The Bruton Tyrosine Kinase (BTK) Inhibitor ACP-196 Demonstrates Clinical Activity in Two Mouse Models of Chronic Lymphocytic Leukemia

Introduction: BTK is involved in B-cell receptor (BCR) signal transduction and is an established target for the treatment of chronic lymphocytic leukemia (CLL) (Byrd, NEJM, 2013). ACP-196 is a novel, potent second generation BTK inhibitor, which binds covalently to Cys481 in the ATP-binding pocket of BTK. IC₅₀determinations on nine kinases with a cysteine in the same position as BTK showed ACP-196 to be more selective than the first-in-class BTK inhibitor, ibrutinib (Covey, AACR, 2015). We present data evaluating the anti-tumor effects of ACP-196 in established murine models of CLL.

Methods: Two distinct murine models were used for these studies. In the TCL1 adoptive transfer model, leukemic cells from Eμ-TCL1 transgenic mice were transplanted into C57BL/6 mice, resulting in a CD5+/CD19+ leukemia with peripheral blood, spleen and nodal involvement. ACP-196 treatment in drinking water (0.16 mg/mL) commenced when recipient mice had > 10% CD5+/CD19+ leukocytes in the peripheral blood. Mice were followed for survival. Separate cohorts were sacrificed for pharmacodynamic analyses after 1 and 4 weeks of treatment. In the second model, NSG mice received primary human CLL cells. The xenografted human CLL cells have comparable tumor biology (including active BCR signaling) to activated human lymph node resident CLL cells (Herman, Leukemia, 2013). PBMCs harvested from CLL patients were adoptively transferred at 1 x 10⁸ cells per mouse. ACP-196 was initiated on day -1 (at the time of busulfan priming) at multiple doses ranging from 0.006 to 0.3 mg/mL in drinking water.

Results: In the TCL1 model, treatment with ACP-196 showed > 90% BTK occupancy of BTK after 1 and 4 weeks of therapy. ACP-196 inhibited BCR signaling as shown by decreased autophosphorylation of BTK and reduction in surface expression of the BCR activation markers CD86 and CD69. After 1 week of ACP-196 inhibited BCR signaling as shown by a 6-fold reduction of autophosphorylation of BTK in the presence of anti-IgM, and surface expression of the BCR activation markers CD69 and CD86 were decreased by 47% and 57% respectively. Inhibition of BTK and downstream BCR activation was maintained through at least day 28 of treatment. Most notably, ACP-196 treatment resulted in a significant increase in survival compared with mice receiving vehicle (median 81 vs 59 days, respectively; P =0.02). In the NSG xenograft model, ACP-196 at the times examined did not cause a significant treatment-induced lymphocytosis in the patients evaluated (n=6). After 4 weeks of treatment with ACP-196, the NSG mice were sacrificed, and BCR signaling activity and tumor burden in the spleen were evaluated. ACP-196 treatment showed decreases in phosphorylation of PLCγ2 and ERK (P <0.02) as expected with BTK inhibition. Additionally, a significant reduction was observed in the percentage of proliferating cells in mice treated with ACP-196 compared to vehicle control (as determined by Ki67 staining; median 18% (Intraquartile Range (IQR): 13-38) and 6% (IQR: 2-15), respectively, P =0.02). Lastly, a significant reduction in total tumor burden in the spleen was observed in the mice treated with ACP-196 (median reduction of 33%) compared with the vehicle treated mice (P =0.04).

Conclusions: ACP-196 is a potent inhibitor of BTK as measured by inhibition of BCR activity, reduced tumor proliferation and increased survival. Overall, ACP-196 showed statistically significant efficacy in two murine models of CLL and is currently in Phase 3 trials for treatment-naive (ClinicalTrials.gov NCT0247568) and previously treated high-risk CLL (ClinicalTrials.gov NCT02477696).

This work was supported by the Intramural Research Program of NHLBI, NIH, R01CA197870, K23 CA178183-02, and Acerta Pharma.