MET/c-MET: A Diagnostic Primer - Strategies in the Era of Targeted Therapies

MET²⁷ proto-oncogene, located on chromosome 7q31, encodes a receptor tyrosine kinase (RTK, c-MET) that binds hepatocyte growth factor (HGF) and regulates cellular proliferation, motility, and survival.¹ Aberrant MET signaling has been implicated in oncogenesis across multiple tumor types, most notably non-small cell lung carcinoma (NSCLC), where MET alterations serve as both primary oncogenic drivers and mechanisms of acquired resistance.² Advances in the therapeutic landscape, particularly the emergence of MET-directed tyrosine kinase inhibitors (TKIs) and antibody-drug conjugates (ADCs), have necessitated equally nuanced diagnostic strategies. For pathologists, ensuring analytically- and clinically-validated identification of MET alterations is of high importance in biomarker-driven diagnostics.

Biological Basis for MET Oncogenicity

MET dysregulation in cancer occurs via four principal mechanisms: exon 14 skipping mutations, gene amplification, and increased protein expression (Figures 1-2). Each carries distinct biological implications and therapeutic eligibility criteria. 

1. MET exon 14 (METex14) skipping mutations, observed in approximately 2% of unselected NSCLC cases, disrupt the Casitas B-lineage lymphoma (CBL) E3 ubiquitin ligase binding site, leading to reduced receptor degradation and sustained oncogenic signaling.² These alterations result in pathway activation independent of HGF and are predictive of response to selective MET TKIs.^3,9,12,19
2. MET gene amplification is seen in approximately 5% of NSCLC cases, and particularly at high levels (eg, MET:CEP7 ratio > 5), leads to increased receptor expression and ligand-independent dimerization, driving proliferation through canonical signaling cascades (PI3K/AKT, RAS/MAPK).⁴ This amplification often emerges as a resistance mechanism to treatment with EGFR TKIs in EGFR-mutant NSCLC.
3. Increased c-MET protein expression, typically evaluated by immunohistochemistry (IHC), most commonly arises from transcriptional upregulation or other regulatory changes. Importantly, while increased expression alone does not confirm oncogene addiction, it forms the basis for patient selection in ADC trials.⁵
4. MET fusions occur in approximately 0.3% of lung cancer patients and approximately 50% are intragenetic. As with many oncogenic fusions, the tyrosine kinase domain-encoding portion of MET creates a fusion gene with a wide variety of partners like KIF5b.¹⁴

The MET-directed Treatment Landscape is Evolving: From TKIs to ADCs

Two selective TKIs, capmatinib and tepotinib, have received Food and Drug Administration (FDA) approval in NSCLC with METex14 skipping mutations as detected by approved companion diagnostics and based on robust responses in the GEOMETRY mono-1 and VISION trials, respectively.^3,9 These agents exhibit activity in both treatment-naïve and previously treated settings, although resistance eventually emerges via secondary MET mutations or bypass pathway activation.

Crizotinib, originally developed as an ALK and ROS1 inhibitor, also exhibits MET inhibition and has shown modest efficacy in MET-amplified and METex14 skipping mutant tumors.^10,11 However, its use is off-label, even though noted in the National Comprehensive Cancer Network (NCCN) Clinical Practice Guidelines in Oncology (NCCN Guidelines 4.2025).

Savolitinib is a selective MET inhibitor approved in China for patients with NSCLC harboring METex14 skipping mutations.¹² While not approved in the United States or Europe currently, savolitinib is under active investigation in combination with osimertinib for EGFR-mutant NSCLC with acquired MET-driven resistance.¹³ Gumarontinib is another MET inhibitor approved in China for METex14 skipping mutations and also in clinical development.¹⁹

The recent accelerated approval by FDA of the ADC, telisotuzumab vedotin-tllv, a c-Met-directed antibody and microtubule inhibitor conjugate, and associated companion diagnostic is the latest addition to the landscape. Unlike TKIs, ADCs exploit surface expression on tumor cells for selective drug delivery and cytotoxicity. In the LUMINOSITY phase 2 trial, this drug demonstrated activity in strong (≥ 50%, 3+) c-MET overexpressing NSCLCs by IHC with responses most pronounced in non-squamous histology and EGFR wildtype tumors.⁵ This modality thus expands the therapy options to patients.

In addition, the horizon for more approvals is promising with more than 30 ongoing TKI and 10 antibody drug trials directed to MET/c-MET in NSCLC alone with investigation into other indications currently planned or underway.¹⁴

Diagnostic Modalities: Strengths and Limitations

Selection of the appropriate diagnostic assay for MET alterations depends on the underlying molecular event. For METex14 skipping mutations, DNA-based next-generation sequencing (NGS) panels are widely used; however, their sensitivity is variable (amplicon- versus hybrid-based). Canonical splice site mutations (eg, c.3028+1G>T, c.3028-2A>G) are typically detected if the assay includes sufficient intronic padding, but noncanonical and deep intronic variants are often missed.^6,20 RNA-based NGS, by directly analyzing the transcriptome, provides higher sensitivity for detecting exon skipping regardless of the causative genomic alteration.²¹

Table 1: METex14 Mutation Types and Detection Challenges by Assay.^20,21 Reused and adapted under Creative Commons license CC-BY-4.0.

MET copy number gain can be detected by several techniques including fluorescence in situ hybridization (FISH), NGS, and polymerase chain reaction (PCR) for example. MET amplification is most accurately assessed via FISH, which remains the clinically validated standard for determining MET:CEP7 ratios and copy number gains. Currently, no consensus exists for an appropriate diagnostic cutoff. While some NGS platforms estimate MET copy number from read depth, these results may be confounded by tumor purity, ploidy, and bioinformatic thresholds, leading to false-positive/negative results.^4,8 Therefore, high-level MET amplification detected by NGS should ideally be confirmed by FISH, especially when used to guide MET-directed therapy.⁹

The most common reason for increased c-MET protein expression is transcriptional upregulation in the absence of gene amplification.¹⁴ Increased protein expression is assessed using IHC with SP44 (accelerated approval as a CDx) and D1C2 being the most validated antibody clones. While IHC offers logistical advantages, being faster, cheaper, and tissue-sparing, it provides limited understanding in the causal event of increased protein expression. Recognizing IHC does not reliably correlate with underlying mutations or amplification, it cannot serve as a standalone predictive biomarker for treatment with TKIs for example; likewise, MET amplification and exon 14 mutations cannot serve as surrogate biomarkers for c-MET expression.^4,22

Digital pathology platforms and AI-based algorithms are increasingly being explored to enhance MET biomarker assessment. Some studies have shown reproducibility of semi-quantification scoring between conventional light microscopy versus whole slide images.¹⁴ In parallel, machine learning models applied to histomorphologic and genomic features have shown early promise in predicting METex14 skipping mutations directly from H&E-stained slides, potentially offering a prescreening tool when molecular resources are limited.^15,16 As these tools evolve, they may help standardize MET testing workflows and triage patients for confirmatory molecular assays.

Inter-Assay (Dis)concordance and Diagnostic Pitfalls

The frequent discordance between different assay modalities is a diagnostic challenge. While each assay interrogates distinct biological elements - protein expression, gene copy number, and broader genomic context - inter-assay concordance remains modest. This diagnostic variability introduces uncertainty into therapeutic decision-making, particularly in borderline or discordant cases.

The rate of increased c-MET expression as assessed by IHC across multiple studies ranges from approximately 15% - 78% and MET amplification as measured mainly by FISH ranges from approximately 2% - 29% inclusive of de novo and secondary types. The correlation between IHC and FISH is poor to moderate with a significant number of cases that can be positive for one modality and not the other; for example, positive for amplification but negative for IHC. A recent literature review summarizing data across studies showed correlation coefficients around 0.4.²³ However, one recent study showed a high concordance between modalities in a MET-amplification-enriched cohort with an IHC-FISH overall percent agreement of approximately 79%.²⁴ Multiple differences are noted across these studies including antibody clone types, staining platform variability, scoring interpretative differences, amplification definitions, type of ISH (chromogenic versus fluorescent), cross-vendor ISH assays, cases with polysomy, borderline cases, clinical stage of cohort, mutation status like EGFR, and of course pre-analytic challenges.²³

In contrast, NGS shows a higher concordance rate with FISH in the setting of high-level MET amplification, between 65% to 92%.^24-26 However, NGS results may diverge from FISH at low-to-intermediate copy number gains, and cross-platform variability driven by differences in coverage depth, tumor purity, and analytic thresholds, which complicate standardization.

Other diagnostic pitfalls persist across modalities: IHC is subject to pre-analytical artifacts (eg, cold ischemia time, fixative types/time, block storage conditions, and cut slide stability)²⁸; analytic variables (eg, clone-dependent variability, antigen retrieval conditions, detection systems, and staining platforms); and interpretive subjectivity (pathologist training and reproducibility, degree of case heterogeneity).¹⁵ FISH may yield equivocal results in borderline cases or in the presence of polysomy. NGS, while powerful, lacks universal amplification cutoffs and may not be reliable in detecting focal high-level gains. In addition, biological dissociation between protein expression and gene amplification further contributes to assay discordance where increased c-MET expression may result from transcriptional activation or post-translational modification in the absence of gene-level alterations.

Together, this assay discordance in MET diagnostics underscores the current challenges and highlights the importance of multi-modality testing.

Best Practices and Interpretive Reporting

Pathology reports should specify the method used (DNA and/or RNA or amplicon versus hybrid capture-based NGS, PCR, FISH, and/or IHC); the mutation or amplification metrics; and the assay's sensitivity limits. For NGS, clear annotation of intronic coverage and splice region analysis is critical. When MET amplification is reported from NGS, concurrent or reflex FISH is advisable to better support decisions. The correct companion diagnostic should be used when considering the respective therapy.

For IHC, pathologists should adhere to validated scoring systems, noting both intensity (0 - 3+) and percentage of positive tumor cells. Reports should note the antibody clone used and clarify that increased protein expression does not imply gene amplification and mutational activation.⁵ In addition, pathologists who will be scoring clinical cases should be trained and incorporate regular cross-pathologist scoring alignment as well as use consensus review, particularly in borderline cases. 

In ambiguous cases, especially with specific clinical characteristics (eg, nonsmoker with adenocarcinoma and no actionable drivers), RNA-based sequencing should be considered. The use of molecular tumor boards,¹⁸ can help enhance interpretive accuracy and improve stakeholder alignment in these settings.

Future Directions and Conclusion

New MET-directed agents are already available in the clinic with many clinical trials starting and ongoing. Combinations with immunotherapy and other targeted agents are being explored to overcome resistance mechanisms. At the same time, a better understanding of MET biology, particularly increased protein expression in the absence of genomic alterations, is needed to refine patient selection.

Pathologists are essential to this evolving paradigm. As biomarker complexity increases, the role of the diagnostic laboratory will not only be technical but interpretive requiring fluency across platforms, mutation classes, and drug eligibility. 

References

1. Sattler M, Salgia R. The expanding role of the receptor tyrosine kinase MET as a therapeutic target in non-small cell lung cancer. Cell Rep Med. 2025;6(3):101983. doi:10.1016/j.xcrm.2025.101983. Epub 2025 Feb 27.
2. Ji M, Ganesan S, Xia B, Huo Y. Targeting c-MET alterations in cancer: a review of genetic drivers and therapeutic implications. Cancers. 2025;17(9):1493. doi:10.3390/cancers17091493
3. Wolf J, Seto T, Han JY, Reguart N, et al. Capmatinib in MET Exon 14-mutated or MET-amplified non-small-cell lung cancer. N Engl J Med. 2020;383(10):944-957. doi:10.1056/NEJMoa2002787
4. Yang M, Mandal E, Liu FX, O’Hara RM, Lesher B, Sanborn RE. Non-small cell lung cancer with MET amplification: review of epidemiology, associated disease characteristics, testing procedures, burden, and treatments. Front Oncol. 2024;13:1241402. doi:10.3389/fonc.2023.12414025.    
5. Camidge DR, Bar J, Horinouchi H, Goldman J, et al. Telisotuzumab vedotin monotherapy in patients with previously treated c-Met protein-overexpressing advanced nonsquamous EGFR-wildtype non-small cell lung cancer in the phase II LUMINOSITY trial. J Clin Oncol. 2024;42(25):3000-3011. doi:10.1200/JCO.24.00720. Epub 2024 Jun 6.
6. Frampton GM, Ali SM, 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. Cancer Discovery. 2015;5(8):850–859. doi:10.1158/2159-8290.CD-15-0285
7. Schildhaus HU, Schultheis AM, Rüschoff J, et al. MET amplification status in therapy-naïve adeno- and squamous cell carcinomas of the lung. Clin Cancer Res. 2015;21(4):907-915. doi:10.1158/1078-0432.CCR-14-0450
8. Lindeman NI, Cagle PT, Aisner DL, et al. Updated molecular testing guideline for the selection of lung cancer patients for treatment with targeted tyrosine kinase inhibitors: guideline from the College of American Pathologists, the International Association for the Study of Lung Cancer, and the Association for Molecular Pathology. Arch Pathol Lab Med. 2018;142(3):321-346. doi:10.5858/arpa.2017-0388-CP
9. Paik PK, Felip E, Veillon R, et al. Tepotinib in non–small-cell lung cancer with MET exon 14 skipping mutations. N Engl J Med. 2020;383(10):931–943. doi:10.1056/NEJMoa2004407
10. Drilon A, Clark JW, Weiss J, et al. Antitumor activity of crizotinib in lung cancers harboring a MET exon 14 alteration. Nat Med. 2020;26(1):47-51. doi:10.1038/s41591-019-0716-8. Epub 2020 Jan 13.
11. Camidge DR, Otterson GA, Clark JW, et al. Crizotinib in patients with MET-amplified NSCLC. J Thorac Oncol. 2021;16(6):1017-1029. doi:10.1016/j.jtho.2021.02.010. Epub 2021 Mar 4
12. Gu Y, Sai Y, Wang J, et al. Preclinical pharmacokinetics, disposition, and translational pharmacokinetic/pharmacodynamic modeling of savolitinib, a novel selective cMet inhibitor. Eur J Pharm Sci. 2019;136:104938. doi:10.1016/j.ejps.2019.05.016. Epub 2019 May 24.
13. Sequist LV, Han JY, Ahn MJ, et al. Osimertinib plus savolitinib in patients with EGFR mutation-positive, MET-amplified, non-small-cell lung cancer after progression on EGFR tyrosine kinase inhibitors: interim results from a multicentre, open-label, phase 1b study. Lancet Oncol. 2020;21(3):373-386. doi:10.1016/S1470-2045(19)30785-5. Epub 2020 Feb 3.
14. Spagnolo CC, Ciappina G, Giovannetti E, et al. Targeting MET in non-small cell lung cancer (NSCLC): a new old story? Int J Mol Sci. 2023;24(12):10119. doi:10.3390/ijms241210119
15. Bontoux C, Hofman V, Chamorey E, et al. Reproducibility of c-Met immunohistochemical scoring (clone SP44) for non-small cell lung cancer using conventional light microscopy and whole slide imaging. Am J Surg Pathol. 2024;48(9):1072-1081. doi:10.1097/PAS.0000000000002274. Epub 2024 Jul 8.
16. Ingale K, Hong SH, Bell JSK, et al. Prediction of MET overexpression in lung adenocarcinoma from hematoxylin and eosin images. Am J Pathol. 2024;194(6):1020-1032. doi:10.1016/j.ajpath.2024.02.015. Epub 2024 Mar 15.
17. Joshi RP, Osinski BL, Beig N, Sha L, Ingale K, Stumpe M. Imaging-based histological features are predictive of MET alterations in non-small cell lung cancer. arXiv:2203.10062. 2022. doi:10.48550/arXiv.2203.10062
18. Barnes M, Cobain E. Pathologists at the Forefront: Shaping the Future of Cancer Treatment with Molecular Tumor Boards. College of American Pathologists. November 15, 2024. Accessed June 4, 2025. https://www.cap.org/member-resources/articles/pathologists-at-the-forefront-shaping-the-future-of-cancer-treatment-with-molecular-tumor-boards.
19. Yu Y, Zhou J, Li X, Goto K, et al. Gumarontinib in patients with non-small-cell lung cancer harbouring MET exon 14 skipping mutations: a multicentre, single-arm, open-label, phase 1b/2 trial. EClinicalMedicine. 2023;59:101952. doi:10.1016/j.eclinm.2023.101952.
20. Poirot B, Doucet L, Benhenda S, Champ J, Meignin V, Lehmann-Che J. MET exon 14 alterations and new resistance mutations to tyrosine kinase inhibitors: risk of inadequate detection with current amplicon-based NGS panels. J Thorac Oncol. 2017;12(10):1582-1587. doi:10.1016/j.jtho.2017.07.026. Epub 2017 Aug 2.
21. Davies KD, Lomboy A, Lawrence CA, et al. DNA-based versus RNA-based detection of MET exon 14 skipping events in lung cancer. J Thorac Oncol. 2019;14(4):737-741. doi:10.1016/j.jtho.2018.12.020. Epub 2019 Jan 9.
22. Baldacci S, Figeac M, Antoine M, et al. High MET overexpression does not predict the presence of MET exon 14 splice mutations in NSCLC: results from the IFCT PREDICT.amm study. J Thorac Oncol. 2020;15(1):120-124. doi:10.1016/j.jtho.2019.09.196. Epub 2019 Oct 9.
23. Yin W, Guo M, Tang Z, et al. MET expression level in lung adenocarcinoma loosely correlates with MET copy number gain/amplification and is a poor predictor of patient outcome. Cancers (Basel). 2022;14(10):2433. doi:10.3390/cancers14102433.
24. Xiang C, Lv X, Chen K, et al. Unraveling the significance of MET focal amplification in lung cancer: integrative NGS, FISH, and IHC investigation. Mod Pathol. 2024;37(4):100451. doi:10.1016/j.modpat.2024.100451.
25. Cabello-Aguilar S, Vendrell JA, et al. An optimized next-generation sequencing method for detecting de novo MET amplification in non-small cell lung cancer: prognostic and therapeutic implications. Lab Invest. 2025;105(5):104117. doi:10.1016/j.labinv.2025.104117. Epub 2025 Feb 20.
26. Peng LX, Jie GL, Li AN, et al. MET amplification identified by next-generation sequencing and its clinical relevance for MET inhibitors. Exp Hematol Oncol. 2021;10(1):52. doi:10.1186/s40164-021-00245-y.
27. Religa P, Łazarczyk M, Mickael ME, Skiba DS. Misleading interpretations of the MET gene acronym: over 15 years of confusion in scientific publications and a call for reaffirming the original gene nomenclature. J Thorac Oncol. 2025;20(8):1032-1034. doi:10.1016/j.jtho.2025.03.044. Epub 2025 Apr 18.
28. Miller R, Thorne-Nuzzo T, Loftin I, McElhinny A, Towne P, Clements J. Impact of pre-analytical conditions on the antigenicity of lung markers: ALK and MET. Appl Immunohistochem Mol Morphol. 2020;28(5):331-338. doi:10.1097/PAI.0000000000000730

Michael Barnes, MD, FCAP, is a board-certified pathologist director at AbbVie, focused on clinical trial and companion diagnostic development. He completed his medical studies at Ohio State University and AP Residency, Surgical Pathology, Neuropathology, and Post-Doctoral Research Fellowships at University of California, San Francisco. Subsequently, he spent 10 years at Roche in their Ventana and Genentech organizations and more recently at PathAI. He is a member of CAP's PHC Committee. His research interests are integrating digital pathology into clinical trials and efficient biomarker-driven trial approaches.

Amita Mistry, MD, FCAP, is a board-certified pathologist director at AbbVie, supporting clinical trials and companion diagnostic development. She completed medical education and AP/CP residency at the University of Texas Medical Branch, Bone and Soft Tissue and Bioinformatics Post-Doctoral Fellowships at University of Pittsburgh Medical Center, and Surgical Pathology and Registry Pathology Fellowship at the California Tumor Tissue Registry and Loma Linda University Medical Center. Subsequently, she worked directly at Roche/Ventana as part of their companion diagnostics pathology group developing companion diagnostics for multiple pharmaceutical partners.