The Bruton's Tyrosine Kinase (BTK) Inhibitor PCI-32765 Inhibits Growth of ABC DLBCL Tumors In Vivo and in Vitro by Preventing Activation of Pro-Survival NF-κB pathways

Abstract 4969

Bruton's tyrosine kinase (BTK), a Tec family kinase essential to signaling via the B-cell Receptor (BCR) has emerged as a very promising drug target in lymphoma. PCI-32765, a potent and selective covalently-bound small molecule inhibitor of BTK (Honigberg, Proc. Natl Acad. Sci. USA 107:13075, 2010), has demonstrated promising clinical activity in a number of Phase I and II clinical trials in patients with chronic lymphocytic leukemia (CLL) (Byrd et al., ASCO 2011 Annual Meeting Abstracts, JCO29 (15): 8012, 2010) and several subtypes of B-cell Non-Hodgkin's Lymphoma (NHL), including diffuse large B-cell lymphoma (DLBCL). Many cell lines derived from B-cell malignancies do not show in vitro growth inhibition by BTK inhibitors, perhaps because most established B-cell lines no longer depend on BCR stimulation for survival. However, some cell lines derived from the aggressive ABC (activated B-cell) subtype of DLBCL such as OCI-Ly10 and TMD8 are very sensitive to BTK inhibition by PCI-32765 (Davis et al, Nature463: 88, 2010). We show here that both of these cell lines are potently and irreversibly inhibited by PCI-32765 (EC₅₀ continuous exposure < 10 nM and EC₅₀ following a 1-hr pulse < 50 nM). Interestingly, 2 other ABC-DLBCL cell lines, OCI-Ly3 and HBL-1, in which the NF-κB pathway is activated by a mutation in CARD11 which is downstream of BTK, are not inhibited by PCI-32765. Both sensitive and resistant cells express Btk at similar levels, and the active site of BTK is fully occupied in the presence of low concentrations of the inhibitor as shown using a fluorescently-labeled affinity probe. We show that the sensitive TMD8 and OCI-Ly10 cells have chronically active BCR signaling to NF-κB which is dose-dependently inhibited by PCI-32765, as measured by comparing the phosphorylation and nuclear relocation of NF-κB p65 subunit. These cells also show inhibition of other major signaling pathways downstream of the BCR, including p-ERK, p-AKT, p-PLCγ and intracellular calcium flux, ultimately leading to NF-κB activation. The non-responsive OCI-Ly3 cells do not show inhibition of NF-κB p65 phosphorylation or nuclear relocation, although Btk is present and active, as shown by the probe assay and inhibition of p-AKT by PCI-32765. Furthermore, whole genome expression analysis reveals inhibition of mRNA expression of several NF-κB target genes in OCI-Ly10, particularly c-Myc, NF-κB subunits and several chemokines and cytokines, leading to downregulation of STAT3 signaling, NFAT signaling, cell cycle and cytokine-chemokine signaling pathways, none of which was observed in OCI-Ly3 cells. In vivo, PCI-32765 dose-dependently inhibited tumor growth in mice carrying OCI-Ly10 (and TMD8, ongoing) xenografts. Analysis of these tumors exhibited dose-dependent occupancy of the BTK active site as well as inhibition of signaling pathways. These results demonstrate the mechanistic basis of PCI-32765 inhibitory activity in ABC-DLBCL and support the ongoing clinical investigation of the therapeutic use of PCI-32765 in patients with this devastating disease.