Ibrutinib and Rituximab in Waldenström's Macroglobulinemia

Kinase-deficient BTK mutants confer ibrutinib resistance through activation of the kinase HCK

The Bruton's tyrosine kinase (BTK) inhibitor ibrutinib irreversibly binds BTK at Cys⁴⁸¹, inhibiting its kinase activity and thus blocking transduction of B cell receptor (BCR) signaling. Although ibrutinib is durably effective in patients with B cell malignancies, many patients still develop ibrutinib-resistant disease. Resistance can arise because of mutations at the ibrutinib-binding site in BTK. Here, we characterized the mechanism by which two BTK mutations, C481F and C481Y, may lead to ibrutinib resistance. Both mutants lacked detectable kinase activity in in vitro kinase assays. Structural modeling suggested that bulky Phe and Tyr side chains at position 481 sterically hinder access to the ATP-binding pocket in BTK, contributing to loss of kinase activity. Nonetheless, BCR signaling still propagated through BTK C481F and C481Y mutants to downstream effectors, the phospholipase PLCγ2 and the transcription factor NF-κB. This maintenance of BCR signaling was partially achieved through the physical recruitment and kinase-independent activation of hematopoietic cell kinase (HCK). Upon BCR activation, BTK C481F or C481Y was phosphorylated by Src family kinases at Tyr⁵⁵¹, which then bound to the SH2 domain of HCK. Modeling suggested that this binding disrupted an intramolecular autoinhibitory interaction in HCK. Activated HCK subsequently phosphorylated PLCγ2, which propagated BCR signaling and promoted clonogenic cell proliferation. This kinase-independent mechanism could inform therapeutic approaches to CLL bearing either the C481F or C481Y BTK mutants.

Ibrutinib plus rituximab for the treatment of adult patients with Waldenström's macroglobulinemia: a safety evaluation

Introduction: Waldenström's macroglobulinemia (WM), an orphan disease, is a rare low-grade B-cell lymphoplasmacytic lymphoma with unique clinical features and monoclonal IgM production. Rituximab remains to this date the backbone of most commonly used treatment combinations. The FDA/EMA approval of Ibrutinib, the first-in-class BTK inhibitor, either as monotherapy or in combination with rituximab, changed the treatment landscape of the disease.Areas covered: Clinical trial data that demonstrate mode of action, efficacy, and the safety profile of each agent will be covered. A safety analysis of the combination treatment will also be performed to point out its high efficacy and overall favorable toxicity profile. The disadvantages and treatment gaps that still exist in the treatment of WM which relate to the need for long-term ibrutinib administration and the lack of deep remissions and subsequent disease relapse, will also be reviewed.Expert opinion: The ibrutinib-rituximab combination is both effective and safe, in the newly-diagnosed and relapsed-refractory disease setting. The optimal therapeutic approach for WM patients remains however to be established. The question of which combinatory (or synergistic) regimen can allow for a fixed-treatment duration, deep and durable responses with a safe toxicity profile is being addressed in ongoing clinical trials.

Current and novel BTK inhibitors in Waldenström's macroglobulinemia

The current therapeutic approach in Waldenström’s macroglobulinemia (WM) is being driven by insights in disease biology and genomic landscape. Bruton’s tyrosine kinase (BTK) plays a key role in signaling pathways for the survival of WM clone. BTK inhibition has changed the treatment landscape of the disease. Ibrutinib has resulted in deep and durable responses both as an upfront and salvage treatment with a manageable toxicity profile. However, the need for fewer off-target effects and deeper responses has resulted in the clinical development of second-generation BTK inhibitors. Zanubrutinib has resulted in clinically meaningful antitumor activity, including deep and durable responses, with a low discontinuation rate due to treatment-related toxicities. Cardiovascular adverse events seem to be milder compared with ibrutinib. Interestingly, the efficacy of zanubrutinib in WM is significant both for MYD88^L265P and MYD88^WT patients. Although the randomized, phase III ASPEN clinical trial did not meet its primary endpoint in terms of showing a superiority of zanubrutinib in deep responses compared with ibrutinib, secondary efficacy and safety endpoints underscore the potential clinical role of zanubrutinib in the treatment algorithm of WM independent of the MYD88 mutational status. Combination regimens and non-covalent BTK inhibitors are emerging as promising treatment strategies. Long-term data will determine whether next-generation BTK inhibitors are more potent and safer compared with ibrutinib, and whether they are able to overcome resistance to ibrutinib, either alone or in combination with inhibitors of other interrelated molecular pathways.

Comparison of Access to Novel Drugs for Lymphoma and Chronic Lymphocytic Leukemia Between India and the United States

This review will compare and contrast the costs and access to novel drugs for treating chronic lymphocytic leukemia (CLL) and lymphoma in the United States and India during the last 5 years. Clinical outcomes for patients with hematologic malignancies have improved significantly since the approval of immunotherapeutic and targeted therapies. These new treatments have had an impact on overall outcomes and have helped determine the design for translational research and future trials. Although most of these novel drugs called “innovators” are initially approved and marketed in the United States, several have also become available in countries such as India. With the expiration of patents, generic versions of innovator drugs have increased and accessibility has improved for patients. The advent of biosimilars is another route for expanding access to biologic compounds. As a result, the development costs for developing these drugs are lower, and consequently, the costs for the patient are often lower. Although the delivery of cancer care is not the same in India as it is in the United States, the introduction of biosimilars and generics has helped bridge the gap. This has made treatment of CLL and lymphoma similar in both countries and has had the same impact on patient outcomes and quality of life. Compulsory licensing for essential medications, as stipulated by the Doha Declaration, and capping of drug prices could improve global access to treatments for CLL and lymphoma.