with serine. We identify three potential ibrutinib resistance scenarios for cysteine 481 replacement: (1) Serine, being catalytically active and therefore predominating among patients. (2) Threonine, also being catalytically active, but predicted to be scarce, because two nucleotide changes are needed. (3) As BTK variants replaced with other residues are catalytically inactive, they presumably need compensatory mutations, therefore being very scarce. Glycine and tryptophan variants were not yet reported but likely also provide resistance.

Ibrutinib for mantle cell lymphoma

Mantle cell lymphoma (MCL) is a rare and aggressive form of non-Hodgkin lymphoma. Ibrutinib is a first-in-class, oral inhibitor of Bruton's tyrosine kinase which acts by downstream inhibition of the B-cell receptor. Early clinical trials have demonstrated excellent tolerability and a modest side-effect profile in relapsed/refractory MCL. Although the majority of disease responses are partial, efficacy data are impressive with more than two-thirds of patients demonstrating a durable response. This article focuses on all aspects of ibrutinib in the context of MCL, including a summary of the basic pharmacology and pharmacokinetics; a review of the safety and efficacy data published to date and a discussion of the future implications in MCL.

Cytoplasmic myosin-exposed apoptotic cells appear with caspase-3 activation and enhance CLL cell viability

The degree of chronic lymphocytic leukemia (CLL) B-cell antigen receptor (BCR) binding to myosin-exposed apoptotic cells (MEACs) correlates with worse patient outcomes, suggesting a link to disease activity. Therefore, we studied MEAC formation and the effects of MEAC binding on CLL cells. In cell line studies, both intrinsic (spontaneous or camptothecin-induced) and extrinsic (FasL- or anti-Fas-induced) apoptosis created a high percent of MEACs over time in a process associated with caspase-3 activation, leading to cytoplasmic myosin cleavage and trafficking to cell membranes. The involvement of common apoptosis pathways suggests that most cells can produce MEACs and indeed CLL cells themselves form MEACs. Consistent with the idea that MEAC formation may be a signal to remove dying cells, we found that natural IgM antibodies bind to MEACs. Functionally, co-culture of MEACs with CLL cells, regardless of immunoglobulin heavy-chain variable region gene mutation status, improved leukemic cell viability. Based on inhibitor studies, this improved viability involved BCR signaling molecules. These results support the hypothesis that stimulation of CLL cells with antigen, such as those on MEACs, promotes CLL cell viability, which in turn could lead to progression to worse disease.

Targets for Ibrutinib Beyond B Cell Malignancies

Ibrutinib (Imbruvica^™) is an irreversible, potent inhibitor of Bruton's tyrosine kinase (BTK). Over the last few years, ibrutinib has developed from a promising drug candidate to being approved by FDA for the treatment of three B cell malignancies, a truly remarkable feat. Few, if any medicines are monospecific and ibrutinib is no exception; already during ibrutinib's initial characterization, it was found that it could bind also to other kinases. In this review, we discuss the implications of such interactions, which go beyond the selective effect on BTK in B cell malignancies. In certain cases, the outcome of ibrutinib treatment likely results from the combined inhibition of BTK and other kinases, causing additive or synergistic, effects. Conversely, there are also examples when the clinical outcome seems unrelated to inhibition of BTK. Thus, more specifically, adverse effects such as enhanced bleeding or arrhythmias could potentially be explained by different interactions. We also predict that during long‐term treatment bone homoeostasis might be affected due to the inhibition of osteoclasts. Moreover, the binding of ibrutinib to molecular targets other than BTK or effects on cells other than B cell‐derived malignancies could be beneficial and result in new indications for clinical applications.

Bruton's tyrosine kinase is a potential therapeutic target in prostate cancer

Bruton's tyrosine kinase (BTK) is a non-receptor tyrosine kinase that has mainly been studied in haematopoietic cells. We have investigated whether BTK is a potential therapeutic target in prostate cancer. We find that BTK is expressed in prostate cells, with the alternate BTK-C isoform predominantly expressed in prostate cancer cells and tumors. This isoform is transcribed from an alternative promoter and results in a protein with an amino-terminal extension. Prostate cancer cell lines and prostate tumors express more BTK-C transcript than the malignant NAMALWA B-cell line or human lymphomas. BTK protein expression is also observed in tumor tissue from prostate cancer patients. Down regulation of this protein with RNAi or inhibition with BTK-specific inhibitors, Ibrutinib, AVL-292 or CGI-1746 decrease cell survival and induce apoptosis in prostate cancer cells. Microarray results show that inhibiting BTK under these conditions increases expression of apoptosis related genes, while overexpression of BTK-C is associated with elevated expression of genes with functions related to cell adhesion, cytoskeletal structure and the extracellular matrix. These results are consistent with studies that show that BTK signaling is important for adhesion and migration of B cells and suggest that BTK-C may confer similar properties to prostate cancer cells. Since BTK-C is a survival factor for these cells, it represents both a potential biomarker and novel therapeutic target for prostate cancer.

Development of the Bruton's tyrosine kinase inhibitor ibrutinib for B cell malignancies

Ibrutinib is a first-in-class oral covalent inhibitor of Bruton's tyrosine kinase that has demonstrated clinical benefit for many patients with B cell malignancies. Positive results in initial trials led the U.S. Food and Drug Administration to grant ibrutinib three breakthrough therapy designations for mantle cell lymphoma (MCL), del17p chronic lymphocytic leukemia (CLL), and Waldenström's macroglobulinemia (WM). Ibrutinib was approved for these three cancers within 14 months of the original U.S. approval. Additionally, ibrutinib is approved for patient subsets with MCL and/or CLL in >45 other countries. Via a unique mechanism of action, ibrutinib inhibits B cell signaling pathways that regulate the survival, proliferation, adhesion, and homing of cancerous cells. This marks a paradigm shift from the conventional cytotoxic chemotherapy approach to treating B cell malignancies. Ibrutinib continues to be evaluated across a range of B cell malignancies, either as single-agent therapy or in combination with other therapies, and continues to transform the lives of these patients.

The pre-B-cell receptor checkpoint in acute lymphoblastic leukaemia

The B-cell receptor (BCR) and its immature form, the precursor-BCR (pre-BCR), have a central role in the control of B-cell development, which is dependent on a sequence of cell-fate decisions at specific antigen-independent checkpoints. Pre-BCR expression provides the first checkpoint, which controls differentiation of pre-B to immature B-cells in normal haemopoiesis. Pre-BCR signalling regulates and co-ordinates diverse processes within the pre-B cell, including clonal selection, proliferation and subsequent maturation. In B-cell precursor acute lymphoblastic leukaemia (BCP-ALL), B-cell development is arrested at this checkpoint. Moreover, malignant blasts avoid clonal extinction by hijacking pre-BCR signalling in favour of the development of BCP-ALL. Here, we discuss three mechanisms that occur in different subtypes of BCP-ALL: (i) blocking pre-BCR expression; (ii) activating pre-BCR-mediated pro-survival and pro-proliferative signalling, while inhibiting cell cycle arrest and maturation; and (iii) bypassing the pre-BCR checkpoint and activating pro-survival signalling through pre-BCR independent alternative mechanisms. A complete understanding of the BCP-ALL-specific signalling networks will highlight their application in BCP-ALL therapy.

Preventing clonal evolutionary processes in cancer: Insights from mathematical models

Clonal evolutionary processes can drive pathogenesis in human diseases, with cancer being a prominent example. To prevent or treat cancer, mechanisms that can potentially interfere with clonal evolutionary processes need to be understood better. Mathematical modeling is an important research tool that plays an ever-increasing role in cancer research. This paper discusses how mathematical models can be useful to gain insights into mechanisms that can prevent disease initiation, help analyze treatment responses, and aid in the design of treatment strategies to combat the emergence of drug-resistant cells. The discussion will be done in the context of specific examples. Among defense mechanisms, we explore how replicative limits and cellular senescence induced by telomere shortening can influence the emergence and evolution of tumors. Among treatment approaches, we consider the targeted treatment of chronic lymphocytic leukemia (CLL) with tyrosine kinase inhibitors. We illustrate how basic evolutionary mathematical models have the potential to make patient-specific predictions about disease and treatment outcome, and argue that evolutionary models could become important clinical tools in the field of personalized medicine.

A critical appraisal of ibrutinib in the treatment of mantle cell lymphoma and chronic lymphocytic leukemia

Although chemo-immunotherapy remains at the forefront of first-line treatment for mantle cell lymphoma (MCL) and chronic lymphocytic leukemia (CLL), small molecules, such as ibrutinib, are beginning to play a significant role, particularly in patients with multiply relapsed or chemotherapy-refractory disease and where toxicity is an overriding concern. Ibrutinib is a first-in-class, oral inhibitor of Bruton's tyrosine kinase, which functions by irreversible inhibition of the downstream signaling pathway of the B-cell receptor, which normally promotes cell survival and proliferation. Early clinical trials have demonstrated excellent tolerability and a modest side-effect profile even in elderly and multiply pretreated patient cohorts. Although the majority of disease responses tend to be partial, efficacy data have also been encouraging with more than two-thirds of patients with CLL and MCL demonstrating a durable response, even in the high-risk disease setting. Resistance mechanisms are only partially understood and appear to be multifactorial, including the binding site mutation C481S, and escape through other common cell-signaling pathways. This article appraises the currently available data on safety and efficacy from clinical trials of ibrutinib in the management of MCL and CLL, both as a single agent and in combination with other therapies, and considers how this drug is likely to be used in future clinical practice.

Ibrutinib enhances IL-17 response by modulating the function of bone marrow derived dendritic cells

Ibrutinib (PCI-32765) is an irreversible dual Btk/Itk inhibitor shown to be effective in treating several B cell malignancies. However, limited studies have been conducted to study the effect of this drug on myeloid cell function. Hence, we studied the effect of ibrutinib treatment on TLR-4 mediated activation of bone marrow derived dendritic cell culture (DCs). Upon ibrutinib treatment, LPS-treated DCs displayed lower synthesis of TNF-α and nitric oxide (NO) and higher induction of IL-6, TGF-β, IL-10 and IL-18. While ibrutinib dampened MHC-II and CD86 expression on DCs, CD80 expression was upregulated. Further, ibrutinib-treated DCs promoted T cell proliferation and enhanced IL-17 production upon co-culture with nylon wool enriched T cells. Taken together, our results indicate that ibrutinib modulates TLR-4 mediated DC activation to promote an IL-17 response. We describe a novel mode of action for ibrutinib on DCs which should be explored to treat other forms of cancer besides B cell malignancies.

Macrophages eat cancer cells using their own calreticulin as a guide: roles of TLR and Btk

Macrophage-mediated programmed cell removal (PrCR) is an important mechanism of eliminating diseased and damaged cells before programmed cell death. The induction of PrCR by eat-me signals on tumor cells is countered by don't-eat-me signals such as CD47, which binds macrophage signal-regulatory protein α to inhibit phagocytosis. Blockade of CD47 on tumor cells leads to phagocytosis by macrophages. Here we demonstrate that the activation of Toll-like receptor (TLR) signaling pathways in macrophages synergizes with blocking CD47 on tumor cells to enhance PrCR. Bruton's tyrosine kinase (Btk) mediates TLR signaling in macrophages. Calreticulin, previously shown to be an eat-me signal on cancer cells, is activated in macrophages for secretion and cell-surface exposure by TLR and Btk to target cancer cells for phagocytosis, even if the cancer cells themselves do not express calreticulin.

Bruton tyrosine kinase is a therapeutic target in stem-like cells from multiple myeloma

Ibrutinib (Imbruvica), a small-drug inhibitor of Bruton tyrosine kinase (BTK), is currently undergoing clinical testing in patients with multiple myeloma, yet important questions on the role of BTK in myeloma biology and treatment are outstanding. Using flow-sorted side population cells from human myeloma cell lines and multiple myeloma primary samples as surrogate for the elusive multiple myeloma stem cell, we found that elevated expression of BTK in myeloma cells leads to AKT/WNT/β-catenin-dependent upregulation of key stemness genes (OCT4, SOX2, NANOG, and MYC) and enhanced self-renewal. Enforced transgenic expression of BTK in myeloma cells increased features of cancer stemness, including clonogenicity and resistance to widely used myeloma drugs, whereas inducible knockdown of BTK abolished them. Furthermore, overexpression of BTK in myeloma cells promoted tumor growth in laboratory mice and rendered side population-derived tumors that contained high levels of BTK more sensitive to the selective, second-generation BTK inhibitor, CGI1746, than side population-derived tumors that harbored low levels of BTK. Taken together, these findings implicate BTK as a positive regulator of myeloma stemness and provide additional support for the clinical testing of BTK-targeted therapies in patients with myeloma.

BCR pathway inhibition as therapy for chronic lymphocytic leukemia and lymphoplasmacytic lymphoma

Chronic lymphocytic leukemia (CLL) and lymphoplasmacytic lymphoma (LPL) are malignancies of mature B cells. In LPL, mutations of the adaptor protein MYD88 (L265P) in the Toll-like receptor pathway have been recognized recently as being a hallmark of the disease and indicate a dependence of the tumor on this pathway. In CLL, functional studies have implicated BCR activation in the tissue microenvironment as a pivotal pathway in the pathogenesis. Bruton's tyrosine kinase (BTK) and the PI3Kδ isoform are essential for BCR signaling and also seem to be required for signal transduction in LPL cells, even if the role of BCR signaling in this disease remains less well defined. Ibrutinib, a covalent inhibitor of BTK approved by the Food and Drug Administration as a second-line treatment for CLL, and idelalisib, a selective inhibitor of PI3Kδ, achieve excellent clinical responses in both diseases irrespective of classic markers indicating high-risk disease. Several additional inhibitors targeting BTK and PI3Kδ, as well as the spleen tyrosine kinase, have entered clinical trials. This review discusses the biologic basis for kinase inhibitors as targeted therapy for CLL and LPL and summarizes the clinical experience with these agents.

Breakthrough cancer medicine and its impact on novel drug development in China: report of the US Chinese Anti-Cancer Association (USCACA) and Chinese Society of Clinical Oncology (CSCO) Joint Session at the 17th CSCO Annual Meeting

The US Chinese Anti-Cancer Association (USCACA) teamed up with Chinese Society of Clinical Oncology (CSCO) to host a joint session at the17th CSCO Annual Meeting on September 20th, 2014 in Xiamen, China. With a focus on breakthrough cancer medicines, the session featured innovative approaches to evaluate breakthrough agents and established a platform to interactively share successful experiences from case studies of 6 novel agents from both the United States and China. The goal of the session is to inspire scientific and practical considerations for clinical trial design and strategy to expedite cancer drug development in China. A panel discussion further provided in-depth advice on advancing both early and full development of novel cancer medicines in China.

Ibrutinib inhibits SDF1/CXCR4 mediated migration in AML

Pharmacological targeting of BTK using ibrutinib has recently shown encouraging clinical activity in a range of lymphoid malignancies. Recently we reported that ibrutinib inhibits human acute myeloid leukemia (AML) blast proliferation and leukemic cell adhesion to the surrounding bone marrow stroma cells. Here we report that in human AML ibrutinib, in addition, functions to inhibit SDF1/CXCR4-mediated AML migration at concentrations achievable in vivo. It has previously been shown that SDF1/CXCR4-induced migration is dependent on activation of downstream BTK in chronic lymphocytic leukaemia (CLL) and multiple myeloma. Here we show that SDF-1 induces BTK phosphorylation and downstream MAPK signalling in primary AML blast. Furthermore, we show that ibrutinib can inhibit SDF1-induced AKT and MAPK activation. These results reported here provide a molecular mechanistic rationale for clinically evaluating BTK inhibition in AML patients and suggests that in some AML patients the blasts count may initially rise in response to ibrutinib therapy, analgous to similar clinical observations in CLL.

Ibrutinib interferes with the cell-mediated anti-tumor activities of therapeutic CD20 antibodies: implications for combination therapy

The novel Bruton tyrosine kinase inhibitor ibrutinib and phosphatidyl-4-5-biphosphate 3-kinase-δ inhibitor idelalisib are promising drugs for the treatment of chronic lymphocytic leukemia and B-cell non-Hodgkin lymphoma, either alone or in combination with anti-CD20 antibodies. We investigated the possible positive or negative impact of these drugs on all known mechanisms of action of both type I and type II anti-CD20 antibodies. Pretreatment with ibrutinib for 1 hour did not increase direct cell death of cell lines or chronic lymphocytic leukemia samples mediated by anti-CD20 antibodies. Pre-treatment with ibrutinib did not inhibit complement activation or complement-mediated lysis. In contrast, ibrutinib strongly inhibited all cell-mediated mechanisms induced by anti-CD20 antibodies rituximab, ofatumumab or obinutuzumab, either in purified systems or whole blood assays. Activation of natural killer cells, and antibody-dependent cellular cytotoxicity by these cells, as well as phagocytosis by macrophages or neutrophils were inhibited by ibrutinib with a half maximal effective concentration of 0.3-3 μM. Analysis of anti-CD20 mediated activation of natural killer cells isolated from patients on continued oral ibrutinib treatment suggested that repeated drug dosing inhibits these cells in vivo. Finally we show that the phosphatidyl-4-5-biphosphate 3-kinase-δ inhibitor idelalisib similarly inhibited the immune cell-mediated mechanisms induced by anti-CD20 antibodies, although the effects of this drug at 10 μM were weaker than those observed with ibrutinib at the same concentration. We conclude that the design of combined treatment schedules of anti-CD20 antibodies with these kinase inhibitors should consider the multiple negative interactions between these two classes of drugs.

Evolution of ibrutinib resistance in chronic lymphocytic leukemia (CLL)

The Bruton tyrosine kinase inhibitor (BTKi) ibrutinib is a new targeted therapy for patients with chronic lymphocytic leukemia (CLL). Ibrutinib is given orally on a continuous schedule and induces durable remissions in the majority of CLL patients. However, a small proportion of patients initially responds to the BTKi and then develops resistance. Estimating the frequency, timing, and individual risk of developing resistance to ibrutinib, therefore, would be valuable for long-term management of patients. Computational evolutionary models, based on measured kinetic parameters of patients, allow us to approach these questions and to develop a roadmap for personalized prognosis and treatment management. Our kinetic models predict that BTKi-resistant mutants exist before initiation of ibrutinib therapy, although they only comprise a minority of the overall tumor burden. Furthermore, we can estimate the time required for resistant cells to grow to detectable levels. We predict that this can be highly variable, depending mostly on growth and death rates of the individual CLL cell clone. For a specific patient, this time can be predicted with a high degree of certainty. Our model can thus be used to predict for how long ibrutinib can suppress the disease in individual patients. Furthermore, the model can suggest whether prior debulking of the tumor with chemo-immunotherapy can prolong progression-free survival under ibrutinib. Finally, by applying the models to data that document progression during ibrutinib therapy, we estimated that resistant mutants might have a small (<2%) mean fitness advantage in the absence of treatment, compared with sensitive cells.

Myeloma bone disease: pathogenesis, current treatments and future targets

INTRODUCTION

Patients with myeloma develop localized and generalized bone loss leading to hypercalcaemia, accelerated osteoporosis, vertebral wedge fractures, other pathological fractures, spinal cord compression and bone pain. Bone loss is mediated by a variety of biological modifiers including osteoclast-activating factors (OAF) and osteoblast (OB) inhibitory factors produced either directly by malignant plasma cells (MPCs) or as a consequence of their interaction with the bone marrow microenvironment (BMM). Raised levels of OAFs such as receptor activator of nuclear factor-kappa B ligand (RANKL), macrophage inflammatory protein 1 alpha, tumour necrosis factor-alpha and interleukin 6 stimulate bone resorption by recruiting additional osteoclasts. Via opposing mechanisms, increases in OB inhibitory factors, such as dickkopf-1 (Dkk-1), soluble frizzled-related protein-3 and hepatocyte growth factor (HGF), suppress bone formation by inhibiting the differentiation and recruitment of OBs. These changes result in an uncoupling of physiological bone remodelling, leading to myeloma bone disease (MBD). Moreover, the altered BMM provides a fertile ground for the growth and survival of MPCs. Current clinical management of MBD is both reactive (to pain and fractures) and preventive, with bisphosphonates (BPs) being the mainstay of pharmacological treatment. However, side effects and uncertainties associated with BPs warrant the search for more targeted treatments for MBD. This review will summarize recent developments in understanding the intimate relationship between MBD and the BMM and the novel ways in which they are being therapeutically targeted.

SOURCES OF DATA

All data included were sourced and referenced from PubMed.

AREAS OF AGREEMENT

The clinical utility of BP therapy is well established. However, there is general acknowledgement that BPs are only partially successful in the treatment of MBD. The number of skeletal events attributable to myeloma are reduced by BPs but not totally eliminated. Furthermore, existing damage is not repaired. It is widely recognized that more effective treatments are needed.

AREAS OF CONTROVERSY

There remains controversy concerning the duration of BP therapy. Whether denosumab is a viable alternative to BP therapy is also contested. Many of the new therapeutic strategies discussed are yet to translate to clinical practice and demonstrate equal efficacy or superiority to BP therapy. It also remains controversial whether reported anti-tumour effects of bone-modulating therapies are clinically significant.

GROWING POINTS

The potential clinical utility of bone anabolic therapies including agents such as anti-Dkk-1, anti-sclerostin and anti-HGF is becoming increasingly recognized.

AREAS TIMELY FOR DEVELOPING RESEARCH

Further research effectively targeting the mediators of MBD, targeting both bone resorption and bone formation, is urgently needed. This should translate promptly to clinical trials of combination therapy comprising anti-resorptives and bone anabolic therapies to demonstrate efficacy and improved outcomes over BPs.

Safety and activity of ibrutinib plus rituximab for patients with high-risk chronic lymphocytic leukaemia: a single-arm, phase 2 study

BACKGROUND

Ibrutinib, an orally administered covalent inhibitor of Bruton's tyrosine kinase (BTK), is an effective treatment for relapsed chronic lymphocytic leukaemia (CLL). We investigated the activity and safety of the combination of ibrutinib with the monoclonal antibody rituximab in patients with high-risk CLL.

METHODS

In this single-arm phase 2 study, we enrolled adult patients with high-risk CLL at the MD Anderson Cancer Center (Houston, TX, USA). All enrolled participants had high-risk cytogenetic abnormalities (deletion 17p, TP53 mutation, or deletion 11q) or a short progression-free survival (PFS <36 months) after previous first-line chemoimmunotherapy. Patients with symptomatic disease requiring therapy received 28-day cycles of once-daily ibrutinib 420 mg together with rituximab (375 mg/m(2), intravenously, every week during cycle 1, then once per cycle until cycle 6), followed by continuous daily single-agent ibrutinib 420 mg until disease progression or until toxicities or complications precluded further treatment. The primary endpoint was progression-free survival in the intention-to-treat population. This study is registered with ClinicalTrials.gov number NCT01520519, and is no longer accruing patients.

FINDINGS

Between Feb 28, 2012, and Sept 11, 2012, we enrolled 40 patients with CLL with high-risk disease features, 20 of whom had deletion 17p (del[17p]) or TP53 mutations (16 previously treated, four untreated), 13 had relapsed CLL with deletion 11q (del[11q]), and seven a PFS less than 36 months after first-line chemoimmunotherapy. 18-month PFS in all patients was 78·0% (95% CI 60·6-88·5), whereas in those with a del(17p) or TP53 mutation it was 72·4% (45·6-87·6) Toxicity was mainly mild to moderate in severity (grade 1-2). Diarrhoea occurred in ten (25%) patients (grade 1 in nine patients and grade 2 in one), bleeding events in 14 (33%) patients (eight grade 1 and five grade 2), nausea or vomiting in 15 patients (38%) (ten grade 1 and five grade 2), and fatigue in seven (18%) patients (four grade 1 and three grade 2). Five patients (13%) had grade 3 infections (two lung infections, one upper respiratory tract infection, one sepsis, and one mucositis), and no grade 4 or 5 infections occurred. One patient had grade 4 neutropenia.

INTERPRETATION

The encouraging safety and activity of ibrutinib and rituximab in this population of patients with high-risk CLL merits further investigation of this combination.

FUNDING

Pharmacyclics Inc, Cancer Prevention and Research Institute of Texas, Leukemia and Lymphoma Society, National Cancer Institute, MD Anderson Cancer Center.

Bruton's tyrosine kinase inhibitors and their clinical potential in the treatment of B-cell malignancies: focus on ibrutinib

Aberrant signaling of the B-cell receptor pathway has been linked to the development and maintenance of B-cell malignancies. Bruton's tyrosine kinase (BTK), a protein early in this pathway, has emerged as a new therapeutic target in a variety of such malignancies. Ibrutinib, the most clinically advanced small molecule inhibitor of BTK, has demonstrated impressive tolerability and activity in a range of B-cell lymphomas which led to its recent approval for relapsed mantle cell lymphoma and chronic lymphocytic leukemia. This review focuses on the preclinical and clinical development of ibrutinib and discusses its therapeutic potential.

Current understanding of tyrosine kinase BMX in inflammation and its inhibitors

Tec family kinases, which include tyrosine kinase expressed in hepatocellular carcinoma (TEC), Bruton’s tyrosine kinase (BTK), interleukin (IL)-2-inducible T-cell kinase (ITK), tyrosine-protein kinase (TXK), and bone marrow tyrosine kinase on chromosome X (BMX), are the second largest group of non-receptor tyrosine kinases and have a highly conserved carboxyl-terminal kinase domain. BMX was identified in human bone marrow cells, and was demonstrated to have been expressed in myeloid hematopoietic lineages cells, endothelial cells, and several types of cancers. Significant progress in this area during the last decade revealed an important role for BMX in inflammation and oncologic disorders. This review focuses on BMX biology, its role in inflammation and possible signaling pathways, and the potential of selective BMX inhibitors.

Mouse models in the study of chronic lymphocytic leukemia pathogenesis and therapy

Mouse models that recapitulate human malignancy are valuable tools for the elucidation of the underlying pathogenetic mechanisms and for preclinical studies. Several genetically engineered mouse models have been generated, either mimicking genetic aberrations or deregulated gene expression in chronic lymphocytic leukemia (CLL). The usefulness of such models in the study of the human disease may potentially be hampered by species-specific biological differences in the target cell of the oncogenic transformation. Specifically, do the genetic lesions or the deregulated expression of leukemia-associated genes faithfully recapitulate the spectrum of lymphoproliferations in humans? Do the CLL-like lymphoproliferations in the mouse have the phenotypic, histological, genetic, and clinical features of the human disease? Here we compare the various CLL mouse models with regard to disease phenotype, penetrance, and severity. We discuss similarities and differences of the murine lymphoproliferations compared with human CLL. We propose that the Eμ-TCL1 transgenic and 13q14-deletion models that have been comprehensively studied at the levels of leukemia phenotype, antigen-receptor repertoire, and disease course show close resemblance to the human disease. We conclude that modeling CLL-associated genetic dysregulations in mice can provide important insights into the molecular mechanisms of disease pathogenesis and generate valuable tools for the development of novel therapies.

The bruton tyrosine kinase inhibitor ibrutinib (PCI-32765) blocks hairy cell leukaemia survival, proliferation and B cell receptor signalling: a new therapeutic approach

B cell receptor (BCR) signalling plays a critical role in the progression of several B-cell malignancies, but its role in hairy cell leukaemia (HCL) is ambiguous. Bruton tyrosine kinase (BTK), a key player in BCR signalling, as well as B cell migration and adhesion, can be targeted with ibrutinib, a selective, irreversible BTK inhibitor. We analysed BTK expression and function in HCL and analysed the effects of ibrutinib on HCL cells. We demonstrated uniform BTK protein expression in HCL cells. Ibrutinib significantly inhibited HCL proliferation and cell cycle progression. Accordingly, ibrutinib also reduced HCL cell survival after BCR triggering with anti-immunoglobulins and abrogated the activation of kinases downstream of the BCR (PI3K and MAPK). Ibrutinib also inhibited BCR-dependent secretion of the chemokines CCL3 and CCL4 by HCL cells. Interestingly, ibrutinib inhibited also CXCL12-induced signalling, a key pathway for bone marrow homing. Collectively, our data support the clinical development of ibrutinib in patients with HCL.

Bruton's tyrosine kinase: from X-linked agammaglobulinemia toward targeted therapy for B-cell malignancies

Discovery of Bruton's tyrosine kinase (BTK) mutations as the cause for X-linked agammaglobulinemia was a milestone in understanding the genetic basis of primary immunodeficiencies. Since then, studies have highlighted the critical role of this enzyme in B-cell development and function, and particularly in B-cell receptor signaling. Because its deletion affects mostly B cells, BTK has become an attractive therapeutic target in autoimmune disorders and B-cell malignancies. Ibrutinib (PCI-32765) is the most advanced BTK inhibitor in clinical testing, with ongoing phase III clinical trials in patients with chronic lymphocytic leukemia and mantle-cell lymphoma. In this article, we discuss key discoveries related to BTK and clinically relevant aspects of BTK inhibitors, and we provide an outlook into clinical development and open questions regarding BTK inhibitor therapy.

Kinetics of CLL cells in tissues and blood during therapy with the BTK inhibitor ibrutinib

The Bruton tyrosine kinase (BTK) inhibitor ibrutinib has excellent clinical activity in patients with chronic lymphocytic leukemia (CLL). Characteristically, ibrutinib causes CLL cell redistribution from tissue sites into the peripheral blood during the initial weeks of therapy. To better characterize the dynamics of this redistribution phenomenon, we correlated serial lymphocyte counts with volumetric changes in lymph node and spleen sizes during ibrutinib therapy. Kinetic parameters were estimated by applying a mathematical model to the data. We found that during ibrutinib therapy, 1.7% ± 1.1% of blood CLL cells and 2.7% ± 0.99% of tissue CLL cells die per day. The fraction of the tissue CLL cells that was redistributed into the blood during therapy was estimated to be 23.3% ± 17% of the total tissue disease burden. These data indicate that the reduction of tissue disease burden by ibrutinib is due more to CLL cell death and less to egress from nodal compartments.

Inhibition of ER stress-associated IRE-1/XBP-1 pathway reduces leukemic cell survival

Activation of the ER stress response is associated with malignant progression of B cell chronic lymphocytic leukemia (CLL). We developed a murine CLL model that lacks the ER stress-associated transcription factor XBP-1 in B cells and found that XBP-1 deficiency decelerates malignant progression of CLL-associated disease. XBP-1 deficiency resulted in acquisition of phenotypes that are disadvantageous for leukemic cell survival, including compromised BCR signaling capability and increased surface expression of sphingosine-1-phosphate receptor 1 (S1P1). Because XBP-1 expression requires the RNase activity of the ER transmembrane receptor IRE-1, we developed a potent IRE-1 RNase inhibitor through chemical synthesis and modified the structure to facilitate entry into cells to target the IRE-1/XBP-1 pathway. Treatment of CLL cells with this inhibitor (B-I09) mimicked XBP-1 deficiency, including upregulation of IRE-1 expression and compromised BCR signaling. Moreover, B-I09 treatment did not affect the transport of secretory and integral membrane-bound proteins. Administration of B-I09 to CLL tumor-bearing mice suppressed leukemic progression by inducing apoptosis and did not cause systemic toxicity. Additionally, B-I09 and ibrutinib, an FDA-approved BTK inhibitor, synergized to induce apoptosis in B cell leukemia, lymphoma, and multiple myeloma. These data indicate that targeting XBP-1 has potential as a treatment strategy, not only for multiple myeloma, but also for mature B cell leukemia and lymphoma.

Bruton's tyrosine kinase (BTK) inhibitors in clinical trials

BTK is a cytoplasmic, non-receptor tyrosine kinase that transmits signals from a variety of cell-surface molecules, including the B-cell receptor (BCR) and tissue homing receptors. Genetic BTK deletion causes B-cell immunodeficiency in humans and mice, making this kinase an attractive therapeutic target for B-cell disorders. The BTK inhibitor ibrutinib (PCI-32765, brand name: Imbruvica) demonstrated high clinical activity in B-cell malignancies, especially in patients with chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL), and Waldenstrom's macroglobulinemia (WM). Therefore, ibrutinib was granted a 'breakthrough therapy' designation for these indications and was recently approved for the treatment of relapsed MCL by the U.S. Food and Drug Administration. Other BTK inhibitors in earlier clinical development include CC-292 (AVL-292), and ONO-4059. In CLL and MCL, ibrutinib characteristically induces redistribution of malignant B cells from tissue sites into the peripheral blood, along with rapid resolution of enlarged lymph nodes and a surge in lymphocytosis. With continuous ibrutinib therapy, growth- and survival-inhibitory activities of ibrutinib result in the normalization of lymphocyte counts and remissions in a majority of patients. This review discusses the clinical advances with BTK inhibitor therapy, as well as its pathophysiological basis, and outlines perspectives for future use of BTK inhibitors.

Identification of Bruton's tyrosine kinase as a therapeutic target in acute myeloid leukemia

Bruton's tyrosine kinase (BTK) is a cytoplasmic protein found in all hematopoietic cell lineages except for T cells. BTK mediates signaling downstream of a number of receptors. Pharmacologic targeting of BTK using ibrutinib (previously PCI-32765) has recently shown encouraging clinical activity in a range of lymphoid malignancies. This study reports for the first time that ibrutinib inhibits blast proliferation from human acute myeloid leukemia (AML) and that treatment with ibrutinib significantly augmented cytotoxic activities of standard AML chemotherapy cytarabine or daunorubicin. Here we describe that BTK is constitutively phosphorylated in the majority of AML samples tested, with BTK phosphorylation correlating highly with the cell's cytotoxic sensitivity toward ibrutinib. BTK-targeted RNAi knockdown reduced colony-forming capacity of primary AML blasts and proliferation of AML cell lines. We showed that ibrutinib binds at nanomolar range to BTK. Furthermore, we showed ibrutinib's antiproliferative effects in AML are mediated via an inhibitory effect on downstream nuclear factor-κB survival pathways. Moreover, ibrutinib inhibited AML cell adhesion to bone marrow stroma. Furthermore, these effects of ibrutinib in AML were seen at comparable concentrations efficacious in chronic lymphocytic leukemia. These results provide a biological rationale for clinical evaluation of BTK inhibition in AML patients.

The microenvironment in chronic lymphocytic leukemia (CLL) and other B cell malignancies: insight into disease biology and new targeted therapies

Over the last decade, the active role of the microenvironment in the pathogenesis of B cell lymphomas has been recognized, delivering signals that favor clonal expansion and drug resistance. We are only beginning to understand the complex cross talk between neoplastic B cells and the tissue microenvironment, for example in secondary lymphoid organs, but some key cellular and molecular players have emerged. Mesenchymal stromal cells, nurselike cells (NLC) and lymphoma-associated macrophages (LAM), in concert with T cells, natural killer cells and extracellular matrix components participate in the dialog with the neoplastic B cells. B cell receptor signaling, activation via TNF family members (i.e. BAFF, APRIL), and tissue homing chemokine receptors and adhesion molecules are important in the interaction between malignant B cells and their microenvironment. Disrupting this cross talk is an attractive novel strategy for treating patients with B cell malignancies. Here, we summarize the cellular and molecular interactions between B cell lymphoma/leukemia cells and their microenvironment, and the therapeutic targets that are emerging, focusing on small molecule inhibitors that are targeting B cell receptor-associated kinases SYK, BTK, and PI3Ks, as well as on immunomodulatory agents and T cell mediated therapies. Clinically relevant aspects of new targeted therapeutics will be discussed, along with an outlook into future therapeutic strategies.