TRKing down an old oncogene in a new era of targeted therapy

Abstract

The employment of high-throughput next-generation sequencing techniques in multiple tumor types during the last few years has identified NTRK1, 2, and 3 gene rearrangements encoding novel oncogenic fusions in 19 different tumor types to date. These recent developments have led us to revisit an old oncogene, Trk, (originally identified as OncD), which encodes the TPM3-NTRK1 gene fusion and was one of the first transforming chromosomal rearrangements identified 32 years ago. However, no drug has yet been approved by the US FDA for cancers harboring this oncogene. This review will discuss the biology of the Trk family of receptors, their role in human cancer, the types of oncogenic alterations, and drugs that are currently in development for this family of oncogene targets.

Introduction

The identification of dominant oncogenic mutations, and our ability to specifically inhibit these genetic abnormalities with targeted inhibitors has altered the therapeutic approach for many cancer patients, particularly those with non-small cell lung cancer (NSCLC). Activating point mutations, in-frame insertions/deletions, gene amplification, and gene rearrangements can serve as predictive biomarkers for oncogene-targeted therapies and thus help select patients that have a high likelihood of benefiting from a particular therapy. There are currently two well-established paradigms of this targeted therapy approach in NSCLC, both of which highlight the potential success of this strategy for other oncogene targets. The treatment of epidermal growth factor receptor (EGFR) mutation positive NSCLC patients (comprising approximately 18% of lung adenocarcinomas) and anaplastic lymphoma kinase, ALK, gene rearrangement positive NSCLC patients (encompassing approximately 5% of lung adenocarcinomas), respond significantly better to the targeted therapies erlotinib and crizotinib, respectively, compared with the standard-of-care chemotherapy (1). EGFR mutation positive patients who are treated with an EGFR tyrosine kinase inhibitor (TKI) have an objective response rate (ORR) of about 70%, and a progression free survival (PFS) time of approximately 10 months, both of which are superior to chemotherapy (2). ALK gene rearrangement positive patients showed a response rate of approximately 65%, and a PFS of approximately 8 months when treated with crizotinib, also superior to chemotherapy (3).

The paradigm of cancer treatment is shifting towards precision oncology. In this model, patients are selected for therapy using predicted biomarkers, such as oncogenic mutations, rather than using empiric chemotherapy. Many of the actionable or potentially actionable oncogenes that represent molecular subtypes in NSCLC involve genomic rearrangements with genes encoding receptor tyrosine kinases (RTKs) such as ALK, ROS1, RET, and most recently NTRK1 (4–7). The unprecedented improvement in patient outcomes with oncogene-targeted therapies suggest that even rare oncogenes, such as ROS1 gene rearrangements (which occur at a frequency of ~1–2%) should be investigated as therapeutic targets, as this molecular subset represents approximately 2,500 patients in the U.S. each year (8, 9). Indeed a recent study of crizotinib in ROS1+ NSCLC patients highlights the ability to successfully accrue rare oncogene subtypes (10). The study of these low frequency oncogenes not only applies to NSCLC, but is also directly relevant to the treatment of numerous other cancer types: ALK, ROS1, RET, and NTRK1 gene rearrangements have also been observed in other malignancies, expanding the relevance of this work to colorectal cancer, thyroid cancer, cholangiocarcinoma, glioblastoma, inflammatory myofibroblastic tumors (IMT), ovarian cancer, bladder cancer, sarcomas, and others (11–17). Indeed, isolated reports show the success of targeting oncogenes across multiple tumor types (15, 18). It was estimated in 2007 that gene fusions were reported in approximately 20% of all cancers accounting for a significant proportion of cancer morbidity and mortality (19). The emergence of high-throughput genomics technologies and programmatic sequencing efforts such as the NCI/NHGRI Cancer Genome Atlas Network and the Sanger Cancer Genome Project have generated the molecular profiles of numerous cancers, and this emergent technology has enabled the identification of many additional gene fusions that are putative oncogenes and predicted to be conserved as drivers across breast, glioblastoma, lung, colorectal cancer, and others tumors (16, 17, 20–22). This article describes the emergence of an increasingly described class of potential oncogene targets in cancer, the Trk family of kinases.

Trk Family Biology

The NTRK1 gene encodes the TrkA receptor tyrosine kinase, the TRK proto-oncogene, which is a member of the Trk (tropomyosin-receptor kinase) family of RTKs that includes TrkB (encoded by NTRK2) and TrkC (encoded by NTRK3) proteins (12, 23). TrkA, B, and C 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 many other non-neuronal cell types and tissues, including monocytes, the lung, bone, and pancreatic beta cells (24). TrkA, B, and C are most frequently activated by their primary ligands nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3), respectively (25, 26). However, overexpression studies in cell lines suggest the possibility of promiscuity among these neurotrophins ligands and all 3 receptors that may be cell-type and neurotrophin concentration dependent (23). Additional studies have shown NT-3 may activate TrkA and TrkB, and NT-4/5 may activate TrkB (27). The binding of each of these ligands to its cognate receptor, such as binding of NGF to TrkA, induces receptor homodimerization and transphosphorylation of five critical tyrosine (Y) residues (Y496, Y676, Y680, Y681, and Y791). Y496 and Y791 serve as phosphorylation-dependent binding sites for various adaptor proteins that contain SH2 or PTB domains, primarily SHC1, PLCγ, and GAB1, but others include FRS2, GRB2, IRS-1, IRS-2, SH2B (12, 28–30) (Fig. 1). Evidence from several studies points to activation of the PI3K signaling pathway from RAS or GAB1, although it may also be activated from other mechanisms (29, 31–33). Once activated, the three wild-type Trk family members most frequently signal through several downstream signaling pathways including SHC/RAS/MAPK, PI3K/AKT, or PLC-γ/PKC, depending on which docking protein(s) bind to the critical phosphorylated tyrosines Y496, and Y791 (34). Activation of these signaling cascades results in transcriptional and other cell programs that mediate cellular proliferation, synaptic plasticity, neurite outgrowth and repair, prevention or repair of neurodegeneration, sensory neuron maintenance, or apoptosis (12, 34–37).

Figure 1. Trk fusion signaling.

Schematic showing common signaling mechanisms for an example of a cytoplasmic (non-membrane bound) chimeric Trk gene fusion are shown. Gene fusions are constitutively activated, or phosphorylated, often as a result of dimerization mediated by sequences in the 5′ gene. SH2 and PTB domain containing adaptors compete for binding at specific tyrosine residues, which most frequently results in propagation of the downstream signaling pathways shown.

It is expected most Trk fusions would employ many or all of the same downstream signaling cascades as the full-length receptors given the preservation of the kinase domain and the critical tyrosine docking sites, however this is a relatively unexplored area compared to decades of detailed studies on the signaling mechanisms employed by the full-length receptors, particularly in the rat pheochromocytoma cell line PC12 (Fig. 1). The ETV6-NTRK3 fusion might be an exception, because it lacks the critical Y845 docking site for the preferential adaptor SHC1 due to the location of the breakpoint in the fusion and evidence points to the use of an alternate adaptor, IRS-1 (38). Cell-type context and differential subcellular localization of fusions might alter the signaling program of the oncogenic fusion kinases.

Studies of TrkA fusions in thyroid cancer have revealed the Trk oncogenes (Trk, Trk-T1-T3) are capable of binding a number of different adaptor molecules, similar to full-length TrkA, but predominantly engaged in signaling through the RAS/RAF/MAPK pathway (39–41) (Fig. 1). The STAT3 signaling pathway was identified for playing a role in NIH-3T3 transformation by TRK oncogenes (42). Interestingly, the constitutive signaling induced by Trk oncogenes has also been shown to result in neuronal differentiation of PC12 cells (43). It was also elegantly demonstrated that the Trk oncogenes are capable of transforming the more relevant in vivo model of cellular transformation, thyroid epithelial cells, not just NIH-3T3 fibroblasts, a more commonly utilized model system for studies of oncogenic transformation (44). Similarly, in vivo transformation of mammary epithelial was shown using the ETV6-NTRK3 fusion, clearly demonstrating the potency of these oncogenes in multiple model systems (45). Studies of TrkA fusion signaling in endogenous colorectal (KM12) and lung cancer (CUTO-3) have been conducted recently by our lab. The TPM3-NTRK1, MPRIP-NTRK1, and CD74-NTRK1 fusions seem to signal predominantly through the SHC/RAS/MAPK pathway in endogenous colorectal and lung cancer cell lines, but can engage PI3K/AKT or STAT3 signaling in certain cell-types (4). Studies of the ETV6-NTRK3 fusion have demonstrated oncogenic signaling is engaged through IRS-1, but due to the limited availability of cell lines expressing the ETV6-NTRK3, most studies were conducted using a cDNA of the fusion expressed in various cell-lines, such as fibroblasts, resulting predominantly in activation of the RAS-MAPK signaling pathway, but also PI3K/AKT, often simultaneously (38) (Fig. 1). The potential for simultaneous, dual activation of multiple downstream pathways may result in a potent oncogene, as this enables the activation of both pro-proliferative and anti-apoptotic pathways (46). Novel studies by the same group have also demonstrated a critical role for upstream RTK signaling, through the Insulin-like Growth Factor Receptor (IGF1R) in ETV6-NTRK3 fusion driven tumorigenesis (38). Studies in mice in select NTRK1 and NTRK3 fusions have suggested these fusions likely play an important, early role in tumor progression (45, 47). Although P75^NTR can modulate the activity and signaling of the full length Trk receptors, its interaction with oncogenic Trk fusions have not been studied to date (23).

Trk Family in Cancer

Mutations in Trk family members have been reported in numerous malignancies, including ovarian cancer, colorectal cancer, melanoma, and lung cancer, but among TrkA alterations other than gene fusions, only an in-frame deletion of NTRK1 (ΔTrkA) in acute myeloid leukemia (AML) and a splice variant of NTRK1 (TrkAIII) in neuroblastoma have been functionally characterized as oncogenic to date (48–54) (Fig. 2A). The deletion in ΔTrkA results in the loss of 75 amino acids in the extra-cellular domain of TrkA, removes four glycosylation sites adjacent to the transmembrane domain, and transforms both fibroblasts and epithelial cells (53). The TrkAIII splice variant, which was identified in a neuroblastoma cell line, results in the loss of exons 6, 7, and 9, and the corresponding loss of the extracellular domain IG-C2 (Ig-like C2-type 1) as well as multiple glycosylation sites (51). TrkAIII is constitutively active (ligand-independent), and its expression is promoted by hypoxic conditions (51). Although not yet identified in human tumors samples to date, mutations in the extra-cellular domain of TrkA, P203A and C345S, have both been characterized as transforming (55, 56). These studies may point to regions of interest where mutations have been identified in human tumors; for example, a relative cluster of mutations occur in NTRK1 at the R342 position in close proximity to the C345 site identified by mutagenesis (57). The most common mechanism of oncogenic activation of TrkA is through genomic rearrangement, and the creation of a gene fusion (12). Interestingly, all of these different mechanisms of oncogenic activation of TrkA (gene rearrangements, deletion, and splice variant) contain the loss of some of the extracellular domain of TrkA. The loss of these common sequences suggests the presence of critical regulatory domain(s) in the extracellular domain of TrkA (and potentially B and C) that when lost, results in constitutive activation of the kinase domain and thus its oncogenic capacity and is supported by mutagenesis studies where Ig-like domains in the extracellular region of TrkA were deleted (58).

Figure 2. TrkA oncogenic variants and Trk gene fusion partners.

A. Schematic of TrkA isoforms, deletions, and mutations are shown. Mutations are shown in the TrkAI isoform. Amino acid position numbers are shown in black. B. Schematic showing the known NTRK1 (blue), NTRK2 (red), and NTRK3 (green) fusions and the tumor types in which they have been identified. It is important to note that not all of these gene fusions have yet been characterized functionally, but each one occurred in-frame with an intact Trk kinase domain and are thus potentially oncogenic. Known 5′ dimerization domains are shown in grey, and 3′ domains shown in blue (NTRK1), red (NTRK2), or green (NTRK3). No protein domains are shown for fusions that lack a reported breakpoint. Fusion proteins are not drawn to scale.

Autocrine and paracrine signaling by Trk receptors have been implicated to be pro-tumorigenic in several different tumor types. An autocrine loop involving TrkA and NGF is associated with pro-tumorigenic activity in both breast and prostate carcinoma; similarly, TrkB and BDNF have been shown to play a pro-tumorigenic role in several malignancies, including both breast and prostate cancers (59, 60). TrkB signaling has also been shown to promote anoikis resistance and induce metastatic programs in numerous cancers (61). Expression of TrkA and TrkC wild-type receptors are associated with a positive prognosis in neuroblastoma patients (excluding expression of the splice variant TrkAIII), while TrkB expression is correlated with a poorer prognosis (62, 63).

Trk family oncogenic fusions

The typical gene structure for an oncogenic fusion is that the 3′ region of a proto-oncogene (encoding the kinase-domain) is juxtaposed to 5′ sequences from an unrelated gene via an intra- or interchromosomal rearrangement. The resultant novel oncogene is both aberrantly expressed and has constitutive activation of the kinase domain. In 1982, the same year that the BCR and ABL genes were implicated in the first oncogenic translocation on the Philadelphia chromosome in chronic myelogenous leukemia (CML), the first NTRK1 gene fusion was identified in a colon cancer sample and contained sequences from TPM3 (non-muscle tropomyosin) (64, 65). The incidence and therapeutic potential of TPM3-NTRK1 in colorectal cancer was recently revisited after 32 years by Isacchi and colleagues, reaffirming that this NTRK1 fusion is indeed a recurrent, albeit infrequent, oncogene in colon cancer (22). Each of the identified colorectal cases harboring NTRK1 fusions identified thus far express the TPM3-NTRK1 oncogene, suggesting a preference for TPM3 as the partner gene in this particular tissue, similar to EML4 with ALK in lung cancer (4, 11, 16, 22, 64). Additionally, TrkC and very recently TrkB have also been shown to form oncogenic chimeras in multiple tumor types (66, 67) (Fig. 2B). The ETV6-NTRK3 fusion has been identified as the dominant oncogene in several malignancies, including secretory breast carcinoma, mammary analogue secretory carcinoma (MASC) of the salivary gland, congenital fibrosarcoma, congenital mesoblastic nephroma, acute myeloid leukemia, and radiation-associated papillary thyroid cancer (45, 67–74). ETV6-NTRK3 fusions can vary slightly with regard to the breakpoint in different cancer types, but always retain the SAM dimerization domain from ETV6 and the kinase domain of TRKC. Chromosomal rearrangements have been observed between NTRK1 and TFG, TPM3, or TPR in papillary thyroid carcinoma (PTC), the most common malignancy of the thyroid (12). Interestingly, many of these activating 5′ gene fusion partners are promiscuous amongst various kinase fusion classes (12, 16). While each of the in-frame NTRK1 and NTRK3 fusions identified thus far fit the paradigm and contain a 5′ gene partner with a dimerization domain, at least two of the NTRK2 partners do not (Fig. 2B). 5′ gene partners often contain one or more dimerization domains, such as the stereotypical coiled-coil domain(s), and the corresponding constitutive tyrosine kinase activity that occurs results in uninterrupted downstream signaling messages for the cell to proliferate aberrantly and survive (12). Oncogenic gene rearrangements involving ROS1 may be an exception to this paradigm, as many of the 5′ gene partners have no known dimerization domains, but have still been shown to possess transforming properties (16). One can speculate based on this that activation of ROS1 may only require loss of 5′ sequences that act as an autoinhibitory signal in the full-length RTK. A similar mechanism may be worth investigating in Trk family fusions. VCL-NTRK2 and AGBL4–NTRK2 have not been functionally characterized for transforming properties, but may be activated without a 5′ dimerization domain through the loss of a regulatory domain or by a different unknown mechanism, similar to many ROS1 fusions.

NGS identification of NTRK1, 2, and 3 fusions

In the last year, many next generation sequencing (NGS) efforts, including programmatic, disease-oriented whole genome and/or transcriptome projects and also targeted clinical NGS platforms have resulted in the identification of NTRK family fusions in numerous tumor types. NTRK1 fusions were recently identified in lung adenocarcinoma, intrahepatic cholangiocarcinoma, spitzoid neoplasms, glioblastoma, and pontine glioma (4, 75–79). These findings were further validated in lung adenocarcinoma, as well as the discovery of novel fusions in papillary thyroid cancer, and glioblastoma using a novel, targeted technique known as anchored multiplex PCR (80). It is also important to note that while Trk fusions were not detected initially by the TCGA, revisiting these data in multiple tumor types with a different bioinformatics algorithm resulted in the identification of multiple new oncogenic fusions (9, 17). The first evidence of gene fusions involving the NTRK2 gene came in pilocytic astrocytoma and very soon afterwards in pontine glioma (66, 74, 75). New tumor types with NTRK3 fusions were also identified, including both PTC, pontine glioma, and Philadelphia chromosome like acute lymphoblastic leukemia (Ph-ALL) (68, 75, 81). Each of the three NTRK family genes can rearrange with multiple 5′ gene partners (Fig. 2). A recent study, mentioned briefly above, revisited TCGA data from 20 different cancers with a more efficient computational pipeline for the detection gene fusions, and was able to identify Trk fusions in 8 additional tumor types (17) (Table 1).

Table 1. Oncogenic Trk fusions are found across multiple tumor types.

The frequency of NTRK1 (blue), NTRK2 (red), and NTRK3 (green) gene fusions indicating the tumor type and the detection method that was employed in each study. Only positive studies are listed and thus the actual prevalence may be lower than reported.

Oncogene	Cancer	Frequency	Detection method(s)
NTRK1 gene fusions	Lung adenocarcinoma	3/91 = 3.3%	Targeted NGS (FMI), FISH4
NTRK1 gene fusions	Intrahepatic cholangiocarcinoma	1/28 = 3.6%	Targeted NGS (FMI)77
NTRK1 gene fusions	Colorectal cancer	3 isolated reports 1/66= 1.5%	cDNA library, FISH, PCR4, 11, 63 PCR, IHC22
NTRK1 gene fusions	Papillary thyroid cancer	28/228 = 12.3%	PCR12
NTRK1 gene fusions	Spitzoid neoplasms	23/140 = 16.4%	Targeted NGS (FMI), FISH, IHC78
NTRK1 gene fusions	Glioblastoma	2/185 = 1.1% 4/162 = 2.5% 1/157	NGS75 NGS/PCR76
NTRK1 gene fusions	Sarcoma (TCGA)	1/103=1%	RNA-SEQ17
NTRK2 gene fusions	Astrocytoma	3/96 = 3.1%	NGS65
NTRK2 gene fusions	Lung adenocarcinoma (TCGA)	1/513=0.2%	RNA-SEQ17
NTRK2 gene fusions	Head and neck squamous cell carcinoma (TCGA)	1/411=0.2%	RNA-SEQ17
NTRK2 gene fusions	Brain lower grade glioma (TCGA)	2/461=0.4%	RNA-SEQ17
NTRK3 gene fusions	Secretory breast carcinoma	12/13 = 92%	FISH, PCR44
NTRK3 gene fusions	Mammary analogue secretory carcinoma	15/15 = 100%	FISH69
NTRK3 gene fusions	Papillary thyroid cancer	9/62 = 14.5%* 7/243= 2.9%+	RNA-SEQ67, 73
NTRK3 gene fusions	Acute myeloid leukemia	2 case reports	PCR, FISH68, 72
NTRK3 gene fusions	Congenital mesoblastic nephroma	5/6 = 83%	PCR and FISH66
NTRK3 gene fusions	Congenital fibrosarcomas	10/11 = 91% 5/5 = 100%	PCR71 PCR and FISH66
NTRK3 gene fusions	Ph-like Acute Lymphoblastic Leukemia	1/154=0.7%	NGS79
NTRK3 gene fusions	Colon adenocarcinoma (TCGA)	2/286=0.7%	RNA-SEQ17
NTRK3 gene fusions	Thyroid Carcinoma (TCGA)	7/498=1.5%	RNA-SEQ17
NTRK3 gene fusions	Skin Cutaneous Melanoma (TCGA)	1/374=0.3%	RNA-SEQ17
NTRK3 gene fusions	Head and neck squamous cell carcinoma (TCGA)	1/411=0.2%	RNA-SEQ17
NTRK1/NTRK2/NTRK3	Pediatric gliomas	8/112 = 7.1%	NGS74

^*post-Chernobyl

⁺sporadic

A unique biological aspect of the ETV6-NTRK3 fusion is that it was the first oncogenic gene fusion to be identified in numerous different cancer tumor tissues. In each of those different tumor types, all of which are relatively rare malignancies, ETV6-NTRK3 is the dominant oncogene. For example 100% of mammary analogous secretory carcinoma of salivary glands (MASC) and 93% of secretory breast cancers harbor ETV6-NTRK3 fusions (45, 70). This observation is similar to CML where BCR-ABL is found in the vast majority of cases. However, in most tumors where Trk fusions are identified, they represent only a small proportion of patients (Table 1). Collectively, the Trk family represents a sizeable number of cases distributed across multiple tumor types.

Trk inhibitors

Given the long history of oncogenic Trk alterations, one might ask why it has taken so long to develop drugs for this target in cancer. Several reasons likely contribute to the slow development of this target including the lack of selective inhibitors and the relative difficulty in screening large tumor cohorts when this oncogene was first identified in the early 1980s. Similar to Trk, oncogenic ALK gene rearrangements were found to be important in cancer long before the first ALK inhibitor was FDA approved. The first ALK gene rearrangement was identified in anaplastic large cell lymphoma in 1994, but no ALK-targeted therapies were developed in this disease until many years later. The critical moment for ALK inhibitor development came in 2007 with the discovery of ALK gene rearrangements in NSCLC (5). The corresponding FDA approval of crizotinib for ALK+ metastatic NSCLC was exceedingly fast taking only 4 years from the time of first identification of ALK in this patient population(5). This rapid approval highlights the successful strategy of precision oncology by matching targeted therapies with biomarker-selected patients. Neurotrophins and Trk receptors, particularly TrkA, have been pursued in the past as drug targets for the treatment of chronic pain (35), and a few studies have pursued the Trk family as an therapeutic target in cancer, (60, 82, 83). High levels of homology between TrkA, B, and C within the intracellular kinase domains have resulted in the synthesis of small molecule inhibitors that target all three Trk family members (pan-Trk inhibitors).

Clinical trials of Trk inhibitors will need to investigate potential side-effects that may arise from inhibition of the full length Trk receptors in normal tissues. Loss of normal regulation of TrkA, B or C receptor activity can result in numerous human diseases. Trk receptors are known for mediating pain sensation and can play a role in chronic pain (35, 84). TrkA loss-of-function mutations are seen in class IV hereditary sensory and autonomic neuropathies (HSAN), such as the genetic disorder congenital insensitivity to pain with anhidrosis (CIPA) (36, 84). Loss-of-function mutations in TrkB result in energy imbalances, the loss of appetite control and obesity, and neuronal defects such as memory impairment (37). Similarly, loss of BDNF expression in the cerebellum of the mutant stargazer mouse is associated with a severe ataxia phenotype (85) and TrkB homozygous mutant mice have severe neurologic deficits (37), suggesting a critical role for the BDNF/TrkB signaling axis in normal neurologic development. However, it remains unclear if inhibition of the full-length TRK receptors will produce symptoms that mimic developmental loss of signaling is this receptor family. A more detailed evaluation of Trk receptors in non-cancer related diseases is beyond the scope of this review, but can be found in other reviews (86, 87). Interestingly, one of the potential beneficial side effects of targeting Trk receptors in cancer might be a decrease in pain sensation, a frequent symptom among cancer patients. Conversely, it will important to monitor for potential neurologic side effects in clinical trials given the expression pattern of the Trk family of receptors. Currently, little data exists on the toxicities of the more selective Trk inhibitors. A phase I study of PHA-848125AC with significant blood-brain penetration produced dose-limiting (grade 3) ataxia and tremors, but as this drug inhibits cyclin-dependent kinases in addition to TrkA, it is unclear which drug target was responsible for these side effects (88). One of the largest reported studies of a drug with TrkA inhibition was a randomized study of lestaurtinib in FLT3 mutant AML (89). Only one neurologic adverse event was noted, a death due to cerebellar toxicity, however given the multiple kinase targets of this drug, no conclusions can be drawn about the relationship of this toxicity to TrkA inhibition. Monoclonal antibodies against TrkA or NGF have been developed for the treatment of pain (35); however, antibodies to TrkA or other Trk family members would not be effective against Trk fusions as the extracellular domains for these fusions are routinely lost in the gene rearrangement.

Currently, several TKIs with activity against the Trk family are being explored in clinical trials (Table 2). Two patients with identified NTRK1 fusions were recently treated with inhibitors that demonstrated the potential clinical benefit of targeting this family of oncogenes. The first was a lung cancer patient harboring the MPRIP-NTRK1 fusion, who was treated off-label with the multi-kinase inhibitor crizotinib (4). Crizotinib has only modest activity against TrkA and only produced a proportionally transient, minor radiographic response emphasizing the need for more potent Trk inhibitors in the clinic. More recently, a colorectal cancer patient with a TPM3-NTRK1 gene fusions was treated in the phase I portion of a clinical trial investigating an interrupted dosing schedule of RXDX-101 (Ignyta) (90). RXDX-101 is a pan-Trk inhibitor that also has activity against two other gene fusion targets, ALK and ROS1. The TrkA+ (TPM3-NTRK1) patient experienced a partial response and provides the first evidence of clinical activity of a Trk inhibitor in a patient with an oncogenic Trk alteration (90). A phase I/II study entitled STARTRK-1 of RXDX-101 is currently accruing patients with Trk alterations (NCT02097810). LOXO-101 (Loxo Oncology) is a selective pan-TRK inhibitor that has no significant activity outside of the Trk family and is currently being investigated in a phase Ia/Ib trial across multiple tumor types (NCT02122913). TSR-011 (Tesaro) is a TrkA and ALK inhibitor that is currently in phase I study (NCT02048488). PLX-7486 (Plexxikon) is a pan-TRK inhibitor that also targets Fms. It is currently being investigated as monotherapy or in combination with nab-paclitaxel in pancreatic cancer, but there are plans to explore activity in patients with oncogenic Trk alterations in the future (NCT01804530). DCC-701 (Deciphera), XL-184 (Exelexis), and MGCD516 (Mirati) multi-kinase inhibitors all have trials are all also currently open for several different molecular sub-types, including Trks. Lestaurtinib (CEP-701; Cephalon) showed promising pre-clinical activity in NTRK1 fusion models, but its clinical development remains unclear (4).

Table 2. Trk inhibitors under development.

A chart listing the Trk inhibitors that are currently in clinical trials, additional non-Trk targets, the current stage of clinical development, and the identifier for the relevant clinical trial at clinicaltrials.gov.

Drug	Stage of Development	Targets	Clinical Trial Identifier
DCC-2701	Phase 1A/1B in advanced solid tumors	TRKA/B/C, MET, TIE2, VEGFR	NCT02228811
LOXO-101	Phase Ia/Ib in patients with genetic alterations in TRKA, TRKB, or TRKC	TRKA/B/C	NCT02122913
MGCD516	Phase 1/1b in patients with advanced NSCLC with genetic alterations in MET, AXL, RET, TRK, DDR2, KDR, PDGFRA or KIT. or HNSCC with alterations in MET	TRK, MET, AXL, RET, DDR2, KDR, PDGFRA, and KIT	NCT02219711
PLX7486	Phase I as single agent and in combination with gemcitabine & nab-paclitaxel in solid tumors (pancreatic cancer expansion cohort)	TRKA/B/C, Fms	NCT01804530
RXDX-101	Phase Ia/Iia in patients with genetic alterations in TRKA, TRKB, TRKC, ROS1, and ALK	TRKA/B/C, ALK, ROS1	NCT02097810
TSR-011	Phase I/Iia in solid tumors and hematologic malignancies (ALK or TRKA positive)	TRKA, ALK	NCT02048488
XL-184	Phase II in advanced NSCLC with NTRK, RET, or ROS1+ fusions, or increased MET or AXL activity	TRKA, RET, AXL	NCT01639508

Summary

Oncogenic Trk is one of the first oncogenes identified more than 3 decades ago. The Trk oncogenes occur across a broad array of tumor types. Oncogenic fusions involving NTRK1, NTRK2, and NTRK3 and in-frame deletions or splice variants of NTRK1 are likely to be actionable oncogenes based on pre-clinical data. The first clinical evidence of tumor response in a patient with a TrkA fusion suggests that this family of oncogenes will represent a new valid drug target in cancer.

Key concepts.

• NTRK1 gene fusions were first identified in colon cancer in 1982, but have since been identified in 8 solid tumor types in recent years; gene fusions involving the highly homologous, NTRK2 and NTRK3 genes have also been identified in 11 different tumor types.

• Drugs with activity against the Trk family of receptor tyrosine kinases are currently in development for patients with oncogenic alterations in NTRK1, 2, and 3.