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

Pathways to therapy resistance: The sheltering effect of the bone marrow microenvironment to multiple myeloma cells

Multiple Myeloma (MM) is a malignant expansion of plasma cells in the bone marrow (BM), resulting in a disease characterized by symptoms of end organ damage from light chain secretion, crowding of the BM, and bone lesions. Although the past two decades have been characterized by numerous novel therapies emerging, the disease remains incurable due to intrinsic or acquired drug resistance. A major player in MM's drug resistance arises from its intimate relationship with the BM microenvironment (BMME). Through stress-inducing conditions, soluble messengers, and physical adhesion to BM elements, the BMME activates numerous pathways in the myeloma cell. This not only propagates myeloma progression through survival and growth signals, but also specific mechanisms to circumvent therapeutic actions. In this review, we provide an overview of the BMME, the role of individual components in MM survival, and various therapy-specific resistance mechanisms reported in the literature.

Ibrutinib, Lenalidomide and Dexamethasone in Patients with Relapsed and/or Refractory Multiple Myeloma: Phase I Trial Results

Therapeutic strategies that target novel pathways are urgently needed for patients with relapsed/refractory multiple myeloma (RRMM). Ibrutinib is an oral covalent inhibitor of Bruton tyrosine kinase, which is overexpressed in MM cells. This phase 1 dose-escalation study examined various doses of ibrutinib in combination with standard doses of lenalidomide (25mg) and dexamethasone (40mg) using a standard 3+3 design in RRMM patients. The primary objective was to determine the maximum tolerated dose (MTD) of ibrutinib in combination with lenalidomide and dexamethasone. Patients (n=15) had received a median of 4 prior regimens, 53% were triple-class exposed, 33% were penta-exposed, and 54% were lenalidomide-refractory. The MTD of ibrutinib was 840mg (n=6) and only 1 dose-limiting toxicity (DLT); a grade 3 rash possibly related to ibrutinib was noted. The most common ≥grade 3 adverse events (AEs) were rash in 2 (13%), lymphopenia in 2(13%), leukopenia, neutropenia, thrombocytopenia, and anemia all occurring in 3 (20%) patients each. One patient achieved a partial response for an overall response rate of 7%. The clinical benefit rate was 80%. The median time to progression was 3.8 months. Ibrutinib, lenalidomide and dexamethasone appears to be a safe and well-tolerated regimen with reasonable efficacy in heavily pretreated RRMM patients. This article is protected by copyright. All rights reserved.

The Mucolipin TRPML2 Channel Enhances the Sensitivity of Multiple Myeloma Cell Lines to Ibrutinib and/or Bortezomib Treatment

Multiple myeloma (MM) is a haematological B cell malignancy characterised by clonal proliferation of plasma cells and their accumulation in the bone marrow. The aim of the present study is the evaluation of biological effects of Ibrutinib in human MM cell lines alone or in combination with different doses of Bortezomib. In addition, the relationship between the expression of TRPML2 channels and chemosensitivity of different MM cell lines to Ibrutinib administered alone or in combination with Bortezomib has been evaluated. By RT-PCR and Western blot analysis, we found that the Ibrutinib-resistant U266 cells showed lower TRPML2 expression, whereas higher TRPML2 mRNA and protein levels were evidenced in RPMI cells. Moreover, TRPML2 gene silencing in RPMI cells markedly reverted the effects induced by Ibrutinib alone or in combination with Bortezomib suggesting that the sensitivity to Ibrutinib is TRPML2 mediated. In conclusion, this study suggests that the expression of TRPML2 in MM cells increases the sensitivity to Ibrutinib treatment, suggesting for a potential stratification of Ibrutinib sensitivity of MM patients on the basis of the TRPML2 expression. Furthermore, studies in vitro and in vivo should still be necessary to completely address the molecular mechanisms and the potential role of TRPML2 channels in therapy and prognosis of MM patients.

Covalent Cysteine Targeting of Bruton's Tyrosine Kinase (BTK) Family by Withaferin-A Reduces Survival of Glucocorticoid-Resistant Multiple Myeloma MM1 Cells

Multiple myeloma (MM) is a hematological malignancy characterized by plasma cells' uncontrolled growth. The major barrier in treating MM is the occurrence of primary and acquired therapy resistance to anticancer drugs. Often, this therapy resistance is associated with constitutive hyperactivation of tyrosine kinase signaling. Novel covalent kinase inhibitors, such as the clinically approved BTK inhibitor ibrutinib (IBR) and the preclinical phytochemical withaferin A (WA), have, therefore, gained pharmaceutical interest. Remarkably, WA is more effective than IBR in killing BTK-overexpressing glucocorticoid (GC)-resistant MM1R cells. To further characterize the kinase inhibitor profiles of WA and IBR in GC-resistant MM cells, we applied phosphopeptidome- and transcriptome-specific tyrosine kinome profiling. In contrast to IBR, WA was found to reverse BTK overexpression in GC-resistant MM1R cells. Furthermore, WA-induced cell death involves covalent cysteine targeting of Hinge-6 domain type tyrosine kinases of the kinase cysteinome classification, including inhibition of the hyperactivated BTK. Covalent interaction between WA and BTK could further be confirmed by biotin-based affinity purification and confocal microscopy. Similarly, molecular modeling suggests WA preferably targets conserved cysteines in the Hinge-6 region of the kinase cysteinome classification, favoring inhibition of multiple B-cell receptors (BCR) family kinases. Altogether, we show that WA's promiscuous inhibition of multiple BTK family tyrosine kinases represents a highly effective strategy to overcome GC-therapy resistance in MM.

Role of Bruton's tyrosine kinase in B cells and malignancies

Bruton’s tyrosine kinase (BTK) is a non-receptor kinase that plays a crucial role in oncogenic signaling that is critical for proliferation and survival of leukemic cells in many B cell malignancies. BTK was initially shown to be defective in the primary immunodeficiency X-linked agammaglobulinemia (XLA) and is essential both for B cell development and function of mature B cells. Shortly after its discovery, BTK was placed in the signal transduction pathway downstream of the B cell antigen receptor (BCR). More recently, small-molecule inhibitors of this kinase have shown excellent anti-tumor activity, first in animal models and subsequently in clinical studies. In particular, the orally administered irreversible BTK inhibitor ibrutinib is associated with high response rates in patients with relapsed/refractory chronic lymphocytic leukemia (CLL) and mantle-cell lymphoma (MCL), including patients with high-risk genetic lesions. Because ibrutinib is generally well tolerated and shows durable single-agent efficacy, it was rapidly approved for first-line treatment of patients with CLL in 2016. To date, evidence is accumulating for efficacy of ibrutinib in various other B cell malignancies. BTK inhibition has molecular effects beyond its classic role in BCR signaling. These involve B cell-intrinsic signaling pathways central to cellular survival, proliferation or retention in supportive lymphoid niches. Moreover, BTK functions in several myeloid cell populations representing important components of the tumor microenvironment. As a result, there is currently a considerable interest in BTK inhibition as an anti-cancer therapy, not only in B cell malignancies but also in solid tumors. Efficacy of BTK inhibition as a single agent therapy is strong, but resistance may develop, fueling the development of combination therapies that improve clinical responses. In this review, we discuss the role of BTK in B cell differentiation and B cell malignancies and highlight the importance of BTK inhibition in cancer therapy.