Combination of Ibrutinib and BCL-2 or Spleen Tyrosine Kinase Inhibitors in Ibrutinib-Resistant Diffuse Large B-Cell Lymphoma
Diffuse large B-cell lymphoma (DLBCL) is the most frequent subtype of non-Hodgkin lymphoma, accounting for approximately 30% of newly diagnosed cases in the United States. The activated B-cell (ABC)-like subtype of DLBCL is considered especially high risk and is characterized by chronic active B-cell receptor signaling. In a 2012 phase 2 clinical trial, results with ibrutinib as a single agent in patients with DLBCL showed an ORR of 41% in the ABC subtype. The responses of various cancers to single kinase-targeted therapies are often limited by the cell’s ability to bypass the target via alternative pathways or acquired mutations in the target or its pathway. Such mechanisms may be overcome by combinations of targeted agents.
Through the screening of wild-type and acquired ibrutinib-resistant ABC DLBCL cell lines (eg, expressing BTK C481S), Kuo and colleagues reported on B-cell lymphoma (BCL)-2, such as ABT-199, and spleen tyrosine kinase (Syk), such as R406, inhibitors that synergize with ibrutinib in vitro and in vivo (Blood. 2014;124. Abstract 505).
DLBCL cell lines with higher BCL-2 expression were more sensitive to single-agent ABT-199 than those with lower expression. The treatment of DLBCL cells with ibrutinib alone increased BCL-2 expression as well as their sensitivity to BCL-2 inhibitors. Combination treatment with BCL-2 inhibitors and ibrutinib completely inhibited tumor growth in murine models of ABC DLBCL. Increased apoptotic cell populations were detected in the tumors that were treated with a combination compared with either treatment alone.
Clinically, pretreatment tissue samples from 28 patients with ABC DLBCL who experienced objective responses to ibrutinib had lower BCL-2 gene expression. A high BCL-2 mutation rate was observed in patients with poor response to ibrutinib; however, none of these mutations occurred in the BH3 or BH1 domains, both of which appear to interact with ABT-199 based on a 3-dimensional cocrystal structure of the inhibitor with BCL-2 (Protein Data Bank code 4MAN) and further molecular simulation results. These findings suggest the potential benefit from combination therapy.
Syk is another downstream mediator of B-cell receptor signaling. The pretreatment of DLBCL cells with R406 increased their sensitivity to ibrutinib. Moreover, ibrutinib-resistant BCL with either C481S BTK or R665W PLCG2 mutations were resensitized to ibrutinib in combination with the BCL-2 or Syk inhibitors, inhibiting cell growth, immunoglobulin M–induced calcium flux, cell adhesion, or migration in mutation-containing cells.
These results show that human B-cell lymphomas harboring ibrutinib-resistant C481S BTK or R665W PLCG2 mutations may be resensitized by BCL-2 or Syk inhibitors, which provides a rationale for the design of combination therapies for the ABC subtype of DLBCL.