Anti-Leukemic Activity of Axitinib Against Cells Harboring the BCR-ABL Point Mutation
Authors: Okabe Seiichi , Tauchi Tetsuzo , Tanaka Yuko , Sakuta Juri , Ohyashiki Kazuma 
Published: October 2019

Abstract
Introduction: The BCR-ABL1 fusion gene is a causative oncogene in chronic myeloid leukemia (CML) and 30-50% of acute lymphoblastic leukemia cases. Although ABL tyrosine kinase inhibitors (ABL TKI) such as imatinib have improved CML treatment, such therapies cannot cure patients with Philadelphia chromosome (Ph)-positive leukemia because of leukemia stem cells. Moreover, some patients develop BCR-ABL point mutations and become resistant to ABL TKI therapy. In particular, the ABL kinase domain mutation T315I is resistant to imatinib and second-generation ABL TKIs (e.g., nilotinib, dasatinib, and bostinib). Accordingly, this mutation is often found in patients with TKI-resistant disease. A third-generation ABL TKI, ponatinib, was recently developed. Ponatinib is a potent oral tyrosine kinase inhibitor that affects both unmutated and mutated BCR-ABL; it is effective against T315I mutant cells and has been approved for TKI-resistant or intolerant CML and Ph-positive ALL patients.Recently, the vascular endothelial growth factor receptor (VEGFR) inhibitor axitinib was found to exhibit anti-leukemic activity against T315I-mutant disease. Axitinib is an orally active and potent TKI of VEGFRs 1, 2, and 3 and inhibits BCR-ABL1, especially the T315I variant, via a distinct binding conformation.
Materials and methods: In this study, we investigated whether axitinib could suppress ponatinib-resistant compound mutant cells harboring the T315I mutation and primary samples.
Results: A 72 h axitinib treatment inhibited the growth of Ba/F3 T315I cells (Figure 1A). Immunoblot analysis of axitinib-treated cells revealed dose-dependent decreases in BCR-ABL, the downstream molecule CrkL, and ribosomal S6 protein phosphorylation and increases in caspase 3 and Poly (ADP-ribose) polymerase (PARP) activity. Ponatinib and axitinib also induced apoptosis, significantly increased caspase activity, and reduced Akt activity. In contrast, clinically available concentrations of axitinib did not inhibit the growth of ponatinib-resistant Ba/F3 cells. Immunoblot analysis revealed that BCR-ABL, Crk-L, and S6 kinase phosphorylation were not inhibited by axitinib or ponatinib. Similarly, no increase in caspase activity or decrease in Akt activity was observed following axitinib treatment, and neither ponatinib nor axitinib affected apoptosis in these cells. We next evaluated primary T315I-mutant and ponatinib-resistant compound mutant samples. Axitinib potently inhibited the growth of T315I mutant primary cells in a dose-dependent manner. Immunoblot analysis further revealed reduced Crk-L and S6 kinase phosphorylation after axitinib or ponatinib treatment. In contrast, the growth of ponatinib-resistant primary cells was not affected by ponatinib or axitinib. Immunoblotting revealed that neither ponatinib nor axitinib affected the phosphorylation of Crk-L and S6 kinase in ponatinib-resistant cells.
Conclusion: In CML, ABL TKI resistance is frequently caused by ABL kinase domain mutations. The T315I mutation is resistant to all ABL TKIs except ponatinib. Although axitinib, which is currently being investigated for efficacy in patients with Ph-positive T315I-mutant leukemia, induced apoptosis in T315I-mutant cells, it was ineffective against cells with a compound mutation including T315I. Current evidence to direct the management of ABL TKI-resistant disease, particularly those harboring T315I and compound mutations, is limited. New molecular-targeted drugs and an understanding of ABL TKI resistance mechanisms are required to apply an appropriate therapeutic approach.

Disclosures
No relevant conflicts of interest to declare.