The insulin receptor is a receptor tyrosine kinase, but it breaks the rules that govern many of its relatives. Rather than dimerizing upon ligand binding, IR exists as a preformed dimer at the cell surface, held in an autoinhibited conformation in which the ectodomain geometry keeps the intracellular kinase domains separated and poorly oriented for the trans-phosphorylation that ignites signaling. Insulin activates the receptor through a multistep trajectory involving two binding sites: site 1, formed by the leucine-rich repeat domain L1 of one protomer together with the alpha-chain C-terminal segment of the opposing protomer, and site 2, located on the lateral surface of the first fibronectin type III domain. When the receptor is fully liganded, up to four insulin molecules can engage it, stabilizing a T-shaped conformation that brings the kinase domains into productive proximity.

Once activated, the receptor autophosphorylates across its activation loop, juxtamembrane region, and C-terminal tail, creating a docking platform for adaptor proteins. The classical branches follow: insulin receptor substrate proteins recruit PI3K to drive the AKT pathway and metabolic regulation, while the adaptor Shc channels signaling into the MAPK cascade, more closely tied to mitogenic and proliferative outputs. Crucially, these are not simply parallel wires but competing engagement routes, and their relative utilization shapes the overall signaling balance. That balance, the review emphasizes, can be distorted in disease, most famously in selective insulin resistance, where the metabolic AKT arm fails while the mitogenic MAPK arm persists, a combination implicated in endothelial dysfunction and in the uncoupling of hepatic glucose production from lipogenesis in type 2 diabetes.

Beyond pathway choice, the review highlights two underappreciated dimensions of IR regulation: trafficking and spatial organization. Internalized receptors were long thought to be simply silenced and recycled, but accumulating evidence shows that endosomal IR can remain signaling-competent, and that insulin-stimulated AKT2 phosphorylation depends on specific endosomal compartments enriched in the protein WDFY2. Genetic mouse models drive the point home. Disrupting the interaction between IR and the spindle checkpoint protein MAD2 delays receptor endocytosis, reduces insulin clearance, elevates circulating insulin, and reshapes both membrane and intracellular signaling, with measurable consequences for lipid metabolism. Conversely, hepatic overexpression of EphB4 drives receptor degradation and insulin resistance, while loss of the beta-cell scavenger inceptor improves glucose tolerance. Precise control of receptor traffic, these studies show, is required for systemic metabolic homeostasis.

Super-resolution imaging adds yet another layer: IR dimers are organized into nanoclusters roughly 74 nanometers across, and insulin stimulation modestly enlarges these assemblies without fundamentally reorganizing them. To test whether ligand architecture matters, researchers built a DNA origami platform presenting insulin at defined valency and spacing. Multivalent insulin NanoRods bound the receptor more tightly, dissociated more slowly, and induced stronger autophosphorylation and downstream transcriptional responses than monovalent insulin. Receptor activation, in other words, is sensitive to nanoscale geometry, not merely to ligand identity, and even the temporal pattern of insulin exposure matters: pulsatile delivery elicits stronger hepatic signaling than constant infusion at an equivalent dose.

The most striking section of the review concerns engineered insulin mimetics, which the authors treat not as insulin substitutes but as molecular probes exposing signaling states inaccessible to the native hormone. Phage display screens originally identified the site 1 and site 2 hotspots, and fusing peptides targeting them in different orientations produced either agonists or antagonists. The 31-amino-acid peptide S597 activates the receptor through a non-native geometry, cross-linking site 1 and site 2 across protomers, and preferentially stimulates metabolic over mitogenic signaling, with in vivo data suggesting possible benefits in atherosclerosis. Reversing the fusion orientation yields antagonists such as S961, which cryo-electron microscopy reveals trap the receptor in an intermediate conformation between the inactive and active states. Fusion geometry, not binding affinity, determines whether a synthetic ligand stabilizes an active, biased, or antagonistic configuration.

Computational protein design has now made that principle fully explicit. De novo designed binders, roughly 7 kilodaltons each and directed against individual extracellular domains, were fused in defined orientations with tunable linker flexibility. Reversing the topology flipped the function: site 2 to site 1 constructs act as agonists, while site 1 to site 2 constructs act as antagonists. Linker rigidity further tunes the output. Rigid constructs such as RF-405 stabilize a compact, fully active conformation and signal in a balanced, insulin-like fashion, whereas flexible constructs such as S2-F1-S1 produce partial autophosphorylation, attenuated mitogenic signaling, and altered trafficking. In animals, these designer agonists lower glucose with slower onset but longer duration than insulin, bypass certain disease-causing IR mutations, and show reduced stimulation of cancer cell proliferation compared with the native hormone.

Aptamers and antibodies round out the modulator toolkit. The DNA aptamer IR-A43 does not activate the receptor alone but potentiates insulin signaling by stabilizing an asymmetric, singly occupied conformation, while IR-A62 behaves as a concentration-dependent allosteric modulator that can lower glucose even under insulin-deficient conditions. Monoclonal antibodies such as 83-7 and 83-14, and the engineered XMet series, demonstrate that epitope selection can tune the receptor from activation to inhibition: XMetA acts as a partial agonist with preferential AKT signaling, XMetS potentiates insulin action, and XMetD reverses hypoglycemia in hyperinsulinemic mouse models. Together these ligands show that IR activation can be synthetically programmed.

The disease implications are broad. In cancer, hyperinsulinemia is associated with increased risk and progression, tumors preferentially express the IR-A isoform that binds IGF2 and sustains mitogenic signaling, and the super-mitogenic analog Asp(B10), which induced mammary tumors in rodents, stands as a cautionary example of what mitogenic bias can do. In polycystic ovary syndrome, skeletal muscle shows post-receptor defects centered on the IRS-PI3K-AKT axis, while ovarian adaptor remodeling promotes androgen excess. In Alzheimer disease, postmortem tissue reveals brain insulin resistance with preserved signaling proteins but elevated inhibitory IRS1 serine phosphorylation, and the genetic risk factor APOE4 impairs neuronal signaling by trapping the receptor in endosomes. Severe insulin resistance syndromes caused by germline IR mutations, meanwhile, may one day be treated with mimetics that activate the receptor through mechanisms distinct from insulin.

The authors are candid about the challenges ahead: it remains unclear which signaling biases are beneficial versus pathological, how bias is maintained in complex physiological environments, and how trafficking, ligand geometry, and adaptor selection integrate in vivo. Translational hurdles, from peptide stability to CNS delivery, are substantial, though the engineering playbook proven by GLP-1 receptor agonists offers a template. Still, the conceptual shift is hard to miss. If insulin action is a programmable network rather than a simple switch, then the future of therapy may lie not in replacing insulin but in reprogramming its receptor, selectively and precisely, in the tissues and disease contexts where it matters most.

Subject of Research: Non-canonical insulin receptor signaling and the design of insulin mimetics

Article Title: Insulin mimetics and non-canonical insulin receptor signaling: mechanisms and disease relevance

Article References: Park, J., & Choi, E. (2026). Insulin mimetics and non-canonical insulin receptor signaling: mechanisms and disease relevance. Experimental & Molecular Medicine. https://doi.org/10.1038/s12276-026-01862-5

Image Credits: AI Generated

DOI: 10.1038/s12276-026-01862-5

Keywords: insulin receptor, insulin mimetics, biased agonism, receptor trafficking, endosomal signaling, protein design, type 2 diabetes, insulin resistance, cancer, PCOS, Alzheimer disease, receptor tyrosine kinase

The patient first came to medical attention at birth with a striking constellation of findings. He presented with hypotonia, meaning diminished muscle tone, along with bilateral talipes equinovarus, the clubfoot deformity in which both feet are twisted inward and downward. His hands showed adducted thumbs, held drawn in toward the palm, and camptodactyly of the index fingers, a permanent flexion contracture of those fingers. Craniofacial dysmorphic features completed the picture of a multisystem congenital syndrome. Taken together, these findings fall within the spectrum of arthrogryposis-like presentations, in which multiple joints are fixed in abnormal positions from before birth, typically reflecting reduced fetal movement or abnormalities of the neuromuscular system during development.

The clinical course that followed was demanding. The boy experienced delayed motor and speech milestones, meaning he reached the stages of sitting, walking, and speaking significantly later than typically developing children. He developed an abnormal gait and suffered from chronic constipation severe enough to require long-term laxative use, a detail that hints at involvement of the autonomic or enteric nervous system. Over the years he underwent multiple orthopedic and ophthalmologic surgeries to address the structural consequences of his condition, and he continues to receive supportive therapies. Notably, his cognitive development remains preserved, a feature that distinguishes his presentation from many other multisystem developmental syndromes in which intellectual disability is a core component.

The genetic explanation emerged through whole exome sequencing, the technique at the heart of modern rare disease diagnosis. Rather than reading the entire three-billion-letter genome, exome sequencing targets the roughly one to two percent of DNA that encodes proteins, where the majority of known disease-causing variants reside. This approach allows laboratories to scan thousands of genes simultaneously in a single test, and it has transformed the diagnosis of congenital disorders by uncovering variants in genes that no one would have suspected from the clinical picture alone. In this patient, sequencing revealed not one but two different variants in ERBB2, one inherited on each of the two copies of the gene, a configuration known as compound heterozygosity.

The two variants are molecularly distinct in ways that matter for interpreting their effects. The first, designated c.1018C>T, converts a cytosine to a thymine at position 1018 of the coding sequence, changing the codon for arginine at position 340 into a premature stop signal, an alteration annotated as p.Arg340Ter. A premature termination codon of this kind typically triggers nonsense-mediated mRNA decay, a cellular quality-control pathway that destroys transcripts containing early stop signals, or else produces a truncated protein lacking essential functional domains. The second variant, c.3412+1_3412+13del, is a deletion that removes the first nucleotide of intron 3412 together with twelve following intronic nucleotides, destroying the canonical splice donor site at that exon-intron boundary. Splice donor mutations of this type generally cause the splicing machinery to skip the preceding exon or retain intronic sequence, either outcome disrupting the reading frame and abolishing functional protein production. The predicted consequence of this second alteration is annotated as uncertain, denoted p.?, reflecting the difficulty of predicting the exact aberrant transcript.

ERBB2 is far better known to the public under another name: HER2, the growth factor receptor famous as a driver and drug target in certain breast cancers. The gene encodes a receptor tyrosine kinase that sits in the cell membrane and partners with the other three members of the ERBB family, EGFR, ERBB3, and ERBB4. When signaling molecules called neuregulins or other ligands engage these receptors, ERBB2 forms dimers with its partners and switches on intracellular cascades, including the MAP kinase and PI3K-AKT pathways, that tell cells to grow, differentiate, and survive. Unlike its relatives, ERBB2 is a preferred dimerization partner with no known soluble ligand of its own, making it a central hub of the signaling network. In developing tissues, particularly the nervous system and the heart, this signaling is essential, which helps explain why germline variants that cripple the gene can produce effects across multiple organ systems.

Before this report, the clinical literature on ERBB2-related congenital disease rested on a remarkably thin foundation: a single family in which two siblings carried a homozygous missense variant, meaning an identical single-letter protein-altering change on both gene copies. That first description established ERBB2 as a gene implicated in rare multisystem developmental disorders, but a phenotype defined by one family is inherently provisional. The new case changes the picture in two ways at once. First, it adds a second, unrelated family, confirming that ERBB2-associated disease is not confined to a single pedigree. Second, it demonstrates that the disorder can arise from a different genetic mechanism, compound heterozygosity for a nonsense variant and a splice-site deletion, rather than a homozygous missense change, and that the resulting phenotype can include features not observed in the original siblings.

The classification of the variants followed established standards. The report references the criteria jointly published by the American College of Medical Genetics and Genomics and the Association for Molecular Pathology, the framework used worldwide to grade sequence variants as pathogenic, likely pathogenic, uncertain, or benign. Under this system, evidence from population frequency databases such as dbSNP, curated mutation collections such as the Human Gene Mutation Database, computational predictions, and the observed segregation of variants within families all feed into the final classification. The authors’ conclusion that this case represents a possible phenotypic extension of the known disorder is deliberately measured: with only a handful of patients described, distinguishing which features are core to the syndrome and which are variable remains an ongoing challenge that each new case helps to resolve.

The diagnostic journey also illustrates why exome sequencing has become the decisive tool for conditions like this one. A presentation combining clubfoot, contractures, hypotonia, and dysmorphic features could plausibly be attributed to dozens of different genes, and for many years such children accumulated diagnoses of unspecified arthrogryposis or syndromic developmental delay. Sequencing collapses that diagnostic odyssey into a single analysis, and in this case it pinpointed a gene whose connection to congenital disease had been established only once before. The Lithuanian team, based at Vilnius University and Vilnius University Hospital Santaros Klinikos, conducted the work with ethics approval from the Vilnius Regional Biomedical Research Ethics Committee and with written informed consent from the patient’s parents for both participation and publication.

For the emerging field of ERBB2-related medicine, the case carries implications in two directions. In oncology, ERBB2 is one of the most intensively targeted genes in human disease, with engineered antibodies and small-molecule inhibitors transforming outcomes for HER2-positive cancers. In genetics, the same gene is now firmly on the map of recessive congenital disorders, and the new report suggests that the clinical spectrum may be wider than the first family implied, potentially including preserved cognition, chronic gastrointestinal involvement, and a pattern of contractures and deformities requiring serial surgical intervention. Every additional patient described sharpens the genotype-phenotype correlations that clinicians will need to counsel families, interpret future sequencing results, and, eventually, evaluate whether therapies aimed at ERBB2 pathways might one day extend from cancer wards to the care of children with rare developmental disease.

Subject of Research: ERBB2-associated multisystem congenital disorder and the expansion of its phenotypic and genetic spectrum through a novel case report

Article Title: Broadening the phenotypic and genetic spectrum of ERBB2-associated multisystem congenital disorder

Article References: Jakubėnaitė, A., Gurskaitė, G., Morkūnienė, A., & Burnyte, B. (2026). Broadening the phenotypic and genetic spectrum of ERBB2-associated multisystem congenital disorder. BMC Pediatrics. https://doi.org/10.1186/s12887-026-07773-8

Image Credits: AI Generated

DOI: 10.1186/s12887-026-07773-8

Keywords: ERBB2, HER2, whole exome sequencing, congenital anomalies, arthrogryposis, compound heterozygosity, rare disease, craniofacial dysmorphic features, receptor tyrosine kinase, hypotonia, genetics, pediatrics

The logic behind the study is deceptively simple but clinically consequential. CAR-T therapy works by engineering a patient’s own T cells to recognize a specific antigen on the surface of tumor cells. The strategy has produced remarkable results in blood cancers, but solid tumors have proven far more resistant, in part because tumor cells can simply stop displaying the antigen the therapy targets, a phenomenon known as target escape. One established countermeasure is to engineer CAR-T cells that recognize two different membrane molecules simultaneously, so that tumor cells remain visible to the immune system as long as they express either one of the two antigens. To justify such a design, researchers first need to know how often the two candidate antigens appear together in the same patients’ tumors, data that had been largely missing for GPC-3 and AXL in liver cancer.

The research team, led by Chenli Qiu, Jieyi Shi and Su Yan, who contributed equally, with senior authors Xiaoyan Zhang, Xiaowu Huang and Jianqing Xu, analyzed surgical specimens from 140 patients with hepatocellular carcinoma who underwent hepatectomy at Zhongshan Hospital in 2019. All patients had complete clinical information, and the samples were obtained following Institutional Review Board approval and written informed consent. The investigators used immunohistochemistry on 5-micrometer formalin-fixed, paraffin-embedded tissue sections, a standard pathology technique that uses antibodies to reveal where specific proteins sit within tissue. Sections were subjected to heat-induced antigen retrieval in Tris/EDTA buffer at 99 degrees Celsius, blocked with bovine serum albumin, and incubated overnight at 4 degrees Celsius with primary antibodies against AXL and GPC-3 before signal detection with HRP-labeled secondary antibodies and DAB chromogen.

Quantification followed a rigorous semiquantitative scheme. Only cytoplasmic or membranous staining counted as positive, and two pathologists independently scored each slide on a four-point scale, from 0 for no expression through weak, moderate and strong expression. The slides were digitized on a TissueFAXS platform, and categorical comparisons were analyzed with Pearson’s chi-square test, ordered data with the Goodman–Kruskal Gamma test, and survival outcomes with Kaplan–Meier curves and the log-rank test. This level of methodological detail matters because immunohistochemical scoring is inherently interpretive, and the discrepancy between different studies often traces back to the antibodies used and the thresholds applied, a point the authors themselves acknowledge.

The headline numbers are striking. GPC-3 was positive in 108 of 140 tumors, a rate of 77.1 percent, with strong expression accounting for the largest share, 42.1 percent of all cases. AXL was even more prevalent, positive in 120 of 140 tumors, or 85.7 percent, with strong expression in 48.6 percent. Critically, only 9 of the 140 samples, 6.4 percent, were negative for both proteins, meaning that 93.6 percent of hepatocellular carcinomas expressed at least one of the two antigens and 69.3 percent expressed both simultaneously. In adjacent non-tumor liver tissue, by contrast, GPC-3 was weakly positive in just one of 140 samples, and AXL staining, while present at low levels in some surrounding tissue, was consistently weaker than in the tumors themselves.

That contrast between tumor and normal tissue is what makes these two proteins attractive drug targets. GPC-3 is an oncofetal protein, a glycosylphosphatidylinositol-anchored membrane molecule that drives embryonic morphogenesis through the canonical Wnt/beta-catenin and Hedgehog pathways. It is essentially silent in the healthy adult liver but reactivated in hepatocellular carcinoma, where it appears in the cytoplasm and on the cell membrane in 70 to 100 percent of cases in previous analyses. Mechanistically, GPC-3 acts as a co-receptor that recruits Wnt, insulin-like growth factors and growth factors such as HGF, FGF-2 and TGF to their respective receptors, fueling proliferation, migration and survival. Overexpression also disrupts the Bax/Bcl-2 axis to increase resistance to apoptosis, promotes epithelial–mesenchymal transition through the ERK pathway, and modulates the CXCL12/CXCR4 axis to drive angiogenesis and metastasis.

AXL tells a complementary story. It is a receptor tyrosine kinase of the TAM family, spanning TYRO3, AXL and MERTK, and it is implicated in cancer cell proliferation, stemness, invasion, angiogenesis and therapeutic resistance. When its ligand GAS6 binds, AXL dimerizes and activates downstream PI3K/Akt and MAPK/ERK signaling, pathways central to tumor growth and spread. AXL is overexpressed across many malignancies, at roughly 47 percent in renal cancer, 69 percent in lung cancer, 57.6 percent in breast cancer and 76.7 percent in colon cancer, and prior studies had placed its expression in hepatocellular carcinoma at 22 to 43 percent. The higher rate observed in the new study may reflect differences in patient populations, detection antibodies and scoring criteria. Notably, AXL positivity was associated with microvascular invasion in this cohort, though neither protein correlated with overall survival over the 50-month follow-up, possibly because the negative sample groups were small and follow-up relatively short.

The therapeutic implications extend beyond CAR-T design. A phase I dose-escalation study of autologous CAR-T cells targeting GPC-3 in advanced hepatocellular carcinoma already showed a manageable safety profile, with an overall objective response rate of 50 percent that reached 57.1 percent in the highest dose cohort. On the AXL side, the selective inhibitor bemcentinib has received US FDA fast-track designation in combination with a PD-1/PD-L1 agent, and AXL-directed CAR-T cells have been reported in non-small cell lung cancer and triple-negative breast cancer. Intriguingly, the authors highlight reports that anti-PD-L1 antibodies can promote liver cancer cell proliferation by activating a PD-L1–AXL signal relay, which suggests that the high AXL expression documented in this study may partly explain the suboptimal performance of immune checkpoint inhibitors in some hepatocellular carcinoma patients, and that AXL-targeted therapy could sensitize tumors to checkpoint blockade.

Dual-targeting is not a panacea, however, and the authors are candid about the challenges. AXL is also present on macrophages and dendritic cells, raising the specter of on-target, off-tumor toxicity if engineered T cells attack healthy immune populations. Proposed engineering countermeasures include optimizing CAR structural design, incorporating logic gates that require combinations of antigens to trigger killing, and building suicide gene systems that can eliminate the infused cells if toxicity emerges. Antigen escape, limited persistence of CAR-T cells in the hostile solid tumor microenvironment, manufacturing cost and toxicity management remain the central obstacles that any GPC-3/AXL dual-target program would need to address in preclinical and clinical testing.

The study’s limitations are equally clear: it was a single-center, retrospective analysis with a relatively modest sample size and short follow-up, and it found no correlation between either protein and most clinicopathological features such as cirrhosis, macrovascular invasion, tumor size, BCLC stage, recurrence or mortality. Yet the core result stands on firm ground. Nearly 94 percent of hepatocellular carcinomas in this cohort displayed at least one of the two antigens, and more than two-thirds displayed both, making GPC-3 and AXL together a coverage profile that few single or paired targets in liver cancer can match. As immunotherapy increasingly becomes a pillar of hepatocellular carcinoma treatment alongside surgery, locoregional and systemic options, this co-expression map offers exactly the kind of evidence base needed to move dual-target CAR-T cells for liver cancer from concept toward the clinic.

Subject of Research: Co-expression of Glypican-3 and AXL in hepatocellular carcinoma and their potential as dual targets for CAR-T cell immunotherapy

Article Title: GPC-3 and AXL are essential factors and therapeutic targets for hepatocellular carcinoma

Article References: Qiu, C., Shi, J., Yan, S., Zhu, C., Zhang, S., Feng, Y., Zhang, X., Huang, X., & Xu, J. (2025). GPC-3 and AXL are essential factors and therapeutic targets for hepatocellular carcinoma. Clinical Cancer Bulletin, 4(1), Article 22. https://doi.org/10.1007/s44272-025-00051-1

Image Credits: AI Generated

DOI: 10.1007/s44272-025-00051-1

Keywords: hepatocellular carcinoma, Glypican-3, AXL, CAR-T cell therapy, immunotherapy, biomarkers, liver cancer, targeted therapy, immunohistochemistry, receptor tyrosine kinase, oncofetal protein, dual-target therapy

The biology behind the excitement is genuine. c-MET, the mesenchymal-epithelial transition factor, is a receptor that becomes activated when hepatocyte growth factor binds its extracellular domain, triggering a cascade of downstream signalling through partners such as Gab1, PI3K, MAPK and STAT3. This signalling drives cell motility, proliferation and survival, the hallmarks of invasive growth. In healthy tissue, c-MET expression is low and restricted mainly to epithelial and endothelial cells involved in repair and development. In cancer, however, the pathway is frequently dysregulated, producing receptor overexpression that is tightly linked to metastasis, therapy resistance and poorer survival across multiple tumour types. Reported overexpression reaches 26 to 82 percent in oral cavity squamous cell carcinoma, up to 87 percent in penile cancer, 15 to 67 percent in colorectal cancer, 17 to 81 percent in non-small cell lung cancer, 56 to 80 percent in renal cell carcinoma and 14 to 54 percent in breast cancer.

Critically for imaging, tumour tissue can show c-MET levels up to eleven times higher than surrounding epithelium, with a median ratio of 3.4. Imaging scientists generally consider a target diagnostically useful when it can be detected with a signal-to-background ratio above 1.5 to 2, placing c-MET squarely within the range worth pursuing. The review team conducted their search of PubMed, Embase and Scopus according to PRISMA 2020 guidelines, with the protocol registered on PROSPERO, and ultimately included 63 eligible reports: 50 preclinical animal studies published between 2002 and 2026 and 13 clinical reports from 2015 to 2026. The preclinical literature described 62 unique tracers, of which 44 carried a radioactive label and 18 a fluorescent one, distributed across three broad compound classes: monoclonal antibodies, peptides and small molecules.

Each scaffold carries its own pharmacokinetic personality. Monoclonal antibodies, at roughly 150 kilodaltons, showed the highest binding affinities and, in selected studies, the highest tumour-to-background ratios, but their slow circulation meant tumour uptake peaked three to four days after injection, forcing reliance on long-lived radionuclides such as zirconium-89, used in nearly 62 percent of antibody designs. Radiolabelled antibodies achieved tumour accumulation of 3 to 47 percent of injected dose per gram, with reported tumour-to-background ratios ranging from a dismal 0.1 to an impressive 43. Onartuzumab-based designs dominated this category, accounting for 38 percent of antibody tracers. Fluorescent antibody variants conjugated to IRDye800CW delivered comparable affinities of 1.0 to 1.3 nanomolar and tumour-to-background ratios near 5, peaking four days after injection.

Peptides, by contrast, occupy a pharmacokinetic sweet spot. Weighing between 0.5 and 5 kilodaltons, they combine rapid tumour targeting with fast systemic clearance, producing high contrast within hours and permitting short-lived isotopes such as technetium-99m and fluorine-18. Sixteen radiolabelled peptide tracers were identified, dominated by cMBP-derived designs, with binding affinities spanning 0.9 to 326 nanomolar and the best variants, such as the macrocyclic HiP-8, approaching antibody-like binding at around 1 nanomolar. Tumour uptake generally peaked within one to two hours, with reported tumour accumulation of 0.7 to 9.4 percent of injected dose per gram and tumour-to-background ratios of 1.7 to 20. Fluorescent peptide tracers followed a similar pattern, with Cy5-analogues the most popular fluorophores and reported ratios reaching as high as 33. Small molecules, though fast and capable of crossing cell membranes, showed generally lower tumour accumulation and contrast, with ratios of just 0.2 to 3.3, and remain the least mature class.

Yet when the authors traced which designs actually reached the clinic, the pattern was unambiguous: all four translated tracers were peptides. EMI-137, a macrocyclic 26-amino-acid peptide engineered with intramolecular cyclisation for subnanomolar-range affinity and renal clearance, was the most extensively studied, evaluated across eight clinical reports involving 102 participants in five tumour types. cMBP-ICG, a minimalist 12-amino-acid linear peptide conjugated to indocyanine green, was tested in 60 patients with oral cavity cancer. The PET tracers gallium-68-EMP-100 and gallium-68-MetP together accounted for three reports and 25 participants. The reasons others stalled remain unclear, but the authors point to a combination of affinity, stability, manufacturability, regulatory feasibility and the clinical relevance of the animal models used, factors that tracer performance metrics alone cannot predict.

The clinical data reveal both promise and frustration. EMI-137 enabled fluorescence-guided tumour visualisation with tumour-to-background ratios of 1.3 to 9.7, and in its landmark colorectal application, second-pass fluorescence endoscopy identified nine additional adenomatous lesions, roughly 19 percent, that white-light colonoscopy had missed. In papillary thyroid cancer, fluorescence reclassified disease from unifocal to multifocal in four of five patients by detecting foci as small as 1.4 millimetres invisible on preoperative ultrasound. But specificity suffered badly: benign c-MET-expressing tissues lit up too, yielding a sample-size weighted average sensitivity of 81.7 percent but a specificity of just 39.3 percent across EMI-137 studies. In laparoscopic colorectal surgery, fluorescence discriminated only 4 of 9 primary tumours and detected no nodal metastases despite histological confirmation in over half the patients.

The standout performer was cMBP-ICG, applied topically in oral cavity squamous cell carcinoma. Because the tracer was rinsed into the mouth rather than injected, systemic exposure was minimal and imaging immediate. A randomised controlled trial of 50 patients compared fluorescence-guided biopsy-site selection against conventional white-light inspection in the same patients, and fluorescence won decisively on every metric: sensitivity of 88 versus 65 percent, specificity of 93 versus 76 percent, positive predictive value of 90 versus 68 percent, negative predictive value of 91 versus 74 percent, and overall diagnostic accuracy of 91 versus 72 percent, with statistical significance across the board. Weighted averages across both cMBP-ICG studies showed 85.5 percent sensitivity and 91.1 percent specificity, with tumour-to-background ratios of 2.7 to 4.1. The two PET tracers demonstrated feasible whole-body c-MET imaging, and gallium-68-MetP produced the first clinical evidence of a quantitative correlation between tracer uptake and immunohistochemical c-MET expression, with a correlation coefficient of 0.71. All four tracers showed favourable safety, with only mild adverse events reported.

The review’s authors are candid about the caveats. Risk-of-bias assessment using the ROBINS-I tool judged the clinical evidence to carry serious methodological limitations, stemming from small sample sizes, absent control groups, unblinded outcome assessment and the predominance of single-arm early-phase feasibility studies. Preclinical work leaned heavily on a handful of high c-MET-expressing cell lines in subcutaneous xenografts, models that poorly recapitulate the heterogeneous expression and microenvironmental complexity of real tumours. Only one radiolabelled tracer report and seven fluorescent tracer reports used orthotopic models. Heterogeneity in study designs, outcome definitions and reporting was so great that a formal meta-analysis proved impossible, forcing the team into narrative synthesis with weighted averages that they themselves caution should be interpreted cautiously.

What emerges is a field at an inflection point. The biological rationale for c-MET imaging is solid, the safety profile of the leading tracers is reassuring, and the single randomised trial in oral cancer demonstrates that molecular imaging can genuinely outperform conventional assessment. The path forward, the authors argue, lies in larger, standardised prospective studies designed to prove that c-MET-targeted imaging improves clinically meaningful endpoints: lesion detection, staging accuracy, treatment selection and surgical decision-making. They also point toward synergy with therapy, noting that c-MET status already guides first-line treatment with MET tyrosine kinase inhibitors and antibody-drug conjugates in non-small cell lung cancer, raising the prospect of matched imaging and therapeutic agents in a theranostic paradigm. Until such evidence arrives, the 62-tracer graveyard of preclinical promise stands as a warning that in molecular imaging, a beautiful animal study is only the beginning of a very long road.

Subject of Research: Systematic review of preclinical and clinical molecular imaging tracers targeting the c-MET receptor in cancer

Article Title: Molecular imaging tracers targeting c-MET: a systematic review of preclinical evidence and clinical translation

Article References: Verdijk, R. W. A., van der Mierde, S. M., Berehova, N., van Meerbeek, M. P., Brouwer, O. R., van der Poel, H. G., van Leeuwen, F. W. B., & Buckle, T. (2026). Molecular imaging tracers targeting c-MET: a systematic review of preclinical evidence and clinical translation. European Journal of Nuclear Medicine and Molecular Imaging. https://doi.org/10.1007/s00259-026-08174-w

Image Credits: AI Generated

DOI: 10.1007/s00259-026-08174-w

Keywords: c-MET, molecular imaging, fluorescence-guided surgery, PET imaging, EMI-137, cMBP-ICG, clinical translation, systematic review, tumour tracers, image-guided surgery, receptor tyrosine kinase, cancer diagnostics