Abl1 proto-oncogene encodes a cytoplasmic and nuclear protein tyrosine kinase. Specifically, it is relevant to cell differentiation, cell division, cell adhesion, and stress response. Deletion of SH3 domain makes Abl1 become oncogene. The new fusion gene Bcr-Abl encodes an unregulated, cytoplasmic targeted tyrosine kinase that enables cell proliferation without being regulated by cytokines. Besides, Bcr-Abl transcripts encode tyrosine kinases, which activate mediators of the cell cycle regulation system, leading to clonal myeloproliferative diseases. The abnormal interaction between Bcr-Abl oncoprotein and other cytoplasmic molecules leads to the destruction of key cellular processes.

Moreover, the N-terminal BCR coding sequence of BCR-ABL contains a tyrosine phosphorylation site. It binds to the SH2 domain of adaptor protein Grb2. By collocating BCR, ABL tyrosine kinase activity can be constitutively activated, thus facilitating dimerization or tetramerization and subsequent autophosphorylation. Furthermore, this increases the number of phosphotyrosine residues on Bcr-Abl. Thus it increases the binding sites of other protein SH2 domains. Meanwhile, Resistance to Abl tyrosine kinase inhibitors (TKIs) acquired through mutations especially in the mutant T315I, is a major problem for CML. Here, we will introduce an orally active, non-ATP-competitive Bcr-Abl inhibitor, Rebastinib.

Rebastinib is an orally active, non-ATP-Competitive Bcr-Abl Inhibitor.

First of all, Rebastinib is a Bcr-Abl inhibitor for Abl1^WT and Abl1^T315I with IC₅₀ of 0.8 nM and 4 nM, respectively. Nonetheless, Rebastinib also inhibits SRC, KDR, FLT3, and Tie-2, and has low activity to seen towards c-Kit.

In the second place, Rebastinib potently inhibits both u-ABL1T315I (IC50=5 nM) and p-ABL1T315I (IC50=4 nM). More significantly, they exist predominately in the Type I conformation due to stabilization of an activating hydrophobic spine by the T315I mutation. Particularly, Rebastinib also potently inhibits ABL1H396P (IC50=1.4 nM). Notably, Rebastinib spared c-KIT (IC50=481 nM). Rebastinib effectively inhibits the proliferation of Ba/F3 cells expressing native BCR-ABL1native (IC50=5.4 nM). Interestingly, Rebastinib inhibits the proliferation of the Ph+ cell line K562 (IC50=5.5 nM). Rebastinib inhibits the proliferation of TKI-resistant mutants, including G250E, Q252H, Y235F, E255K, V299L, F317L, and M351T. It has IC50s ranging from 6-150 nM. Importantly, Rebastinib effectively inhibits autophosphorylation of BCR-ABL1native (IC50=29 nM) and BCR-ABL1T315I (IC50=18 nM).

Last but not the least, a single oral dose of Rebastinib (100 mg/kg) affords circulating plasma levels that exceed 12 μM for up to 24 hours. Obviously, it effectively inhibits BCR-ABL1 signaling for up to 8 hours in Ba/F3-BCR-ABL1^T315I leukemia cells isolated from BM and spleen of tumor-bearing mice. Additionally, treatment of mice bearing Ba/F3-BCR-ABL1^T315I leukemia cells with Rebastinib at 100 mg/kg/day by oral gavage significantly prolonged their survival. In an aggressive allograft model, Rebastinib is as effective for the treatment of BCR-ABL^T315I leukemia as STI571 at 100 mg/kg twice daily in BCR-ABL1^native leukemia. At the same time, they reduced the leukemia cell burden in the spleens of treated mice.

All in all, Rebastinib is an orally active, non-ATP-competitive Bcr-Abl inhibitor.

References:

Chan WW, et al. Cancer Cell. 2011, 19(4), 556-568.