David S. Hong, MD

Deputy Chair

Department of Investigational Cancer Therapeutics Division of Cancer Medicine

The University of Texas MD Anderson Cancer Center

Houston, TX

Targeted therapy has transformed cancer treatment, and gene fusions were one of the earliest harbingers of the movement when the FDA approved imatinib (Gleevec)— which targets the BCR-ABL fusion protein—for the treatment of chronic myelogenous leukemia in 2001.

Gene fusions are rare, occurring when part of the DNA from one chromosome detaches and moves to another. The 2 segments are transcribed as a single entity, often producing an abnormal protein that can drive cancer progression.

In recent years, fusions involving the neurotrophic receptor tyrosine kinase (NTRK) family of genes across multiple solid tumors have attracted considerable interest.¹ The family includes the genes NTRK1, NTRK2, and NTRK3, which encode the tropomyosin receptor kinase (TRK) family of receptor tyrosine kinases (TRKA, TRKB, and TRKC).2 NTRK fusions occur in several rare cancers, including the vast majority of cases of mammary analogue secretory carcinoma of the breast, mammary secretory carcinoma, fibrosarcoma, and congenital mesoblastic nephroma.¹ Such fusions are rare in common cancers, but they do occur. One systematic review of 160 studies found that NTRK fusions occurred in 0.03% to 0.70% of adult cancers,¹ whereas a study of 25,792 patient samples found an overall rate of 1.6%, with the highest rates in thyroid cancers (17.2%) and salivary gland cancers (15.3%).²

Emergence of TRK Receptors

TRKA is a receptor for the neurotrophic nerve growth factor. Researchers later identified the closely related TRKB and TRKC receptors, and all 3 are crucial to nervous system development and differentiation. Other binding partners for TRKs include brain-derived neurotrophic factor or neurotrophin-4 for TRKB and neurotrophin-3 for TRKC.²

In their natural state, TRK receptors form dimers when activated by their natural ligands, and this leads to structural changes that cause them to activate the RAS/MAPK pathway, leading to increased cellular proliferation and growth via signaling, or prevention of apoptosis.² TRK receptors are essential to embryonic development but also play a key role in survival of sensory nerves that relay information on blood pressure and pH. They also occur and function in nonneural tissues (eg, vasculature, ovaries, immune system). Mutations in TRKA have been associated with congenital insensitivity to pain, energy imbalances, loss of appetite control, and memory problems.²

When TRK proteins are part of a fusion, they may interact with and be activated by ligands other than their usual partners. Fusions can also cause deactivation of the protein’s tyrosine kinase function. TRK inhibition prevents adenosine triphosphate, the energy storage molecule, from binding to the fusion protein, which prevents protein activation. This in turn may cut off signals that otherwise induce cell growth, survival, and differentiation, leading to tumor regression.^3,4

The wide range of activity of TRK receptors led to several therapeutic development programs. One was focused on pain relief in the hopes that TRK inhibitors could be an alternative to opioids; this included a monoclonal antibody drug that Pfizer Inc and Eli Lilly and Company took through phase 3 clinical trials.⁵

NTRK Fusions in Cancers

In the 2000s, David S. Hong, MD, deputy chair of the Department of Investigational Cancer Therapeutics in the Division of Cancer Medicine at The University of Texas MD Anderson Cancer Center in Houston, said researchers discovered NTRK fusions in cancers, especially in rare tumors. However, early efforts produced drug candidates that were too toxic, according to Hong. In addition to the expected adverse effects (AEs) such as weight gain and dizziness, the drugs sometimes caused a strange AE. “Some of these patients were committing suicide,” said Hong. That eventually contributed to a halt of TRK inhibitor development, but Hong was not dissuaded, especially when a colleague approached him about the belief that NTRK fusions occur in a wide variety of tumor types.

Hong went on to coauthor an article published in the New England Journal of Medicine that described findings from phase 1/2 studies of the TRK inhibitor larotrectinib (Vitrakvi).⁶ The FDA approved larotrectinib in 2018 and then entrectinib (Rozlytrek) in 2019 for adult and pediatric tumors with NTRK fusions that are metastatic or inoperable, making them the first-generation TRK inhibitors.^6,7

Larotrectinib is a selective inhibitor of TRKA^8,9; entrectinib is a multikinase inhibitor that also targets oncogenic rearrangements in ROS1 and ALK and is additionally approved for ROS1-positive non–small cell lung cancer (NSCLC).⁹

NTRK fusions represent an important and exploitable vulnerability in a tumor. Some patients have gone into remissions that have lasted nearly 10 years (so far), according to Hong. “I always tell patients that [looking for an NTRK fusion] is kind of like a lottery ticket,” he said.

The FDA approval of larotrectinib was based in part on data from the phase 2 NAVIGATE basket study (NCT02576431), which recruited patients with NTRK fusions regardless of tumor type. The study is still ongoing, and Hong reported updates on 100 patients with 14 tumor types at the European Society for Medical Oncology Congress 2024. The results included a 77% overall response rate and 47% complete response rate.¹⁰ The median duration of response was 59 months, and the median overall survival had not been reached.¹⁰

“Many of these patients we enrolled have never received their [frontline standard care], and we can see from data sets now that they’re doing amazingly great,” Hong said.

Rare and Exquisite

To what do TRK inhibitors owe their efficacy? According to Hong, cancers with fusion proteins, although rare, are also biologically unique. “These cancer cells are addicted to fusions, and we see that with ALK-ROS1 [and with other fusions like FGFR and RET],” he said. “They’re kind of oncogenically addicted to the protein that is expressed from the fusion, and oftentimes they don’t have other alterations. They are exquisitely driven by the fusion alone, so if you can block the protein that’s driving the cancer, you can have some pretty amazing effects.”

The fusions are rare but are more common in specific subpopulations. Hong cited the example of a young patient with lung cancer who was a nonsmoker, for whom DNA sequencing doesn’t show any fusions. He pointed out that fusion proteins are often missed by DNA sequencing because the fusion can make the DNA sequence too long for next-generation sequencing to discover, adding that he “wouldn’t be surprised if [RNA sequencing showed] they have a fusion. Same thing with patients with colorectal cancer.”

NTRK fusion proteins are also common in patients with microsatellite instability–high tumor types, according to Hong. “So little clues like that” can prompt a decision to look for fusion proteins, Hong said.

Ideally, all tumors would be sequenced to determine whether they harbor TRK or other fusion proteins. However, tests can be cumbersome; methods such as immunohistochemistry can be difficult to interpret, and fluorescence in situ hybridization is highly sensitive but complex, requiring individual assays for each gene, and slower to return results. DNA- and RNA-based next-generation sequencing can detect NTRK fusions, but they are expensive and limited in availability.

“Bayer [HealthCare Pharmaceuticals Inc] and Genentech bought [larotrectinib and entrectinib, respectively,] I think with anticipation that everybody in the community was going to get [DNA sequencing], but it is not as prevalent as we thought it was going to be in the community,” Hong said.

He added that it remains unclear why NTRK fusions are more common in some tumor types than others.

“There hasn’t been a whole lot of in-depth science as to what is going on here,” Hong said.

Instead, research is more focused on mechanisms of resistance to first-line TRK inhibitors.

“There are now clear mechanisms of resistance with these molecules in the context of certain mutations,” he said.

Emergence of TRK Receptors

Gatekeeper mutations occur in the binding pocket of a protein and prevent an inhibitor from binding to it, and these drive resistance to TRK inhibitors. A solvent front mutation occurs at a place where the protein interacts with surrounding fluids but also near the binding site. These mutations lead to structural changes that interfere with the ability of inhibitors to bind to the proteins. Another possibility is that mutations to BRAF, KRAS, or MET may be off-target resistance mechanisms that allow cells to bypass TRK inhibitors.¹¹

Developers also predicted on-target mutations, and in anticipation, developed second-generation TRK inhibitors selitrectinib and repotrectinib (Augtyro) alongside larotrectinib and entrectinib to overcome resistance.¹¹ The 2 drugs entered their own clinical trials in 2017, a year before larotrectinib received FDA approval. Selitrectinib had an overall response rate of 34% in a phase 1 trial and is undergoing development.¹² Repotrectinib received FDA approval in June 2024 for NTRK gene fusion–positive solid tumors13 and had previously received approval for ROS1 fusion–positive NSCLC.¹⁴

Another resistance possibility is that the epidermal growth factor receptor (EGFR) protein is involved in a feedback loop, according to Hong. Together with colleagues at MD Anderson Cancer Center, Hong is designing a clinical trial to test the combination of larotrectinib and an EGFR inhibitor in preventing resistance. “It’s a challenge,” he said, “because NTRK gene fusion–positive tumors are so rare.” Hong pointed out that these trials are testing resistance, and evaluating patients who are not doing well. If most patients are doing well, there's a challenge to recruit patients who are not doing well.

At an earlier stage of development, the next-generation TRK inhibitor zurletrectinib has shown activity in cell-based assays and good brain penetration in rats.¹⁵ SIM1803-1A has demonstrated activity in wild-type and solvent front–mutant NTRK fusion–positive cancers in preclinical studies,¹⁶ and PBI-200 has shown improved brain penetrance vs other TRK inhibitors¹⁷; both drugs are in phase 1 clinical trials.

Another means to overcoming resistance is combination therapy. In the TRK space, the addition of the MET inhibitor crizotinib (Xalkori) countered progression that occurred due to MET amplification after entrectinib treatment that selitrectinib monotherapy had failed to control.¹⁸ NTRK fusion is itself a mechanism of resistance to other targeted therapies, so these strategies may be applicable to situations where TRK inhibitors are brought in to overcome resistance to other targeted therapies, according to Hong.

Development of resistance to next-generation TRK inhibitors can also occur but is poorly understood. One study showed that both selitrectinib and repotrectinib had reduced activity against TRKA/B/C xDFG mutations, which suggests a possible convergent mechanism of resistance. These mutations have also been identified in samples from patients with NTRK fusion– positive sarcoma, breast cancer, and colorectal cancer. They cause the protein to shift to a different structure, potentially making them susceptible to inhibition by type II kinase inhibitors, which bind to inactive kinases and trap them in the inactive state. And this could provide an avenue to the design of third-generation TRK inhibitors.¹⁹

“If you can find a TRK fusion in a patient with non–small cell lung cancer or a patient with fibroid sarcoma or whoever, these drugs can really be transformative and change the direction of [the disease],” Hong said.

REFERENCES:

1. O’Haire S, Franchini F, Kang YJ, et al. Systematic review of NTRK 1/2/3 fusion prevalence pan-cancer and across solid tumours. Sci Rep. 2023;13(1):4116. doi:10.1038/s41598-02331055-3

2. Amatu A, Sartore-Bianchi A, Bencardino K, Pizzutilo EG, Tosi F, Siena S. Tropomyosin receptor kinase (TRK) biology and the role of NTRK gene fusions in cancer. Ann Oncol. 2019;30(suppl 8):viii5-viii15.

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4. Theik NWY, Muminovic M, Alvarez-Pinzon AM, Shoreibah A, Hussein AM, Raez LE. NTRK therapy among different types of cancers, review and future perspectives. Int J Mol Sci. 2024;25(4):2366. doi:10.3390/ijms25042366

5. Pfizer and Lilly announce top-line results from long-term phase 3 study of tanezumab in patients with osteoarthritis. News release. Pfizer Inc and Eli Lilly and Company. April 18, 2019. Accessed October 17, 2024. https://tinyurl.com/yfss46xx

6. Drilon A, Laetsch TW, Kummar S, et al. Efficacy of larotrectinib in TRK fusion-positive cancers in adults and children. N Engl J Med. 2018;378(8):731-739. doi:10.1056/ NEJMoa1714448

7. FDA approves larotrectinib for solid tumors with NTRK gene fusions. FDA. Updated December 14, 2018. Accessed October 17, 2024. https://tinyurl.com/yeu9pzvm

8. FDA approves entrectinib for NTRK solid tumors and ROS-1 NSCLC. FDA. Updated August 16, 2019. Accessed October 17, 2024. https://tinyurl.com/4tkxpscf

9. Federman N, McDermott R. Larotrectinib, a highly selective tropomyosin receptor kinase (TRK) inhibitor for the treatment of TRK fusion cancer. Expert Rev Clin Pharmacol. 2019;12(10):931-939. doi:10.1080/17512433.2019.1661775

10. Hong, DS, Xu, R, Shen, L, et al. MYE Symphony: a first-inhuman study to investigate the safety, pharmacokinetics, pharmacodynamics and preliminary efficacy of the in vivo mRNA CAR therapy, MT-302, targeting TROP2 in adults with advanced epithelial tumors. Ann Oncol. 2024;35(suppl 2):S482-S535. doi:10.1016/annonc/annonc1589

11. Cocco E, Scaltriti M, Drilon A. NTRK fusion-positive cancers and TRK inhibitor therapy. Nat Rev Clin Oncol. 2018;15(12):731-747. doi:10.1038/s41571-018-0113-0

12. Hyman, D, Kummar, S, Farago, A, et al. Abstract CT127: phase I and expanded access experience of LOXO-195 (BAY 2731954), a selective next-generation TRK inhibitor (TRKi). Cancer Res. 2019;79(suppl 13):CT127. doi:10.1158/1538-7445. AM2019-CT127

13. FDA grants accelerated approval to repotrectinib for adult and pediatric patients with NTRK gene fusion-positive solid tumors. FDA. Updated June 13, 2024. Accessed October 17, 2024. https://tinyurl.com/3tzy362c

14. FDA approves repotrectinib for ROS1-positive non-small cell lung cancer. FDA. Updated November 16, 2023. Accessed October 17, 2024. https://tinyurl.com/33rjepyx

15. Roa P, Foglizzo V, Harada G, et al. Zurletrectinib is a next-generation TRK inhibitor with strong intracranial activity against NTRK fusion-positive tumours with on-target resistance to first-generation agents. Br J Cancer. 2024;131(3):601-610. doi:10.1038/s41416-024-02760-1

16. Wang J, Yu X, Zhu S, et al. Preclinical evaluation of SIM1803-1A, a small molecule Trk/ROS1 dual inhibitor for wild and mutate NTRK/ROS1 fusion solid malignancies. J Clin Oncol. 2020;38(suppl 15):e21663. doi:10.1200/ JCO.2020.38.15_suppl.e21663

17. Regina A, Elagoz A, Albert V, et al. Abstract 2198: PBI-200: a novel, brain penetrant, next generation pan-TRK kinase inhibitor. Cancer Res. 2019;79(suppl 13):2198. doi:10.1158/15387445.AM2019-2198

18. Cocco E, Schram AM, Kulick A et al. Resistance to TRK inhibition mediated by convergent MAPK pathway activation. Nat Med. 2019;25(9):1422-1427. doi:10.1038/s41591-019-0542-z

19. Cocco E, Lee JE, Kannan S, et al. TRK xDFG mutations trigger a sensitivity switch from type I to II kinase inhibitors. Cancer Discov. 2021;11(1):126-141. doi:10.1158/2159-8290. CD-20-0571