B-Cell Receptor Signaling Pathway Emerges as Ripe Target in Many Cancers

Despite the availability of numerous effective treatment options, most patients with B-cell malignancies still experience frequent relapses and progressively shorter remissions, creating a pressing need for new drugs to add to the therapeutic arsenal.

The B-cell receptor (BCR) signaling pathway, as a central regulator of B-cell function, has emerged as an exciting new anticancer drug target. Dysregulation of the pathway is a driver of many types of B-cell malignancies, and efforts to target it predominantly take aim at the abundance of readily druggable kinases that coordinate signal transmission from the BCR to downstream effectors.

The FDA’s approval of drugs targeting Bruton tyrosine kinase (BTK) and phosphoinositide-3-kinase (PI3K) provide confirmation that targeting the BCR pathway is feasible and fruitful. Although numerous challenges and questions remain unresolved, understanding of the pathway is growing and many next-generation drugs are in development (Table).

^aThis study is ongoing, but not recruiting participants. ^bThis study is not yet open for participant recruitment. AML indicates acute myeloid leukemia; BCR, B-cell receptor; BTK, Bruton tyrosine kinase; CLL, chronic lymphocytic leukemia; DLBCL, di use large B-cell lymphoma; FL, follicular lymphoma; MCL, mantle cell lymphoma; NHL, non-Hodgkin lymphoma; PI3K, phosphoinositide 3-kinase; SYK, spleen tyrosine kinase; SLL, small lymphocytic lymphoma;

The Players in B-Cell Biology

As the B cells of the immune system mature, each acquires a unique BCR on its surface. BCRs are made up of 2 parts: (1) an antigen-binding subunit composed of a membrane-bound antibody with randomly determined antigen-binding sites that allow each BCR to recognize different foreign antigens, forming the basis of the humoral immune response, and (2) a signaling subunit that triggers a signaling cascade within the B cell, regulating many important cellular functions.

The latter is a heterodimer of the CD79A and CD79B proteins, which spans the plasma membrane and contains an immunoreceptor tyrosine-based activation motif (ITAM) within the portion that protrudes into the cell.

When an antigen binds to the antibody subunit, the ITAM domains within the signaling subunit are phosphorylated by members of the SRC family of kinases, including the LYN protein. These phosphorylated domains then act as a docking platform for proteins with an SH2 domain, such as spleen tyrosine kinase (SYK).

The signal is then propagated further downstream through a complex network of interacting proteins. Central among them are PI3K and BTK. (Figure). Ultimately, the pathway culminates in the activation of transcriptional programs within the nucleus, such as those governed by nuclear factor kappa B (NFκB) transcription factors.

Taking Aim at BCR Signaling

This process can induce a variety of cellular responses, including survival, proliferation, and apoptosis, depending upon the nature of the antigen that initiated signaling and the stage of B-cell development, among other factors.Given how important the BCR pathway is to the functioning of normal B cells, it is unsurprising to find that its dysregulation has been implicated in the development of numerous B-cell malignancies, including chronic lymphocytic leukemia (CLL), diffuse large B-cell lymphoma (DLBCL), mantle cell lymphoma (MCL), hairy cell leukemia (HCL), and Burkitt lymphoma (BL).

There are numerous mechanisms by which cancer cells co-opt this pathway to promote B-cell growth, proliferation, and survival. The precise molecular mechanisms involved vary among cancer types. For example, CLL cancers rarely display somatic mutations, but the BCR has been shown to be overexpressed in 30% of cases and increased expression of SYK is also common. In contrast, mutations in CD79A/B are frequently associated with DLBCL.

BTK Inhibitors

The first clinical successes in targeting the BCR pathway came from drugs aimed at the BTK protein, culminating in the approval of ibrutinib (Imbruvica) in 2013 for the treatment of patients with MCL who have received at least 1 prior therapy.

BTK is a member of the TEC family of nonreceptor tyrosine kinases and is activated by SYK following BCR engagement. It is recruited to the plasma membrane by the lipid phosphatidylinositol-3,4,5-trisphosphate (PIP₃) via a part of the protein known as its pleckstrin homology domain and is then activated by LYN and SYK.

Initial approval of ibrutinib in MCL was based on the results of a multicenter, international, single-arm trial in 111 patients with previously treated disease. Ibrutinib resulted in an overall response rate (ORR) of 66%, including complete responses (CR) in 17% of patients, with a median duration of response (DOR) of 17.5 months.

The following year, ibrutinib was granted accelerated approval in patients with CLL, based on the demonstration of response rates exceeding 50% among 48 patients enrolled in a multicenter, single-arm trial. This was rapidly translated into full approval following the phase III RESONATE trial, in which ibrutinib significantly improved both overall (OS) and progression-free survival (PFS), resulting in a 57% reduction in the risk of progression or death, compared with the CD20-targeting monoclonal antibody ofatumumab (Arzerra).

In conjunction with the full approval, the FDA also expanded ibrutinib’s indication to include patients with CLL who had a chromosome 17p deletion. Then in early 2016, the label was further expanded to include patients with treatment-naïve CLL, based on the findings of the RESONATE-2 study, in which ibrutinib significantly prolonged PFS in comparison with chlorambucil, reducing the risk of progression or death by more than 80%.

Ibrutinib is also approved for the treatment of Waldenstrӧm macroglobulinemia (WM) and, most recently, for the treatment of marginal zone lymphoma (MZL), both in the second-line setting. In MZL, patients enrolled in the pivotal phase II PCYC-1121 trial experienced an ORR of 46%, including CR of 3.2%, and a median PFS of 14.2 months.

Ibrutinib continues to be evaluated in more than 100 ongoing clinical trials, in efforts to further clarify its role in various disease settings and to test it in new types of cancer. These include phase III trials in patients with relapsed or refractory DLBCL, treatment-naïve FL and young adult or pediatric patients with relapsed or refractory mature B-cell non-Hodgkin lymphoma (NHL).

Results from the ongoing phase III iNNOVATE study of ibrutinib in combination with rituximab (Rituxan) in patients with WM were recently published. A substudy analysis identied a sustained benefit for the combination, with an estimated 18-month PFS rate of 86% and an OS rate of 97% among 31 patients with rituximab-refractory disease.

The phase III GENUINE study is evaluating the combination of the CD20-targeted antibody ublituximab and ibrutinib in patients with previously treated, high-risk CLL. Recently reported results suggest that the combination significantly improves ORR compared with ibrutinib alone (80% vs 47%, respectively).

Despite promising efficacy across multiple different types of B-cell malignancies, ibrutinib faces several challenges. Its lack of specificity— it inhibits several other kinases in addition to BTK—results in some significant toxicity. Several next-generation BTK inhibitors with greater specificity are being developed in an effort to overcome this issue. As with other kinase inhibitors, the emergence of resistant disease is also problematic and researchers are attempting to unravel the mechanisms underlying this issue.

PI3K-delta Inhibitors

PI3K is another central kinase in the BCR pathway and a promising target for anticancer therapy. Activated by SYK, it catalyzes the formation of the PIP₃ lipid in the plasma membrane, thus playing an important role in the recruitment of BTK, in addition to activating other signaling pathways such as the AKT/mTOR pathway.

PI3K has several isoforms, but the delta isoform is expressed only in hematopoietic cells and has been shown to play a critical role in B-cell function. For this reason, the development of PI3K inhibitors for the treatment of B-cell cancers has focused on targeting PI3K-delta.

Idelalisib (Zydelig) is a highly selective inhibitor of PI3K-delta that was shown to block BCR-induced activation of PI3K in preclinical models of B-cell cancer. Following promising clinical trial results, idelalisib received regulatory approval in 2014 for the treatment of patients with relapsed CLL, when used in combination with rituximab, in addition to the treatment of patients with relapsed FL and small lymphocytic lymphoma (SLL) who have received at least 2 prior systemic therapies.

Idelalisib remains under study as a single agent and in combination with other drugs in a variety of settings, including phase II studies as monotherapy after autologous stem cell transplant in patients with relapsed CLL, SLL, or NHL and in combination with obinutuzumab in patients with relapsed or refractory WM.

The results of a phase III study of idelalisib in combination with bendamustine (Treanda) and rituximab in patients with relapsed or refractory CLL demonstrated a significant improvement in PFS compared with placebo. Median PFS was 20.8 months with the combination compared with 11.1 months for the placebo (HR, 0.33). However, there was a greater incidence of grade 3 or worse neutropenia, febrile neutropenia, infections and infestations with the idelalisib-containing regimen.

Next-Generation Drugs

Although clinical trials of idelalisib continue in patients with relapsed and refractory disease, the drug has faced a major setback in the frontline setting. Gilead Sciences, Inc, the drug developer, halted 6 clinical trials of idelalisib as first-line therapy in 2016 amid an FDA warning about reports of increased rates of adverse events, including deaths, in combination studies.A number of novel BCR-targeting drugs that are attempting to build on the success of ibrutinib and idelalisib by boosting efficacy and reducing toxicity are in development. There are 3 more specific BTK inhibitors in development, the most advanced being acalabrutinib, which is currently being evaluated in phase III trials in patients with relapsed or refractory CLL and in treatment-naïve MCL.

The results of the phase I/II ACE-CL-OO1 study of acalabrutinib in patients with CLL who could not tolerate ibrutinib were recently presented at the 2016 American Society for Hematology (ASH) Annual Meeting. Among 33 patients treated with either a 100-mg twice-daily or 200-mg once-daily dose of acalabrutinib, the ORR was 79% and only one-third of patients had a recurrent AE. In a separate presentation of results from the same trial, it was reported that acalabrutinib had an ORR of 38.1% in patients who had Richter transformation, a complication that occurs in 5% to 10% of patients with CLL in which it transforms into a fast-growing form of lymphoma.

Another novel BTK inhibitor in development is BGB-3111, which is undergoing phase III testing in patients with WM. A phase I dose-escalation study in patients with CLL/SLL achieved an ORR of 96%, although there were no CRs, according to data presented at 2016 ASH meeting. The results were from 46 patients treated with doses ranging from 400-mg daily to 160-mg twice daily who had at least 12 weeks of follow-up. In patients with high-risk molecular characteristics, such as deletion of chromosome 17p, the ORR was 100%.

In the PI3K inhibitor space, the PI3K-delta inhibitors duvelisib and TGR-1202 are following in the wake of idelalisib. Findings from clinical trials performed to date have suggested that these drugs have increased potency and reduced toxicity, but it is not known precisely why. These novel PI3K inhibitors are not necessarily more specific than idelalisib, but they do have different targeting pro les that could partly explain their improved properties.

For example, duvelisib targets both the delta and gamma isoforms of PI3K, both of which are expressed in immune cells. Although PI3K-delta predominantly functions in B cells and PI3K- gamma in T cells, there is significant crosstalk between the 2 and blocking signaling through both isoforms could help to further inhibit malignant B-cell growth and survival. Meanwhile, TGR-1202 targets casein kinase epsilon 1 in addition to PI3K-delta.

Despite promise in early clinical trials and meeting its primary endpoint in the phase II DYNAMO study in patients with indolent NHL, the future of duvelisib is somewhat uncertain since it did not live up to expectations in terms of the magnitude of its benefit.

TGR-1202, on the other hand, was awarded an Orphan Drug designation in 2016 and several phase III trials are underway, including the UNITY-CLL trial in patients with untreated and previously treated CLL and UNITY-DLBCL in those with previously treated DLBCL.

Alternative Targets Arise

The results of a phase I/Ib study of TGR-1202 in combination with ibrutinib in patients with relapsed or refractory CLL and MCL generated ORRs of 88% and 73%, respectively. One-year PFS and OS rates were each 94% for patients with CLL and were 37% and 52%, respectively, for patients with MCL.The SYK protein, another key kinase in the BCR signaling cascade, offers an alternative for therapeutic targeting, but clinical development of SYK inhibitors has not progressed as rapidly as it has for other elements of the pathway. The SYK inhibitor fostamatinib was the first BCR-targeted drug to enter clinical development, but was limited by toxicities such as hypertension and neutropenia, thought to result from off-target effects.

Several novel SYK inhibitors with improved selectivity and, as a result, improved toxicity profiles, are now in development and a variety of phase II clinical trials are underway. Interim results from a phase I/II study of entospletinib as monotherapy and in combination with vincristine in patients with previously untreated acute myeloid leukemia (AML) were recently reported. In 12 patients treated with entospletinib monotherapy for 14 days followed by entospletinib in combination with daunorubicin and cytarabine, there was significant clinical activity and doses up to 400 mg were well-tolerated.

Another drug that has recently had significant success in the treatment of B-cell malignancies, particularly in patients with CLL, is venetoclax (Venclexta). Venetoclax targets the BCL-2 protein, which is part of the p53 pathway and involved in the regulation of apoptosis. One of the outcomes of the BCR signaling cascade is the activation of the NFκB transcription factors and the BCL-2 gene is among their targets. BCL-2 is commonly overexpressed in patients with CLL as a means for the cancerous cells to become resistant to apoptosis.

The FDA approved venetoclax for the treatment of patients with relapsed/refractory CLL with a chromosome 17p deletion in 2016 on the basis of a single-arm, dose-escalation trial demonstrating an ORR of nearly 80%, with responses lasting for a year or more among 85% of those patients.

Clinical trials of combination therapy incorporating venetoclax are also underway, including with ibrutinib, since their distinct mechanisms of impacting the BCR pathway in CLL may result in synergistic efficacy. Recently reported results from a phase I trial of venetoclax plus rituximab in patients with CLL indicated that the combination produced an ORR of 85%, with a CR rate of 51%. Most noteworthy was that a proportion of responding patients were able to discontinue treatment and still maintain their responses.

References

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