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

Abstract

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.

B-cell receptor signaling pathway

The B-cell receptor (BCR) signaling pathway is a central determinant of B-cell fate and function [Wiestner, 2012]. The receptor is a multiprotein structure consisting of a surface transmembrane immunoglobulin (Ig) noncovalently associated with the Igα (CD79A) and Igβ (CD79B) chains [Dal Porto et al. 2004]. In antigen-dependent signaling, antigenic binding to the BCR causes receptor aggregation and initiation of signal transduction via phosphorylation of the receptor’s cytoplasmic tyrosine-based activation motifs (ITAMs) by recruited SRC-family kinases, including LYN, FYN, BLK, and LCK [Dal Porto et al. 2004]. SYK further transmits the signal downstream through activation of phosphoinositide 3-kinase (PI3Kδ), which mediates the conversion of phosphatidylinositol 4,5 bisphosphate to phosphatidylinositol 3,4,5 triphosphate and ultimately recruits Bruton’s tyrosine kinase (BTK) [Wiestner, 2013]. BTK phosphorylates downstream target phospholipase C γ2 (PLCγ2), eventually leading to activation of nuclear factor κB (NFκB), nuclear factor of activated T cells (NFAT), and mitogen-activated protein kinase pathways [Wiestner, 2013; Brown, 2012b] (Figure 1).

Figure 1.

The B cell receptor signalling pathway controls B-cell fate and function. Reprinted from Aalipour and Advani [2013] with permission from Wiley. BTK, Bruton’s tyrosine kinase; IKK, IκB Kinase MAPK, mitogen-activated protein kinase; NFκB, nuclear factor κB; PI3Kδ, phosphoinositide 3-kinase; PIP2, phosphatidylinositol 4,5 bisphosphate; PIP3, phosphatidylinositol 3,4,5 triphosphate; PKC, protein kinase C; PLCγ2, phospholipase C γ2.

In contrast to antigen-dependent signaling in normal B cells, antigen-independent signaling has been implicated in B-cell malignancies involving constitutive or aberrant BCR signaling [de Rooij et al. 2012; Davids and Brown, 2012; Monroe, 2006; Chen et al. 2008; Rinaldi et al. 2006; Dühren-von Minden et al. 2012]. Overactive signaling facilitates the development of a supportive tumor microenvironment by altering cell chemo-taxis and adhesion while lack of signaling results in cell apoptosis [Dal Porto et al. 2004; Wiestner, 2012; de Rooij et al. 2012]. In accordance with these findings, significant clinical activity has been seen with small molecule inhibitors of protein kinases in the BCR pathway (including LYN, SYK, and PI3Kδ) [Robak and Robak, 2013].

Rationale for therapeutic targeting of Bruton’s tyrosine kinase

BTK, a nonreceptor tyrosine kinase member of the Tec kinase family, plays a significant role in B-cell development and is a unique therapeutic target in B-cell malignancies. In humans, loss of function mutations in BTK result in X-linked agammaglobulinemia (XLA), which is characterized by low peripheral blood B cells, low levels of Ig, and recurring infections. This clinical phenotype is suggestive of BTK’s exclusive role in B-cell development and immunoglobulin production [Maas and Hendriks, 2001]. This specificity to B cells (as opposed to T cells) combined with the nonlethality of XLA make BTK a promising therapeutic target [Herman et al. 2011].

The therapeutic potential of BTK inhibition is supported by its effects on malignant cell lines in vitro. In chronic lymphocytic leukemia (CLL), inhibition of BTK inhibits NFκB DNA binding, reduces cell migration, proliferation, and survival, disrupts integrin-mediated adhesion to fibronectin, inhibits DNA synthesis, diminishes cellular response to tissue homing chemokines (CXCL12, CXCL13, CCL19), and induces apoptosis [de Rooij et al. 2012; Cheng et al. 2014; Herman et al. 2011; Ponader et al. 2012; Jain and O’Brien 2013]. In mantle cell lymphoma (MCL), inhibition of BTK induces apoptosis and decreases levels of the antiapoptotic proteins Bcl-2, Bcl-xL, and Mcl-1 [Cinar et al. 2013]. siRNA-mediated knockdown of BTK in MCL cells significantly reduces phospho-STAT3 and inhibits the NFκB pathway, which are necessary for MCL growth and cellular migration [Ou et al. 2013].

This encouraging preclinical framework has made BTK inhibition a rich field in pharmacology with several small molecule inhibitors in development including LFM-A13, dasatinib, ONO-4059, CC-292, and ibrutinib [Akinleye et al. 2013; Burger, 2014; Aalipour and Advani, 2013]. Of these agents, ibrutinib is the furthest in clinical development and the focus of this review.

Preclinical and early clinical data with ibrutinib

Preclinical data

Ibrutinib (developed by Pharmacyclics, Inc., Sunnyvale, CA, USA and Janssen Pharmaceuticals, Inc., Beerse, Belgium) is an orally available, potent [inhibitory concentration 50 (IC₅₀) = 0.5 nM], selective irreversible inhibitor of BTK that covalently binds to Cys481 [Burger and Buggy, 2013; Bhatt et al. 201; Honigberg et al. 2010]. Ibrutinib was shown to lack significant activity against a panel of 19 BTK-related kinases. In DOHH2 cells, ibrutinib prevented autophosphorylation of BTK, phosphorylation of BTK’s immediate substrate PLCγ2, and phosphorylation of downstream kinase ERK while not affecting the upstream kinase SYK. Using a fluorescently tagged derivative of ibrutinib, PCI-33380, it was shown that 10 nM of ibrutinib was sufficient to achieve full BTK occupancy and completely prevent upregulation of CD69, a surrogate for B-cell activation, in primary B-cell cultures. In primary T cells, upregulation of CD69 was prevented only at concentrations of 10 μM, suggesting that ibrutinib maintains an over 1000-fold selectivity for B cells over T cells.

In vivo, ibrutinib achieved clinical responses in both mice and canine models [Honigberg et al. 2010]. In mice with collagen-induced arthritis, levels of BTK inhibition correlated with a reduction in circulating anticollagen autoantibodies and diminished signs of arthritic disease. In dogs with both naïve and previously treated spontaneously occurring B-cell non-Hodgkin’s lymphoma (B-NHL), treatment with ibrutinib led to objective clinical responses and full BTK occupancy for 24 h at doses ranging from 2.5 to 20 mg/kg/day.

Ibrutinib’s unique biochemistry and in vivo activity paved the way for human clinical trials. In 2012, the US Food and Drug Administration (FDA) designated ibrutinib as a breakthrough therapy for MCL, CLL with 17p deletion, and Waldenström’s macroglobulinemia (WM). Clinical data leading to its subsequent approval and ongoing trials are discussed below.

Early clinical data

Ibrutinib was evaluated in a phase Ia dose-escalation trial to determine the maximum tolerable dose (MTD), safety, pharmacokinetics (PK), pharmacodynamics, and tumor response [Advani et al. 2013]. Fifty-six patients with relapsed/refractory (R/R) B-NHL of varying histologies [16 CLL/small lymphocytic lymphoma (SLL), 9 MCL, 16 follicular lymphoma (FL), 7 diffuse large -cell lymphoma (DLBCL), 4 marginal zone lymphoma (MZL), and 4 WM) were enrolled. Two schedules were evaluated: 28 days on and 7 days off versus once daily continuous dosing. In the absence of MTD, dose escalation continued to three levels above the dosage achieving full BTK occupancy. The median age of patients was 65 years (range 41–82) and the median number of prior therapies 3 (range 1–10). No dose-limiting events were observed and ibrutinib was well tolerated at doses of 1.0–12.5 mg/kg/day without reaching MTD. Using a fluorescent probe to determine BTK engagement, ibrutinib was shown to achieve full occupancy of the BTK active site at 4 and 24 h post dosing with 2.5 mg/kg/day dosing and sustained BTK engagement over 24 h with 1.25 mg/kg/day dosing. The initial mean half life was approximately 2–3 h. Continuous dosing was as effective as intermittent dosing in terms of BTK occupancy, PK, and toxicity profile. A fixed dose of 560 mg/day, which achieved full BTK occupancy in a range of individual weights, was selected for phase II studies. Ibrutinib was well tolerated with the majority of adverse events (AEs) being grade 1 or 2. Grade 3 and 4 toxicities were infrequent, independent of dose, and included neutropenia (12.5%), thrombocytopenia (7.2%), and anemia (7.1%). Only two dose-limiting toxicities (DLTs) occurred: a grade 3 allergic reaction in a patient with a history of drug sensitivity and one dose interruption for more than 7 days because of transient grade 2 neutropenia.

Fifty patients evaluable for tumor response achieved an objective response rate (ORR) of 60% [complete response (CR) 16%] [Advani et al. 2013]. Objective responses were observed across all histologies: CLL/SLL (11 of 16 patients, 2 CRs), MCL (7 of 9 patients, 3 CRs), FL (6 of 16 patients, 3 CRs), DLBCL (2 of 7 patients), WM (3 of 4 patients), and MZL (1 of 4 patients). The median progression-free survival (PFS) was 13.6 months and 20 patients remained on the study at the time of publication because of continued clinical benefit.

Transient lymphocytosis, a class effect of agents targeting BCR signaling in CLL and MCL, was observed with nodal shrinkage in all 11 patients with CLL during cycles 1 and 2 of treatment and slowly resolved over time. In patients with interrupted dosing schedules, absolute lymphocyte counts (ALCs) rapidly declined while ibrutinib was not administered and rose again when ibrutinib was resumed. Corroborating data from other studies suggest that these alternating spikes in ALC are due to a reversible egress of CLL cells from solid lymphoid tissue into the peripheral circulation [Brown, 2014; Herman et al. 2011; Cheng et al. 2014]. Ibrutinib also inhibits integrin- and chemokine-mediated adhesion and homing of malignant cells, therefore reducing retention within the tumor microenvironment and increasing recirculation of malignant cells in the peripheral blood [Herman et al. 2011].

Updated results on the 16 patients with FL in the phase I study have also been reported [Fowler et al. 2012]. The median age of patients was 60 years (range 41–71) and the median number of prior therapies was 3 (range 1–5). The Follicular Lymphoma International Prognostic Index scores were low in 19%, intermediate in 38%, and high in 44% of patients. The most frequency treatment-emergent AEs included diarrhea (50%), fatigue (44%), nausea (38%), cough (31%), and myalgia (25%). Of the 11 patients who received treatment with a dose of 2.5 mg/kg or higher (the minimum dose necessary for full BTK occupancy), the ORR was 55% (27% CR). The duration of response (DOR) was 12.3 months and the median PFS was 13.4 months. The nine patients treated at a dose of 5 mg/kg or higher had a similar ORR and DOR as patients treated with the lower doses but with a slightly improved median PFS of 19.6 months. Two patients remained on the study at 25 and 29 months.

In summary, the phase I study demonstrated that ibrutinib was well tolerated, had significant, durable activity in R/R B-NHL and CLL, and was worthy of further investigation in phase II trials of specific B-cell histologies [Advani et al. 2013; Brown, 2014; Barrientos and Rai, 2013].

Phase II studies of ibrutinib in specific disease subtypes

Mantle cell lymphoma

In a pivotal phase II trial, 111 patients were enrolled to determine the safety and efficacy of 560 mg/day ibrutinib in continuous 28-day cycles [Wang et al. 2013]. The median age was 68 years (range 40–84) and the median number of prior therapies 3 (range 1–5). Sixty-five patients had no prior treatment with bortezomib. Forty-five percent of patients had refractory disease and 72% had advanced disease, defined as either bone marrow or extranodal site involvement (or both). Thirty-four percent of patients experienced a transient increase in ALC which decreased toward the end of the second cycle and tapered off by cycles 4 and 5. At a median follow up of 15.3 months, the ORR was 68% (21% CR). The CR rate among 51 patients who had received treatment for the longest period was 37%. There was no significant efficacy difference between bortezomib-naïve and bortezomib-exposed patients. Sixty-three percent of patients previously treated with lenalidomide responded to ibrutinib. The median DOR of the 75 patients who responded to treatment was 17.5 months and the median time to response 1.9 months. The estimated median PFS was 13.9 months and the estimated median overall survival (OS) at 18 months was 58%. These impressive results led the FDA to approve ibrutinib (Imbruvica; Pharmacyclics, Inc.) for patients with MCL who have received at least one prior therapy (Table 1). Phase III trials with ibrutinib include temsirolimus versus ibrutinib in R/R MCL and bendamustine rituximab (BR) + ibrutinib versus BR + placebo for frontline treatment of MCL in patients >= 65 years (Table 2).

Table 1.

Clinical data from phase II studies with ibrutinib.

Regimen	Disease	n	% ORR (CR)	Grade 3/4 AEs	Outcome
Ibrutinib	CLL, TN	31	71 (13)	Infection (9.7%)	24-month PFS: 96%
				Neutropenia (3.2%)
				Thrombocytopenia (3.2%)
	CLL, R/R	85	71 (3)	Pneumonia (19.7%)	26-month PFS: 75%
				Neutropenia (15%)
				Dehydration (6%)
				Thrombocytopenia (6%)
				Anemia (6%)
	MCL, R/R	111	68 (21)	Neutropenia (11%)	Median PFS: 13.9 months
				Thrombocytopenia (11%)
				Anemia (10%)
				Pneumonia (6%)
	GCB DLBCL, R/R	20	5.3 (0)	Unreported	Median OS: 3.35 months
	ABC DLBCL, R/R	29	40 (8)	Unreported	Median OS: 9.76 months
	WM, R/R	63	57 (0)	Neutropenia (19.1%)	94% remain on study
				Thrombocytopenia (14.3%)
Ibrutinib + R-CHOP	NHL (MCL, DLBCL, FL), TN	17	100 (73)	Neutropenia (61%)	Unreported
				Thrombocytopenia (21%)
	DLBCL, TN	16	100 (64)	Anemia (18%)	Unreported
Ibrutinib + ofatumumab	CLL, R/R	27	100 (4)	Anemia (11%)	89% remain on study
				Pneumonia (11%)
				UTI (7%)
				Hyponatremia (7%)
Ibrutinib + bendamustine rituximab	CLL, R/R	30	93 (17)	Neutropenia (40%)	12-month PFS: 90%
				Muculopapular rash (10%)
				Fatigue (10%)
				Thrombocytopenia (6.7%)
				Febrile neutropenia (6.7%)
				Cellulitis (6.7%)
Ibrutinib + rituximab	CLL, high risk	40	95(8)	Peripheral neuropathy (2.5%)	80% remain on study
				Mucositis (2.5%)

ABC, activated B cell; AE, adverse event; CLL, chronic lymphocytic leukemia; CR, complete response; DLBCL, diffuse large B-cell lymphoma; FL, follicular lymphoma; GCB, germinal center B cell; MCL, mantle cell lymphoma; NHL, non-Hodgkin’s lymphoma; ORR, objective response rate; OS, overall survival; PFS, progression-free survival; R-CHOP, rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone; R/R: relapsed/refractory; TN, treatment naïve; UTI, urinary tract infection; WM, Waldenström’s macroglobulinemia.

Table 2.

Phase III trials with ibrutinib.

Disease	Trial	Eligibility	Regimen
DLBCL (non-GCB)	NCT01855750	Newly diagnosed	Ibrutinib + R-CHOP versus placebo + R-CHOP
Indolent NHL	NCT01974440	R/R, ≥1 prior therapy	(R-CHOP or BR) + ibrutinib versus (R-CHOP or BR) + placebo
MCL	NCT01776840 (SHINE)	Untreated ≥ 65 years old	BR + ibrutinib versus BR + placebo
	NCT01646021 (RAY)	R/R, ≥1 prior therapy	Ibrutinib versus temsirolimus
CLL	NCT01578707 (RESONATE)	R/R not eligible for purine analogues	Ibrutinib versus ofatumumab
	NCT01722487 (RESONATE 2)	Untreated ≥ 65 years old	Ibrutinib versus chlorambucil
	NCT01611090 (HELIOS)	R/R, ≥1 prior therapy	BR + ibrutinib versus BR + placebo
	NCT01886872	Untreated ≥ 65 years old	BR versus ibrutinib versus ibrutinib + rituximab
	NCT01973387	R/R Chinese adults	Ibrutinib versus rituximab
	US Intergroup ECOG 1912 (Protocol in development)	Untreated fit patients	Ibrutinib-based therapy versus fludarabine + cyclophosphamide + rituximab
	UK NCRI CLL10	Untreated fit patients	Ibrutinib + rituximab versus fludarabine + cyclophosphamide + rituximab

BR, bendamustine rituximab; CLL, chronic lymphocytic leukemia; GCB-DLBCL, nongerminal center B-cell diffuse large B-cell lymphoma; MCL, mantle cell lymphoma; NHL, non-Hodgkin’s lymphoma; R-CHOP, rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone; R/R, relapsed/refractory.

Relapsed/refractory chronic lymphocytic leukemia and small lymphocytic lymphoma

Eighty-five patients with R/R CLL/SLL were enrolled in three cohorts as part of a phase Ib/II study [Byrd et al. 2013]. Patients received either 420 mg/day or 840 mg/day in 28-day cycles. The median age was 64 years (range 37–82) and the median number of prior therapies was 4 (range 1–12). At treatment initiation, patient characteristics were 65% Rai stage III or IV, 33% 17p deletion, 81% unmutated IGHV, and 52% bulky adenopathy (>5 cm). At a median follow up of 20.9 months, the ORR was 71% (4% CR) in the 420 mg cohort and 71% (0% CR) in the 820 mg cohort. An additional 20% and 15% of patients achieved nodal partial response (PR) with transient lymphocytosis in the 420 mg and 840 mg cohorts respectively. Thirteen percent of patients had progressive disease (PD), the majority of whom (10 of 11) had high-risk cytogenetic abnormalities. The 26-month estimated PFS and OS were 75% and 83% respectively. Similarity in response between the two dose levels led to 420 mg as the dose for future clinical evaluation.

The results of a long-term extension trial of an expanded cohort of 117 patients have also been reported [O’Brien et al. 2013]. The median age was 65 years (range 37–82), 71% had at least three prior therapies, and 53% were Rai stage III or IV. At a median follow up 20.5 months (range 0.3–42.0), the ORR was 88.3% with an additional 5.4% of patients achieving PR with lymphocytosis. Eighteen percent of patients had treatment discontinuation due to disease progression. In patients achieving a PR or better, median DOR had not been reached at a median follow up of 27.2 months.

The remarkable single-agent activity and tolerability of ibrutinib in CLL/SLL led to its approval in CLL for patients who have received at least one prior therapy and have inspired numerous trials of ibrutinib in combination with chemotherapy or immunotherapy. The interim results of several such phase Ib/II trials have been reported.

The combination of ibrutinib with ofatumumab was studied in a phase Ib/II study [ClinicalTrial.gov identifier: NCT01217749] of 27 patients with R/R CLL/SLL with a median age of 66 years (range 51–85) and three median prior therapies (range 2–10) [Jaglowski et al. 2012]. Ninety-six percent were immediate- or high-risk Rai, 91% had unmutated IGHV, and 37% had 17p deletion. At a median follow up of 9.8 months the ORR was 100% (4% CR) and 89% of patients remained on the study.

Ibrutinib in combination with BR reported impressive results in a phase Ib trial [ClinicalTrial.gov identifier: NCT01292135] of 30 patients with R/R CLL/SLL [Brown et al. 2013a]. The median age was 62 years (range 41–82) and the median number of prior therapies was 2 (range 1–3). At a median follow up of 16 months, the ORR was 93% (17% CR). The estimated 12-month PFS was 90%. The spike in ALC associated with single-agent ibrutinib therapy was significantly diminished in the combination therapy. No additional toxicities resulted from adding ibrutinib to the standard BR regimen.

Preliminary response data from a phase II trial [NCT01520519] evaluating the combination of ibrutinib and rituximab in high-risk (defined as 17p deletion or TP53 mutation) CLL has been reported [Burger et al. 2013]. Forty patients were enrolled with a median age of 65 years (range 35–82) and with a median of 2 prior therapies. At a median follow up of 14 months, the ORR was 95% (8% CR). Another study evaluating the activity of ibrutinib in patients with CLL with or without 17p deletion reported no efficacy variation between the two groups [Farooqui et al. 2013].

Recently, a phase III trial (RESONATE) [ClinicalTrial.gov identifier: NCT01578707] comparing ibrutinib with ofatumumab in patients with R/R CLL not eligible for purine analogues was stopped early due to significant improvement in PFS and OS with ibrutinib [Pharmacyclics, Inc., 2014]. Other ongoing phase III trials include the HELIOS trial [ClinicalTrial.gov identifier: NCT01611090] evaluating the combination of ibrutinib with BR against a standard BR regimen.

Therapy-naïve chronic lymphocytic leukemia and small lymphocytic lymphoma

In a phase Ib/II study, 31 patients received 28-day cycles of ibrutinib at either 420 mg/day or 840 mg/day to assess its safety and activity in therapy-naïve CLL/SLL [O’Brien et al. 2014]. The 840 mg dose was discontinued due to comparable activity of the 420 mg dose. The median age was 71 years (range 65–84), 54% were Rai stage III or IV, and 48% had unmutated IGHV. At a median follow up of 22.1 months, the ORR was 71% (13% CR). The estimated PFS and OS at 24 months were 96.3% and 96.6% respectively. The significant tolerability and activity of ibrutinib in patients with CLL have led to an ongoing phase III study comparing ibrutinib with chlorambucil in untreated patients older than 65 years (RESONATE 2) [ClinicalTrial.gov identifier: NCT01722487].

Diffuse large B-cell lymphoma

The preliminary results of 70 patients in a phase II trial of R/R germinal center B cell-like (GCB) and activated B cell-like (ABC) subtypes of DLBCL have been reported [Wilson et al. 2012]. In contrast to the GCB subtype, the ABC subtype of DLBCL commonly harbors gain-of-function mutations in the CD79B BCR subunit, constitutively active MYD88 mutations, and is sustained by chronically active BCR signaling [Davis et al. 2010; Ngo et al. 2011]. Thus, the study tested the hypothesis that ibrutinib would be more active in the ABC subtype. The median age was 63 years (range 28–92) and the median number of prior therapies was 3 (range 1–7). Sixty-three percent had stage IV disease, 54% had refractory disease, and 23% had a prior stem cell transplant. All patients received 560 mg/day of ibrutinib. Ibrutinib showed preferential activity in the ABC subtype with an ORR of 40% (8% CR) compared with 5.3% (0% CR) in the GCB subtype. Responses were seen in both patients with mutated and wild-type CD79B, suggesting alternate mechanisms of BCR signaling. Patients harboring both CARD11 and MYD88 mutations along with wild-type CD79B did not respond to ibrutinib, suggesting dominance of CD79B-driven BCR signaling.

Ibrutinib has also been evaluated in a combination with rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) in a phase Ib/II study of therapy-naïve patients with CD20-positive NHL (DLBCL, MCL, or FL) [Younes et al. 2013]. In part 1 of the study, patients with all histologies (n = 17) received ibrutinib at 280, 420, or 560 mg/day along with standard doses of R-CHOP to determine the recommended phase II dose (RP2D). In part 2 of the study, which was restricted to DLBCL, 16 patients were treated with the RP2D (560 mg/day) in combination with a standard R-CHOP regimen. The median age was 61 years (range 22–81). The ORR in DLBCL was 100% (60% CR). Phase III studies evaluating ibrutinib as a frontline therapy for the non-GCB subtype of DLBCL are ongoing [ClinicalTrial.gov identifier: NCT01855750].

Waldenström’s macroglobulinemia

MYD88 L265P is present in over 90% of patients with WM and ibrutinib induces apoptosis of WM cells bearing the mutation in vitro [Yang et al. 2013]. In contrast, WHIM-like CXCR4 mutations, present in one-third of patients with WM, induce BTK activity and reduce sensitivity of WM cells to ibrutinib in vitro [Cao et al. 2013]. Sixty-three patients were enrolled in a study to determine the safety and efficacy of ibrutinib in R/R WM and to determine the impact of MYD88 L265P and WHIM-like CXCR4 mutations on ibrutinib response [Treon et al. 2013]. The median age was 63 years (range 44–86) and the median number of prior therapies was 2 (range 1–6). At a median follow up of six cycles, the best response rate [minor response (MR) or better] was 81% (6% very good PR, 51% PR, 24% MR). Consistent with in vitro findings, the major response rate for patients with WHIM-like CXCR4 mutations was 30% as opposed to 77% for patients with unmutated CXCR4. Patients with unmutated CXCR4 also experienced increased peripheral lymphocytosis, larger decreases in serum IgM, and greater improvements in hemoglobin. The efficacy of ibrutinib was not impacted by the presence or absence of MYD88 L265P.

Adverse events with ibrutinib

Overall, ibrutinib is well tolerated as a single agent. In phase Ib/II trials most AEs were grade 1 or 2. The most common (>20%) AEs independent of grade were diarrhea (49–50%), fatigue (32–41%), nausea (18–31%), cough (31%), peripheral edema (21–28%), dyspnea (27%), arthralgia (27%), rash (27%), pyrexia (27%), constipation (18–25%), upper respiratory tract infection (23–33%), vomiting (16–23%), decreased appetite (21%), and muscle spasms (20%) [Wang et al. 2013; Byrd et al. 2013]. AEs of grade 3 or higher from all studies are summarized in Table 1. In MCL, the most common infection of grade 3 or higher was pneumonia (6%), and hematologic AEs of grade 3 or higher included neutropenia (16%), thrombocytopenia (11%), and anemia (10%). The long-term extension study in R/R CLL reported that of grade 3 or higher serious adverse events (SAEs) occurred in 62% of patients and pneumonia in 19.7% [O’Brien et al. 2013]. Grade 3 and 4 hematologic toxicities in CLL were infrequent and included anemia (6%), neutropenia (15%), and thrombocytopenia (6%). Notably, pneumonia was most commonly observed in patients with CLL heavily pretreated with nucleoside analogues. Most toxicities were reversible with discontinuation of ibrutinib. Grade 3 or higher bleeding events occurred in 5% and 6% of patients with MCL and CLL respectively. Overall, 48% and 63% of patients experienced bleeding of any grade in MCL and CLL respectively. The mechanism for bleeding events is currently not well understood. Based on these experiences, the benefit–risk of ibrutinib in patients requiring anticoagulant therapies as well as the benefit–risk of withholding ibrutinib for 3–7 days pre and post surgery should be considered as per the prescription guidelines (Pharmacyclics, Inc.).

In combination therapies, the reported toxicities were no more than would be expected with each agent alone. The most common treatment-emergent AEs of any grade resulting from the combination of ibrutinib with BR were diarrhea (70%), nausea (66.7%), fatigue (46.7%), neutropenia (40%), and upper respiratory tract infection (36.7%) [Brown et al. 2013a]. Grade 3 or higher hematologic toxicities resulting from this combination included neutropenia (40%) and thrombocytopenia (6.7%). The combination of ibrutinib and rituximab was also well tolerated, with only two grade 3 AEs (one case of each mucositis and peripheral neuropathy across 40 patients) [Burger et al. 2013]. The most common infectious complications were pneumonia (15%) and upper respiratory infection (8%). When ibrutinib was administered in combination with a standard R-CHOP regimen, no excess toxicities were noted and most side effects were R-CHOP related. The most common (≥25%) all-grade complications were neutropenia (67%), nausea (67%), thrombocytopenia (61%), vomiting (48%), anemia (36%), fatigue (30%), diarrhea (30%), headache (27%), constipation (27%), and alopecia (27%). Seventy-three percent of patients experienced grade 3 or higher AEs, including neutropenia (61%), thrombocytopenia (21%), and anemia (18%).

As ibrutinib is primarily metabolized by cytochrome P450 enzyme 3A (CYP3A), coadministration with CYP3A inhibitors or inducers can produce complications (see package insert; Pharmacyclics, Inc.). In healthy volunteers, coadministration of a strong CYP3A inhibitor (ketoconazole) increased the C_max of ibrutinib 29-fold and decreased its AUC 24-fold. In contrast, ibrutinib plasma concentrations were reduced 10-fold when administered together with strong CYP3A inducers. Based on these studies, prescribing information for ibrutinib recommends avoiding concomitant administration with strong CYP3A inhibitors (e.g. ritonavir, indinavir, nelfinavir, saquinavir, boceprevir, telaprevir, and nefazodone) or strong CYP3A inducers (e.g. carbamazepine, rifampin, phenytoin, and St John’s Wort). In the case of short-term (7 days or less) use of strong CPY3A inhibitors (such as antifungals or antibiotics, including ketoconazole, itraconazole, voriconazole, posaconazole, clarithromycin, and telithromycin), the prescribing guidelines suggest that interruption of ibrutinib for the duration of inhibitor use should be considered.

The future of ibrutinib therapy

Innovative small-molecule combinations of ibrutinib that block adaptive signaling responses are being investigated in the preclinical setting. Combinatorial drug screens have identified the proteasome inhibitors bortezomib and carfilzomib as having robust in vitro cytotoxic synergies with ibrutinib in MCL and DLBCL cell lines and primary samples from patients [Axelrod et al. 2014; Dominici et al. 2013; Zhang et al. 2013; Dasmahapatra et al. 2013]. In MCL-bearing SCID-hu mice, the combination of ibrutinib with carfilzomib increased survival threefold compared with either agent administered alone [Axelrod et al. 2014]. Carfilzomib’s superior safety profile and greater apoptotic activity (compared with bortezomib) have led to the design of a phase I/II clinical trial evaluating it in combination with ibrutinib in R/R MCL. Synergy with ibrutinib has also been demonstrated in vitro with the BCL-2 pathway inhibitor (ABT-199) and IRAK4 inhibitors [Zhao et al. 2013; Tesar et al. 2013].

Aside from their therapeutic potential, these combinations are useful in elucidating mechanisms of resistance to ibrutinib and how genomic differences are correlated with response to ibrutinib, both of which are currently poorly understood [Brown, 2014]. Understanding these mechanisms and correlations will likely lead to more personalized approaches of treating disease that cause fewer undesirable toxicities [Kharfan-Dabaja et al. 2013; Garraway and Jänne, 2012]. Early studies in CLL have begun to address these questions. In vitro, NFκB pathway mutations RIPK1 Q375*, and KRAS Q61H significantly diminish the efficacy of ibrutinib [Improgo et al. 2013]. The effect of MYD88 L265P on ibrutinib response in CLL is, in contrast, still uncertain [Tesar et al. 2013; Improgo et al. 2013]. A preclinical investigation of ibrutinib combined with rituximab reported that rituximab’s antitumor efficacy in vitro is diminished when adding ibrutinib and suggested that dosing schedules and treatment sequence may influence the efficacy of this combination [Kohrt et al. 2013]. New evidence shows that ibrutinib irreversibly targets interleukin-2-inducible kinase and therefore promotes a T helper 1 response that inhibits malignant cell growth by altering the tumor microenvironment. This finding suggests that ibrutinib’s impressive clinical efficacy may be a result of multikinase inhibition and may expand the pool of diseases in which ibrutinib is used as a therapy [Dubovsky et al. 2013; Ansell, 2013].

Other Bruton’s tyrosine kinase inhibitors in development

In addition to ibrutinib, several other BTK inhibitors are in various stages of development.

LFM-A13 is a reversible inhibitor of BTK (IC₅₀ = 7.5 µM) that binds to the kinase’s catalytic domain without affecting the activity of related kinases [Uckun et al. 2002; Mahajan et al. 1999]. It was shown to increase apoptotic activity of ceramide and vincristine in B-lineage leukemic cells [Mahajan et al. 1999]. In vitro, LFM-A13 in combination with chemotherapy can overcome chemoresistance in relapsed acute lymphoblastic leukemia (ALL) [Uckun et al. 2011]. No clinical data have been reported to date.

Dasatinib was originally approved for treatment of chronic myelogenous leukemia (CML) and ALL but was later found to have BTK inhibiting properties (IC₅₀ = 5 nM) in vitro. In a phase I/II trial enrolling 27 patients with R/R NHL, dasatinib achieved an ORR of 32% (11% CR) across all histologies at a median follow up of 24 months [William et al. 2010]. In a phase II trial evaluating dasatinib in 15 patients with R/R CLL, the ORR was 20% (0% CR) at a median follow up of 14 weeks [Amrein et al. 2011]. Grades 3 and 4 hematologic toxicities were frequent and included neutropenia (67%) and thrombocytopenia (40%).

ONO-4059 (formerly known as ONO-WG-307) is a potent (IC₅₀ = 2 nM) reversible BTK inhibitor that blocks autophosphorylation of Tyr223 [Yasuhiro et al. 2012]. In vitro, ONO-4059 diminishes proliferation or FL, MCL, and CLL cell lines and shows therapeutic promise in combination with rituximab [Kozaki et al. 2011, 2012]. In vivo studies in mouse xenograft models indicate activity against the ABC subtype of DLBCL [Kozaki et al. 2011]. A phase I study enrolling 14 patients evaluated ONO-4059 in R/R B cell lymphomas [Rule et al. 2013]. ONO-4059 was found to be well tolerated with no DLTs. The best ORR was 42% across all histologies and in MCL the ORR was 50% (0% CR). Data from a separate phase I trial enrolling 16 patients with R/R and high-risk CLL have also been reported [Salles et al. 2013]. The majority of AEs were grades 1 and 2 and ONO-4059-related hematologic toxicities of grade 3 or higher were limited. The 10 evaluable patients achieved an ORR of 70% (2 PR, 5 PR with lymphocytosis, 2 SD, and 1 PD).

CC-292, formerly known as AVL-292, is a potent (IC₅₀ = 2 nM) irreversible inhibitor of BTK that forms a covalent bond with Cys481 [Evans et al. 2011b]. In vitro, CC-292 inhibits autophosphorylation in primary B cells, achieves dose-dependent reduction in CD69 expression, and reduces CLL cell migration and response to chemokines CXCL12 and CXCL13 [Evans et al. 2011a; Pierce et al. 2013]. In vivo, CC-292 reduces signs of arthritic disease in a manner proportional to percentage BTK occupancy [Evans et al. 2013]. In a phase Ia trial, CC-292 was well tolerated over the 0.5–7.0 mg/kg dosage range and there were no reported AEs or SAEs [Evans et al. 2013]. A phase Ib trial of 86 patients with R/R B-NHL (n = 23), R/R WM (n = 6), and R/R CLL/SLL (n = 57) evaluated CC-292 at doses of 125, 250, 400, 625, 750, and 1000 mg once daily, and 375 and 500 mg twice daily [Brown et al. 2013c]. The majority of treatment-emergent AEs were grades 1 and 2. There were three cases of DLTs. The study reported preliminary ORRs of 31% at 750 mg once daily, 50% at 1000 mg once daily, and 66.7% at 375 mg twice daily. Updated results on 83 patients with R/R CLL/SLL have also been reported [Brown et al. 2013b]. The study reported ORRs of 31% at 750 mg once daily, 57% at 1000 mg once daily, 67% at 375 mg twice daily, and 38% at 500 mg twice daily. No patient achieved a CR with any regimen of CC-292.

Conclusion

BCR signaling is central to the development and maintenance of B-cell malignancies and BTK inhibitors have shown immense promise as targeted agents of this pathway. Ibrutinib, the most advanced BTK inhibitor in clinical trials, has demonstrated impressive clinical efficacy and safety in several B-cell malignancies both as a single agent and in combination therapy. In the future ibrutinib will likely complement traditional immunotherapy as a frontline therapy and it has the potential to obviate the need for chemoimmunotherapy. A better understanding of mechanisms of resistance to ibrutinib will enable development of rational combinations and treatment sequences with other inhibitors of the BCR pathway to achieve better outcomes with fewer unnecessary toxicities.