ACP-196 Is a Second Generation Inhibitor of Bruton Tyrosine Kinase (BTK) with Enhanced Target Specificity

Background:  Signaling through the B-cell receptor (BCR) results in cell survival and proliferation, and this pathway is critical for the progression of many B-cell malignancies, such a chronic lymphocytic leukemia (CLL). BTK, a TEC family kinase, is essential to the BCR signaling cascade.  The recent approval of the first-in-class BTK inhibitor for the treatment certain B-cell malignancies, ibrutinib, has validated BTK as a clinical target.  ACP-196 is a novel, second generation inhibitor of BTK with an IC₅₀ of 3 nM against purified BTK and EC₅₀ of 8 nM in a human whole-blood CD69 B-cell activation assay. In vivo, oral administration of ACP-196 inhibits BCR-induced CD69 expression in murine splenic lymphocytes with an ED₅₀ of 1.3 mg/kg compared with 2.9 mg/kg for ibrutinib. ACP-196 also has improved target specificity over ibrutinib with 323-, 94-, 19- and 9-fold selectivity over the other TEC kinase family members (ITK, TXK, BMX, and TEC, respectively) and no activity against EGFR (Covey AACR, 2015). TXK and ITK are critical for T-cell receptor activation and development.  ITK is also necessary for natural killer (NK)-cell mediated antibody dependent cell cytotoxicity (ADCC), which is important for the efficacy of monoclonal antibody therapy, eg, obinutuzumab.  Additionally, therapeutic blockade of EGFR signaling is employed for treatment of epithelial cancers and is associated with adverse events, such as diarrhea and skin rash.  Here, we describe results from signaling and functional assays to further elucidate the differential effects of ACP-196 on primary CLL cells compared with T-cells, NK-cells and epithelial cells.

Methods:  CLL patient samples were evaluated for pathway activation in the presence and absence of ACP-196 using phospho-protein immunoblotting. Helper T-cell (Th) skewing assays were performed using purified naive CD4+ T-cells from C57BL/6, Itk knock out (KO) and Itk/Txk double KO mice in the presence ACP-196 or ibrutinib.  CD8+ T-cell activity was assessed by cytolytic activity (CTL) assays using T-cells isolated from C57BL/6 mice.  The CTL response was initiated with various effector:T-cell ratios using C57BL/6 T-cells and murine BALB/c lymphoblasts. To evaluate NK-cell ADCC function, standard chromium release assays were done using purified NK-cells isolated from healthy volunteer peripheral blood mononuclear cells and cultured with obinutuzumab-coated (10 µg/mL) CLL cells in the presence and absence of ACP-196.  Immunoblotting of H460 lung cancer cells for phospho-EGFR was used to evaluate the effect of ACP-196 on EGFR signaling.

Results:  ACP-196 treatment of primary human CLL cells showed a dose-dependent (0.01μM-1μM) decrease in p-BTK (Y223), p-ERK (T202/Y204) and p-AKT (T308) and p-IκB-α (S32), without impact on total protein levels. These findings demonstrate ACP-196 inhibits autophosphorylation of BTK as well as down-stream effectors of BCR signaling.

Phospho-protein immunoblotting of primary human T-cells and Jurkat cells showed that ACP-196 does not inhibit T-cell receptor signaling molecules downstream of ITK and TXK such as IκBα, NFAT, or JunB.  In in vitro helper T cell skewing assays, ACP-196, unlike ibrutinib, showed no effect on differentiation of Th subsets (Th1/Th2/Th17) or regulatory T cells (Tregs). The effect of ibrutinib on Th and Treg differentiation was similar to that observed in cells from Itk KO and Itk/Txk KO mice.  In addition, ACP-196 had no effect on CD8+ T-cell viability or cytotoxicity as compared with ibrutinib-treated CD8+ T cells (46% and 36% reduction, respectively). 

In the in vitro ADCC assay, ACP-196 had no effect on NK cell function (chromium release of 34.8% vs 39.6% for the vehicle control).  Lastly, at clinically relevant concentrations ACP-196 did not attenuate autophosphorylation of EGFR in H460 lung epithelial cells demonstrating no effect of ACP-196 on EGFR signaling.

Conclusions: ACP-196 selectively inhibits BTK and BCR signaling at pharmacologic concentrations without off-target effects on T-cells, NK-cells and lung epithelial cells. ACP-196 is currently in Phase 3 trials for the treatment of CLL (ClinicalTrials.gov NCT0247568 and NCT02477696).

This work was supported by the NIH NCI R01CA197870 and Acerta Pharma.