Abstract C258: BTK inhibitor ibrutinib inhibits breast cancer growth by inhibiting ErbB2 kinase.
Authors: Elias Laurence , Chen Jun , Buggy Joseph 
Published: August 2014

Abstract
Ibrutinib is a BTK (Bruton tyrosine kinase) targeting inhibitor that has been developed for the treatment of B cell malignancies. Ibrutinib's specificity is enhanced by its ability to bind covalently to a cysteine residue in the BTK active site, which is conserved among 9 other kinases. Although BTK is the most sensitive of these to ibrutinib inhibition, with IC50 = 0.4 nM, some of the other 9 kinases may also be inhibited by ibrutinib at clinically achievable plasma levels. IC50s of ibrutinib for ErbB1, ErbB2 and ErbB4 in particular are 18, 22, and 0.6 nM, respectively.
Among 13 breast cancer lines screened for sensitivity, significant inhibition of in vitro growth at ibrutinib concentrations &lt;100 nM was noted of the ErbB2 over-expressing and gene amplified lines MDA-MB-453, BT-474, UACC-893 and SK-BR-3. In vivo growth of MB-453 in mice was inhibited &gt;60% by 5 mg/kg po QD of ibrutinib, and nearly completely by 50 mg/kg/day. Ibrutinib inhibited in vitro phosphorylation of ErbB2 and ErbB4, accompanied by downstream phosphorylation of AKT, MEK and ERK, by Western blot. Neither CC-292, another BTK inhibitor which lacks significant ErbB1 or 2 inhibitory activity, nor the ErbB1 inhibitor gefitinib, were inhibitory at concentrations comparable to ibrutinib. Although ErbB4 is the most sensitive of the ErbB family kinases to ibrutinib, comparison of effects with several other congeners with varying relative specificities were most consistent with dependence of growth inhibition of these lines on ErbB2 inhibition. The growth inhibitory effect of ibrutinib at concentrations &lt;100 nM was markedly decreased in the presence of HRG-1 (1 μg/mL) and partly blocked by EGF (0.2 μg/mL). Ibrutinib did not significantly inhibit of cell lines MBA-MD-231, MCF-7, and SK-OV-3 which overexpress ErbB2 but without genomic amplification. These lines generally showed lower basal phosphorylation of MEK, ERK and AKT with lower sensitivity to inhibition by ibrutinib.
Covalent binding of ibrutinib to BTK appears to be rapidly established, within ½ hour, as evidenced by persistent inhibition of auto-phosphorylation and persistent labeling by a fluorescently tagged ibrutinib congener, following wash-out into ibrutinib free media. Covalent binding to ErbB family members in breast cancer lines was slightly less robust, requiring longer incubations to maximally bind probe or establish resistance of phosphorylation to washout. This parallels kinetics of enzyme inhibition in cell-free systems. A431 EGFR+ cells exhibited inhibition of EGFR phosphorylation by ibrutinib, which was easily reversed by washout.
Conclusion: The BTK inhibitor Ibrutinib also inhibits the ErbB receptor kinases in cell free assays and is now shown to be capable of inhibiting growth of ErbB2 amplified breast cancer cell lines in vivo and in vitro. Inhibition was paralleled by inhibition of AKT, MEK and ERK phosphorylation downstream. Ibrutinib thus has properties of a pan-ErbB family inhibitor.
Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C258.
Citation Format: Laurence Elias, Jun Chen, Joseph Buggy. BTK inhibitor ibrutinib inhibits breast cancer growth by inhibiting ErbB2 kinase. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C258.