Synergistic Targeting of BTK and E-Selectin/CXCR4 in the Microenvironment of Mantle Cell Lymphomas

Mantle cell lymphoma (MCL) is a rare subtype of aggressive B-cell non-Hodgkin lymphoma that remains incurable with standard therapy. Overexpression of B-cell receptor signaling through Bruton tyrosine kinase (BTK) is a hallmark of MCL (Pal Singh et al., 2018). Inactivation of BTK signaling with the small molecule inhibitor ibrutinib is currently the most broadly used treatment of B cell lymphoma. However, it induces only low rates of apoptosis in vitro at clinically achievable concentrations. Frequently, primary and acquired resistance is observed (Chiron et al., 2014; Wang et al., 2013). One of the molecular mechanisms of acquired resistance is the development of BTKC481S mutations (Martin et al., 2016). In addition, the tumor microenvironment (TME), in which mesenchymal stroma cells (MSC) and vascular endothelial cells (ECs) are specialized components, has increasingly been recognized as a central determinant of drug resistance, subclonal evolution and late progression/transformation of B-cell lymphomas (Balsas et al., 2017; Weis and Cheresh, 2011). Although the pro-tumoral ecosystem that supports MCL is still poorly understood, it has been reported that MCL cells express high levels of functional CXCR4 and CXCR5 chemokine receptors and VLA-4 adhesion molecules (Kurtova et al., 2009) . Lymphoma cells also display high levels of CD44, one of E-selectin ligands, in co-culture with ECs (Cao et al., 2014). These findings strongly suggest the association of acquired BTK mutations and the TME with resistance to BTK-targeted therapy in MCL. Therefore, we hypothesize that disrupting the crosstalk of MCL cells and TME by blocking CXCR4/CXCL12 or E-selectin/CD44 might benefit BTK-targeted therapy against MCL. In this study, we investigated the anti-lymphoma effect of a novel small-molecule multi-kinase inhibitor CG-806 which exerts promising enzymatic inhibitory activity against the C481S mutation and wild type BTK at extremely low doses (IC50s were 2.52 and 5 nM, respectively). CG-806 demonstrated impressive anti-lymphoma effects in MCL cell lines Z138, MINO, Jeko-1 and JVM2 (IC50s of 2.7, 3.87, 3.79 and 8.27 nM, respectively), all of which were much less sensitive to ibrutinib (IC50s ≈ 10,000 nM). Mechanistically, CG-806 not only suppressed BTK activation, but also its downstream signaling targets phospho-Stat3,-AKT,-ERK and -Src, as well as NF-κB and c-Myc, and surprisingly upregulated p53 in MCL cells but exerted no suppression of phospho-FLT3 and aurora kinas