Briefly, neurotrophin growth factors bind and activate TRKs in a specific manner: nerve growth factor neurotrophin (NGF) to TRKA; brain-derived neurotrophic factor (BDNF) and neurotrophin 4 (NT-4) that bins to TRKB; and neurotrophin 3 (NT-3) to all three TRK proteins, although it has a higher kinship for TRKC (21–26).

The RAS/MAPK, PI3K/AKT, and PLC/PKC signaling pathway is triggered by the bond between ligand to the extracellular domain that causes the homodimerization and transactivation of TRK receptors via autophosphorylation of tyrosine residues ( Figure 1 ). Activation of the above pathways promotes cell proliferation, differentiation, and survival (5, 6, 27, 28).

In CNS tumors, significant frequencies of NTRK fusions have been identified and their detection has become a cornerstone in the diagnostic evaluation of these cancers (3).

Several studies including large cohorts of pediatric CNS tumors found that NRTK1-3 alterations occur mostly in very young children and tumors localized to the hemispheric lobs (29, 30). These results converged in the 2021 WHO Classification of CNS Tumors, in which NTRK alterations contribute to defining novel entities among both HGGs and LGGs in children, namely infant-type hemispheric glioma and diffuse LGG, MAPK pathway‐altered, respectively (20). Despite the high-grade histology, the first subgroup benefits from a better outcome compared to its counterpart without tyrosine kinase fusions (29, 30).

NTRK fusions found in several studies are depicted in Figure 2 .

There have been only limited in vitro or preclinical studies of signaling performed to illuminate the effect of TRKi on downstream cascade signaling or the time span of inhibition, but meaningful clinical responsiveness to these drugs has been shown in several types of tumors such as soft tissue sarcomas, childhood fibrosarcoma, lung cancer, colon cancer, melanoma (40–45).

First-generation TRKi were developed in 2015 that included larotrectinib and entrectinib. The recruiting clinical trials of either larotrectinib or entrectinib are listed in Supplementary Table 1 .

Larotrectinib, developed simultaneously for pediatric and adult cancer, is the first oral treatment with a “tumor-agnostic” indication: discovered in 2015, it obtained accelerated approval from FDA in 2017. It is a small-sized competitive inhibitor of ATP and selective pan-TRK, with a 50% inhibitory concentration (IC50) of 5-11 nm in vitro and a specificity >100 times for TRK (46, 47). Inhibition of RAF-MEK-ERK or PI3K-AKT pathways, caused by larotrectinib, inhibits the growth of some cell lines that contained targeted NTRK fusions, as well TPM3-NTRK1, TRIM24-NTRK2, and ETV6-NTRK3 (48, 49). An awesome overall response rate (ORR - the percentage of patients who experienced a complete or partial response) of 79% and a well-tolerated profile of toxicity were found in phase I/II clinical trials enrolling both adult and pediatric patients (50). In brain tumors, ORR was 30% and in particular, a retrospective study showed the results on the efficacy and safety for patients (n=33) with progressive or refractory CNS tumors enrolled in the SCOUT (NCT02637687) and NAVIGATE (NCT02576431) trials; among the 26 pediatric patients (79%), 13 pediatric HGGs and 7 pediatric LGGs were included. The observed ORR was 38% (38% in HGGs and 43% in LGGs, respectively), with three complete responses and seven partial responses. Importantly, the disease control rate at 24 weeks was 77% for pediatric HGGs and 100% for pediatric LGGs (43). In Supplementary Table 2 are reported results on patents with CNS tumor and treated with Larotectinb.

The Food and Drug Administration approved entrectinib in August 2019 to treat adult and pediatric populations with NTRK fusion tumors (51).

Robinson and colleagues published the first interim results based on 29 enrolled patients, aged 5 months to 20 years. The ORR was 100% in 11 pts [(high-grade CNS tumors (n=5) and extracranial solid tumors (n=6)] (52). In 2020, an expanded cohort of 39 patients confirmed an ORR of 77%. CNS tumors were in 14 patients, of which 11 displayed NTRK fusions. Notably, the ORR in this subgroup reached 64% (53). Desai et al. demonstrated that entrectinib had a rapid and durable responses in pediatric patients with solid tumors harboring NTRK1/2/3 or ROS1 fusions (54). In Supplementary Table 2 are reported results on patents with CNS tumor and treated with entrectinib. In addition, Liu et al. reported weight gain, dizziness and withdrawal pain in a several patients who were treated with TRKi (55).

Usually, both Larotrectinib and Entrectinib are administered until disease progression or unacceptable toxicity occurs (42, 43, 54). Treatment discontinuation is reported in extracranial tumors in which tumor size reduction has made complete resection possible; interestingly, patients who discontinued treatment following an initial response and subsequently experienced disease progression may still benefit from restart of therapy (41).

Mutations called on target and off target, respectively, on the NTRK gene or in genes associated with the MAPK pathway, are responsible for resistance to those drugs in several type of cancers (5, 56–59). In the NTRK3 gene, acquired variants p.G623E and p.G623R have been identified to confer resistance to either larotrectinib or entrectinib (48, 57, 59, 60). Additionally, acquired variants p.F617L and p.G696A specifically confer resistance to larotrectinib (50, 57, 61). In the NTRK1 gene, acquired variants p.V573M and p.G667S have been found to induce resistance to both larotrectinib and entrectinib, whereas the acquired variant p.F589L in the same gene only confers resistance to larotrectinib (50, 57, 62–64).

As a result, the need for second-generation TRKi, such as selitrectinib (loxo-195), taletrectinib (DS-6051b, AB-106), and repotrectinib (tpx-0005), has arisen (5, 30, 56). Taletrectinib works as a multi-kinase inhibitor that can overcome resistance from solvent-front replacements involving TRKA, TRKB and TRKC such as others involving ROS1 (65). Selitrectinib is a selective TRKi studied in a phase I trial involving both children and adults with tumors that have developed resistance mediated by TRK kinase mutations, in which a preliminary efficacy was found (66). Repotrectinib functions as a kinase inhibitor encoded by the NTRK, ROS1, and ALK genes. It effectively binds to the ATP-binding pocket of the kinase, preventing steric hindrance caused by various clinically resistant mutations (57). A clinical trial investigating its use in pediatric patients with solid tumors that include CNS neoplasms is currently ongoing (NCT04094610).

On the other hand, mutations that involved other RTKs or downstream pathway mediators can result in off-target resistance to TRKi. Specifically, MET amplification, BRAF^V600E mutation, or KRAS alterations have been found in patients with TRK fusion and who show a progression of the tumor during the treatment of TRKi (56). Of note, the TRKi monotherapy was not effective for resistance mediated to overcome the mutational pathway, while a dual blockade of TRK and other pathways involved in the resistance mechanism could effectively control tumor growth (67). For instance, the combination of the inhibitors of TRK and MET has been found to be effective in a patient carried a TRK fusion and MET amplification that drives the resistance to the TRKi alone (56).