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Targeting MET in Lung Cancer: Will Expectations Finally Be MET?

Open ArchivePublished:October 26, 2016DOI:https://doi.org/10.1016/j.jtho.2016.10.014

      Abstract

      The hepatocyte growth factor receptor (MET) is a potential therapeutic target in a number of cancers, including NSCLC. In NSCLC, MET pathway activation is thought to occur through a diverse set of mechanisms that influence properties affecting cancer cell survival, growth, and invasiveness. Preclinical and clinical evidence suggests a role for MET activation as both a primary oncogenic driver in subsets of lung cancer and as a secondary driver of acquired resistance to targeted therapy in other genomic subsets. In this review, we explore the biology and clinical significance behind MET proto-oncogene receptor tyrosine kinase (MET) exon 14 alterations and MET amplification in NSCLC, the role of MET amplification in the setting of acquired resistance to EGFR tyrosine kinase inhibitor therapy in EGFR-mutant NSCLC, and the history of MET pathway inhibitor drug development in NSCLC, highlighting current strategies that enrich for biomarkers likely to be predictive of response. Whereas previous trials that focused on MET pathway–directed targeted therapy in unselected or MET-overexpressing NSCLC yielded largely negative results, more recent investigations focusing on MET exon 14 alterations and MET amplification have been notable for meaningful clinical responses to MET inhibitor therapy in a substantial proportion of patients.

      Keywords

      Introduction

      Phase III randomized trials of tyrosine kinase inhibitor (TKI) therapy for EGFR-mutant and anaplastic lymphoma receptor tyrosine kinase (ALK)-rearranged lung cancers have documented improvements in response and progression-free survival (PFS),
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      Phase III study of afatinib or cisplatin plus pemetrexed in patients with metastatic lung adenocarcinoma with EGFR mutations.
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      First-line crizotinib versus chemotherapy in ALK-positive lung cancer.
      and seven TKIs have gained regulatory approval for the treatment of patients with these tumors. The treatment landscape continues to evolve as durable responses to targeted therapy have been reported in a growing number of other genomic subsets.
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      Phase II study of cabozantinib for patients with advanced RET-rearranged lung cancers.
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      Durable clinical response to entrectinib in NTRK1-rearranged non-small cell lung cancer.
      The path to approval of targeted therapy for lung cancers with alterations of the MET proto-oncogene receptor tyrosine kinase (MET), however, has not been straightforward. First discovered in the mid-1980s, the hepatocyte growth factor receptor (MET) pathway was found in the 1990s to be dysregulated in lung cancer (Fig. 1A).
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      Expression of c-met/HGF receptor in human non-small cell lung carcinomas in vitro and in vivo and its prognostic significance.
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      The clinical significance of hepatocyte growth factor for non-small cell lung cancer.
      More than 20 agents targeting MET or its ligand, hepatocyte growth factor (HGF), have undergone preclinical and clinical study, but findings have ranged from relatively high response rates in molecularly preselected subtypes of NSCLC in single-arm trials to the prominent failure of large phase III studies in different trial populations.
      Figure thumbnail gr1
      Figure 1(A) Time line of discovery in lung cancers harboring alterations of the hepatocyte growth factor receptor (MET) pathway. (B) The MET receptor and selected MET pathway–directed targeted therapies. MET, MET proto-oncogene receptor tyrosine kinase; HGF, hepatocyte growth factor; mAb, monoclonal antibody; PSI, plexin-semaphorin-integrin; IPT, immunoglobulin-plexin transcription; TKI, tyrosine kinase inhibitor.
      This review summarizes MET pathway dysregulation in lung cancers and critiques different scientific methods and clinical trial approaches taken for translating these into predictive biomarkers of benefit from MET inhibition.

      The MET Pathway and Targeted Therapy

      The MET gene, located on chromosome 7q21–q31, is approximately 125 kilobases long, with 21 exons.
      • Liu Y.
      The human hepatocyte growth factor receptor gene: complete structural organization and promoter characterization.
      • Skead G.
      • Govender D.
      Gene of the month: MET.
      The 150-kDa MET polypeptide undergoes glycosylation to a 190-kDa glycoprotein that functions as a transmembrane receptor tyrosine kinase.
      • Skead G.
      • Govender D.
      Gene of the month: MET.
      The extracellular region of MET contains semaphorin, cysteine-rich, and immunoglobulin domains; the intracellular region consists of a juxtamembrane domain, a tyrosine kinase catalytic domain, and a carboxy terminal docking site (Fig. 1B).
      • Organ S.L.
      • Tsao M.S.
      An overview of the c-MET signaling pathway.
      • Comoglio P.M.
      • Giordano S.
      • Trusolino L.
      Drug development of MET inhibitors: targeting oncogene addiction and expedience.
      MET is activated when the HGF ligand binds to the MET receptor, inducing homodimerization and phosphorylation of intracellular tyrosine residues.
      • Skead G.
      • Govender D.
      Gene of the month: MET.
      This activates the downstream RAS/ERK/MAPK, PI3K/AKT, Wnt/β-catenin, and STAT signaling pathways. Depending on the cellular context, these pathways can drive cell proliferation, survival, migration, motility, invasion, angiogenesis, and the epithelial-to-mesenchymal transition.
      • Organ S.L.
      • Tsao M.S.
      An overview of the c-MET signaling pathway.
      • Finocchiaro G.
      • Toschi L.
      • Gianoncelli L.
      • et al.
      Prognostic and predictive value of MET deregulation in non-small cell lung cancer.
      In embryonic development, MET and HGF are important in placental trophoblast and hepatocyte formation.
      • Cui J.J.
      Targeting receptor tyrosine kinase MET in cancer: small molecule inhibitors and clinical progress.
      In adults, both are broadly expressed in a variety of tissues and can be up-regulated in response to tissue injury.
      • Skead G.
      • Govender D.
      Gene of the month: MET.
      Dysregulation of the MET pathway in lung cancer occurs through a variety of mechanisms, including gene mutation, amplification, rearrangement, and protein overexpression. MET was first discovered as an oncogene with the identification of a translocated promoter region, nuclear basket protein gene (TPR)-MET fusion in a mutagenized osteosarcoma cell line. The fusion oncoprotein lacked the juxtamembrane Y1003 and was unaffected by c-Cbl recruitment and ubiquitination.
      • Peschard P.
      • Fournier T.M.
      • Lamorte L.
      • et al.
      Mutation of the c-Cbl TKB domain binding site on the Met receptor tyrosine kinase converts it into a transforming protein.
      A kinesin family member 5B gene (KIF5B)-MET fusion has since been detected by The Cancer Genome Atlas by RNA sequencing in a sample from a patient with lung adenocarcinoma
      • Stransky N.
      • Cerami E.
      • Schalm S.
      • et al.
      The landscape of kinase fusions in cancer.
      ; however, MET rearrangements are likely rare events in lung cancers.
      Several agents have been developed to target MET or HGF (see Fig. 1B). These are divided into small molecule inhibitors and monoclonal antibodies. The small molecule TKIs are further subdivided into multikinase and selective MET inhibitors. Examples of multikinase MET inhibitors include crizotinib, cabozantinib, MGCD265, AMG208, altiratinib, and golvatinib. Selective MET inhibitors include the adenosine triphosphate–competitive agents capmatinib and tepotinib (MSC2156119J)
      • Wu Y.-L.
      • Yang J.C.-H.
      • Kim D.-W.
      • et al.
      Safety and efficacy of INC280 in combination with gefitinib (gef) in patients with EGFR-mutated (mut), MET-positive NSCLC: a single-arm phase lb/ll study.
      • Bladt F.
      • Friese-Hamim M.
      • Ihling C.
      • et al.
      The c-Met inhibitor MSC2156119J effectively inhibits tumor growth in liver cancer models.
      and the adenosine triphosphate–noncompetitive agent tivantinib.
      • Yoshioka H.
      • Azuma K.
      • Yamamoto N.
      • et al.
      A randomized, double-blind, placebo-controlled, phase III trial of erlotinib with or without a c-Met inhibitor tivantinib (ARQ 197) in Asian patients with previously treated stage IIIB/IV nonsquamous nonsmall-cell lung cancer harboring wild-type epidermal growth factor receptor (ATTENTION study).
      Monoclonal antibody therapy is divided into anti-MET antibodies (e.g., onartuzumab and emibetuzumab [LY2875358])
      • Banck M.S.
      • Chugh R.
      • Natale R.B.
      • et al.
      Phase 1 results of emibetuzumab (LY2875358), a bivalent MET antibody, in patients with advanced castration-resistant prostate cancer, and MET positive renal cell carcinoma, non-small cell lung cancer, and hepatocellular carcinoma.
      • Spigel D.R.
      • Edelman M.J.
      • O'Byrne K.
      • et al.
      Onartuzumab plus erlotinib versus erlotinib in previously treated stage IIIb or IV NSCLC: results from the pivotal phase III randomized, multicenter, placebo-controlled METLung (OAM4971g) global trial.
      • Spigel D.R.
      • Ervin T.J.
      • Ramlau R.A.
      • et al.
      Randomized phase II trial of onartuzumab in combination with erlotinib in patients with advanced non-small-cell lung cancer.
      and anti-HGF antibodies (e.g., ficlatuzumab [AV-299] and rilotumumab [AMG 102]).
      • Comoglio P.M.
      • Giordano S.
      • Trusolino L.
      Drug development of MET inhibitors: targeting oncogene addiction and expedience.
      • Mok T.S.K.
      • Park K.
      • Geater S.L.
      • et al.
      A randomized phase (Ph) 2 study with exploratory biomarker analysis of ficlatuzumab (F) a humanized hepatocyte growth factor (HGF) inhibitory MAB in combination with gefitinib (G) versus G in Asian patients (pts) with lung adenocarcinoma (LA).
      In recognition of the diversity of putative alterations resulting in MET pathway activation in NSCLC, the challenge has been to determine the best way to distinguish a true sensitizing MET signature, either as a primary driver state or as a codriver state in the setting of acquired resistance to EGFR-directed therapy. For diagnostic purposes, this would involve selection from a combination of continuous and potentially overlapping MET-related biomarkers.

      MET as a Primary Driver in NSCLC

      By analogy with ALK rearrangements and EGFR mutations, it is conceivable that some NSCLCs may be primarily driven by, and therefore addicted to, the MET pathway alone. In the presence of an active MET inhibitor, precedent from other driver states suggests that monotherapy against MET should display clear evidence of anticancer activity. To date, two partially overlapping MET-related states in NSCLC have shown promise: MET exon 14 (METex14) alterations and MET gene amplification.

      METex14-Altered Lung Cancers

      Whereas tumors such as sporadic and hereditary renal cell carcinomas harbor activating mutations of the MET kinase domain,
      • Schmidt L.
      • Duh F.M.
      • Chen F.
      • et al.
      Germline and somatic mutations in the tyrosine kinase domain of the MET proto-oncogene in papillary renal carcinomas.
      lung cancers frequently harbor mutations in the extracellular/juxtamembrane domains.
      • Krishnaswamy S.
      • Kanteti R.
      • Duke-Cohan J.S.
      • et al.
      Ethnic differences and functional analysis of MET mutations in lung cancer.
      The extracellular semaphorin domain is thought to be required for receptor activation and dimerization
      • Kong-Beltran M.
      • Stamos J.
      • Wickramasinghe D.
      The Sema domain of Met is necessary for receptor dimerization and activation.
      ; however, the relevance of mutations in this domain remains unclear. In contrast, juxtamembrane domain mutations often result in METex14 alterations.
      Cancers with METex14 alterations, a prime example of the association between aberrant splicing and oncogenesis, were initially reported in SCLC and NSCLC in 2003 and 2005, respectively.
      • Ma P.C.
      • Kijima T.
      • Maulik G.
      • et al.
      c-MET mutational analysis in small cell lung cancer: novel juxtamembrane domain mutations regulating cytoskeletal functions.
      • Ma P.C.
      • Jagadeeswaran R.
      • Jagadeesh S.
      • et al.
      Functional expression and mutations of c-Met and its therapeutic inhibition with SU11274 and small interfering RNA in non-small cell lung cancer.
      Normally, introns flanking METex14 in pre-mRNA are spliced out, resulting in mRNA containing METex14 that is translated into a functional MET receptor (Fig. 2A). METex14 encodes part of the juxtamembrane domain containing Y1003, the c-Cbl E3 ubiquitin ligase binding site.
      • Onozato R.
      • Kosaka T.
      • Kuwano H.
      • et al.
      Activation of MET by gene amplification or by splice mutations deleting the juxtamembrane domain in primary resected lung cancers.
      Ubiquitination tags the MET receptor for degradation. Juxtamembrane domain mutations that disrupt splice sites flanking METex14 result in aberrant splicing (Fig. 2B). These mutations result in METex14 skipping, producing a truncated MET receptor lacking the Y1003 c-Cbl binding site. Losing this binding site results in decreased ubiquitination and degradation of the MET protein, sustained MET activation, and oncogenesis.
      • Kong-Beltran M.
      • Seshagiri S.
      • Zha J.
      • et al.
      Somatic mutations lead to an oncogenic deletion of met in lung cancer.
      Decreased degradation of the MET receptor is thought to potentially cause MET overexpression on some tumors that is detectable by methods such as immunohistochemistry (IHC).
      Figure thumbnail gr2
      Figure 2The pathobiology of MET proto-oncogene receptor tyrosine kinase (MET) exon 14 alterations and MET amplification. MET, hepatocyte growth factor receptor; CEP7, centromeric portion of chromosome 7.
      METex14 alterations are extremely diverse. Base substitutions or indels disrupt several gene positions important for splicing out introns flanking METex14,
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      including the branch point, the polypyrimidine tract, the 3′ splice site of intron 13, and the 5′ splice site of intron 14.
      • Onozato R.
      • Kosaka T.
      • Kuwano H.
      • et al.
      Activation of MET by gene amplification or by splice mutations deleting the juxtamembrane domain in primary resected lung cancers.
      • Kong-Beltran M.
      • Seshagiri S.
      • Zha J.
      • et al.
      Somatic mutations lead to an oncogenic deletion of met in lung cancer.
      • Awad M.M.
      • Oxnard G.R.
      • Jackman D.M.
      • et al.
      MET exon 14 mutations in non-small-cell lung cancer are associated with advanced age and stage-dependent MET genomic amplification and c-Met overexpression.
      The Cancer Genome Atlas project identified METex14 alterations resulting in incomplete splicing from the mature mRNA, leaving low-level expression of untruncated MET.
      The Cancer Genome Atlas Research Network
      Comprehensive molecular profiling of lung adenocarcinoma.
      Notably, point mutations or deletions within METex14 can affect the Y1003 residue, resulting in c-Cbl binding site loss of function without necessarily causing METex14 skipping.
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      • Awad M.M.
      • Oxnard G.R.
      • Jackman D.M.
      • et al.
      MET exon 14 mutations in non-small-cell lung cancer are associated with advanced age and stage-dependent MET genomic amplification and c-Met overexpression.
      The Cancer Genome Atlas Research Network
      Comprehensive molecular profiling of lung adenocarcinoma.
      The diversity of METex14 alterations presents challenges for diagnostic testing.
      • Cui J.J.
      Targeting receptor tyrosine kinase MET in cancer: small molecule inhibitors and clinical progress.
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      Algorithms for molecular profiling will need to rapidly move toward comprehensive clinical sequencing platforms permitting routine detection of these mutations.
      • Paik P.K.
      • Drilon A.
      • Fan P.D.
      • et al.
      Response to MET inhibitors in patients with stage IV lung adenocarcinomas harboring MET mutations causing exon 14 skipping.
      Currently, DNA-based broad, hybrid capture next-generation sequencing (NGS) represents the most frequently used tool. RNA-based sequencing using anchored multiplex polymerase chain reaction
      • Zheng Z.
      • Liebers M.
      • Zhelyazkova B.
      • et al.
      Anchored multiplex PCR for targeted next-generation sequencing.
      or NanoString technology (NanoString Technologies, Inc., Seattle, WA) provide complementary tools.
      • Paik P.K.
      • Drilon A.
      • Fan P.D.
      • et al.
      Response to MET inhibitors in patients with stage IV lung adenocarcinomas harboring MET mutations causing exon 14 skipping.
      It should be noted that NGS is a platform, not a standardized test, and detection of specific genomic alterations crucially depends on the primers within the NGS panel. It cannot be assumed that the wide array of METex14 variants will be equally detected (or detected at all) by every NGS panel used in clinical practice. Similarly, RNA-based testing, although a means of getting around the underlying variety of DNA-based changes by focusing on the more uniform resultant RNA-related splice-altered message, is not routinely performed in the clinic. Furthermore, the amount of tissue available after DNA-based NGS can be scant and inadequate for further RNA-based testing. Future diagnostic investigation must explore tests that will detect these changes in a manner suitable for widespread clinical use.
      Lung cancers harboring METex14 alterations have been found to overexpress MET by means of IHC (3+ in 100% of cells in select cases).
      • Paik P.K.
      • Drilon A.
      • Fan P.D.
      • et al.
      Response to MET inhibitors in patients with stage IV lung adenocarcinomas harboring MET mutations causing exon 14 skipping.
      MET overexpression is not found in all cases documented in the literature. In one series, stage IV METex14-altered lung cancers were more likely to display strong MET IHC expression compared with stage IA–IIIB METex14-altered lung cancers.
      • Awad M.M.
      • Oxnard G.R.
      • Jackman D.M.
      • et al.
      MET exon 14 mutations in non-small-cell lung cancer are associated with advanced age and stage-dependent MET genomic amplification and c-Met overexpression.
      Rapid initial IHC screening has been proposed to narrow the population to undergo more comprehensive molecular profiling. To estimate the validity of this approach, better data on the prevalence of MET IHC 3+ cases that contain METex14 variants are required.
      • Tong J.H.
      • Yeung S.F.
      • Chan A.W.
      • et al.
      MET amplification and exon 14 splice site mutation define unique molecular subgroups of non-small cell lung carcinoma with poor prognosis.
      METex14 alterations are detected in 3% to 4% of lung adenocarcinoma samples (Table 1),
      • Onozato R.
      • Kosaka T.
      • Kuwano H.
      • et al.
      Activation of MET by gene amplification or by splice mutations deleting the juxtamembrane domain in primary resected lung cancers.
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      The Cancer Genome Atlas Research Network
      Comprehensive molecular profiling of lung adenocarcinoma.
      • Tong J.H.
      • Yeung S.F.
      • Chan A.W.
      • et al.
      MET amplification and exon 14 splice site mutation define unique molecular subgroups of non-small cell lung carcinoma with poor prognosis.
      • Okuda K.
      • Sasaki H.
      • Yukiue H.
      • et al.
      Met gene copy number predicts the prognosis for completely resected non-small cell lung cancer.
      • Cappuzzo F.
      • Marchetti A.
      • Skokan M.
      • et al.
      Increased MET gene copy number negatively affects survival of surgically resected non-small-cell lung cancer patients.
      a prevalence comparable to that in ALK-rearranged lung cancers.
      • Shaw A.T.
      • Engelman J.A.
      ALK in lung cancer: past, present, and future.
      These mutations occur in tumors from older patients, with a lower percentage of never-smokers than in the case of patients with tumors harboring other oncogenes.
      • Awad M.M.
      • Oxnard G.R.
      • Jackman D.M.
      • et al.
      MET exon 14 mutations in non-small-cell lung cancer are associated with advanced age and stage-dependent MET genomic amplification and c-Met overexpression.
      In a series of 687 Asian patients with resected NSCLC, METex14 alterations were poor prognostic factors for overall survival (OS).
      • Tong J.H.
      • Yeung S.F.
      • Chan A.W.
      • et al.
      MET amplification and exon 14 splice site mutation define unique molecular subgroups of non-small cell lung carcinoma with poor prognosis.
      Table 1Prevalence of MET Exon 14 Alterations and MET Amplification in NSCLC Using Different Testing Methods
      StudyGenomic AlterationDiagnostic MethodPrevalence
      METex14 alterations
       The Cancer Genome Atlas. 2014
      The Cancer Genome Atlas Research Network
      Comprehensive molecular profiling of lung adenocarcinoma.
      Exon 14 alterationsWES4.3% (10 of 230)
       Frampton et al. 2015
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      Exon 14 alterationsParallel DNA sequencing3% (131 of 4402)
       Okuda et al. 2008
      • Okuda K.
      • Sasaki H.
      • Yukiue H.
      • et al.
      Met gene copy number predicts the prognosis for completely resected non-small cell lung cancer.
      Exon 14 alterationsDirect sequencing1.7% (3 of 178)
       Onozato et al. 2009
      • Onozato R.
      • Kosaka T.
      • Kuwano H.
      • et al.
      Activation of MET by gene amplification or by splice mutations deleting the juxtamembrane domain in primary resected lung cancers.
      Exon 14 alterationsDirect sequencing3.3% (7 of 211)
       Tong et al. 2016
      • Tong J.H.
      • Yeung S.F.
      • Chan A.W.
      • et al.
      MET amplification and exon 14 splice site mutation define unique molecular subgroups of non-small cell lung carcinoma with poor prognosis.
      Exon 14 alterationsDirect sequencing2.6% (10 of 392)
      MET amplification
       Cancer Genome Atlas. 2014
      The Cancer Genome Atlas Research Network
      Comprehensive molecular profiling of lung adenocarcinoma.
      Somatic copy numberWES5.2% (12 of 230)
       Capuzzo et al. 2009
      • Cappuzzo F.
      • Marchetti A.
      • Skokan M.
      • et al.
      Increased MET gene copy number negatively affects survival of surgically resected non-small-cell lung cancer patients.
      MET copy number ≥5 (polysomy + gene amplification)FISH11.1% (48 of 435)
      MET copy number ≥5 (gene amplification only)4.1% (18 of 435)
       Okuda et al. 2008
      • Okuda K.
      • Sasaki H.
      • Yukiue H.
      • et al.
      Met gene copy number predicts the prognosis for completely resected non-small cell lung cancer.
      MET copy-number >3qRT-PCR5.6% (12 of 213)
       Tong et al. 2016
      • Tong J.H.
      • Yeung S.F.
      • Chan A.W.
      • et al.
      MET amplification and exon 14 splice site mutation define unique molecular subgroups of non-small cell lung carcinoma with poor prognosis.
      MET/CEP7 ratio ≥5FISH1.0% (4 of 392)
       Onozato et al. 2009
      • Onozato R.
      • Kosaka T.
      • Kuwano H.
      • et al.
      Activation of MET by gene amplification or by splice mutations deleting the juxtamembrane domain in primary resected lung cancers.
      MET amplification

      MET splice mutations
      qRT-PCR

      Direct sequencing
      1.4% (2 of 148)

      3.3% (7 of 211)
      MET, MET proto-oncogene receptor tyrosine kinase; METex14, MET exon 14; WES, whole-exome sequencing; FISH, fluorescence in situ hybridization; qRT-PCR, quantitative real-time polymerase chain reaction.
      METex14 alterations are mutually exclusive with other lung cancer drivers, suggesting they represent a true oncogenic driver state.
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      In a study of 933 patients with nonsquamous NSCLC,
      • Awad M.M.
      • Oxnard G.R.
      • Jackman D.M.
      • et al.
      MET exon 14 mutations in non-small-cell lung cancer are associated with advanced age and stage-dependent MET genomic amplification and c-Met overexpression.
      no patients with METex14 alterations had activating mutations in KRAS, EGFR, or erb-b2 receptor tyrosine kinase 2 gene (ERBB2), or rearrangements involving ALK, ROS1, or ret proto-oncogene (RET).
      • Awad M.M.
      • Oxnard G.R.
      • Jackman D.M.
      • et al.
      MET exon 14 mutations in non-small-cell lung cancer are associated with advanced age and stage-dependent MET genomic amplification and c-Met overexpression.
      In contrast, METex14 alterations can overlap with other alterations such as MET and MDM2 proto-oncogene (MDM2) amplification. METex14 alterations can co-occur with MET copy number gain/amplification, with the frequency of overlap being heavily influenced by the definition of amplification used.
      • Tong J.H.
      • Yeung S.F.
      • Chan A.W.
      • et al.
      MET amplification and exon 14 splice site mutation define unique molecular subgroups of non-small cell lung carcinoma with poor prognosis.
      Although many cases of METex14 alterations are found in lung adenocarcinomas, these events have a much higher incidence in pulmonary sarcomatoid carcinomas. Approximately 20% to 30% of sarcomatoid carcinomas harbor METex14 alterations.
      • Tong J.H.
      • Yeung S.F.
      • Chan A.W.
      • et al.
      MET amplification and exon 14 splice site mutation define unique molecular subgroups of non-small cell lung carcinoma with poor prognosis.
      • Liu X.
      • Jia Y.
      • Stoopler M.B.
      • et al.
      Next-generation sequencing of pulmonary sarcomatoid carcinoma reveals high frequency of actionable MET gene mutations.
      In one series, these were more likely to be associated with sarcomatoid carcinomas with an adenocarcinoma component,
      • Liu X.
      • Jia Y.
      • Stoopler M.B.
      • et al.
      Next-generation sequencing of pulmonary sarcomatoid carcinoma reveals high frequency of actionable MET gene mutations.
      suggesting the possibility of a shared tumor origin. The therapeutic implications of METex14 alterations in sarcomatoid carcinomas are discussed later.
      METex14 alterations are likely to be highly predictive of response to MET inhibition (Table 2).
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      • Awad M.M.
      • Oxnard G.R.
      • Jackman D.M.
      • et al.
      MET exon 14 mutations in non-small-cell lung cancer are associated with advanced age and stage-dependent MET genomic amplification and c-Met overexpression.
      • Paik P.K.
      • Drilon A.
      • Fan P.D.
      • et al.
      Response to MET inhibitors in patients with stage IV lung adenocarcinomas harboring MET mutations causing exon 14 skipping.
      • Liu X.
      • Jia Y.
      • Stoopler M.B.
      • et al.
      Next-generation sequencing of pulmonary sarcomatoid carcinoma reveals high frequency of actionable MET gene mutations.
      • Jenkins R.W.
      • Oxnard G.R.
      • Elkin S.
      • et al.
      Response to crizotinib in a patient with lung adenocarcinoma harboring a MET splice site mutation.
      • Jorge S.E.
      • Schulman S.
      • Freed J.A.
      • et al.
      Responses to the multitargeted MET/ALK/ROS1 inhibitor crizotinib and co-occurring mutations in lung adenocarcinomas with MET amplification or MET exon 14 skipping mutation.
      • Lee C.
      • Usenko D.
      • Frampton G.M.
      • et al.
      MET 14 Deletion in sarcomatoid non-small-cell lung cancer detected by next-generation sequencing and successfully treated with a MET inhibitor.
      • Mahjoubi L.
      • Gazzah A.
      • Besse B.
      • et al.
      A never-smoker lung adenocarcinoma patient with a MET exon 14 mutation (D1028N) and a rapid partial response after crizotinib.
      • Mendenhall M.A.
      • Goldman J.W.
      MET-mutated NSCLC with major response to crizotinib.
      • Waqar S.N.
      • Morgensztern D.
      • Sehn J.
      MET mutation associated with responsiveness to crizotinib.
      Dramatic and durable partial responses (PRs) to crizotinib were first reported in mid-2015 in patients with advanced lung cancers with METex14 alterations.
      • Paik P.K.
      • Drilon A.
      • Fan P.D.
      • et al.
      Response to MET inhibitors in patients with stage IV lung adenocarcinomas harboring MET mutations causing exon 14 skipping.
      The same authors reported a complete metabolic response (according to the Positron Emission Tomography Response Criteria in Solid Tumors) to cabozantinib therapy (stable disease by the Response Evaluation Criteria in Solid Tumors). Durable PRs to capmatinib or crizotinib have been reported in patients with advanced METex14-altered lung cancers.
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      Subsequent case reports have confirmed these observations with use of different MET TKIs and in all NSCLC histologic types.
      • Awad M.M.
      • Oxnard G.R.
      • Jackman D.M.
      • et al.
      MET exon 14 mutations in non-small-cell lung cancer are associated with advanced age and stage-dependent MET genomic amplification and c-Met overexpression.
      • Jenkins R.W.
      • Oxnard G.R.
      • Elkin S.
      • et al.
      Response to crizotinib in a patient with lung adenocarcinoma harboring a MET splice site mutation.
      • Jorge S.E.
      • Schulman S.
      • Freed J.A.
      • et al.
      Responses to the multitargeted MET/ALK/ROS1 inhibitor crizotinib and co-occurring mutations in lung adenocarcinomas with MET amplification or MET exon 14 skipping mutation.
      • Lee C.
      • Usenko D.
      • Frampton G.M.
      • et al.
      MET 14 Deletion in sarcomatoid non-small-cell lung cancer detected by next-generation sequencing and successfully treated with a MET inhibitor.
      • Mahjoubi L.
      • Gazzah A.
      • Besse B.
      • et al.
      A never-smoker lung adenocarcinoma patient with a MET exon 14 mutation (D1028N) and a rapid partial response after crizotinib.
      Table 2Case Reports of NSCLCs with MET Exon 14 Alterations Responding to MET Inhibitors
      ReferenceAge/SexSmoking HistoryMETex14 AlterationMET IHCMET AmplificationAgentBest Response
      Awad et al. 2016
      • Awad M.M.
      • Oxnard G.R.
      • Jackman D.M.
      • et al.
      MET exon 14 mutations in non-small-cell lung cancer are associated with advanced age and stage-dependent MET genomic amplification and c-Met overexpression.
      64 FNeverSplice donor mutationNAYesCrizotinibPR
      Frampton et al. 2015
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      82 FFormerSplice donor mutation3+YesCapmatinibPR
      Frampton et al. 2015
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      66 FFormerSplice donor mutation3+Not testedCapmatinibPR
      Jenkins et al. 2015
      • Jenkins R.W.
      • Oxnard G.R.
      • Elkin S.
      • et al.
      Response to crizotinib in a patient with lung adenocarcinoma harboring a MET splice site mutation.
      86 MNeverSplice acceptor deletion2+NACrizotinibPR
      Jorge et al. 2015
      • Jorge S.E.
      • Schulman S.
      • Freed J.A.
      • et al.
      Responses to the multitargeted MET/ALK/ROS1 inhibitor crizotinib and co-occurring mutations in lung adenocarcinomas with MET amplification or MET exon 14 skipping mutation.
      68 FFormerSplice donor mutationNANACrizotinibPR
      Lee et al. 2015
      • Lee C.
      • Usenko D.
      • Frampton G.M.
      • et al.
      MET 14 Deletion in sarcomatoid non-small-cell lung cancer detected by next-generation sequencing and successfully treated with a MET inhibitor.
      61 MNeverSplice donor deletionNANACrizotinibPR
      Liu et al. 2015
      • Liu X.
      • Jia Y.
      • Stoopler M.B.
      • et al.
      Next-generation sequencing of pulmonary sarcomatoid carcinoma reveals high frequency of actionable MET gene mutations.
      74 FFormerSplice site mutationNANACrizotinibPR
      Mahjoubi et al. 2016
      • Mahjoubi L.
      • Gazzah A.
      • Besse B.
      • et al.
      A never-smoker lung adenocarcinoma patient with a MET exon 14 mutation (D1028N) and a rapid partial response after crizotinib.
      67 FNeverSplice donor mutationNANACrizotinibPR
      Mendenhall et al. 2015
      • Mendenhall M.A.
      • Goldman J.W.
      MET-mutated NSCLC with major response to crizotinib.
      76 FFormerSplice donor mutationNANACrizotinibPR
      Paik et al. 2015
      • Paik P.K.
      • Drilon A.
      • Fan P.D.
      • et al.
      Response to MET inhibitors in patients with stage IV lung adenocarcinomas harboring MET mutations causing exon 14 skipping.
      65 MFormerSplice donor mutationNANACrizotinibPR
      Paik et al. 2015
      • Paik P.K.
      • Drilon A.
      • Fan P.D.
      • et al.
      Response to MET inhibitors in patients with stage IV lung adenocarcinomas harboring MET mutations causing exon 14 skipping.
      78 MFormerSplice donor deletion3+NACrizotinibPR (lung)

      PD (liver)
      Paik et al. 2015
      • Paik P.K.
      • Drilon A.
      • Fan P.D.
      • et al.
      Response to MET inhibitors in patients with stage IV lung adenocarcinomas harboring MET mutations causing exon 14 skipping.
      80 FNeverSplice donor mutation3+YesCabozantinibCR (PERCIST)
      Paik et al. 2015
      • Paik P.K.
      • Drilon A.
      • Fan P.D.
      • et al.
      Response to MET inhibitors in patients with stage IV lung adenocarcinomas harboring MET mutations causing exon 14 skipping.
      90 FNeverSplice donor mutationNANACrizotinibPR
      Waqar et al. 2015
      • Waqar S.N.
      • Morgensztern D.
      • Sehn J.
      MET mutation associated with responsiveness to crizotinib.
      71 MFormerSplice donor mutationNANoCrizotinibPR
      MET, MET proto-oncogene receptor tyrosine kinase; METex14, MET exon 14; MET, mesenchymal epithelial transition receptor; IHC, immunohistochemistry; F, female; NA, not applicable/available; PR, partial response; M, male; PD, progressive disease; CR, complete response; PERCIST, Positron Emission Tomography Response Criteria in Solid Tumors.
      Pulmonary sarcomatoid carcinomas were thought to be relatively refractory to cytotoxic chemotherapies; however, a dramatic PR was reported in a patient with advanced pulmonary sarcomatoid carcinoma harboring both a METex14 alteration and MET amplification. No responses to an anti-MET or anti-HGF monoclonal antibody in a patient with lung cancer with a METex14 alteration have been reported, although such a response is not unlikely given our knowledge of these tumors’ biology, coupled with preclinical data supporting the use of these agents.
      • Kong-Beltran M.
      • Seshagiri S.
      • Zha J.
      • et al.
      Somatic mutations lead to an oncogenic deletion of met in lung cancer.
      Reports of response to MET inhibitors have prompted drug development plans focused on molecular enrichment for METex14 alterations. The phase I trial that resulted in approval of crizotinib for ALK- and ROS1-rearranged lung cancers (NCT00585195) is currently treating patients with advanced lung cancer with METex14 alterations in an enriched cohort.
      • Lee C.
      • Usenko D.
      • Frampton G.M.
      • et al.
      MET 14 Deletion in sarcomatoid non-small-cell lung cancer detected by next-generation sequencing and successfully treated with a MET inhibitor.
      Of the 18 response-evaluable patients at the latest available data cutoff, eight experienced a confirmed PR (overall response rate 44% [95% confidence interval: 22–69]) with tumor shrinkage in 14 of 18 patients.
      • Drilon A.
      • Camidge D.R.
      • Ou S.H.
      • et al.
      Efficacy and safety of crizotinib in patients (pts) with advanced MET exon 14-altered non-small cell lung cancer (NSCLC).
      We look forward to studies of potential mechanisms of acquired resistance to MET TKIs, but already MET D1228N has been reported as a putative mechanism.
      • Heist R.S.
      • Sequist L.V.
      • Borger D.
      • et al.
      Acquired resistance to crizotinib in NSCLC with MET exon 14 skipping.

      MET-Amplified Lung Cancers

      MET amplification is thought to dysregulate MET pathway signaling through protein overexpression and constitutive kinase activation. Identification of MET copy number gains in the setting of acquired resistance to EGFR TKI therapy in lung cancer stimulated interest in these alterations.
      MET copy number gains arise from two distinct processes: polysomy and amplification.
      • Kawakami H.
      • Okamoto I.
      • Okamoto W.
      • et al.
      Targeting MET amplification as a new oncogenic driver.
      High polysomy occurs when there are multiple copies of chromosome 7 in tumor cells secondary to factors such as chromosomal duplication.
      • Albertson D.G.
      • Collins C.
      • McCormick F.
      • et al.
      Chromosome aberrations in solid tumors.
      True amplification occurs in the setting of focal or regional gene duplication through processes such as breakage-fusion-bridge mechanisms.
      • Hellman A.
      • Zlotorynski E.
      • Scherer S.W.
      • et al.
      A role for common fragile site induction in amplification of human oncogenes.
      As opposed to polysomy, amplification is thought to represent a state of true biologic selection for MET activation as an oncogenic driver. Additionally, each type of MET gene copy number change represents a continuous variable. Placing a cutpoint to define positivity may dramatically alter the reported frequency, overlap with other NSCLC subtypes, and ultimately affect its potential to act as a predictive biomarker for benefit from MET inhibition.
      With the use of fluorescence in situ hybridization (FISH), the ratio of MET to the centromeric portion of chromosome 7 (CEP7) can be used to distinguish between polysomy and true amplification. In polysomy, each copy of MET is associated with a corresponding centromere, preserving the MET/CEP7 ratio as copy number increases.
      • Kawakami H.
      • Okamoto I.
      • Okamoto W.
      • et al.
      Targeting MET amplification as a new oncogenic driver.
      In true MET amplification, copy number increases without an increase in CEP7 and the MET/CEP7 ratio increases.
      • Kawakami H.
      • Okamoto I.
      • Okamoto W.
      • et al.
      Targeting MET amplification as a new oncogenic driver.
      Broad, hybrid capture NGS assays are able to detect amplification events. Copy number changes can be identified by comparing sequence coverage of targeted regions in tumors relative to a diploid normal sample, and select platforms have been validated against tumor samples that previously tested positive for amplification of other genes such as ERBB2 by FISH.
      • Cheng D.T.
      • Mitchell T.
      • Zehir A.
      • et al.
      MSK-IMPACT: a hybridization capture-based next-generation sequencing clinical assay for solid tumor molecular oncology.
      • Frampton G.M.
      • Fichtenholtz A.
      • Otto G.A.
      • et al.
      Development and validation of a clinical cancer genomic profiling test based on massively parallel DNA sequencing.
      As with FISH, copy number gains detected by NGS are reported as continuous variables, and cutoffs can vary significantly between assays. In contrast to FISH, NGS and anchored multiplex polymerase chain reaction may provide additional information on other potentially clinically relevant, concurrent genomic alterations.
      • Zheng Z.
      • Liebers M.
      • Zhelyazkova B.
      • et al.
      Anchored multiplex PCR for targeted next-generation sequencing.
      No consensus on the definition of MET positivity based on gene copy number has yet been reached. Examples of a positive MET FISH result include five or more MET signals per cell (Cappuzzo scoring system),
      • Cappuzzo F.
      • Marchetti A.
      • Skokan M.
      • et al.
      Increased MET gene copy number negatively affects survival of surgically resected non-small-cell lung cancer patients.
      and a MET/CEP7 ratio of 2 or higher (PathVysion kit [Abbott Molecular, Des Plaines, IL]).
      • Tong J.H.
      • Yeung S.F.
      • Chan A.W.
      • et al.
      MET amplification and exon 14 splice site mutation define unique molecular subgroups of non-small cell lung carcinoma with poor prognosis.
      • Tanaka A.
      • Sueoka-Aragane N.
      • Nakamura T.
      • et al.
      Co-existence of positive MET FISH status with EGFR mutations signifies poor prognosis in lung adenocarcinoma patients.
      MET amplification has also been classified by using the MET/CEP7 ratio as low (≥1.8 to ≤2.2), intermediate (>2.2 to <5), and high (≥5), as summarized in Table 3. Variation of classification thresholds between studies complicates comparisons of reported MET amplification/copy number gain relative to the underlying frequency, associated factors, and outcomes from therapy, although more rigorous data are now emerging.
      • Finocchiaro G.
      • Toschi L.
      • Gianoncelli L.
      • et al.
      Prognostic and predictive value of MET deregulation in non-small cell lung cancer.
      Table 3MET/CEP7 Ratio and Classification of MET Amplification
      MET/CEP7 RatioMET Amplification ClassificationPercentage of Total
      <1.8Negative92.6
      ≥1.8 to ≤2.2Low3.6
      >2.2 to <5.0Intermediate3.0
      ≥5.0High0.8
      Total100.0
      Note: Data from a personal communication from L. Garcia (University of Colorado).
      MET, MET proto-oncogene receptor tyrosine kinase; CEP7, centromeric portion of chromosome 7.
      The reported prevalence of de novo MET amplification in NSCLC ranges from 1% to 5%, depending on the level of preselection, the assay, and the positivity cutpoint used (see Table 1).
      • Onozato R.
      • Kosaka T.
      • Kuwano H.
      • et al.
      Activation of MET by gene amplification or by splice mutations deleting the juxtamembrane domain in primary resected lung cancers.
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      • Okuda K.
      • Sasaki H.
      • Yukiue H.
      • et al.
      Met gene copy number predicts the prognosis for completely resected non-small cell lung cancer.
      • Cappuzzo F.
      • Marchetti A.
      • Skokan M.
      • et al.
      Increased MET gene copy number negatively affects survival of surgically resected non-small-cell lung cancer patients.
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Comprehensive genomic profiling (CGP) of advanced cancers to identify MET exon 14 alterations that confer sensitivity to MET inhibitors.
      In adenocarcinoma, because most true oncogenic drivers are mutually exclusive, so-called oncogene overlap analysis was used in 1164 cases to see whether there was a level of MET copy number gain by using either the mean number of copies of MET per cell (which would include high-polysomy cases) or the MET/CEP7 ratio, which could define a group in which the degree of overlap with other known oncogenic drivers (EGFR, KRAS, ALK, ERBB2, BRAF, NRAS, ROS1, or RET) disappeared.
      • Noonan S.A.
      • Berry L.
      • Lu X.
      • et al.
      Identifying the appropriate FISH criteria for defining MET copy number driven lung adenocarcinoma through oncogene overlap analysis.
      Across all levels of mean increase in MET per cell (low, ≥5 to <6; intermediate, ≥6 to <7; and high, ≥7) oncogene overlap occurred in 41% to 63% of cases. Similarly, when the MET/CEP7 ratio was used, at low (≥1.8 to ≤2.2) and intermediate (>2.2 to <5) levels of MET amplification, oncogene overlap occurred in 52% and 50% of cases, respectively. However, zero oncogene overlap was seen in the high-MET amplification category (MET/CEP7 ratio ≥5). Only this high-level amplification category was associated with a dramatic rate of response to crizotinib. These data suggest that high MET copy number (MET/CEP7 ratio ≥5) represents the best case for a true MET copy number gain–dependent MET-driven state, whereas lower or different MET copy number definitions of positivity may more likely represent MET as a coincident event.
      • Noonan S.A.
      • Berry L.
      • Lu X.
      • et al.
      Identifying the appropriate FISH criteria for defining MET copy number driven lung adenocarcinoma through oncogene overlap analysis.
      There are two important issues related to exploring MET amplification as a predictive biomarker for benefit from MET inhibition. The first is that a MET/CEP7 ratio of 5 or higher represented only 0.34% of adenocarcinomas in a large series,
      • Noonan S.A.
      • Berry L.
      • Lu X.
      • et al.
      Identifying the appropriate FISH criteria for defining MET copy number driven lung adenocarcinoma through oncogene overlap analysis.
      which is approximately 10% of the frequency of METex14 variants in the same population. The second is that the degree of benefit in this population independent of METex14 mutations remains under investigation. METex14 alterations harbor concurrent high-level MET copy number gain in approximately 20% of cases, with the degree of overlap increasing (just as with other known oncogenes) as less stringent definitions of MET amplification are used.
      • Awad M.M.
      • Oxnard G.R.
      • Jackman D.M.
      • et al.
      MET exon 14 mutations in non-small-cell lung cancer are associated with advanced age and stage-dependent MET genomic amplification and c-Met overexpression.
      • Paik P.K.
      • Drilon A.
      • Fan P.D.
      • et al.
      Response to MET inhibitors in patients with stage IV lung adenocarcinomas harboring MET mutations causing exon 14 skipping.
      • Tong J.H.
      • Yeung S.F.
      • Chan A.W.
      • et al.
      MET amplification and exon 14 splice site mutation define unique molecular subgroups of non-small cell lung carcinoma with poor prognosis.
      The case for METex14 variants to act as predictive biomarkers in the absence of MET amplification seems to have been made, as responses in this setting have been documented. Whether MET amplification is only a surrogate for some cases of METex14 (in which case testing should focus exclusively on the METex14 approach) or can truly function as an independent MET-addicted state capable of driving clinical responses without METex14 changes (requiring an all-inclusive testing approach for actionable abnormalities in lung cancer in addition to METex14 testing) is undetermined. Therefore, testing for both MET amplification and METex14 changes should be conducted in all MET TKI trials and then used to retrospectively investigate differential responses based on MET amplification status. As both MDM2 and cyclin-dependent kinase 4 gene (CDK4) amplification are strongly coincident with METex14 alterations,
      • Frampton G.M.
      • Ali S.M.
      • Rosenzweig M.
      • et al.
      Activation of MET via diverse exon 14 splicing alterations occurs in multiple tumor types and confers clinical sensitivity to MET inhibitors.
      a similar approach could be taken to investigate MET TKI response with concurrent MDM2 and CDK4 amplification.
      The first report of a response to MET inhibition in a patient with a de novo MET-amplified lung cancer was published in 2011. The patient was a 77-year-old woman with a 45 pack-year history of smoking and advanced lung adenocarcinoma. Her cancer had high-level MET amplification according to FISH (MET/CEP7 ratio >5). She was treated on the phase I trial of crizotinib (NCT00585195) and achieved a dramatic and durable PR.
      • Ou S.H.
      • Kwak E.L.
      • Siwak-Tapp C.
      • et al.
      Activity of crizotinib (PF02341066), a dual mesenchymal-epithelial transition (MET) and anaplastic lymphoma kinase (ALK) inhibitor, in a non-small cell lung cancer patient with de novo MET amplification.
      Preliminary results were presented in 2014 and showed PRs in one of six patients (16.7%) with intermediate-level MET amplification (MET/CEP7 ratio >2.2 to <5) and in three of six patients (50%) with high-level MET amplification (MET/CEP7 ratio ≥5).
      • Camidge D.R.
      • Ou S.H.
      • Shapiro G.
      • et al.
      Efficacy and safety of crizotinib in patients with advanced c-MET-amplified non-small cell lung cancer (NSCLC).
      Responses were not seen in patients with low-level MET amplification (MET/CEP7 ratio ≥1.8 to ≤2.2).

      MET as a Codriver in NSCLC

      There is significant cross-talk between the MET pathway and other signaling pathways. Historically, many investigators have chosen to explore combination MET inhibitor and EGFR inhibitor therapy in clinical trials on patients with NSCLCs (Table 4). This strategy was partially based on the synergy of MET and EGFR in driving oncogenesis in both EGFR wild-type lung and mutant lung cancer models in the setting of acquired resistance to EGFR TKIs.
      • Puri N.
      • Salgia R.
      Synergism of EGFR and c-Met pathways, cross-talk and inhibition, in non-small cell lung cancer.
      • Matsubara D.
      • Ishikawa S.
      • Sachiko O.
      • et al.
      Co-activation of epidermal growth factor receptor and c-MET defines a distinct subset of lung adenocarcinomas.
      In 2007, MET amplification was found to be associated with acquired resistance to first-generation EGFR TKIs.
      • Engelman J.A.
      • Zejnullahu K.
      • Mitsudomi T.
      • et al.
      MET amplification leads to gefitinib resistance in lung cancer by activating ERBB3 signaling.
      Although most EGFR-mutant lung cancers develop resistance to EGFR TKI therapy through acquired T790M mutation, activation of the MET pathway as a bypass tract represents a distinct acquired resistance mechanism driven by ERBB3-dependent phosphoinositide-3 kinase pathway activation. MET exon 14 alterations are generally thought to be mutually exclusive with other major lung cancer drivers and have not been associated with acquired resistance to EGFR TKI therapy in EGFR-mutant lung cancers.
      • Onozato R.
      • Kosaka T.
      • Kuwano H.
      • et al.
      Activation of MET by gene amplification or by splice mutations deleting the juxtamembrane domain in primary resected lung cancers.
      Table 4Clinical Experience with Select MET- and HGF-Directed Targeted Therapies
      AgentTarget(s)PatientsPhaseResults
      Multikinase MET TKIs
      CrizotinibMET, ALK, ROS1Crizotinib monotherapy

      Patients with MET exon 14-altered and MET-amplified NSCLC
      I/IIMET exon 14–altered NSCLC: responses observed in 8 of 18 patients (44%); MET-amplified NSCLC: at data cutoff, partial responses were observed in 1 of 6 patients (16.7%) with a MET/CEP7 ratio of >2.2 to <5 and in 3 of 6 patients (50%) with a MET/CEP7 ratio of ≥5
      • Camidge D.R.
      • Ou S.H.
      • Shapiro G.
      • et al.
      Efficacy and safety of crizotinib in patients with advanced c-MET-amplified non-small cell lung cancer (NSCLC).
      CabozantinibMET, RET, ROS1, VEGFR2Erlotinib ± cabozantinib

      Patients with nonsquamous NSCLC and no EGFR mutation. MET expression assessed by IHC
      IIOverall improvement in PFS with cabozantinib but MET IHC score was not predictive
      • Neal J.W.
      • Dahlberg S.E.
      • Wakelee H.A.
      • et al.
      Cabozantinib (C), erlotinib (E) or the combination (E+C) as second- or third-line therapy in patients with EGFR wild-type (wt) non-small cell lung cancer (NSCLC): A randomized phase 2 trial of the ECOG-ACRIN Cancer Research Group (E1512) [abstract].
      MET-selective TKIs
      TivantinibMETErlotinib ± tivantinib

      MARQUEE: Western cohort of patients with nonsquamous NSCLC. Not selected on the basis of MET analysis
      IIITivantinib was not associated with any improvement in OS, although PFS was increased in the tivantinib group compared with the group receiving erlotinib alone
      • Scagliotti G.
      • von Pawel J.
      • Novello S.
      • et al.
      Phase III multinational, randomized, double-blind, placebo-controlled study of tivantinib (ARQ 197) plus erlotinib versus erlotinib alone in previously treated patients with locally advanced or metastatic nonsquamous non-small-cell lung cancer.
      Erlotinib ± tivantinib

      ATTENTION: East Asian cohort of patients with nonsquamous NSCLC. Not selected on the basis of MET analysis
      IIITivantinib was not associated with any improvement in OS, although PFS was increased in the tivantinib group compared with in the group receiving erlotinib alone. Trial terminated early because of an increase of interstitial lung disease in the tivantinib group
      • Yoshioka H.
      • Azuma K.
      • Yamamoto N.
      • et al.
      A randomized, double-blind, placebo-controlled, phase III trial of erlotinib with or without a c-Met inhibitor tivantinib (ARQ 197) in Asian patients with previously treated stage IIIB/IV nonsquamous nonsmall-cell lung cancer harboring wild-type epidermal growth factor receptor (ATTENTION study).
      CapmatinibMETGefitinib + capmatinib

      Patients with EGFR-mutated NSCLC, refractory to EGFR TKIs, and MET amplification or MET overexpression
      Ib/IIPartial responses in 6 of 41 patients (15%), all with either high MET amplification or MET overexpression
      • Wu Y.-L.
      • Yang J.C.-H.
      • Kim D.-W.
      • et al.
      Safety and efficacy of INC280 in combination with gefitinib (gef) in patients with EGFR-mutated (mut), MET-positive NSCLC: a single-arm phase lb/ll study.
      Anti-MET monoclonal antibody
      OnartuzumabMETErlotinib ± onartuzumab

      Patients with stage IIIB or IV NSCLC. MET expression evaluated at baseline
      IIOnartuzumab plus erlotinib did not show an OS advantage, although an OS advantage was evident in the MET-positive subgroup
      • Spigel D.R.
      • Ervin T.J.
      • Ramlau R.A.
      • et al.
      Randomized phase II trial of onartuzumab in combination with erlotinib in patients with advanced non-small-cell lung cancer.
      Erlotinib ± onartuzumab

      Patients with previously treated MET-positive stage IIIB or IV NSCLC
      IIIStopped for futility as there was no improvement in OS, PFS, or ORR
      • Spigel D.R.
      • Edelman M.J.
      • O'Byrne K.
      • et al.
      Onartuzumab plus erlotinib versus erlotinib in previously treated stage IIIb or IV NSCLC: results from the pivotal phase III randomized, multicenter, placebo-controlled METLung (OAM4971g) global trial.
      Emibetuzumab (LY2875358)METEmibetuzumab monotherapy

      Patients with locally advanced or metastatic CRPC with bone metastasis, RCC, NSCLC, and HCC. Patients with RCC, NSCLC, and HCC were required to have ≥50% of tumor cells to be ≥2+ for MET expression by IHC
      IIn patients with NSCLC, the disease control rate (PR + stable disease) was 26% (5 of 19), and the median duration of disease stabilization was 3.9 months (range 2.5–6.4) in NSCLC
      • Banck M.S.
      • Chugh R.
      • Natale R.B.
      • et al.
      Phase 1 results of emibetuzumab (LY2875358), a bivalent MET antibody, in patients with advanced castration-resistant prostate cancer, and MET positive renal cell carcinoma, non-small cell lung cancer, and hepatocellular carcinoma.
      Anti-HGF monoclonal antibody
      FiclatuzumabHGFGefitinib ± ficlatuzumab

      Asian patients with stage IIIB or IV pulmonary adenocarcinoma. Patients were not selected on the basis of MET analysis
      IIFailed to demonstrate significant improvement in PFS and overall response
      • Mok T.S.K.
      • Park K.
      • Geater S.L.
      • et al.
      A randomized phase (Ph) 2 study with exploratory biomarker analysis of ficlatuzumab (F) a humanized hepatocyte growth factor (HGF) inhibitory MAB in combination with gefitinib (G) versus G in Asian patients (pts) with lung adenocarcinoma (LA).
      RilotumumabHGFErlotinib + rilotumumab

      Patients with recurrent or progressive NSCLC. Not selected on the basis of MET analysis
      IIOngoing (NCT01233687)
      MET, hepatocyte growth factor receptor; HGF, hepatocyte growth factor; TKI, tyrosine kinase inhibitor; ALK, anaplastic lymphoma kinase; MET, MET proto-oncogene receptor tyrosine kinase; CEP7, centromeric portion of chromosome 7; RET, ret proto-oncogene; VEGFR, vascular endothelial growth factor receptor; PFS, progression-free survival; IHC, immunohistochemistry; OS, overall survival; ORR, overall response rate; CRPC, castration-resistant prostate cancer; RCC, renal cell carcinoma; HCC, hepatocellular carcinoma; PR, partial response.
      Unfortunately, significant variation in preselection criteria for identifying those patients who are potentially sensitive to EGFR and MET inhibition has contributed to some confusion over the results of trials combining EGFR and MET inhibition in NSCLC.

      Combination Trials Not Focused on EGFR-Mutant Lung Cancers

      Increased expression of MET alone is sufficient to induce oncogenic transformation in vitro and in vivo.
      • Patane S.
      • Avnet S.
      • Coltella N.
      • et al.
      MET overexpression turns human primary osteoblasts into osteosarcomas.
      • Wang R.
      • Ferrell L.D.
      • Faouzi S.
      • et al.
      Activation of the Met receptor by cell attachment induces and sustains hepatocellular carcinomas in transgenic mice.
      Although overexpression of both MET and HGF have been identified in unselected NSCLC specimens, the role of increased expression alone as a clinically relevant oncogenic driver has come into question.
      • Ichimura E.
      • Maeshima A.
      • Nakajima T.
      • et al.
      Expression of c-met/HGF receptor in human non-small cell lung carcinomas in vitro and in vivo and its prognostic significance.
      • Siegfried J.M.
      • Weissfeld L.A.
      • Luketich J.D.
      • et al.
      The clinical significance of hepatocyte growth factor for non-small cell lung cancer.
      The prevalence of MET overexpression in unselected NSCLCs ranges from 15% to 70%.
      • Park S.
      • Choi Y.L.
      • Sung C.O.
      • et al.
      High MET copy number and MET overexpression: poor outcome in non-small cell lung cancer patients.
      • Ma P.C.
      • Tretiakova M.S.
      • MacKinnon A.C.
      • et al.
      Expression and mutational analysis of MET in human solid cancers.
      • Tsao M.S.
      • Yang Y.
      • Marcus A.
      • et al.
      Hepatocyte growth factor is predominantly expressed by the carcinoma cells in non-small-cell lung cancer.
      • Tsuta K.
      • Kozu Y.
      • Mimae T.
      • et al.
      c-MET/phospho-MET protein expression and MET gene copy number in non-small cell lung carcinomas.
      This frequency depends on the antibody, the assay, and the positivity cutpoint. Although MET protein expression has been associated with poor prognostic outcomes in lung cancer,
      • Park S.
      • Choi Y.L.
      • Sung C.O.
      • et al.
      High MET copy number and MET overexpression: poor outcome in non-small cell lung cancer patients.
      • Dimou A.
      • Non L.
      • Chae Y.K.
      • et al.
      MET gene copy number predicts worse overall survival in patients with non-small cell lung cancer (NSCLC); a systematic review and meta-analysis.
      it has thus far served as a poor predictive biomarker of response to targeted therapy.
      Interest in the treatment of patients with MET-overexpressing lung cancers was initially piqued by a subset analysis of a phase II combination trial of erlotinib and onartuzumab.
      • Spigel D.R.
      • Ervin T.J.
      • Ramlau R.A.
      • et al.
      Randomized phase II trial of onartuzumab in combination with erlotinib in patients with advanced non-small-cell lung cancer.
      In this study, unselected second-line patients with advanced NSCLC were randomized to receive erlotinib with or without onartuzumab. Although the coprimary end points of OS (hazard ratio [HR] = 0.80, p = 0.34) and PFS (HR = 1.09, p = 0.69) were not met in the overall population, patients whose tumors expressed higher levels of MET (IHC result 2 to 3+) showed an improvement in both PFS (HR = 0.53, p = 0.04) and OS (HR = 0.37, p < 0.05).
      • Spigel D.R.
      • Ervin T.J.
      • Ramlau R.A.
      • et al.
      Randomized phase II trial of onartuzumab in combination with erlotinib in patients with advanced non-small-cell lung cancer.
      Disappointingly, a subsequent phase III trial randomizing 499 patients with advanced NSCLC with MET-overexpressing tumors (IHC result 2 to 3+) to erlotinib with or without onartuzumab was terminated early owing to futility.
      • Spigel D.R.
      • Edelman M.J.
      • O'Byrne K.
      • et al.
      Onartuzumab plus erlotinib versus erlotinib in previously treated stage IIIb or IV NSCLC: results from the pivotal phase III randomized, multicenter, placebo-controlled METLung (OAM4971g) global trial.
      The primary end point of OS was not different between groups (HR = 1.27, p = 0.07).
      • Spigel D.R.
      • Edelman M.J.
      • O'Byrne K.
      • et al.
      Onartuzumab plus erlotinib versus erlotinib in previously treated stage IIIb or IV NSCLC: results from the pivotal phase III randomized, multicenter, placebo-controlled METLung (OAM4971g) global trial.
      Median OS was numerically decreased in patients who received combination therapy, suggesting the possibility of harm.
      • Spigel D.R.
      • Edelman M.J.
      • O'Byrne K.
      • et al.
      Onartuzumab plus erlotinib versus erlotinib in previously treated stage IIIb or IV NSCLC: results from the pivotal phase III randomized, multicenter, placebo-controlled METLung (OAM4971g) global trial.
      Two phase III combination studies of tivantinib that had reported anti-MET activity and treated largely unselected patients with NSCLC did not meet their primary end point. The ATTENTION trial randomized 307 patients with advanced, EGFR wild-type, nonsquamous NSCLC to receive erlotinib with or without tivantinib. Although the study was terminated early secondary to an increased incidence of interstitial lung disease in the tivantinib arm, the primary end point of OS was not significantly different between groups (HR = 0.89, p = 0.43).
      • Yoshioka H.
      • Azuma K.
      • Yamamoto N.
      • et al.
      A randomized, double-blind, placebo-controlled, phase III trial of erlotinib with or without a c-Met inhibitor tivantinib (ARQ 197) in Asian patients with previously treated stage IIIB/IV nonsquamous nonsmall-cell lung cancer harboring wild-type epidermal growth factor receptor (ATTENTION study).
      The MARQUEE trial randomized 1048 patients with advanced, nonsquamous NSCLC to receive erlotinib with or without tivantinib. This trial was terminated early owing to an interim analysis revealing futility, and the primary end point of OS did not differ between groups (HR = 0.98, p = 0.81).
      • Scagliotti G.
      • von Pawel J.
      • Novello S.
      • et al.
      Phase III multinational, randomized, double-blind, placebo-controlled study of tivantinib (ARQ 197) plus erlotinib versus erlotinib alone in previously treated patients with locally advanced or metastatic nonsquamous non-small-cell lung cancer.
      Although the secondary end point of PFS was improved by the combination in both trials, the absolute difference compared with single-agent erlotinib was small.
      • Scagliotti G.
      • von Pawel J.
      • Novello S.
      • et al.
      Phase III multinational, randomized, double-blind, placebo-controlled study of tivantinib (ARQ 197) plus erlotinib versus erlotinib alone in previously treated patients with locally advanced or metastatic nonsquamous non-small-cell lung cancer.
      Of note, tivantinib is thought to potentially function as a mitotic spindle poison.
      • Katayama R.
      • Aoyama A.
      • Yamori T.
      • et al.
      Cytotoxic activity of tivantinib (ARQ 197) is not due solely to c-MET inhibition.
      Recently, a phase II study randomizing 118 patients with advanced, EGFR wild-type NSCLC to receive erlotinib, cabozantinib, or both in combination reached its primary end point of PFS (HR = 0.38, p < 0.05 for cabozantinib versus erlotinib; HR = 0.35, p < 0.05 for combination versus erlotinib). Unlike the MET-selective inhibitor tivantinib that was tested in the ATTENTION and MARQUEE studies, cabozantinib is a multikinase inhibitor with activity against several other potentially sensitive subgroups that may have been contained within this trial population, including both ROS1-rearranged and RET-rearranged lung cancers. This cohort of patients did not undergo comprehensive molecular profiling to rule out the presence of these alterations or other events, such as METex14 alterations. The contribution of these potentially undetected cases to these results remains unclear.

      MET Inhibition in EGFR-Mutant Lung Cancers

      The prevalence of MET amplification in EGFR-mutant lung cancers with acquired resistance to EGFR TKI therapy was initially reported at 15% to 20%.
      • Engelman J.A.
      • Zejnullahu K.
      • Mitsudomi T.
      • et al.
      MET amplification leads to gefitinib resistance in lung cancer by activating ERBB3 signaling.
      • Turke A.B.
      • Zejnullahu K.
      • Wu Y.L.
      • et al.
      Preexistence and clonal selection of MET amplification in EGFR mutant NSCLC.
      A subsequent series noted a lower prevalence at 5% and found that MET amplification overlapped with other resistance mechanisms such as EGFR T790M acquisition or small cell transformation.
      • Yu H.A.
      • Arcila M.E.
      • Rekhtman N.
      • et al.
      Analysis of tumor specimens at the time of acquired resistance to EGFR-TKI therapy in 155 patients with EGFR-mutant lung cancers.
      Unsurprisingly, the acquisition of MET amplification has also been reported as a mechanism of resistance to third-generation EGFR TKI therapy in patients with EGFR T790M–positive lung cancer.
      • Planchard D.
      • Loriot Y.
      • Andre F.
      • et al.
      EGFR-independent mechanisms of acquired resistance to AZD9291 in EGFR T790M-positive NSCLC patients.
      Clinical trials preselected or enriched for EGFR-mutant NSCLC exploring combined MET and EGFR inhibition have focused on either the EGFR TKI–naive setting as a means of preventing MET-driven resistance or the acquired resistance setting, with varying degrees of preselection to identify a MET-codriven state at the time of its emergence. The former approach does not depend on having specific biomarkers of MET activation. As an EGFR TKI is associated with significant benefit in an EGFR-mutant TKI-naive population, clinical investigations must rely on randomized data to make the case for combination therapy being superior to monotherapy with an EGFR TKI. In addition, this approach, with PFS as the primary end point, is inherently dependent on the expected underlying frequency of MET activation that would otherwise emerge to size the study to detect a change compared with the benefit from an EGFR TKI alone. The lower the frequency of MET as a predicted mechanism of acquired resistance, the larger the study must be to prove the combination adds unequivocal benefit. In a phase II trial comparing emibetuzumab with or without erlotinib, in patients whose tumors developed acquired resistance to erlotinib and harbored MET overexpression, the overall response rate (ORR) was higher in both the combination and monotherapy arms (3.8% and 4.8%, respectively) for patients with at least 60% of cells determined to be MET positive by IHC (n = 74) than for patients with at least 10% of cells determined to be positive (n = 89), in which case the ORR was 3.0% in the combination arm and 4.3% in the monotherapy arm.
      • Camidge D.R.
      • Moran T.
      • Demedts I.
      • et al.
      A randomized, open-label, phase 2 study of emibetuzumab plus erlotinib (LY+E) and emibetuzumab monotherapy (LY) in patients with acquired resistance to erlotinib and MET diagnostic positive (MET Dx+) metastatic NSCLC.
      In the acquired resistance setting, the same challenges associated with defining the appropriate method and positivity cutpoint for identifying MET gene copy number gain as a primary driver apply to defining MET positivity as a codriver state. Data converging with the literature on primary drivers recently emerged from a small phase II study in EGFR-mutant patients with acquired resistance to an EGFR TKI who were then treated with the combination of gefitinib and capmatinib. When new biopsy specimens obtained at the time of acquired resistance were analyzed, the rate of response to the combination was 40% among those with a MET copy number of 5 or higher (the ratio was not reported), but zero among those with a copy number less than 5.
      • Wu Y.-L.
      • Yang J.C.-H.
      • Kim D.-W.
      • et al.
      Safety and efficacy of INC280 in combination with gefitinib (gef) in patients with EGFR-mutated (mut), MET-positive NSCLC: a single-arm phase lb/ll study.
      Clinical trials focusing on combination MET and EGFR inhibitor therapy for patients with acquired resistance to EGFR TKIs and using differing degrees of MET preselection are ongoing.
      • Thress K.S.
      • Paweletz C.P.
      • Felip E.
      • et al.
      Acquired EGFR C797S mutation mediates resistance to AZD9291 in non-small cell lung cancer harboring EGFR T790M.

      Conclusions

      Although research into the MET pathway as a driver of oncogenesis has stretched well over three decades, advances in technology and appropriate patient selection have reinvigorated the search for an effective targeted therapeutic for lung cancers harboring METex14 alterations and/or MET amplification as their primary oncogenic driver. Attempts to define the criteria for optimal use of a combination of MET and EGFR inhibitors in which MET acts as a targetable codriver, particularly in EGFR-mutant patients, continue. Ongoing and future drug development plans with a strong focus on molecular enrichment are likely to succeed in this arena. Both patients and providers look forward to eventual regulatory approval.

      Acknowledgments

      Medical writing support was provided by Ash Dunne at inScience Communications (Chester, United Kingdom) and Wendy Sacks and Thierry Deltheil at ACUMED (New York, NY, and Tytherington, United Kingdom) and funded by Pfizer Inc.

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