Axl Receptor Axis: A New Therapeutic Target in Lung Cancer

Abstract

Axl belongs to the TAM family of receptor tyrosine kinases, which consists of Tyro3, Axl, and Mer. All three family members have similar structures and share a number of ligands, including the vitamin K–dependent ligands growth arrest protein 6 (Gas6) and protein S. In normal tissues, TAM receptor tyrosine kinases contribute to immune response regulation, including clearance of apoptotic cells and inhibition of cytotoxic immune activation in response to apoptosis. When cells undergo apoptosis, the polarity of the plasma membrane lipid bilayer is altered, externalizing the anionic phospholipid phosphatidylserine (PS). Gas6, which is often prebound to Axl, binds PS via the γ-carboxyglutamic domain. This ligand-dependent Axl activation regulates macrophage-mediated endocytosis and clearance of apoptotic cells by a process termed efferocytosis while inhibiting proinflammatory cytokine response.1Subramanian M. Hayes C.D. Thome J.J. et al.An AXL/LRP-1/RANBP9 complex mediates DC efferocytosis and antigen cross-presentation in vivo.J Clin Invest. 2014; 124: 1296-1308Crossref PubMed Scopus (72) Google Scholar In preclinical models, TAM receptor triple-knockout mice (Tyro3-/-, Mer-/-, and Axl-/-) develop normally, but as the immune system matures, chronic inflammation and autoimmunity tends to develop. TAM receptor tyrosine kinases also participate in platelet activation and clot stability.2Cosemans J.M. Van Kruchten R. Olieslagers S. et al.Potentiating role of Gas6 and Tyro3, Axl and Mer (TAM) receptors in human and murine platelet activation and thrombus stabilization.J Thromb Haemost. 2010; 8: 1797-1808Crossref PubMed Scopus (81) Google Scholar Other less-studied mechanisms of Axl activation include ligand-independent homodimerization of Axl due to receptor overexpression, transcellular homophilic binding of the Axl extracellular domain, heterodimerization with other TAM family receptors such as Tyro3, and dimerization with non-TAM receptor tyrosine kinases, such as epidermal growth factor receptor (EGFR) (Fig. 1).3Lemke G. Rothlin C.V. Immunobiology of the TAM receptors.Nat Rev Immunol. 2008; 8: 327-336Crossref PubMed Scopus (597) Google Scholar, 4Elkabets M. Pazarentzos E. Juric D. et al.AXL mediates resistance to PI3Kalpha inhibition by activating the EGFR/PKC/mTOR axis in head and neck and esophageal squamous cell carcinomas.Cancer Cell. 2015; 27: 533-546Abstract Full Text Full Text PDF PubMed Scopus (226) Google Scholar, 5Bellosta P. Costa M. Lin D.A. Basilico C. The receptor tyrosine kinase ARK mediates cell aggregation by homophilic binding.Mol Cell Biol. 1995; 15: 614-625Crossref PubMed Google Scholar, 6Burchert A. Attar E.C. McCloskey P. Fridell Y.W. Liu E.T. Determinants for transformation induced by the Axl receptor tyrosine kinase.Oncogene. 1998; 16: 3177-3187Crossref PubMed Scopus (48) Google Scholar Complex transcriptional and translational mechanisms regulate Axl expression (see Fig. 1). The AXL receptor tyrosine kinase gene (Axl) is located on chromosome 19 and consists of 20 exons. Different Axl transcripts arise from alternative splicing of exon 10 and utilization of one of the two imperfect polyadenylation termination sites, thereby creating different 3′-untranslated regions. Multiple transcription factors bind to the Axl promoter, including specificity protein 1 and specificity protein 3, myeloid zinc finger 1, and activator protein 1. In cancer, increased Axl expression has been reported at the messenger RNA (mRNA) and protein levels. Transcriptional factors implicated in driving Axl expression include mutant p53, yes-associated protein 1 (in non–small cell lung cancer [NSCLC]), and hypoxia-inducible factor-1 (in renal cell carcinoma).7Cui Z.L. Han F.F. Peng X.H. et al.YES-associated protein 1 promotes adenocarcinoma growth and metastasis through activation of the receptor tyrosine kinase Axl.Int J Immunopathol Pharmacol. 2012; 25: 989-1001Crossref PubMed Scopus (39) Google Scholar, 8Mudduluru G. Ceppi P. Kumarswamy R. Scagliotti G.V. Papotti M. Allgayer H. Regulation of Axl receptor tyrosine kinase expression by miR-34a and miR-199a/b in solid cancer.Oncogene. 2011; 30: 2888-2899Crossref PubMed Scopus (210) Google Scholar, 9Vaughan C.A. Singh S. Windle B. et al.Gain-of-function activity of mutant p53 in lung cancer through up-regulation of receptor protein tyrosine kinase Axl.Genes Cancer. 2012; 3: 491-502Crossref PubMed Scopus (51) Google Scholar Axl expression is also regulated through various epigenetic mechanisms. Axl promoter hypermethylation results in downregulation of Axl expression. Additionally, Axl mRNA is degraded in the presence of microRNAs mir-34 and mir-199a/b. Methylation status of mir-34 and mir-199a/b correlate with Axl expression and are associated with worse survival in NSCLC.8Mudduluru G. Ceppi P. Kumarswamy R. Scagliotti G.V. Papotti M. Allgayer H. Regulation of Axl receptor tyrosine kinase expression by miR-34a and miR-199a/b in solid cancer.Oncogene. 2011; 30: 2888-2899Crossref PubMed Scopus (210) Google Scholar Axl protein folding is dependent on the heat shock protein 90 (HSP90) chaperone such that HSP90 inhibition leads to increased Axl degradation.10Choi Y.J. Kim S.Y. So K.S. et al.AUY922 effectively overcomes MET- and AXL-mediated resistance to EGFR-TKI in lung cancer cells.PLoS One. 2015; 10: e0119832Google Scholar Axl gene amplification has been reported in 5% of colorectal cancer tissue samples and has been described in lung adenocarcinoma as well, but the prevalence of Axl amplification in other cancer types is poorly characterized.11Do KT, MacConaill L, Dubuc A, et al. Evaluation of the MET/AXL receptor tyrosine kinase (RTK) inhibitor MGCD265 in a patient with metastatic non-small cell lung cancer (NSCLC) harboring AXL amplification. Paper presented at: 16th World Conference on Lung Cancer. September 6–9, 2015; Denver, CO.Google Scholar, 12Martinelli E. Martini G. Cardone C. et al.AXL is an oncotarget in human colorectal cancer.Oncotarget. 2015; 6: 23281-23296Crossref PubMed Scopus (44) Google Scholar Transcriptome sequencing of 200 surgical tumor samples of lung adenocarcinoma revealed a new Axl, MAP3K12-binding inhibitory protein fusion gene (MBIP), which preserved the Axl tyrosine kinase domain.13Seo J.S. Ju Y.S. Lee W.C. et al.The transcriptional landscape and mutational profile of lung adenocarcinoma.Genome Res. 2012; 22: 2109-2119Crossref PubMed Scopus (464) Google Scholar The structure of Axl has been well described. Like other members of the TAM family, the extracellular N-terminal portion of the Axl receptor protein consists of two immunoglobulin domains and two fibronectin type 3 domains linked to a single transmembrane domain. The intracellular portion of the receptor contains conserved kinase domains, including a KWIAIES sequence of amino acids unique to this family of receptor tyrosine kinases.14Graham D.K. DeRyckere D. Davies K.D. Earp H.S. The TAM family: phosphatidylserine sensing receptor tyrosine kinases gone awry in cancer.Nat Rev Cancer. 2014; 14: 769-785Crossref PubMed Scopus (426) Google Scholar Axl activation depends on the presence of Gas6 ligand and PS. Evidence suggests that the affinity of Gas6 for Axl is sufficiently high that under physiologic conditions the two may be constitutively bound. However, only in the presence of PS is Gas6 capable of fully activating Axl.15Meyer A.S. Zweemer A.J. Lauffenburger D.A. The AXL Receptor is a sensor of ligand spatial heterogeneity.Cell Syst. 2015; 1: 25-36Abstract Full Text Full Text PDF PubMed Scopus (35) Google Scholar Upon activation, Axl homodimerizes and tyrosine amino acids are autophosphorylated at positions 779, 821, and 866. Their phosphorylation creates binding sites for multiple adaptor proteins, including growth factor receptor-bound protein 2 and phospholipase C, which in turn result in activation of key intracellular pathways responsible for proliferation and survival, such as Ras/MEK/ERK and phosphoinositide-3 (PI3K)/protein kinase B.16Goruppi S. Ruaro E. Varnum B. Schneider C. Requirement of phosphatidylinositol 3-kinase-dependent pathway and Src for Gas6-Axl mitogenic and survival activities in NIH 3T3 fibroblasts.Mol Cell Biol. 1997; 17: 4442-4453Crossref PubMed Scopus (171) Google Scholar, 17Fridell Y.W. Jin Y. Quilliam L.A. et al.Differential activation of the Ras/extracellular-signal-regulated protein kinase pathway is responsible for the biological consequences induced by the Axl receptor tyrosine kinase.Mol Cell Biol. 1996; 16: 135-145Crossref PubMed Scopus (145) Google Scholar Axl can also activate Rac1, leading to cytoskeletal reorganization and a more mesenchymal-like phenotype, characterized by increased migration and invasion.18Abu-Thuraia A. Gauthier R. Chidiac R. et al.Axl phosphorylates Elmo scaffold proteins to promote Rac activation and cell invasion.Mol Cell Biol. 2015; 35: 76-87Crossref PubMed Scopus (55) Google Scholar Axl was initially isolated from chronic myelogenous leukemia cells as a protein capable of transforming the fibrocyte cell line NIH 3T3.19O'Bryan J.P. Frye R.A. Cogswell P.C. et al.Axl, a transforming gene isolated from primary human myeloid leukemia cells, encodes a novel receptor tyrosine kinase.Mol Cell Biol. 1991; 11: 5016-5031Crossref PubMed Scopus (631) Google Scholar Since that time, Axl overexpression has been noted in multiple cancer types, including lung, breast, ovarian, gastric, colon, pancreatic, and prostate. Axl expression and activity are associated with epithelial-to-mesenchymal transition (EMT), higher metastatic potential, therapeutic resistance, and overall worse prognosis.20Byers L.A. Diao L. Wang J. et al.An epithelial-mesenchymal transition gene signature predicts resistance to EGFR and PI3K inhibitors and identifies Axl as a therapeutic target for overcoming EGFR inhibitor resistance.Clin Cancer Res. 2013; 19: 279-290Crossref PubMed Scopus (691) Google Scholar, 21Asiedu M.K. Beauchamp-Perez F.D. Ingle J.N. Behrens M.D. Radisky D.C. Knutson K.L. AXL induces epithelial-to-mesenchymal transition and regulates the function of breast cancer stem cells.Oncogene. 2014; 33: 1316-1324Crossref PubMed Scopus (191) Google Scholar, 22Gjerdrum C. Tiron C. Hoiby T. et al.Axl is an essential epithelial-to-mesenchymal transition-induced regulator of breast cancer metastasis and patient survival.Proc Natl Acad Sci U S A. 2010; 107: 1124-1129Crossref PubMed Scopus (431) Google Scholar, 23Wu F. Li J. Jang C. Wang J. Xiong J. The role of Axl in drug resistance and epithelial-to-mesenchymal transition of non-small cell lung carcinoma.Int J Clin Exp Pathol. 2014; 7: 6653-6661PubMed Google Scholar In NSCLC and head and neck squamous cell cancer, Axl expression is driven by activation of the EGFR/Ras/mitogen-activated protein kinase pathway and leads to increased resistance to anti-EGFR therapy.24Brand T.M. Iida M. Stein A.P. et al.AXL mediates resistance to cetuximab therapy.Cancer Res. 2014; 74: 5152-5164Crossref PubMed Scopus (136) Google Scholar Moreover, in head and neck cancer and esophageal squamous cell cancer, Axl was shown to bind and activate EGFR receptor in a ligand-independent manner. This interaction leads to activation of protein kinase C and downstream mammalian target of rapamycin independently from PI3K-α, thereby rendering tumor cells resistant to PI3K inhibitors.4Elkabets M. Pazarentzos E. Juric D. et al.AXL mediates resistance to PI3Kalpha inhibition by activating the EGFR/PKC/mTOR axis in head and neck and esophageal squamous cell carcinomas.Cancer Cell. 2015; 27: 533-546Abstract Full Text Full Text PDF PubMed Scopus (226) Google Scholar Conversely, Axl inhibition sensitizes resistant cells to cytotoxic agents and to targeted inhibitors across cancer types. In head and neck squamous cell cancer, Axl inhibition decreases cancer cell proliferation, migration, and invasion and increases sensitivity to the anti-EGFR antibody cetuximab.25Brand T.M. Iida M. Stein A.P. et al.AXL Is a logical molecular target in head and neck squamous cell carcinoma.Clin Cancer Res. 2015; 21: 2601-2612Crossref PubMed Scopus (74) Google Scholar In a breast cancer model, sensitivity to taxanes increased 1000-fold with coincident Axl inhibition.26Wilson C. Ye X. Pham T. et al.AXL inhibition sensitizes mesenchymal cancer cells to antimitotic drugs.Cancer Res. 2014; 74: 5878-5890Crossref PubMed Scopus (107) Google Scholar Axl is also expressed on endothelial cells and plays a role in angiogenesis. Preclinical studies have shown that Axl inhibition decreases endothelial cell tube formation in vitro and potentiates the antiangiogenic effect of anti–vascular endothelial growth factor antibodies.27Li Y. Ye X. Tan C. et al.Axl as a potential therapeutic target in cancer: role of Axl in tumor growth, metastasis and angiogenesis.Oncogene. 2009; 28: 3442-3455Crossref PubMed Scopus (221) Google Scholar In lung cancer cell lines, Axl mRNA and protein expression correlate with increased migration and invasion, which can be inhibited with Axl small interfering RNA. In clinical specimens, Axl is expressed in approximately half of lung cancer cases and is associated with lymph node involvement, more advanced clinical stage, and worse survival.28Shieh Y.S. Lai C.Y. Kao Y.R. et al.Expression of axl in lung adenocarcinoma and correlation with tumor progression.Neoplasia. 2005; 7: 1058-1064Abstract Full Text PDF PubMed Scopus (214) Google Scholar The association between Gas6 expression and prognosis is less clear. Gas6 protein expression by immunohistochemical analysis is associated with worse clinical outcomes. However, Gas6 mRNA expression—which is inversely related to protein expression—confers improved 5-year survival.29Ishikawa M. Sonobe M. Nakayama E. et al.Higher expression of receptor tyrosine kinase Axl, and differential expression of its ligand, Gas6, predict poor survival in lung adenocarcinoma patients.Ann Surg Oncol. 2013; 20: S467-S476Crossref PubMed Scopus (69) Google Scholar This discordance may reflect the fact that most ligand expression comes from nonmalignant cells (such as endothelium, fibroblasts, and immune cells) and may not be related to cancer cell expression of Gas6. In lung cancer, Axl has been associated with drug resistance. Analysis of tissue samples from patients who had activating epidermal growth factor receptor gene (EGFR) mutations and in whom resistance to EGFR inhibitors subsequently developed revealed that almost 30% of previously unexplained mechanisms of resistance to EGFR inhibition—that is, without evidence of a characteristic molecular phenotype such as EGFR T790M mutation or MMNG HOS Transforming gene (MET) amplification—could be attributed to EMT.30Zhang Z. Lee J.C. Lin L. et al.Activation of the AXL kinase causes resistance to EGFR-targeted therapy in lung cancer.Nat Genet. 2012; 44: 852-860Crossref PubMed Scopus (905) Google Scholar, 31Sequist L.V. Waltman B.A. Dias-Santagata D. et al.Genotypic and histological evolution of lung cancers acquiring resistance to EGFR inhibitors.Sci Transl Med. 2011; 3 (75ra26)Crossref PubMed Scopus (2691) Google Scholar Screening of tumor samples and 54 different NSCLC cell lines for an EMT gene signature identified Axl expression (protein and mRNA) as highly correlated with mesenchymal phenotype. Analysis of cancer tissues from patients with NSCLC has demonstrated that Axl overexpression can coexist with EGFR T790M mutation and that these two mechanisms of resistance are not mutually exclusive.30Zhang Z. Lee J.C. Lin L. et al.Activation of the AXL kinase causes resistance to EGFR-targeted therapy in lung cancer.Nat Genet. 2012; 44: 852-860Crossref PubMed Scopus (905) Google Scholar In cell lines and xenograft models, administration of the Axl tyrosine kinase inhibitor SGI-7079 reversed mesenchymal phenotype and increased EGFR inhibitor sensitivity.20Byers L.A. Diao L. Wang J. et al.An epithelial-mesenchymal transition gene signature predicts resistance to EGFR and PI3K inhibitors and identifies Axl as a therapeutic target for overcoming EGFR inhibitor resistance.Clin Cancer Res. 2013; 19: 279-290Crossref PubMed Scopus (691) Google Scholar Similar sensitization to anti-EGFR therapy was also achieved by promoting Axl degradation through inhibition of HSP90.10Choi Y.J. Kim S.Y. So K.S. et al.AUY922 effectively overcomes MET- and AXL-mediated resistance to EGFR-TKI in lung cancer cells.PLoS One. 2015; 10: e0119832Google Scholar A key question related to the role of Axl inhibition in the approach to EGFR-mutant NSCLC is whether it would be more advantageous to use such combinations upfront to preempt resistance or to reserve the use of Axl inhibitors for the purpose of reversing resistance after it develops. Both strategies are being investigated in early clinical trials. In some lung cancers, Axl alterations may render tumors highly sensitive to Axl inhibition. For instance, a patient with heavily pretreated advanced NSCLC harboring Axl amplification experienced a partial response to the single-agent MET proto-oncogene/Axl inhibitor MGCD265, with a 48% reduction in tumor dimension and substantial symptomatic improvement.11Do KT, MacConaill L, Dubuc A, et al. Evaluation of the MET/AXL receptor tyrosine kinase (RTK) inhibitor MGCD265 in a patient with metastatic non-small cell lung cancer (NSCLC) harboring AXL amplification. Paper presented at: 16th World Conference on Lung Cancer. September 6–9, 2015; Denver, CO.Google Scholar Additionally, Axl and other TAM receptors have immunomodulatory effects. Activation of Axl in immune cells suppresses proinflammatory cytokine release and decreases inflammation. Therefore, Axl-mediated signaling may aid in creation of an immunotolerant milieu and allow tumor growth. Preclinical studies have shown that targeted inhibition of Axl or its downstream signaling in natural killer cells promotes anticancer immune responses in melanoma and breast cancer mouse models.32Paolino M. Choidas A. Wallner S. et al.The E3 ligase Cbl-b and TAM receptors regulate cancer metastasis via natural killer cells.Nature. 2014; 507: 508-512Crossref PubMed Scopus (307) Google Scholar These properties suggest that Axl inhibition might be incorporated into existing cancer immunotherapy strategies to augment treatment efficacy. To date, most clinical studies of Axl inhibition in lung cancer have used nonspecific multikinase inhibitors, which target Axl among a number of receptor tyrosine kinases (Table 1). The small molecule inhibitor cabozantinib has been approved for treatment of medullary thyroid cancer because of its anti-RET activity. Cabozantinib also has activity against vascular endothelial growth factor receptor, MET, Flt3, Kit, and Axl. There are several ongoing clinical trials investigating the benefit on cabozantinib in NSCLC either as monotherapy or in combination with erlotinib. In addition to its well-known effects on anaplastic receptor tyrosine kinase, MET, and Ros proto-oncogene 1, receptor tyrosine kinase, crizotinib also targets Axl. Other multitargeted kinase inhibitors with Axl activity include ASLAN002, MGCD265, MGCD516, and foretinib. Recently, a number of more specific Axl inhibitors such as BGB324 and BPI-9016M have entered early-phase clinical trials.33Myers S.H. Brunton V.G. Unciti-Broceta A. AXL inhibitors in cancer: a medicinal chemistry perspective.J Med Chem. 2015; 59: 3593-3608Crossref PubMed Scopus (134) Google Scholar The anti-Axl monoclonal antibody YW327.6S2 and Axl decoy receptor and aptamer GL2I.T are undergoing preclinical development.34Ye X. Li Y. Stawicki S. et al.An anti-Axl monoclonal antibody attenuates xenograft tumor growth and enhances the effect of multiple anticancer therapies.Oncogene. 2010; 29: 5254-5264Crossref PubMed Scopus (168) Google Scholar, 35Cerchia L. Esposito C.L. Camorani S. et al.Targeting Axl with an high-affinity inhibitory aptamer.Mol Ther. 2012; 20: 2291-2303Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar For Axl pathway targeting, small molecule and antibody-based approaches have distinct characteristics and advantages. Although antibodies as a class are generally more specific than small molecules, the effect of these drugs will depend on the selected target. Although anti-Gas6 antibodies should inhibit only ligand-induced Axl activation, anti-Axl antibodies and small molecule inhibitors should inhibit ligand-dependent and ligand-independent activity. Finally, because Gas6 binding to PS requires vitamin K–dependent y-carboxylation of the γ-carboxyglutamic domain, the vitamin K antagonist warfarin has been shown to inhibit TAM receptor signaling and reduce malignant potential.36Kirane A. Ludwig K.F. Sorrelle N. et al.Warfarin blocks Gas6-mediated Axl activation required for pancreatic cancer epithelial plasticity and metastasis.Cancer Res. 2015; 75: 3699-3705Crossref PubMed Scopus (98) Google Scholar Further investigation is needed to determine the clinical benefit of warfarin as a cancer therapeutic agent.Table 1Axl Inhibitors in Clinical TrialsTyrosine Kinase InhibitorTarget ProteinAxl IC50Clinical Trial (Disease) (Combination)PhaseCabozantinibAxl, MET, VEGFR2, RET, Kit, Flt-1, Flt-3, Flt-4, Tie27 nMNCT01639508 (NSCLC)NCT00596648 (NSCLC) (+ erlotinib)NCT01708954 (NSCLC) (+ erlotinib)NCT01866410 (NSCLC) (+ erlotinib)21 and 222CrizotinibAxl, Alk, MET, RON, ROS1294 nMNCT02034981 (multiple cancers)2ASLAN002Axl, RON, MET, Tyro3, Mer, Flt-31.1 nMNCT01721148 (multiple cancers)1MGCD265Axl, MET, VEGFR2NCT00697632 (multiple cancers)1MGCD516Axl, RET, TRK, DDR2, MET, Kit, VEGFR, PDGFRNCT02219711 (multiple cancers)1/1BForetinib + erlotinibAxl, MET, VEGFR2, RON11 nMNCT00725764 (HNSCC)NCT01068587 (NSCLC) (+ erlotinib)21 and 2BGB324Axl14 nMNCT02488408 (AML) (+ cytarabine)NCT02424617 (NSCLC) (+ erlotinib)11 and 2BPI-9016Axl, METNCT02478866 (multiple cancers)1AML, acute myeloid leukemia; Alk, anaplastic lymphoma kinase; DDR2, discoidin domain receptor 2; Flt-1, Flt-3, and Flt-4, FMS-related tyrosine kinase 1, 3 and 4; HNSCC, head and neck squamous cell cancer; IC50, concentration that inhibits 50%; NSCLC, non–small cell lung cancer; PDGFR, platelet-derived growth factor receptor; RET, ret proto-oncogene; ROS1, ROS proto-oncogene 1, receptor tyrosine kinase; Tie2, epithelial-specific protein receptor tyrosine kinase; TRK, tyrosine kinase receptor; VEGFR2, vascular endothelial growth factor receptor 2; RON, RON protein tyrosine kinase; MER, MET proto-oncogene. Open table in a new tab AML, acute myeloid leukemia; Alk, anaplastic lymphoma kinase; DDR2, discoidin domain receptor 2; Flt-1, Flt-3, and Flt-4, FMS-related tyrosine kinase 1, 3 and 4; HNSCC, head and neck squamous cell cancer; IC50, concentration that inhibits 50%; NSCLC, non–small cell lung cancer; PDGFR, platelet-derived growth factor receptor; RET, ret proto-oncogene; ROS1, ROS proto-oncogene 1, receptor tyrosine kinase; Tie2, epithelial-specific protein receptor tyrosine kinase; TRK, tyrosine kinase receptor; VEGFR2, vascular endothelial growth factor receptor 2; RON, RON protein tyrosine kinase; MER, MET proto-oncogene. Growing evidence suggests that Axl is critical for cancer cell survival, EMT, metastatic potential, and therapeutic resistance in many cancer types, including NSCLC. A number of clinically available multitargeted kinase inhibitors, such as cabozantinib and crizotinib, inhibit Axl function. Recently, a number of more specific Axl kinase inhibitors have entered early-phase clinical trials, and anti-Axl monoclonal antibodies and decoy receptors are undergoing preclinical development. Results from these studies will give important insights into the potential of Axl targeting as a therapeutic approach, especially for patients with de novo or acquired resistance to existing targeted therapies.