Targeting Btk in Lymphoma: PCI-32765 Inhibits Tumor Growth in Mouse Lymphoma Models and a Fluorescent Analog of PCI-32765 Is an Active-Site Probe That Enables Assessment of Btk Inhibition In Vivo.
Authors: Honigberg Lee A. , Smith Ashley M. , Chen Jun , Thiemann Patti , Verner Erik 
Published: October 2019

Abstract
There is increasing evidence indicating that B-cell-receptor (BCR) signaling is required for survival of non-Hodgkin’s lymphoma (NHL) cells. Bruton’s tyrosine kinase (Btk) is required for BCR signaling and mutations that inactivate human Btk cause X-linked-agammaglobulinemia, a B-cell immunodeficiency. Although Btk functions selectively in B cells, the Btk active site is structurally similar to the active site in several Src and Abl kinases and as a result, there have been few highly selective small molecule inhibitors of Btk. We have developed a series of covalent Btk inhibitors that target Cys-481 in Btk and this approach results in increased potency and selectivity over related kinases that lack a Cys residue at this position (ChemMedChem 15:58). PCI-32765 is a Cys-481 targeting Btk inhibitor that has been optimized for potency, selectivity and pharmacokinetics. In cellular assays, PCI-32765 inhibits BCR-stimulus induced phosphorylation of Phospholipase-C-gamma, a Btk substrate, as well as downstream phosphorylation of Erk (IC50 < 100 nM). In addition, PCI-32765 induces apoptosis and inhibits proliferation in a subset of NHL cell lines including DHL-4, DHL-6, WSU-DLCL2, OCI-Ly10 and DOHH2 (IC50s = 0.6–1.6uM). We have used RNAi knockdown in DOHH2 cells as an independent method to confirm that Btk is required for lymphoma cell proliferation. In vivo, orally dosed PCI-32765 (50mg/kg) inhibits growth of DOHH2 and WSU-DLCL2 xenografts. PCI-32765 also prevents disease progression in a mouse collagen-induced arthritis model (12.5mg/kg PO), indicating that other B cell lineage diseases are sensitive to Btk inhibition. In order to further characterize the selectivity and in vivo potency of PCI-32765, we have developed PCI-33380, an active-site probe consisting of a covalent Btk inhibitor linked to the fluorophore Bodipy-FL. PCI-33380 binds to Btk and can be detected by flow cytometry or by denaturing gel electrophoresis of cell lysates. In cell lysates, the probe labels a single predominant band of the same molecular weight as Btk and this band is absent in cells from xid mice. Labeling of this band is inhibited (IC50=10nM) by a brief pre-treatment of cells with PCI-32765, indicating that the probe can be used to assess occupancy of Btk by a covalent inhibitor. We have used the probe to quantitate the inhibition of Btk by PCI-32765 in vivo. A single oral dose of PCI-32765 (10mg/kg) delivered to mice leads to rapid and complete inhibition of Btk in spleen. In addition, a single oral dose of PCI-32765 fully inhibits Btk in xenograft tumors and peripheral blood cells and this inhibition is maintained for up to 24hr. The Btk probe provides pharmacodynamic measurements that may allow optimization of dosing and schedule for in vivo studies and we are currently adapting the probe assays for use in monitoring the inhibition of Btk in human clinical trials.