Tropomyosin Receptor Kinase as a Target for the Treatment of Solid Tumors

September 19, 2019
Evolving Paradigms, Tropomyosin Receptor Kinase as a Target for the Treatment of Solid Tumors,

NTRK fusions, although rare, have been identified as an important driver of oncogenic activity in a variety of cancer types. Advances in the understanding of TRK family biology, coupled with the growing recognition of TRK alterations as drivers of oncogenesis across tumor types, has fueled interest in the development of small molecule inhibitors of TRK.

Neurotrophic receptor tyrosine kinase(NTRK) fusions, although rare, have been identified as an important driver of oncogenic activity in a variety of cancer types.1First identified in 1986 from a tumor sample in a patient with colorectal cancer,NTRKrearrangements were among the first oncogenic fusion products described in literature.2

In a recent video interview withTargeted Therapies in Oncology(available attargetedonc.com/precision-medicine/trk-inhibitors), Corey Langer, MD, noted thatNTRKgene fusions have been identified “across the board in both adults and kids—some unusual tumors, [including] secretory tumors of the salivary gland and the breast, pediatric tumors, congenital nephromas, infantile fibrosarcomas, GBM [glioblastoma], thyroid carcinoma, and sarcoma in both adults and children. And then in adults, lung cancer, pancreatic cancer, cholangiocarcinoma, colorectal cancer, and melanoma.”

Tropomyosin receptor kinase (TRK) A, B, and C proteins are encoded by neurotrophic receptor tyrosine kinase genes 1, 2, and 3 (NTRK1,NTRK2, andNTRK3), respectively.1,3Members of theTRKgene family are enriched in neural tissues4and play an important role in mediating neurotrophic signaling.4,5Specifically, TRK A, B, and C function to regulate cell proliferation, apoptosis, differentiation, and survival of neurons in the peripheral and central nervous systems (CNS).6Although TRK A, B, and C are most frequently activated by nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3,

respectively, it is likely that these receptor—ligand relationships are not specific, with the receptors harboring potential to be activated by a variety of ligands.6

An inter- or intrachromosomal rearrangement involving the 3’ end of the NTRK proto-oncogene to the 5’ end of an unrelated gene results in the production of aberrant protein products that cause inappropriate activation of the TRK kinase domain; this activity in turn triggers a cascade of signaling pathways, including MAPK, PI3K, and PLCϒ.1Activation of these signaling cascades and other pathways by proliferating tumor cells, in this manner, confers a survival benefit and is ultimately functional in cell proliferation, differentiation, survival, and angiogenesis.1,6Notably, most of the 5’ gene fusion partners within the NTRK family that have been identified to date contain a dimerization domain, which would explain subsequent constitutive activation resulting in uninterrupted downstream signaling. However, at least 2NRTK2partners do not exhibit this pattern. A similar pattern has been recognized inROS1rearrangements, which possess transforming properties, despite the absence of 5’ fusion partners with known dimerization domains. It may be the case in suchROS1rearrangements that the loss of key 5’ sequences acts as an autoinhibitory signal, and a similar pattern may be present in TRK family fusions.6Regardless,NTRKgene fusions cause resultant TRK kinases to stay constantly active, without requiring a ligand to modulate activity.7These constitutively active kinases cause “oncogene addiction,” irrespective of tissue origin, driving cancer cell growth and metastases.7,8Indeed, evidence is emerging that at least some of the knownNTRKgene fusions may be capable of simultaneous, dual activation of multiple downstream pathways, which may enable proproliferative as well as antiapoptotic signaling.6

“Researchers in oncology identified that there were unique translocations, or fusions, of theseNTRKgenes with other partners that led to constitutive or continued signaling in cancer cells, truly driving their growth and their metastases,” said David Hong, MD, in a video interview withTargeted Therapies in Oncology(available attargetedonc.com/precision-medicine/trk-inhibitors). “What’s interesting is that in more common tumors like lung and colorectal, although the prevalence is very low—1% or lower—the patients who have no mutations, other thanNTRKgene fusions, seem to be the ones who probably harbor these fusions. There are, however, a subset of tumors, MSI [microsatellite instability]—high tumors, particularly MSI-high colorectal cancers, that seem to also have a higher prevalence ofNTRKgene fusions.”

TRK proteins A, B, and C may have a high degree of homology within their intracellular kinase domains.6They are each composed of a transmembrane region, an intracellular domain with a kinase domain, and an extracellular domain for the purpose of ligand binding. Ligation triggers oligomerization of the receptors and phosphorylation in the intracytoplasmic kinase domain, resulting in activation of signaling cascades that influence cell proliferation, differentiation, and survival.9Proper regulation of TRK activity is critical for cell functioning; upregulation of TRK receptor levels has been shown in many CNS-related disorders, including epilepsy, neuropathic pain, and depression.9Similarly, autocrine and paracrine signaling by TRK receptors is associated with protumor activity in a number of cancer types.6Research designed to understand the oncogenic activation ofTRK A may serve as model for understanding the causative elements in constitutive activation of all 3 TRK proteins.TRK Agene rearrangements, deletions, and splice variants share a common loss of some of the extracellular domain of TRK A, suggesting a critical regulatory sequence within that domain (and potentially within B and C).6This hypothesis is further supported by study results demonstrating that the alteration of the immunoglobulin-like structure of TRK A responsible for ligand-specific binding results in spontaneous and ligand-independent dimerization.6,10

To date, approximately 80 5’NTRKfusion partners have been identified.11Although the incidence of TRK fusion—positive cancers vary, they may occur in approximately 1% of all solid tumors.12It is estimated that approximately 1500 to 5000 new cases ofNTRK-positive cancers occur each year in the United States.1Among those cancers types most likely to be seen in clinical practice,NTRKgene fusion incidence is generally <5%, although the true incidence is somewhat unknown in the nascent era of next-generation sequencing (NGS) adoption.11In addition,NTRKgene fusions are more common in certain pediatric and adult cancers (TABLE).11

TRK INHIBITION AS A THERAPEUTIC STRATEGY

Advances in the understanding of TRK family biology, coupled with the growing recognition of TRK alterations as drivers of oncogenesis across tumor types, has fueled interest in the development of small molecule inhibitors of TRK. Despite the recognition since the 1980s ofNTRKrearrangements being consequential in tumor growth, development, and survival, the most significant progress in the development of TRK inhibitors have arguably occurred in the past decade.2,6These developments have been propelled, in some measure, by the emergence of NGS techniques and technologies that are capable of identifyingNTRKgene fusions.6These capabilities led directly to the discovery that mostNTRKgene alterations are associated with uncontrolled kinase activation, thereby suggesting a role for inhibitors of said activity as part of a personalized drug approach.3Because the majority of residues of the kinase domain, those that potentially interact with ligands in the ATP binding site, are conserved across members of the TRK family, drug research has largely focused on pan-inhibitors, rather than selective inhibitors, of the 3 distinct TRK receptor isoforms.9To date, most research on TRK inhibition has focused on drugs that competitively bind the ATP binding site to obviate autophosphorylation that would otherwise drive downstream signaling, although other strategies for TRK inhibition, including non—ATP-competitive inhibitors at allosteric binding sites that target the downstream activity of TRK activation, have also been explored.3Currently, 2 NTRK inhibitors are approved for the treatment of patients with solid tumors that haveNTRKgene fusions.

Larotrectinib

Larotrectinib inhibits TRK A, B, and C proteins. Approved in November 2018, larotrectinib is indicated for the treatment of pediatric and adult patients with solid tumors that haveNTRKgene fusions without any known acquired mutations that are metastatic or where surgical resection is likely to result in severe morbidity, have no satisfactory alternative treatments, or have progressed following treatment.13Larotrectinib’s mechanism of action is to bind with and competitively inhibit the ATP-binding site on the TRK molecule, thus interfering with autophosphorylation of the TRK kinase domain, slowing or stopping downstream signaling.1Preclinical study results have shown that larotrectinib demonstrated dose-dependent inhibition of cell proliferation in numerous cell lines.1In July 2016, the FDA granted larotrectinib a breakthrough therapy designation for adult and pediatric patients with advanced solid tumors positive forNTRKgene fusions, becoming the first therapy to receive a tissue-agnostic designation.1

“The approval of larotrectinib was really a mini first,” noted Hong. “It was a first approval of a drug in both pediatrics and adults at the same time. It was not only the very first histology-agnostic approval, but also the first genetic mutation or driver approval for any histology-agnostic indication.” Currently, larotrectinib has the largest initial and follow-up data set in patients withNTRKfusion—positive cancer.

Clinical Trial Data

Larotrectinib is being studied as part of an ongoing clinical trial program involving a phase I study in adults, phase I/II study in children, and phase II basket study among adolescents and adults.7In a report on the first 55 consecutively enrolled patients, the overall response rate (ORR) determined by an independent review committee was 75% among patients ranging in age from 4 months to 76 years with 17 unique TRK fusion—positive tumor types, all identified with NGS or fluorescence in situ hybridization.7The trial investigators determined that the ORR was slightly higher at 80%.7Seven of 55 patients (13%) achieved a complete response. Median duration of response and median progression-free survival (PFS) had not yet been reached at the time of reporting.7The majority of adverse events (AEs) were grade 1 or 2, and clinically significant AEs were uncommon. The most common grade 3 and 4 AEs were anemia (11%), an increase in alanine aminotransferase or aspartate aminotransferase levels (7%), weight increase (7%), and a decrease in the neutrophil count (7%).7In a supplementary data set that included 35 additional patients, the ORR was determined to be 74%, with 14% of patients in the supplemental group (n = 5) achieving a complete response. With a median follow-up of 5.5 months, median duration of response had not been reached.14Based on data integrated from the primary and supplementary groups of patients, the investigators noted that larotrectinib was found to be efficacious regardless of tumor type or patient age.14

“Responses with larotrectinib appear to be independent of tumor type,” Langer observed. “In fact, they seem to be pretty consistent across the board regardless of tumor type. Responses were observed regardless of the number of prior regimens that were administered to the patients, and that is a common theme in the world of personalized therapy or precision medicine, where we are using very specific agents against very specific targets. Prior history to some extent may influence outcome, but not to the extent that we’ve traditionally seen with systemic cytotoxics.”

Phase I results from the phase I/II study in pediatric patients have been reported separately. Among enrolled infants, children, and adolescents aged 1 month to 21 years with locally advanced or metastatic solid tumors or CNS tumors that had relapsed, progressed, or were nonresponsive to available therapies, 14 of 15 patients (93%) with TRK fusion—positive cancers achieved an objective response as per RECIST 1.1, whereas none of the 7 patients with TRK fusion–negative cancers had an objective response.15

Hong et al recently reported results from a multicenter, phase I dose-escalation study among adults with metastatic solid tumors. In the study,NTRKgene fusion status was not used as an eligibility criterion.16Among 67 assessable patients, the objective response rate was 12% (n = 8); however, responses were seen in 7 of 8 patients with tumors harboringNTRKgene fusions and in 1 patient with a tumor harboring anNTRK1gene amplification. No patients with an NTRK point mutation had an objective response. Corresponding reductions in tumor burden among all patients with tumors harboring anNTRKgene fusion were noted.16All 70 patients were evaluated for safety; most treatment-related AEs were grade 1 or 2, and 60% of patients experienced grade 3 or worse treatment-emergent AEs, with the most common being anemia, fatigue, and increase in aspartate aminotransferase. Four percent of patients (none with anNTRKgene fusion) discontinued treatment because of larotrectinib-related AEs. Moreover, the investigators reported no meaningful difference in treatment-related AEs among patients with or withoutNTRKgene fusions.16

Because ORR appears to be particularly robust among pediatric patients, outcomes were analyzed among 83 adults (median age, 57 years; range, 20-80), in new data presented at the 2019 American Society of Clinical Oncology (ASCO) Annual Meeting. Among 74 evaluable patients, the ORR was 76% as assessed by investigators, with 9% achieving a complete response and 57% achieving a partial response; among 65 patients assessed by an independent review committee, the ORR was 68%, with 17% achieving complete response and 51% achieving a partial response. At the time of reporting, median duration of response had not been met after a median of 17.2 months. Following data cutoff, patients experienced mostly grade 1 and 2 AEs.17

Additional findings from the 2019 ASCO Annual Meeting showed meaningful improvements in quality of life in patients treated with larotrectinib. The study evaluated 37 patients (13 pediatric and 24 adolescent or adult) with TRK-positive fusion cancers who were treated with larotrectinib. Patients who completed a baseline questionnaire and at least 1 follow-up quality-of-life questionnaire were eligible for evaluations. Improvement was defined as ≥10-point improvement on the EQ-5D-5L (which measures health-related quality of life) or on the EORTC QLQ-C30 (which measures the physical, psychological, and social functions of patients with cancer), or ≥4.5-point improvement on the PedsQL (which measures health-related quality of life specifically in pediatric patients). Results showed that 58%, 62%, and 77% of the patients demonstrated improvement in EQ-5D-5L, EORTC QLQ-C30, and PedsQL, respectively. Investigators noted that improvements were rapid and were sustained for a minimum of 2 cycles. Although the study size was relatively small, the results showed improvements in overall quality of life in both pediatric and adult patients.18“Almost 70% of these patients had significant quality-of-life improvement[s],” said Hong, a rate that is unique to larotrectinib. Based on its robust clinical profile, Hong observed that “larotrectinib particularly has been the most effective small molecule kinase inhibitor I’ve ever seen.”

Entrectinib

Entrectinib was approved in August 2019 for the treatment of adults and pediatric patients 12 years and older with solid tumors that have aNTRKgene fusion without a known acquired resistance mutation, are metastatic or where surgical resection is likely to result in severe morbidity, and have progressed following treatment or have no satisfactory standard therapy. Entrectinib was initially discovered as a result of a screening program designed to identify ALK kinase inhibitors. During early testing, it was found to be a highly potent inhibitor of ALK as well as of 2 closely related receptor tyrosine kinases, ROS1 and TRK. It was subsequently selected for clinical trials based on its safety and tolerability profile as well as its demonstrated ability to induce tumor regression in human xenograft models.19Similar to larotrectinib, entrectinib competitively binds the ATP-binding site.3

Additional preclinical study results confirmed that entrectinib was a highly potent inhibitor of TRK A, B, and C; ALK; and ROS1 kinases. Testing showed that it abolished autophosphorylation in a colorectal carcinoma cell line. Of note, it also displayed favorable blood—brain barrier penetration in a murine xenograft model, suggesting its potential utility in treating brain metastases after relapse following initial lines of therapy.20Positive clinical results after treatment with entrectinib have also been reported in a patient with stage IV lung adenocarcinoma with a tumor harboring anSQSTM-NTRK1gene fusion21and in a patient with glioblastoma with a tumor harboring aBCAN-NTRK1gene fusion.22

Clinical Trial Data

Entrectinib was studied in 2 phase I trials, ALKA-371-001 and STARTRK-1, with 54 and 65 patients participating, respectively. Patients in the ALKA-372-001 trial were treated on 1 of 3 dosing schedules to establish appropriate dosing: with or without fasting, taken continuously daily, or using drug holidays. Patients in STARTRK-1 were treated with a continuous dosing schedule for 28 days. The majority of treatment-related AEs were grade 1 or 2 in severity and were reversible with dose modifications, with no dose-limiting toxicities reported. Commonly reported treatment-related AEs of any grade included fatigue/asthenia (46%), dysgeusia (42%), paresthesias (29%), nausea (28%), and myalgias (23%). No significant differences in toxicity were noted between the 2 studied dosing schemas.23

“There are some minor differences in toxicity, I believe, between larotrectinib and entrectinib,” noted Langer. “Entrectinib does seem to cause some anemia. Both can cause fluid retention and weight gain, but [with] entrectinib they seem to be a bit more pronounced. And there is some neuropathy—peripheral paresthesias, peripheral sensory neuropathy associated with entrectinib. The total incidence is about 13% to 14%, with maybe about 4% or so that’s grade 3—so, it’s clearly manageable, but a bit more pronounced. How these stack up against other agents or other upcoming agents remains to be seen, but I think for the most part, these patients can be readily managed with the tools that we have currently for [adverse] effects.”

Overall, responses were observed in patients with non—small cell lung cancer (NSCLC), colorectal cancer, mammary analogue secretory carcinoma, melanoma, and renal cell carcinoma as early as 4 weeks after starting treatment and lasting as long as >2 years.23Among 24 patients with extracranial solid tumors who might meet eligibility for enrollment in a phase II study (defined as having a tumor harboringNTRK 1/2/3,ALK, orROS1; no prior tyrosine kinase inhibitor [TKI] exposure; and minimal exposure of 600 mg entrectinib daily), the ORR was 100% in 3NTRK-positive patients, 86% in 14ROS1-positive patients, and 57% in 7ALK-positive patients.23Additionally, treatment responses were noted in 5 of 8 patients with known primary or metastatic brain involvement prior to treatment: patients withNTRK1- (n = 1), ROS1- (n = 2), and ALK- (n = 1) rearranged NSCLC and 1 patient withALK-rearranged colorectal cancer.23

“The nice thing about entrectinib is [that] it hits all, targetingALK,ROS1, and the 3 major variants ofNTRK:NTRKs 1,2, and3,” noted Langer. “The drug that we typically use in ALK, alectinib, has no activity in ROS1 and essentially no activity in NTRK. So as time goes on, we’re going to need to be more cognizant of the various differences among the TKIs and where the bulk of activity exists.”

An integrated analysis of ALKA-372-001 and STARTRK-2 was recently reported.24A blinded independent central review determined an ORR of 57.4%, a median duration of response of 10.4 months, a median PFS of 11.2 months, and a median overall survival of 20.9 months. Of the 54 evaluable patients, 10 patients hadNTRKgene fusion—positive NSCLC, with 6 of them also having CNS involvement. In this subgroup, the ORR was 70% (7 of 10 patients). Among the 6 patients with CNS involvement, 2 patients had a complete intracranial response, 2 had a partial response, 1 had stable disease, and 1 was not evaluable. Sixty-eight patients withNTRKgene fusion—positive tumors who received at least 1 dose of entrectinib were evaluated for safety. Regarding treatment-related AEs, 32.4% of patients experienced grade 3 AEs; 2.9% experienced grade 4 AEs; 4.4% discontinued treatment; and 39.7% had a dose reduction because of an AE.24

“[Both entrectinib and larotrectinib have] high CNS penetrance, so patients with brain involvement will frequently respond,” Langer explained. “In the original phase I and phase II studies, the overall response rate again was on the order of 70%, 75%. It was a 55% intracranial response. We would never see that with standard systemic chemotherapy, but we are seeing that increasingly with our newer generation of tyrosine kinase inhibitors,” said Langer.

OTHER TRK INHIBITORS IN DEVELOPMENT

A number of candidate TRK inhibitors have been identified and have entered various stages of development.3To date, a select few have accumulated data from use in human patients.

BAY2731954 (formerly known as LOXO-195) was specifically designed in parallel with larotrectinib in anticipation of the potential for acquired resistance to TRK inhibitors.25In preclinical studies, BAY2731954 demonstrated inhibitory activity in various TRK kinase—mutated tumor models, in which larotrectinib showed reduced or no inhibitory activity.25In addition to confirmatory studies in mice, the latter authors also reported rapid clinical response to therapy in an adult patient withLMNA-NTRK1—rearranged colorectal cancer soon after starting BAY2731954, as well as visible tumor regression in a previously palpable mass in the head and neck region within 15 days of initiating BAY2731954 treatment in a pediatric patient with aETV6-NTRK3—rearranged infantile fibrosarcoma.25

BAY2731954 was evaluated in 31 patients (n = 20 in a phase I study [NCT03215511] and n = 11 in an FDA—expanded access single-patient protocol) aged ≥4 weeks with a locally identifiedNTRKgene fusion who had progressed or were intolerant to ≥1 prior NTRK inhibitor.26In the phase I study, treatment-emergent AEs that occurred in >3 patients included dizziness/ataxia (65%), nausea/vomiting (50%), anemia (30%), myalgia (20%), abdominal pain (20%), fatigue (20%), and lymphopenia (20%). Five patients, all adults, experienced dose-limiting toxicities, including ataxia/dizziness

(n = 4), and ataxia/vomiting (n = 1).26

“Response rates [for BAY2731954] in those with an established TRK kinase abnormality is close to 50%, 9 patients of 20,” said Langer. “My initial read on the toxicity is that [this] may be somewhat harder to deal with—nausea and vomiting, myalgias, [and] abdominal pain in addition to fatigue and edema and constipation.”

Repotrectinib is a rationally designed, next-generation TKI designed to potently inhibit clinically recalcitrant solvent-front substitutions involving ROS1, TRKA-C, and ALK, in addition to wild-typeROS1,TRKA-C, andALK, and other clinically relevant, nonsolvent-front mutations.27Repotrectinib is smaller in size compared with clinically available ROS1, TRK, and ALK inhibitors, suggesting it will have favorable penetration of the human brain. By targeting the center of the ATP binding site, it circumvents steric hindrance from solvent-front substitutions. In preclinical testing, it demonstrated potent antiproliferative activity against both wild-type and mutatedALK,ROS1, andTRKin cellular inhibitory assays and xenograft models.27The drug is currently being studied in a phase I/II clinical trial (NCT03093116); outcomes in 2 patients have been reported. The first, a Caucasian man aged 44 years who experienced progression on several prior lines of therapy (crizotinib, entrectinib, doxorubicin, and entrectinib plus trametinib), had a “rapid and dramatic” response to repotrectinib within the first few days of treatment and later had a partial response, which was confirmed with radiologic imaging. The second patient was an Asian never-smoker woman, aged

41 years, with stage IV NSCLC. The patient received 1 cycle of carboplatin, pemetrexed, and bevacizumab while awaiting insurance authorization for crizotinib. She had a durable response on the latter for 12 months, but then showed increasing mediastinal lymphadenopathy on imaging. After confirmation of aCD74—ROS1and aROS1 G2032Rsolvent-front mutation, the patient was enrolled in a phase I trial involving repotrectinib. Partial response per RECIST v1.1 was subsequently achieved with a duration of response of 7.4 months, and there was also response in the CNS for 9 months with progression controlled by whole-brain radiation.27“Response rates to date are 80% [in patients withROS1] and 66% in those with prior exposure to other TKIs, including crizotinib,” said Langer.

TREATMENT IMPLICATIONS AND FUTURE DIRECTIONS

The availability of small molecule TRK inhibitors presents both opportunities and challenges in clinical practice. AlthoughTRKgene fusions are considered rare events, they have nonetheless been identified in a variety of cancer types.11Targeted inhibition of TRK has proven to be a highly effective strategy in clinical trials. The ORR associated with larotrectinib is approximately 75%, with data still emerging regarding the clinical effectiveness of entrectinib.12Nevertheless, data from phase I studies of entrectinib provide suggestive evidence of treatment responses in patients who harbor extracranial solid tumors and/or CNS tumors or metastases withNTRK 1/2/3,ALK, orROS1gene fusions.23,24Other TRK-specific and multikinase inhibitors in development with activity against TRK offer additional promise for fulfilling the promise of targeted therapy in the era of precision medicine.12

However, the variability ofNTRKgene fusion incidence across cancer types, coupled with the relative rarity ofNTRKgene fusions,11raises questions about who to test, and when, and how such testing should be conducted. NGS is the gold standard when it comes to identifying patients withNTRKgene fusions. It does, however, have its limitations.NRTK1,NTRK2, and especiallyNTRK3feature long introns that make them difficult to detect in standard NGS panels.28

“You really need what’s called RNAseq, or RNA-based NGS, in order to capture some of that,” said Hong. “It’s important for individual clinicians to talk to their pathologists [and] to talk to molecular pathologists if they’re on staff. And if the tissue is sent out to a commercial outfit, to talk to the vendor who is actually providing the assay and doing the screening, [to find out] whether NTRK is properly covered [and] whether they have DNA- or RNA-based assays,” said Langer.

One inherent challenge with small molecule TRK inhibitors in general, and likely with all small molecule inhibitors approaching or in clinical practice, will be to develop clinical practice guidelines that allow for efficient identification of patients with fusion-positive cancers. Indeed, although mass screening of all solid tumors with a broad NGS panel may seem ideal, such an approach may not be economically viable or practical.11One possible solution may be to approach the idea of targeted testing based on reported incidence ofNTRKgene fusions in the given cancer while also considering the incidence of TRK expression in that cancer type. Thus, cancers associated with a high incidence ofNTRKgene fusions could be targeted for testing, whereas those with a low expected incidence ofNTRKgene fusions but high expression of TRK would be included in a second category and those associated with low incidence of bothNTRKgene fusions and TRK expression would be tested only in specialized circumstances.11,28 &nbsp;What those categories might look like in practical terms, however, remains unclear.

References

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