In the last three years, two TRK inhibitors, larotrectinib and entrectinib, gained the approval for NTRK fusion-positive cancer patients, regardless of tumour histology, by both FDA and EMA thanks to the high response rate observed in the pan-cancer setting of basket trials.

The neurotrophic receptor tyrosine kinase genes NTRK1, NTRK2 and NTRK3 encode the tropomyosin receptor kinase proteins TRKA, TRKB and TRKC, respectively. These three receptors play important roles in nervous system development through their regulation of cell proliferation, differentiation, apoptosis, and survival of neurons in both the central and peripheral nervous systems. The TRK receptors are expressed abundantly in the nervous system, as well as in monocytes, lung, bone and pancreatic beta cells. TRKA, TRKB, and TRKC are most frequently activated by their primary ligands nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT-3), respectively. Once activated, the three TRK family members most frequently signal through several downstream signalling pathways, including SHC–RAS–MAPK, PI3K–AKT or PLC–PKC. Activation of these signalling cascades results in cell programs that mediate cellular proliferation, synaptic plasticity, neurite outgrowth and repair, prevention or repair of neurodegeneration, sensory neuron maintenance or apoptosis.

The most common mechanism of oncogenic activation of TRK proteins is through genomic rearrangement and the creation of a gene fusion. This causes the loss of some of the extracellular domain of TRK, which likely contains the critical regulatory domains, thereby resulting in constitutive activation of the kinase. These oncogenic fusions, indeed, involve the carboxy-terminal kinase domain, or 3’, of TRK and the amino-terminal region, or 5’, of various partner genes. TRK fusions lead to overexpression of the chimeric proteins, resulting in constitutively active upstream, ligand-independent downstream signalling. To date, over 80 different fusion partner genes have been identified. NTRK fusions occur in about 1% of all solid tumours, at high frequencies in some rare cancer types and with a very low prevalence in largely diffuse tumours. NTRK rearrangements can be detected in 80-90% of infantile fibrosarcomas, congenital mesoblastic nephroma, secretory breast cancer, mammary analogue secretory carcinoma (MASC) of the salivary glands, papillary thyroid cancers, spitzoid neoplasms, gastrointestinal stromal tumours (GIST) and certain paediatric gliomas have been shown to harbour NTRK fusions with frequencies of 5–25%. NTRK fusions can be detected in <5% (predominantly <1%) of lung or pancreatic adenocarcinomas, head and neck squamous cell, bile duct, breast, colorectal and renal cell carcinomas, melanomas, primary brain tumours of adulthood (such as astrocytomas or glioblastomas) and non- GIST soft- tissue sarcomas.

In the last few years, NTRK gene fusions have emerged as novel targets for tissue-agnostic anti-cancer drugs. Several molecules have shown a targeted and effective inhibitor activity against TRK kinases, regardless of tumour histology. Approved drugs for NTRK fusion-positive tumours in the market are larotrectinib and entrectinib, whereas other TRK inhibitors, like repotrectinib, selitrectinib and taletrectinib, are currently in clinical development.

Larotrectinib is a potent NTRK inhibitor, highly selective for TRKA, TRKB and TRKC. A combined analysis of three phase 1 and/or 2 clinical trials (LOXO-TRK-14001, SCOUT and NAVIGATE) highlighted a fast and long-term improvement of tumour response and quality of life in patients with a NTRK fusion-positive cancer treated with larotrectinib, regardless of patient’s age and tumour type. In particular, in the first 55 evaluable adult and paediatric patients, with a total of 17 unique cancer types harbouring NTRK fusions, larotrectinib demonstrated a 75% overall response rate (ORR) and predominantly grade 1 and 2 adverse events. On the basis of these results, in November 2018, FDA granted accelerated approval to larotrectinib for the treatment of adult and paediatric patients with solid tumours that have a NTRK gene fusion, are locally advanced or metastatic, or where surgical resection is likely to result in severe morbidity and have no satisfactory alternative treatments. Ten months later, the NTRK inhibitor received a conditional marketing authorisation from the European Medicines Agency (EMA) with the same therapeutic indications. Updated data, from the same three ongoing clinical trials, over a prolonged observation period and a greater number of patients, presented at the ASCO 2021 congress, confirm the clinical efficacy and long-term safety profile of larotrectinib, with rapid and long-lasting responses and with a favourable tolerability. In the dataset consisting of 206 evaluable patients and 21 different types of NTRK fusion-positive tumours, the overall response rate (ORR) was 75%, of which 22% complete responses (CR) and 53% partial responses (PR). The median time to response was 1.8 months and the median duration of response (DoR) was 49.3 months. Median progression-free survival (PFS) was 35.4 months, while median overall survival (OS) was not reached at a median follow-up of 22.3 months. Treatment-related adverse events (TRAEs) were mainly Grade 1 or 2.

Entrectinib is a potent oral multi-kinase inhibitor of TRKA, TRKB and TRKC, as well as of ALK and ROS1. Efficacy and safety of entrectinib have been evaluated in four clinical trials: ALKA-372-001, STARTRK-1, STARTRK-2 and STRATRK-NG. Pooled analysis of the first three studies were performed, including ten different types of NTRK fusion-positive tumour. Among the 54 evaluable patients, 57% had an objective response (ORR), comprising of 7% complete response (CR) and 50% partial responses (PR). The median response duration (DoR) was 10 months and the toxicity profile was manageable. On the basis of these data, in August 2019, entrectinib received accelerated approval from the FDA for the treatment of adult and paediatric patients with solid tumours that have a NTRK gene fusion, are metastatic or where surgical resection is likely to result in severe morbidity, and have progressed following treatment or have no satisfactory standard therapy.
An updated integrated analysis of the same clinical trials, performed in a larger population with longer follow-up, was presented at the ASCO 2020 congress and published in the number of April 2021 of ESMO Open. In the dataset consisting of 74 evaluable patients and 12 different types of NTRK fusion-positive tumours, the overall response rate (ORR) was 63.5%, of which 6.8% complete responses (CR). The median duration of response (DoR) was 12.9 months. Median progression-free survival (PFS) and overall survival (OS) were 11.2 months and 23.9 months respectively. On the basis of these results, end July 2020, the European Commission has granted conditional marketing authorisation for entrectinib for the treatment of adult and paediatric patients with solid tumours expressing a NTRK gene fusion, who have a disease that is locally advanced, metastatic or where surgical resection is likely to result in severe morbidity, and who have not received a prior NTRK inhibitor, who have no satisfactory treatment options.

There are multiple methods for NTRK fusion dete