NTRK genes encode a family of three receptor proteins that play essential roles in brain development, pain signaling, and nerve cell survival. When these same genes break apart and fuse with unrelated partner genes, the resulting proteins can drive cancer growth across dozens of tumor types. That discovery has made NTRK one of the most consequential stories in modern oncology, giving rise to the first cancer drugs approved based on a genetic alteration rather than where in the body the tumor originated.
What NTRK Genes Normally Do
The human genome contains three NTRK genes: NTRK1, NTRK2, and NTRK3. Each encodes a TRK (tropomyosin receptor kinase) protein called TrkA, TrkB, and TrkC, respectively. These proteins sit on the surface of nerve cells and act as receivers for a family of signaling molecules called neurotrophins.
TrkA binds nerve growth factor (NGF), a molecule critical for the development and survival of sensory and sympathetic neurons during embryonic life.1PubMed Central. Congenital insensitivity to pain with anhidrosis: novel mutations in the TRKA (NTRK1) gene encoding a high-affinity receptor for nerve growth factor TrkA is also heavily expressed in pain-sensing neurons, where it mediates most of NGF’s effects on the pain system.2Journal of Pain Research. Nerve Growth Factor Signaling and Its Contribution to Pain
TrkB pairs with brain-derived neurotrophic factor (BDNF) and plays a central role in synaptic plasticity, the process by which connections between neurons strengthen or weaken over time. BDNF-TrkB signaling influences everything from how neural progenitor cells are selected during development to how mature neurons wire their connections.3PubMed Central. Postsynaptic BDNF-TrkB signaling in synapse maturation, plasticity, and disease
TrkC responds to neurotrophin-3 (NT-3) and is expressed early in embryonic development. Animal studies in which the NTRK3 gene was knocked out revealed that the loss leads to the death of specific sensory neuron populations and abnormalities at early stages of sensory nerve development.4PubMed Central. Targeted mutation in the neurotrophin-3 gene results in loss of muscle sensory neurons In short, each TRK receptor partners with a specific neurotrophin to keep particular populations of nerve cells alive and functioning. Problems with any of these receptors can have serious consequences. Mutations in NTRK1, for example, cause a rare condition called congenital insensitivity to pain with anhidrosis, in which a person cannot feel pain or produce sweat.5PubMed Central. Congenital insensitivity to pain with anhidrosis: novel mutations in the TRKA (NTRK1) gene encoding a high-affinity receptor for nerve growth factor
How NTRK Fusions Drive Cancer
Under normal circumstances, TRK receptors switch on only when a neurotrophin binds to them. A fusion changes that arrangement entirely. An NTRK fusion happens when part of an NTRK gene breaks off and attaches to an unrelated gene. The resulting hybrid gene produces a chimeric protein that is stuck in the “on” position, sending constant growth signals to the cell without needing any neurotrophin to trigger it.6PubMed Central. Phase separation underlies signaling activation of oncogenic NTRK fusions Because the growth signal never turns off, the cell keeps dividing, forming a tumor.
The most commonly described fusion partner is ETV6, which pairs with NTRK3 to form the ETV6-NTRK3 fusion. But the roster of known partners keeps growing. A study of pediatric tumors found several novel fusions, including KCTD16-NTRK1 in ganglioglioma and IRF2BP2-NTRK3 in papillary thyroid carcinomas, and concluded that NTRK fusions involve a broader panel of partners and a wider range of tumor types than previously appreciated.7PubMed Central. NTRK Fusions Identified in Pediatric Tumors: The Frequency, Fusion Partners, and Clinical Outcome As more tumors are sequenced, the list of known fusion partners continues to expand.
Where NTRK Fusions Show Up
NTRK fusions appear in two distinct patterns. In a handful of rare cancers, they occur at very high rates, up to or greater than 90%, making them a defining genetic feature of those diseases. Secretory breast carcinoma, mammary analogue secretory carcinoma of the salivary gland, and infantile fibrosarcoma are the best-known examples.8ESMO Open. How I treat NTRK gene fusion-positive cancers
In common cancers, the picture is very different. Fusions occur at low frequencies, typically less than 1%, across tumor types like melanoma, glioma, and carcinomas of the thyroid, lung, and colon.9Annals of Oncology. Identifying patients with NTRK fusion cancer Because these cancers are so much more prevalent overall, a tiny percentage still translates to a meaningful number of patients who could benefit from targeted therapy. The challenge is finding them.
NTRK fusions also appear more frequently in pediatric tumors than in adult cancers.10PubMed Central. NTRK Fusions Identified in Pediatric Tumors: The Frequency, Fusion Partners, and Clinical Outcome This has practical implications for how aggressively children with certain tumor types are screened.
Detecting NTRK Fusions
Several laboratory techniques can identify NTRK fusions, and each comes with trade-offs. Immunohistochemistry (pan-Trk IHC) is a relatively quick and inexpensive screening tool that flags tumors producing excess TRK protein, but it can produce false positives and misses some fusions. FISH and reverse transcription PCR detect specific known fusions but may miss novel or unusual partners. DNA-based next-generation sequencing (NGS) can catch fusions but sometimes struggles with large intronic regions in the NTRK genes. RNA-based NGS is generally considered the most comprehensive approach because it detects the actual fusion transcript regardless of the genomic breakpoint, though it costs more and requires higher-quality tissue samples.11PubMed Central. Detection of NTRK Fusions: Merits and Limitations of Current Diagnostic Platforms
The economic picture is sobering. Because NTRK fusions are rare in common cancers, you have to test many patients to find one who carries a fusion. An analysis of testing costs estimated that the expense of RNA-based NGS to identify a single patient with an NTRK fusion ranged from £377 to over £282,000, depending on the underlying prevalence of the fusion in the cancer type being tested and the testing strategy used.12PubMed. Appraising the Costs of Genomic Testing for Histology-Independent Technologies: An Illustrative Example for NTRK Fusions In rare tumor types where fusions are common, testing is efficient. In common cancers where fusions appear in less than 1% of patients, the cost per identification soars. Many oncology guidelines now recommend broad NGS panels for advanced solid tumors, partly because NTRK fusions are just one of many actionable targets these panels can detect simultaneously, which improves the cost-effectiveness of each test.
First-Generation TRK Inhibitors
The clinical payoff of finding an NTRK fusion lies in the drugs designed to block TRK signaling. Larotrectinib was the first to earn regulatory approval, and the pivotal trial results were striking. Across 55 patients representing 17 different TRK fusion-positive tumor types, ages ranging from four months to 76 years, the overall response rate was 75% by independent review. At one year, 71% of responses were still ongoing and 55% of patients remained progression-free.13PubMed Central. Efficacy of Larotrectinib in TRK Fusion-Positive Cancers in Adults and Children
Those numbers were remarkable not just for their magnitude but for their consistency across wildly different cancer types. This was the proof of concept for “tumor-agnostic” therapy: if the cancer is driven by an NTRK fusion, the drug works regardless of where the tumor originated. Real-world data from a French cohort of children, adolescents, and young adults reported a broadly similar overall response rate of 70%, with a median time to best response of about two months and a two-year overall survival rate of roughly 75%.14PubMed. Real-world experience of larotrectinib in children, adolescents and young adults with TRK fusion solid tumors: The SACHA-France experience
Entrectinib, the other first-generation TRK inhibitor, works against TRK, ROS1, and ALK. It was developed with a particular emphasis on reaching the brain, which most cancer drugs struggle to do.
Treating Brain Metastases
Brain metastases represent one of the hardest problems in oncology. Most targeted therapies cannot reach the brain in meaningful concentrations, leaving patients dependent on radiation or surgery. Entrectinib was specifically engineered to get past this barrier. It is a weak substrate for P-glycoprotein, the cellular pump that normally ejects drugs from the central nervous system. In animal models with intracranial tumors, oral entrectinib at moderate doses produced over 80% tumor inhibition, and higher doses pushed inhibition to nearly 100%, with treated animals surviving longer throughout the dosing window.15PubMed Central. Entrectinib, a TRK/ROS1 inhibitor with anti-CNS tumor activity: differentiation from other inhibitors in its class due to weak interaction with P-glycoprotein
Clinical evidence confirmed the preclinical promise. In one early case of NTRK1-rearranged non-small cell lung cancer, a patient’s brain metastases completely resolved on entrectinib without any radiation treatment, demonstrating potent real-world CNS activity of the drug.16PubMed Central. Durable Clinical Response to Entrectinib in NTRK1-Rearranged Non-Small Cell Lung Cancer The ability to treat both systemic disease and brain metastases with a single oral pill was a meaningful advance for patients who previously faced very limited options.
When TRK Inhibitors Stop Working
Most patients who respond to first-generation TRK inhibitors eventually develop resistance. The mechanisms fall into two broad categories.
On-target resistance involves new mutations within the NTRK gene itself, specifically in the kinase domain where the drug binds. The most well-characterized of these are solvent front mutations like the G595R substitution in TrkA. Structural analysis shows that replacing the small glycine amino acid at position 595 with the much bulkier arginine creates a steric clash inside the binding pocket, disrupting the contacts the drug needs to stay attached and substantially increasing the energy required for binding.17Journal of Molecular Modeling. Mechanistic basis of G595R-mediated resistance to entrectinib in TRK kinase: a structural-energetic perspective Gatekeeper mutations work similarly by blocking drug access to the active site.
Off-target resistance takes a completely different route. Instead of changing the drug’s target, the tumor activates alternative growth pathways that bypass TRK signaling altogether. Research has shown that these bypass mechanisms frequently converge on the MAPK pathway, a major signaling cascade that can sustain tumor growth even when TRK is successfully inhibited.18PubMed Central. Resistance to TRK inhibition mediated by convergent MAPK pathway activation Off-target resistance is harder to combat with next-generation TRK inhibitors alone because the problem is not that the drug cannot hit its target. The problem is that the target no longer matters to the cancer cell.
Next-Generation TRK Inhibitors
Repotrectinib was designed to overcome the on-target resistance mutations that defeat first-generation drugs. In preclinical testing, it was over 10-fold more potent than the second-generation agent LOXO-195 against both wild-type TRK fusions and solvent front mutations, and more than 100-fold more potent against certain gatekeeper mutations. It was also the only TRK inhibitor active against a particularly challenging compound mutation, in which two resistance mutations occur simultaneously in the same gene copy.19Clinical research. Repotrectinib, a next generation TRK inhibitor, overcomes TRK resistance mutations including solvent front, gatekeeper and compound mutations
Whether next-generation inhibitors will fully address off-target resistance remains an open question. Because off-target resistance relies on entirely different pathways, the most promising approaches likely involve combining a TRK inhibitor with drugs that block those escape routes, such as MEK inhibitors targeting the MAPK pathway. Early clinical efforts are exploring these combinations, but the field is still in its early stages.
Side Effects Tied to TRK Biology
Because TRK receptors have important jobs in the normal nervous system, blocking them has predictable consequences. A study characterizing on-target side effects found that about half of patients on TRK inhibitors gained weight, and the percentage increased the longer they stayed on therapy. Dizziness, sometimes accompanied by problems with coordination, affected roughly 40% of patients, typically appearing within the first couple of weeks. Perhaps most telling, about a third of patients experienced increased pain when their TRK inhibitor was temporarily or permanently stopped, a rebound effect consistent with TrkA’s central role in pain signaling.20PubMed Central. Characterization of On-Target Adverse Events Caused by TRK Inhibitor Therapy
These side effects are manageable for most patients but underscore an important point: TRK inhibitors are not surgically precise cancer drugs that leave normal biology untouched. They suppress a signaling system that the nervous system genuinely relies on, and the side-effect profile reflects that reality.
The NGF-TrkA Pain Connection Beyond Cancer
The same pathway that goes haywire in NTRK fusion cancers has become a major target in pain research, independent of oncology. NGF was identified decades ago as a survival factor for developing nerve cells, but it also acts as a potent pain mediator in adults. When tissue is injured or inflamed, NGF levels rise, bind to TrkA on pain-sensing neurons, and amplify pain signals.21Journal of Pain Research. Anti-nerve growth factor in pain management: current evidence
This has driven the development of anti-NGF monoclonal antibodies, drugs that intercept NGF before it can reach TrkA. These antibodies have been tested for chronic pain conditions including osteoarthritis and cancer-related pain, representing a fundamentally different approach from opioids.22PubMed Central. The Role of Anti-Nerve Growth Factor Monoclonal Antibodies in the Control of Chronic Cancer and Non-Cancer Pain Broader reviews of non-opioid analgesics have highlighted NGF antibodies as one of the most promising new drug classes for chronic, inflammatory, and neuropathic pain, with potentially fewer side effects than traditional painkillers.23PubMed Central. Novel non-opioid analgesics in pain management
The connection to TRK inhibitor side effects mentioned earlier is direct: if blocking TRK signaling in cancer patients causes rebound pain upon drug withdrawal, that confirms just how central this pathway is to how the body processes pain. Oncology and pain medicine are, in a sense, studying the same receptor from opposite directions.
BDNF, TrkB, and Neurodegenerative Disease
TrkB and its ligand BDNF have drawn attention from an entirely different medical field. BDNF-TrkB signaling is essential for synaptic plasticity, and disruptions in this pathway have been strongly linked to Alzheimer’s disease. The imbalance between the precursor form of BDNF and its mature, active form appears to play a role in the neurodegeneration that characterizes the disease.24PubMed Central. Unlocking Alzheimer’s Disease: The Role of BDNF Signaling in Neuropathology and Treatment
This puts the NTRK gene family at the intersection of three distinct medical domains: oncology, pain management, and neurodegeneration. The biological tools are different in each case. Small-molecule inhibitors block TRK in cancer, antibodies intercept NGF for pain, and potential BDNF-boosting strategies are being explored for neurological disease. But the underlying receptor family is the same, which means discoveries in any one of these fields can inform the others in unexpected ways.
Evolutionary Origins of the TRK System
The neurotrophin-TRK signaling system is ancient. It was long assumed to have appeared with vertebrates, but the identification of a TRK receptor gene in a cephalochordate, a simple marine animal related to vertebrates, pushed the origin back further. The system already existed at the base of the deuterostome group, the broad animal lineage that includes everything from sea urchins to humans.25Brain, Behavior and Evolution. Formation and Evolution of the Chordate Neurotrophin and Trk Receptor Genes
Within vertebrates, the family expanded through major gene duplication events early in evolutionary history, before the appearance of cartilaginous fish. Having multiple copies of these genes allowed each to specialize: TrkA for pain and sympathetic neurons, TrkB for synaptic plasticity, TrkC for proprioceptive sensory neurons. This diversification enabled the increasingly complex nervous systems that followed.26PubMed. Evolution of the vertebrate neurotrophin and Trk receptor gene families The proposal that subtle mutations and duplications in a primitive neurotrophin-TRK system were instrumental to the development of the complex vertebrate brain remains one of the more compelling hypotheses in comparative neurobiology.27Molecular and Cellular Neuroscience. Origin and evolution of the Trk family of neurotrophic receptors In other words, the same gene family now targeted by cancer drugs helped build the nervous system in the first place, hundreds of millions of years ago.