Accumulating evidence suggests important roles for the Axl receptor tyrosine kinase in cancer progression, invasion, metastasis, drug resistance, and patient mortality, highlighting Axl as an attractive target for therapeutic development [
21,
25]. For example, Axl is highly expressed in invasive breast cancer cells, and Axl knockdown blocks the invasive phenotype. Moreover, high Axl expression in primary breast tumours is a strong independent predictor of poor patient outcomes [
74]. As mentioned in Graham et al.’s review, a wide range of small-molecule kinase inhibitors that target the Axl receptor have been described in several studies, including Foretinib, Cabozantinib, Merestinib, Bosutinib, Gilteritinib, Crizotinib, Amuvatinib, Sunitinib, MGCD265, ASLAN002, NPS-1034, LDC1267, SGI-7079, TP-0903, UNC2025, S49076 and BGB324 [
75]. However, in most cases, Axl was not the intended primary target but a secondary target resulting from the similarities among the kinase domains of Axl and other receptor tyrosine kinases (RTKs), such as MET or Mer. Consequently, these inhibitors often show less potency for Axl than their main target. Intriguingly, BGB324, also known as R428, was found to be an Axl-selective inhibitor, and has advanced to clinical trials [
74]. R428 inhibits Axl with low nanomolar activity and blocks Axl-dependent events, including Akt phosphorylation, breast cancer cell invasion, and proinflammatory cytokine production. Pharmacologic investigations have revealed favourable effects after oral administration, with R428-treated tumours displaying a dose-dependent reduction in expression of the cytokine granulocyte macrophage colony-stimulating factor and the epithelial-mesenchymal transition transcriptional regulator Snail. In agreement with an earlier study, R428 inhibited angiogenesis in corneal micropocket and tumour models. Furthermore, R428 administration reduced the metastatic burden and extended survival in MDA-MB-231 intracardiac and 4 T1 orthotopic mouse models of breast cancer metastasis. Additionally, R428 acted synergistically with cisplatin to enhance suppression of liver micrometastases [
74]. Notably, in addition to breast cancer, R428 has been shown to inhibit Axl signalling in glioblastoma multiforme (GBM), AML and Ewing sarcoma, indicating the effectiveness of R428 for targeting Axl [
76‐
78]. Indeed, R428 is now in clinical development [
77], and several ongoing controlled trials involving R428 at various clinical centres aimed at identifying its maximum tolerated dose are registered at
ClinicalTrails.gov (Identifier: NCT02922777, NCT02488408, NCT02424617 and NCT02872259). These studies include trials of R428 in NSCLC, AML and metastatic melanoma, and results are expected soon. In addition to specific inhibitors, shRNA knockdown of Axl and inhibition of Axl with siRNA are also effective approaches to inhibiting Axl signalling [
22,
79]. In fact, the first clinical trial using R428 for the treatment of acute myeloid leukaemia and non-small cell lung cancer is currently being conducted [
80], and several other new inhibitors specific for TAM receptors are being tested in a preclinical stage [
81,
82]. Stable shRNA knockdown of Axl significantly reduces tumour growth in a xenograft model of breast carcinoma [
65], and inhibition of Axl with siRNA in human umbilical vein endothelial cells blocks endothelial tube formation in vitro, suggesting that inhibition of Axl may restrict the angiogenesis required for breast tumour cell growth [
79]. Several monoclonal antibodies specifically targeting AXL have been reported, including 12A11 [
83], Mab173 [
84], YW327.6S2 [
85] and, more recently, D9 and E8 [
86]. All these results may provide avenues for potential therapeutic targeting of Axl.