Bruton tyrosine kinase represents a promising therapeutic target for treatment of chronic lymphocytic leukemia and is effectively targeted by PCI-32765

B-cell receptor (BCR) signaling is aberrantly activated in chronic lymphocytic leukemia (CLL). Bruton tyrosine kinase (Btk) is essential to BCR signaling and in knockout mouse models its mutation has a relatively B cell-specific phenotype. Herein, we demonstrate that Btk protein and mRNA are significantly over expressed in CLL compared with normal B cells. Although Btk is not always constitutively active in CLL cells, BCR or CD40 signaling is accompanied by effective activation of this pathway. Using the irreversible Btk Inhibitor PCI-32765, we demonstrate modest Apoptosis in CLL cells that is greater than that observed in normal B cells. No influence of PCI-32765 on T-cell survival is observed. Treatment of CD40 or BCR activated CLL cells with PCI-32765 results in inhibition of Btk tyrosine phosphorylation and also effectively abrogates downstream survival pathways activated by this kinase including ERK1/2, PI3K, and NF-κB. In addition, PCI-32765 inhibits activation-induced proliferation of CLL cells in vitro, and effectively blocks survival signals provided externally to CLL cells from the microenvironment including soluble factors (CD40L, BAFF, IL-6, IL-4, and TNF-α), fibronectin engagement, and stromal cell contact. Based on these collective data, future efforts targeting Btk with the irreversible inhibitor PCI-32765 in clinical trials of CLL patients is warranted.