Potency and Selectivity of BTK Inhibitors in Clinical Development for B-Cell Malignancies

Background: Bruton tyrosine kinase (BTK) is a validated target for B-cell malignancies. The BTK inhibitor ibrutinib was approved in chronic lymphocytic leukemia, mantle cell lymphoma (MCL), and Waldenstrom macroglobulinemia. Acalabrutinib is a potent, highly selective, covalent BTK inhibitor with minimal off-target activity; it received accelerated FDA approval in October 2017 for the treatment of patients with MCL having ≥1 prior therapy. In addition to the approved covalent BTK inhibitors ibrutinib and acalabrutinib, clinical data in B-cell malignancies are available for spebrutinib (CC-292), tirabrutinib (ONO/GS-4059) and zanubrutinib (BGB-3111). We performed biochemical and cellular profiling of these 5 BTK inhibitors, investigating potency and selectivity.

Methods: Two biochemical kinase assays assessed BTK inhibitor potency, with IC₅₀ determination at a fixed time point (IMAP; Molecular Devices) or over time (LanthaScreen; Invitrogen); the latter was used to calculate binding kinetics. Kinome profiling was performed at a single dose (1 µM) using KINOMEscan (Eurofins DiscoverX). IC₅₀ determinations were made using kinases with a Cys in the same position as the Cys481 residue in BTK, using assays developed in house (using IMAP and LanthaScreen) or at Thermo Fisher Scientific (Z'-LYTE) with IC₅₀ determination at a fixed time point.

On-target inhibition of BTK in cellular assays was evaluated using B-cell receptor-mediated activation of CD69 expression on peripheral B cells using human peripheral blood mononuclear cells (hPBMCs) or human whole blood (hWB).

Off-target inhibition of epidermal growth factor receptor (EGFR) was evaluated in a cellular assay examining the effect of EGF-induced EGFR phosphorylation in A431 cells.

Off-target inhibition of ITK and/or TXK was evaluated using T-cell receptor (TCR)-mediated activation of interleukin-2 expression in Jurkat T cells and CD25 cell surface expression using primary human peripheral T cells.

Results: Based on biochemical binding kinetics, ibrutinib and zanubrutinib were the most potent BTK inhibitors (Table 1), followed by spebrutinib; acalabrutinib and tirabrutinib had comparable potency. Differences in potency were largely driven by differences in inactivation rates. However, differences in biochemical potency were lost (in part) in cellular assays using hPBMCs or hWB. EC₅₀ values in hWB were <10 nM for ibrutinib, acalabrutinib and zanubrutinib. Spebrutinib had an EC₅₀ of 140 nM in hWB, showing the greatest loss of potency versus biochemical assays and hPBMCs, going to the physiologically most relevant cellular experimental conditions (Table 2).

Differences in overall kinase selectivity were observed among the BTK inhibitors (Figure 1; KINOMEscan). Acalabrutinib had the lowest hit rate; 1.5% of human wild-type kinases were inhibited >65% at 1 µM (excluding BTK). Kinase hit rate was also low for tirabrutinb (2.3%), whereas ibrutinib (9.4%), zanubrutinib (4.3%), and spebrutinib (8.3%) had higher hit rates.

Acalabrutinib had a high selectivity for BTK over kinases with a Cys in the same position as the Cys481 residue in BTK (Table 2). Similar results were observed for tirabrutinib, whereas ibrutinib, spebrutinib and zanubrutinib were less selective in this panel of kinases with potential for off-target covalent binding by BTK inhibitors (Table 2). Acalabrutinib also had a higher selectivity for BTK over Src-family kinases than the other BTK inhibitors tested.

Acalabrutinib and tirabrutinib had EC₅₀ values >10 µM in the cellular assay of off-target inhibition of EGFR; ibrutinib, zanubrutinib, and spebrutinib had EC₅₀ values of 0.07, 0.39, and 4.7 µM, respectively.

No off-target inhibition was observed for acalabrutinib or tirabrutinib on TCR-mediated activation of T cells up to 10 µM. Ibrutinib, zanubrutinib, and spebrutinib had EC₅₀ values <1 µM in Jurkat T cells. Similar results were observed in primary human peripheral T cells.

Conclusion: BTK inhibitors in clinical development for B-cell malignancies had differing potency in biochemical assays, but these differences were lost (in part) in cellular assays, particularly in hWB. Among the BTK inhibitors tested, the greatest differentiation was observed in kinase selectivity profiles; acalabrutinib and tirabrutinib had the highest kinase selectivity.

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