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`OncoTargets and "erapy
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`Open Access Full Text Article
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`Dovepress
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`open access to scientific and medical research
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`R E V I E W
`
`Novel Bruton’s tyrosine kinase inhibitors
`currently in development
`
`Osmond J D’Cruz1
`Fatih M Uckun1,2
`
`1Children’s Center for Cancer and
`Blood Diseases, Children’s Hospital
`Los Angeles, Los Angeles, CA, USA;
`2Department of Pediatrics, University
`of Southern California, Los Angeles,
`CA, USA
`
`Abstract: Bruton’s tyrosine kinase (Btk) is intimately involved in multiple signal-transduction
`
`pathways regulating survival, activation, proliferation, and differentiation of B-lineage lymphoid
`
`cells. Btk is overexpressed and constitutively active in several B-lineage lymphoid malignancies.
`
`Btk has emerged as a new antiapoptotic molecular target for treatment of B-lineage leukemias
`
`and lymphomas. Preclinical and early clinical results indicate that Btk inhibitors may be useful
`
`in the treatment of leukemias and lymphomas.
`Keywords: tyrosine kinase, personalized therapy, kinase inhibitors, Btk, leukemia, lymphoma
`
`Introduction
`Bruton’s tyrosine kinase (Btk) is intimately involved in multiple signal-transduction
`
`pathways regulating survival, activation, proliferation, and differentiation of B-lineage
`lymphoid cells.1,2 Btk is an upstream activator of multiple antiapoptotic signaling
`molecules and networks, including the signal transducer and activator of transcription
`5 (STAT5) protein,3 phosphatidylinositol (PI) 3-kinase/AKT/mammalian target of
`rapamycin (mTOR) pathway,4 and nuclear factor kappa B (NF-(cid:75)B) (Figure 1).5,6 Further,
`Btk associates with the death receptor Fas via its kinase and pleckstrin homology (PH)
`
`domains and prevents the interaction of Fas with Fas-associated protein with death domain
`
`(FADD), which is essential for the recruitment and activation of caspase-8/FLICE by Fas
`
`during the apoptotic signal (Figure 1). This impairment by Btk prevents the assembly of
`a proapoptotic death-inducing signaling complex (DISC) after Fas ligation.7
`Btk is abundantly expressed in malignant cells from patients with B-cell precursor
`
`(BCP)-acute lymphoblastic leukemia (ALL, the most common form of cancer in
`
`children and adolescents), chronic lymphocytic leukemia (CLL), and non-Hodgkin’s
`lymphoma (NHL).8–11 A meta-analysis of cancer-associated gene expression
`changes utilizing the Oncomine database revealed a marked enrichment of the most
`
`discriminating Btk-dependent antiapoptotic gene targets in 17 comparisons for
`diagnostic classes of human leukemias and lymphomas obtained from eight studies.11
`Consequently, Btk has emerged as a new molecular target for treatment of B-lineage
`
`leukemias and lymphomas.
`
`Btk disease targets
`Lymphohematopoietic malignancies
`B-lineage ALL (B-ALL) and B-cell CLL (B-CLL) are the most common childhood
`
`and adult leukemias, respectively. In both ALL and CLL, the resistance of leukemia
`
`Correspondence: Fatih M Uckun
`Translational Research in Leukemia and
`Lymphoma, Children’s Center for Cancer
`and Blood Diseases, Children’s Hospital
`Los Angeles, 4650 Sunset Boulevard,
`Smith Research Tower, Suite 316,
`Los Angeles, CA 90027, USA
`Email fmuckun@chla.usc.edu
`
`submit your manuscript | www.dovepress.com
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`Dovepress
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`http://dx.doi.org/10.2147/OTT.S33732
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`161
`OncoTargets and Therapy 2013:6 161–176
`© 2013 D’Cruz and Uckun, publisher and licensee Dove Medical Press Ltd. This is an Open Access
`article which permits unrestricted noncommercial use, provided the original work is properly cited.
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`D’Cruz and Uckun
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`PI3K
`
`AKT
`
`IKK
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`BAD
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`BTK
`
`MDM2
`
`P53
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`DISC
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`Prevention of
`apoptosis
`
`STAT5
`
`NFkB
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`Figure 1 Btk activates antiapoptotic pathways. Btk is an upstream regulator of
`multiple antiapoptotic pathways, including the PI3K–AKT pathway, STAT5 pathway,
`and NF(cid:75)B pathway. BTK also blocks the Fas-mediated apoptosis. See text for further
`discussion and references.
`Abbreviations: Btk, Bruton’s tyrosine kinase; PI3K, phosphatidylinositol 3-kinase;
`BAD, B-cell lymphoma 2-associated death promoter; IKK, I(cid:75)B kinase; DISC, death-
`inducing signaling complex; STAT, signal transducer and activator of transcription;
`NF(cid:75)B, nuclear factor kappa B.
`
`cells to apoptosis-inducing chemotherapeutic agents hampers
`a more successful outcome.12–15
`
`B-cell chronic lymphocytic leukemia
`CLL is the most common leukemia in adults, accounting
`
`for 25% of all leukemias, with approximately 8000 new
`cases diagnosed each year.16–19 CLL is characterized by
`the accumulation of mature, CD5(cid:11)/CD23(cid:11) monoclonal B
`lymphocytes in the blood, secondary lymphatic tissues, and
`bone marrow (BM).20 It is well established that the tumor
`microenvironment plays a major role in the pathogenesis
`
`of CLL: various cytokines, chemokines, and adhesion
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`Dovepress
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`the importance of BCR signaling in CLL comes from recent
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`comparative gene-expression profiling data that revealed
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`BCR signaling as the most prominent pathway activated in
`CLL cells isolated from lymphatic tissues.24 Because either
`tonic, chronic, or antigen-driven BCR signaling is involved
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`in the pathogenesis of most types of B-cell malignancies,
`
`the BCR signalosome provides a rational therapeutic target,
`including CLL.26
`Although CLL is a disease that is considered to be
`
`incurable with currently available therapy, its clinical course
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`is heterogeneous: some patients have a more stable disease
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`and die after many years from unrelated causes, whereas
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`others progress very quickly and die within a few years. This
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`variability has stimulated the search for prognostic markers
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`with which to predict the outcome of patients and to allow
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`treatments to be adapted to the specific risk. There is an urgent
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`need for apoptosis-promoting new antileukemic agents
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`against B-CLL. Several kinases are expressed at elevated
`
`levels in CLL cells, including Btk, Zeta-chain-associated
`
`protein (ZAP), and Lyn, and therefore have emerged as
`potential molecular targets.27
`
`B-lineage acute lymphoblastic leukemia
`B-ALL is the most common form of cancer in children
`and adolescents.28 Current treatment with ALL can cure
`approximately 80% of children with the disease. 29,30
`Currently, the major challenge in the treatment of B-ALL is
`to cure patients who have relapsed ((cid:94)20%) despite intensive
`multiagent chemotherapy.31–35 The standard approach to
`the treatment of these high-risk patients has been salvage
`
`molecules provided within the lymph nodes (LNs), spleen,
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`chemotherapy to achieve a second remission and subsequent
`
`and BM microenvironment, as well as signaling by the
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`use of very intensive treatment regimens, including high-
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`B-cell receptor (BCR), play a critical role in the localization,
`growth, survival, and drug resistance of CLL cells.21–26
`Proliferating CLL cells, which account for approximately
`0.1%–1% of the CLL clone,21 are typically found within
`microanatomical structures called proliferation centers or
`pseudofollicles,22 where CLL cells interact with accessory
`cells (ie, stromal cells or T cells), thereby receiving survival
`and growth signals.23 Such external signals from the leukemia
`microenvironment can supplement intrinsic oncogenic
`
`lesions, thereby promoting maintenance and expansion of
`the CLL clone.22 Among the various external stimuli in the
`tissue microenvironments, BCR activation and signaling,
`
`particularly in lymphatic tissues, is a central pathologic
`
`mechanism, even though the precise mechanism of BCR
`
`stimulation and the nature of the antigen(s) that activate the
`BCRs remain obscure.20,24,25 The most direct evidence for
`
`dose “supralethal” chemotherapy, often combined with
`
`total-body irradiation (TBI), followed by hematopoietic stem
`
`cell transplantation (SCT). Laboratory evidence indicates
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`that primary clonogenic blasts from a significant portion
`
`of B-ALL patients are among those reported as the most
`radiation-resistant human tumor cells.36,37 Furthermore,
`clonogenic leukemia cells from over two-thirds of the ALL
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`patients exhibit a substantial capacity to repair sublethal
`radiation damage.38 Resistance of B-leukemia cells to the
`proapoptotic effects of radiation-induced oxidative stress
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`hampers the attempts to improve the survival outcome
`
`of patients with B-ALL undergoing TBI and SCT and
`only (cid:12) 20% of high-risk B-ALL patients become long-term
`disease-free survivors even after TBI/SCT, with substantial
`short-term and long-term morbidity and mortality.38,39 These
`preclinical and clinical observations in B-ALL emphasize
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`Novel Btk inhibitors in development
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`the urgent need for identification of new drug candidates
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`capable of potentiating the antileukemic potency of pre-SCT
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`chemosensitizing properties and enhance the chemosensitivity
`of ALL cells.2,56
`
`radiochemotherapy regimens.
`
`In contrast with the marked improvements achieved in the
`
`treatment outcome of pediatric ALL patients, adult patients
`
`with ALL continue to have a poor outcome, with long-term
`leukemia-free survival rates of less than 50%.39–42 This poor
`outcome has been attributed in part to an increased frequency
`of high-risk leukemia with greater drug resistance.2,40–44
`A more rapid and complete reduction of the leukemia cell
`
`B-lineage non-Hodgkin’s lymphoma
`NHL is the third most common group of malignancies
`
`in children and adolescents in the US and accounts for
`approximately 7% of newly diagnosed cancers.57,58 NHL
`and Hodgkin’s lymphomas are represented prominently in
`
`the adolescent and young adult population. NHL constitutes
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`6%–10% of all pediatric malignancies in different parts of
`
`burden by upfront induction chemotherapy is likely to
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`the world. NHL is the fifth most common cause of cancer
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`prevent drug resistance and improve the survival outcome
`in ALL.8,9,43 Furthermore, a more effective postinduction
`intensification therapy may eliminate a higher fraction of
`
`residual leukemia cells, thereby improving the duration
`of remission.9,13,15,43 Consequently, the development of
`new potent anti-ALL drugs and the design of combinative
`
`treatment protocols using these new agents have emerged as
`exceptional focal points for leukemia research.8,13,24
`Btk is the first cytoplasmic non-Janus kinase (JAK)
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`deaths in young adult women aged 20–39 years. Although
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`age-adjusted incidence rates of NHL increase with age, the
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`more aggressive lymphomas are seen more commonly in the
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`younger population, with a transition to low-grade, indolent
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`subtypes as the population ages. Burkitt lymphoma, diffuse
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`large B-cell lymphoma (DLBCL), lymphoblastic lymphoma,
`
`and anaplastic large-cell lymphoma make up the most common
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`subtypes in the young adult population, although within
`
`the subgroup aged 30–39 years, follicular lymphoma (FL)
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`to be identified as a positive regulator of STAT5A in
`
`becomes more prominent. Btk inhibitors have demonstrated
`
`neoplastic B cells and B-cell precursors (BCPs). STAT5,
`
`clinical activity against a variety of B-cell malignancies in
`
`a direct substrate of Btk that is activated by Btk-mediated
`tyrosine phosphorylation of its Y694 residue,40 is an important
`regulator of survival17,44–46 and proliferation27,44,47–49 of BCPs at
`various stages of B-cell ontogeny. Recent studies have further
`
`ongoing phase I/II trials, including mantle-cell lymphoma
`
`(MCL), CLL, FL, and DLBCL, with good tolerability.
`
`DLBCL is one of the most common types of aggressive
`
`B-cell NHL. Pathologically, the tumor has a fast growth
`
`revealed a nucleocytoplasmic shuttling system for Btk, which
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`rate with a high Ki67 index. Without treatment, patients
`
`has implications regarding potential targets inside the nucleus
`
`usually die within 6–24 months, and with cur rent
`
`and which may be critical in gene regulation during B-cell
`development and differentiation as well as apoptosis.50 Btk
`is essential for BCR-mediated activation of the NF-(cid:75)B/Rel
`family of transcription factors, which in turn regulates genes
`controlling B-cell growth.51–53 STAT5 knockout mice suffer
`from leukopenia/lymphopenia and an accelerated rate of
`lymphohematopoietic cell apoptosis in the BM.17 Conversely,
`constitutive activation of STAT5 is capable of causing
`leukemic transformation of lymphohematopoietic cells19 and
`development of BCP leukemia in mice.45 Dominant-negative
`forms of STAT5 induce massive apoptosis in BCP-ALL
`cells.46 Btk is required for pre-BCR-dependent survival
`signals in BCP-ALL cells, including STAT5 activation and
`
`STAT5-mediated upregulation of BCL-xL, which rescues
`BCR-ABL(cid:11) BCP-ALL cells from apoptosis. The Btk-linked
`NF-(cid:75)B and phosphatidylinositol 3-kinase (PI3K)/Akt
`survival pathways are activated by chemotherapeutic agents
`and also contribute to drug resistance of BCP-ALL cells.54,55
`Tyrosine kinase inhibitors (TKIs) targeting Btk are likely
`
`to act as antileukemic agents with apoptosis-promoting and
`
`immunochemotherapeutic regimens 50%–60% of patients
`
`can be cured. However, 40%–50% of patients remain
`
`refractory to the therapy. Recently, BCR signaling has
`
`been recognized as a key pathway in the pathogenesis of
`DLBCL.59 Gene-expression profiling and unsupervised
`consensus clustering for analysis studies have identified a
`
`subset of lymphoma that demonstrates a BCR/proliferation
`signature (BCR-type DLBCL).60 In activated B-cell-
`like (ABC) DLBCL, NF-(cid:75)B activity relies upon chronic
`active BCR signaling, which can be potentially blocked
`
`by kinase inhibitors targeting Btk. Btk inhibitors are highly
`
`active against ABC DLBCL cells in vitro, and demonstrated
`
`clinical activity in a subset of patients with relapsed/
`refractory (R/R) ABC DLBCL.61
`Follicular NHL is the most common of the indolent
`
`lymphomas, accounting for about 70% of them and about
`
`22% of all lymphomas in North America and Europe. One of
`
`the primary concerns for any patient with follicular or other
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`indolent lymphoma is transformation to a more aggressive
`lymphoma, such as DLBCL.62,63 Similarly, patients with
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`recurrent B-lineage NHL or NHL that shows progression
`
`on standard chemotherapy regimens have a dismal prognosis
`
`and are in urgent need for innovative treatments capable of
`
`overcoming the multidrug resistance of their NHL cells. In
`
`the US, approximately 70,000 people will be diagnosed with
`
`NHL and 19,000 people will die of NHL during 2013.
`
`MCL is a malignancy of mature B cells characterized by
`
`the translocation t(11;14) that leads to aberrant expression
`of cyclin D1.64 Response to first-line chemotherapy is good,
`but most patients relapse, resulting in a median survival of
`
`5–7 years. Clinical studies with Btk inhibitors suggest that
`BCR signaling could play a role.65
`
`Multiple myeloma
`Multiple myeloma (MM) is a clonal malignancy of plasma
`
`cells accumulating in the BM. Myeloma cells have a high
`
`capacity to induce osteolytic bone lesions in patients,
`especially in the advanced stages.66 One key clinical feature
`of this cancer is the hyperactive bone resorption and minimal
`
`bone regeneration, partly due to overactive osteoclasts and
`
`inactive osteoblasts via unbalanced regulation of cytokines
`and chemokines in the BM microenvironment.67 MM cells
`are highly dependent on the BM microenvironment for
`
`the activity of NFATc1, the major osteoclast transcriptional
`factor activated following receptor activator of NF-(cid:75)B ligand
`(RANKL) stimulation.75 These findings suggest a potential
`role of Btk in mediating osteolytic bone disease in MM and a
`
`framework for the clinical development of Btk inhibitors as a
`novel therapeutic strategy in MM.76
`
`Solid tumors
`Recently, it was discovered that Btk is expressed not only
`
`in malignant lymphohematopoietic cells but in solid tumor
`cells as well.77–79 Eifert used an RNA interference (RNAi)
`screen to perform a large-scale loss-of-function analysis to
`
`facilitate the identification of individual factors necessary
`
`for the survival of an ErbB2/human epidermal growth factor
`receptor (HER)-2-positive breast cancer cell line.77,78 Notably,
`Btk short interfering RNA (siRNA) caused apoptosis in breast
`cancer cells.77,78 An aberrantly spliced 80 kDa Btk product
`with an amino-terminal extension was abundantly expressed
`in breast cancer cells as a potential molecular target.77,78
`Likewise, Lavitrano et al79 discovered a functional isoform
`of Btk in colorectal cancer cells and showed that its inhibition
`
`enhances cancer cell chemosensitivity.
`
`growth and survival through interactions, particularly with
`
`BM stromal cells, osteoclasts, and osteoblasts, all of which
`
`Btk inhibitor pipeline
`Several Btk inhibitors have been reported by us and others,
`
`secrete important MM growth factors and cytokines.
`
`Btk protein is expressed in plasma cell cancers, including
`
`MM and Waldenström’s macroglobulinemia (WM), with
`
`baseline activation correlating with levels of protein expression.
`
`Btk plays a crucial role in myeloma-associated osteolysis via
`
`regulating a broad panel of cytokines and chemokines both
`
`at transcriptional and protein levels in osteoclast lineage cells
`
`and BMSCs, which are in close contact with MM cells within
`
`the BM microenvironment. These Btk-targeted cytokines
`
`and chemokines include macrophage inflammatory protein
`(MIP)-1(cid:65),68 MIP-1(cid:66),69 stromal cell-derived factor (SDF)-1,69
`transforming growth factor (TGF)-(cid:66)1,70 activin A,71 acidic
`protein rich in leucines (APRIL),72 B-cell-activating factor
`(BAFF),73 and interleukin (IL)-8,74 which have been shown to
`contribute to MM-related bone lesions and disease progression.
`
`Btk activation in the BM milieu promotes MM cell growth,
`
`survival, and interaction with other BM stromal components,
`
`in addition to triggering MM-induced bone lysis. In a genome-
`
`wide screening of mRNA for nonreceptor TKs expressed
`
`and are being developed as therapeutic agents for various
`indications (Figure 2).80–104 Among these, the covalent
`inhibitor ibrutinib/PCI-32765 (Pharmacyclics) was developed
`
`as a selective and irreversible inhibitor of Btk, targeting the
`cysteine-481 residue in the active site.100 PCI-32765 is a potent
`nanomolar inhibitor of Btk, and exhibited promising activity
`in preclinical models of BCR-driven B-lineage lymphoma101
`and clinical testing in lymphoma patients.102 Likewise,
`dianilinopyrimidine-based irreversible Btk inhibitors with
`
`micromolar activity were developed, and two lead compounds,
`
`AVL-101 and AVL-291 (Avila Therapeutics) showed promising
`in vitro activity against lymphoma cells.103 Dasatinib/Sprycel,
`a breakpoint cluster region–Abelson (BCR–Abl) kinase
`
`inhibitor that is FDA-approved for the treatment of chronic
`
`myelogenous leukemia (CML), is a potent inhibitor of
`Btk.104,105 The availability of the coordinates of the Btk kinase
`domain X-ray crystal structures will continue to support
`further development of rationally designed Btk inhibitors.104,106
`
`during osteoclast and osteoblast differentiation in mice, high
`
`expression of Lyn and Syk, which are upstream of Btk, as
`
`Covalent Btk inhibitors
`Conventional small-molecule inhibitors interact with high
`
`well as Src, were identified in osteoclasts. In addition, Btk
`
`affinity in the binding site of a protein target, and the drug-target
`
`was shown to regulate osteoclast maturation by modulating
`
`complex is favored when plasma drug concentration is high.
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`Novel Btk inhibitors in development
`
`N
`
`NH
`
`O
`
`N
`
`O
`
`N
`
`N
`
`H N
`
`S
`
`O
`
`Br
`
`Br
`
`OH
`
`O
`
`N H
`
`CN
`
`H3C
`
`NH2
`
`N
`
`N
`
`N
`
`N
`
`O
`
`N
`
`O
`Ibrutinib (PCI-32765)
`
`LFM-A13
`
`GDC-0834
`
`N
`
`N
`
`N
`
`OH
`
`O
`
`H
`
`H
`
`N
`
`S
`
`NH
`
`N
`
`H N
`
`O
`
`CI
`
`Dasatinib
`
`Figure 2 Chemical structures of Bruton’s tyrosine kinase inhibitors. Ibrutinib (PCI-32765) is a covalent inhibitor currently under phase II and III clinical development for
`B-cell malignancies.
`Note: LFM-A13, GDC-0834, and dasatinib are noncovalent adenosine triphosphate-competitive Bruton’s tyrosine kinase inhibitors.
`
`Covalent inhibitors also form high-affinity interactions with
`
`the binding site of the protein target and bring a low-reactivity
`
`and induces apoptosis in B-cell lymphoma cell lines.107
`PCI-32765 inhibits activation-induced proliferation of
`
`warhead in close proximity to a structurally unique amino
`
`CLL cells and effectively blocks survival signals provided
`
`acid. Through covalent bonding, the compound is locked to
`
`externally to CLL cells from the microenvironment
`
`its target, thereby silencing the target’s activity. As a result,
`
`the pharmacodynamic behavior of a compound is coupled
`
`to protein half-life and turnover rather than pharmacokinetic
`
`properties. The covalent kinase inhibitors include ibrutinib/
`
`PCI-32765 (Figure 2), AVL-101, and AVL-291/292.
`
`Ibrutinib/PCI-32765
`Ibrutinib/PCI-32765 (1-[(3R)-3-[4-amino-3-(4-phenoxy-
`
`phenyl)pyrazolo[3,4-d]pyrimidin-1-yl]piperidin-1-yl]prop-
`
`(CD40L, BAFF, IL-6, IL-4, and tumor necrosis factor
`[TNF]-(cid:65)), fibronectin engagement, and stromal cell contact,
`as well as migration in response to tissue-homing chemokines
`(CXCL12, CXCL13).108,109 PCI-32765 targets not only B
`lymphocytes but also monocytes, macrophages, and mast
`
`cells, which are important Btk-expressing effector cells in
`arthritis.110 PCI-32765 inhibited TNF-(cid:65), IL-1(cid:66), and IL-6
`production in primary monocytes.110
`PCI-32765 is active in multiple in vivo models:
`
`2-en-1-one) is an orally active, small-molecule inhibitor that
`
`(1) transgenic murine model of BCR-driven lymphoma,
`
`forms an irreversible bond with cysteine-481 in the active site
`of Btk and inhibits Btk phosphorylation on Tyr223, resulting
`in Btk inhibition (half maximal inhibitory concentration
`[IC50] (cid:29) 0.5 nM).100,101 However, the cysteine residue is
`also present in a small subset of kinases with the potential
`
`for irreversible binding by PCI-32765. In vitro preclinical
`
`(2) spontaneous B-cell lymphoma in canines, and (3) mouse
`
`models of autoimmune disease. Once-daily dosing resulted in
`
`24-hour sustained target inhibition. PCI-32765 affected disease
`
`progression in an adoptive transfer T-cell leukemia/lymphoma
`1 (TCL1) mouse model of CLL.108 The TCL1 transgenic mouse
`model of CLL has been validated as a model similar to human
`
`studies demonstrated that PCI-32765 blocks BCR-stimulated
`activation of ERK1/2, PI3K, and NF-(cid:75)B, inhibits growth,
`
`CLL, providing an avenue to extend study of BCR signaling
`in the in vivo setting.111,112 These mice spontaneously develop
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`a CD5(cid:11) B-cell leukemia with unmutated immunoglobulin
`heavy-chain variable-region (IgVH) resembling unmutated
`CLL.113 PCI-32765 affected leukemia cell homing and
`disease progression. In this model, PCI-32765 caused a
`
`transient early lymphocytosis and profoundly inhibited
`
`CLL progression, as assessed by weight, development of
`hepatosplenomegaly, and survival.108 PCI-32765 induced
`objective clinical responses in dogs with spontaneous B-cell
`NHL.101 Active site occupancy of Btk was tightly correlated
`with the blockade of BCR signaling and in vivo efficacy.
`
`Clinical studies
`Ibrutinib/PCI-32765 is being investigated in patients with
`
`B-cell malignancies that include CLL/small lymphocytic
`
`lymphoma (SLL), MCL, DLBCL, FL, and MM (Tables 1
`
`and 2). In phase I/II studies, PCI-32765 showed encouraging
`
`clinical activity in patients with several types of B-cell
`lymphoma.114
`
`second phase I study by Fowler et al118,119 in 47 patients with
`various relapsed or refractory B-cell malignancies, a dose
`of 420 mg daily was established based on (cid:14)90% occupancy
`of Btk. In this study, PCI-32765 induced a durable objective
`response (CR (cid:11) PR) in various relapsed or refractory B-cell
`malignancies.
`
`Relapsed/refractory MCL
`An interim analysis of a phase II study in patients with
`relapsed or refractory MCL, Staudt et al,120 showed that PCI-
`32765 (560 mg daily in 28-day cycles) produced an objective
`
`response of 67%; in patients pretreated with bortezomib
`
`(Velcade) the overall response rate was 75%, compared with
`58% in bortezomib-naive patients.121 PCI-32765 was gener-
`ally well tolerated, with hematologic grade 3/4 AEs occur-
`ring in (cid:12)5% of patients. A unique observation of marked
`lymphocytosis, termed a “compartment shift,” was observed
`
`where the lymphocyte count doubled from about day 8 to
`
`28 of cycle 1, and then slowly declined to about 40% above
`
`Relapsed/refractory B-cell malignancies
`Advani et al115–117 reported a dose-escalating phase I study
`of PCI-32765 (1.25 mg/kg/day to 12.5 mg/kg in a 28-day
`
`baseline with subsequent cycles. These lymphocytes were
`identified as being CD5(cid:11)฀and CD45(cid:11), consistent with circu-
`lating, mobilized lymphoma cells. The mechanism may be
`
`cycle) in 56 patients with multiple histologic subtypes of
`
`related to inhibition of tissue-homing cytokines CXCL12 and
`
`B-cell NHL (NCT00849654). The overall response rate was
`
`achieved in 30 (62%) of 50 evaluable patients, including
`
`seven complete responses (CRs) and 23 partial responses
`
`CXCL13, and inhibition of adhesion molecule functioning.
`Wang et al122 reported the results of a phase II study of PCI-
`32765 in relapsed or refractory MCL patients. In this study,
`
`(PRs). Responses were achieved in eleven (79%) patients
`
`111 patients (bortezomib-naive and bortezomib-exposed)
`
`with CLL/SLL, seven (78%) with MCL, six (46%) with FL,
`
`received oral PCI-32765 (560 mg daily). The primary end
`
`three (75%) with WM (Waldenström macroglobulinemia),
`
`point of the study was overall response rate. Duration of
`
`two (29%) with DLBCL, and one (33%) with Marginal
`
`response and safety evaluation were secondary end points.
`
`zone lymphoma (MZL). Median progression-free survival
`
`The median follow-up time was 9.2 months, with a range
`
`in all patients was 13.6 months. Therapy was well tolerated,
`
`of time to response to treatment of 1.4 to 16.4 months.
`
`and grade 3 or higher adverse events (AEs) observed were
`neutropenia, thrombocytopenia, and anemia ((cid:12)20%). In a
`
`The study showed an overall response rate of 68% (22%
`
`CR and 46% PR), with a median progression-free survival
`
`Table 1 Small-molecule Bruton’s tyrosine kinase inhibitors in development
`
`Company
`
`Pharmacyclics, Janssen
`
`Pharmacyclics, Janssen
`
`Avila Therapeutics, Celgene
`Bristol-Myers Squibb
`University of Southern California
`Ono Pharmaceutical
`Genentech, Gilead
`
`Compound
`
`Indications
`
`Stage
`
`Ibrutinib/
`PCL-32765
`PCL-32765 (cid:11) rituximab
`PCL-32765 (cid:11) ofatumumab
`PCL-32765 (cid:11) bendamustine and rituximab
`AVL-292
`Dasatinib (cid:11) fludarabine
`LFM-A13
`ONO-WG-307
`GDC-0834
`
`R/R CLL/SLL, MCL, FL, DLBCL, MM,
`indolent NHL, MALT/MZL
`R/R CLL/SLL
`High-risk CLL
`
`Phase Ib/II
`Phase III (CLL/NHL)
`Phase II
`Phase III
`
`CLL, B-cell NHL, WM
`R/R CLL/SLL
`B-cell malignancies
`B-cell malignancies
`B-cell malignancies
`
`Phase Ib
`Phase II
`Preclinical
`Preclinical
`Preclinical
`
`Abbreviations: DLBCL, diffuse large B-cell lymphoma; R/R CLL/SLL, relapsed/refractory chronic lymphocytic leukemia/small lymphocytic lymphoma; MCL, mantle-cell
`lymphoma; FL, follicular lymphoma; MM, multiple myeloma; NHL, non-Hodgkin’s lymphoma; MALT/MZL, mucosa-associated lymphoid tissue/marginal zone lymphoma;
`WM, Waldenström macroglobulinemia.
`
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`Novel Btk inhibitors in development
`
`Table 2 Summary of Bruton’s tyrosine kinase inhibitor candidates and their stage of clinical development
`
`Compound
`
`Indication
`
`Trial identifier
`
`Phase I
`
`Phase II
`
`Phase III
`
`Ibrutinib/
`PCI-32765
`
`AVL-292
`Dasatinib/
`Sprycel
`
`B-cell lymphoma
`B-cell lymphoma and CLL
`Newly diagnosed B-cell NHL
`CLL/SLL
`CLL/SLL
`CLL/SLL and prolymphocytic leukemia
`CLL/SLL
`MCL
`DLBCL
`R/R MM
`MCL
`R or R MCL
`R or R CLL or SLL
`R or R CLL or SLL
`Treatment-naive CLL or SLL
`R or R B-cell NHL, CLL, WM
`R or R CLL/SLL
`
`NCT00849654
`NCT01109069
`NCT01569750
`NCT01292135
`NCT01105247
`NCT01217749
`NCT01744691
`NCT01236391
`NCT01325701
`NCT01478581
`NCT01599949
`NCT01646021
`NCT01611090
`NCT01578707
`NCT01722487
`NCT01351935
`NCT00438854
`
`X
`X
`X
`X
`X
`X
`
`X
`
`X
`X
`X
`X
`X
`X
`X
`
`X
`
`X
`X
`X
`X
`
`Abbreviations: DLBCL, diffuse large B-cell lymphoma; R/R CLL/SLL, relapsed/refractory chronic lymphocytic leukemia/small lymphocytic lymphoma; MCL, mantle-cell
`lymphoma; FL, follicular lymphoma; MM, multiple myeloma; NHL, non-Hodgkin’s lymphoma; WM, Waldenström macroglobulinemia.
`
`estimated at 13.9 months. Results were similar between
`
`suggested the existence of additional Btk-dependent pathways
`
`the bortezomib-naive and bortezomib-exposed patients.
`
`(perhaps leading to p-ERK) in MCL that regulate their growth
`
`Long-term follow-up of the initial 51 patients showed an
`
`and phosphorylation of downstream kinase, supporting ERK
`
`incremental improvement in the response rate over time. The
`
`as a marker of sensitivity to PCI-32765.
`
`overall response rate increased for this patient subset to 75%.
`
`PCI-32765 resulted in high and durable responses and was
`
`generally well tolerated. Pneumonia was the only grade 3 or
`higher treatment-emergent AE occurring in (cid:4)5% of patients.
`Recently, Janssen has initated a randomized, multicenter
`
`Relapsed/refractory DLBCL
`Wilson et al124 reported the results of a phase II study of
`PCI-32765 in 70 patients with relapsed or refractory DLBCL
`
`in two genetically distinct subtypes of DLBCL: the ABC
`
`phase III trial comparing PCI-32765 versus temsirolimus
`
`subtype and the germinal center (GC) subtype. The ABC
`
`as a monotherapy in relapsed or refractory MCL patients
`
`subtype appears to be more driven by BCR signaling. The
`
`who had a prior rituximab-containing chemotherapy regi-
`
`overall response rate in the heavily pretreated population was
`
`men (NCT01646021). The primary end point of the study is
`
`23% (16 of 70 patients). Responses were primarily in the
`
`progression-free survival when compared to temsirolimus.
`
`ABC subtype, with twelve of 29 patients (41%) responding
`
`The plan is to enroll 280 patients outside the US.
`
`(five CR and seven PR). In the 20 GC patients, only one
`
`MCL cells express surface IgM and Btk and exhibit
`
`patient (5%) had a PR. This study supports the use of ABC
`
`constitutive Btk autophosphorylation, which is inhibited
`by PCI-32765. Ponader et al123 tested the comparative
`sensitivity of PCI-32765 to inhibit BCR-signaling pathways
`
`in nine MCL cell lines, including phosphorylation of
`phospholipase C (PLC)-(cid:71) that leads to calcium flux. In
`resistant cells, IgM stimulation induces p-Btk but not
`pPLC-(cid:71)2 or calcium flux. PCI-32765-sensitive cell lines were
`shown to display constitutive p-ERK activity, which was
`
`increased on IgM stimulation and completely abrogated by
`
`PCI-32765. The more resistant cell lines had low endogenous
`
`levels of p-ERK, which was induced strongly by IgM and only
`
`partially blocked by PCI-32765. Sensitive cell lines showed a
`
`DLBCL molecular subtype as a biomarker for enrichment
`
`of patients for future PCI-32765 studies. Grade 3 or higher
`
`AEs were 5%–10% of the patients.
`
`Relapsed/refractory CLL or SLL
`Daily oral administration of PCI-32765 has been evaluated
`
`in patients with R/R CLL or SLL that had been treated with
`at least two prior therapies, including fludarabine.125 In an
`ongoing phase Ib/II study by O’Brien et al,126,127 61 patients
`with R/R CLL enrolled at two dose levels: 420 mg (n (cid:29) 27) or
`840 mg (n (cid:29) 34). Serious grade 3 or 4 infections were noted
`in 25% of patients in the 420 mg cohort and 29% patients in
`
`dose-dependent inhibition of IgM-induced p-ERK. These data
`
`the 840 mg cohort. Treatment was associated with an early
`
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`D’Cruz and Uckun
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`increase in lymphocytosis that began around 1 week and
`
`estimated that 96% of the treatment-naive and 75% of the
`
`persisted for several months in the majority of patients. With
`
`R/R high-risk patients were without progression.
`
`a median follow-up of 12.6 months in the 420 mg cohort and
`
`9.3 months in the 840 mg cohort