Direct Targeting of RET Aberrations Moves Closer to the Clinic

OncologyLive, Vol. 19/No. 23, Volume 19, Issue 23

Over the past 30 years, from the discovery of a fusion rearrangement to the recent promise of LOXO-292, RET targeting has gone from distant concept toward clinical reality.

Alexander Drilon, MD

Research on the proto-oncogene RET has accelerated exponentially since 1985, when Masahide Takahashi, MD, PhD, and colleagues first reported a fusion rearrangement in lymphoma DNA.1 Early progress was slow. Other investigators noted RET abnormalities—both fusions and mutations—in papillary thyroid cancer (PTC), in multiple endocrine neoplasia type 2 (MEN2), and after radiation exposure, but targeted interventions were still on the distant horizon.2-4

In 2005, the structure of the RET kinase domain was described, and in subsequent years, as small-molecule inhibition gained traction and RET fusions were discovered in patients with lung cancer, research on RET targeting built momentum.5 Initial multikinase inhibitor trials suggested that RET could be a viable target, but “off-target” toxicity limited effective drug design.6

In light of these findings, the focus has shifted to the development of RET-specific inhibitors. In September 2018, the leading prospect in the field, LOXO-292, was granted a breakthrough designation by the FDA for RET fusion—positive metastatic non–small cell lung cancer (NSCLC) and RET-mutant medullary thyroid cancer (MTC).7 Five weeks later, the designation was expanded to RET fusion-positive thyroid cancer.8

Pathways Involving RET

RET-specific inhibition appears well tolerated and effective regardless of gatekeeper mutations or RET fusion partners, and researchers are optimistic about the value of RET inhibition across tumor types. How this inhibition will be accomplished—by selective agents, custom-tailored multikinase inhibitors, or in combination with other therapies—has yet to be determined.RET is a transmembrane glycoprotein receptor tyrosine kinase (RTK) encoded by RET, which is located on chromosome 10.9 During embryogenesis, RET aids in development of the enteric nervous system and kidneys; throughout life, in the homeostasis of multiple tissues.10,11

Receptor activation is achieved through indirect ligand binding; glial cell line—derived neurotrophic factor (GDNF) family ligands (GFLs) bind to GDNF family receptor-α coreceptors, resulting in a GFL–GFRα complex that homodimerizes RET.12

RET Aberrations

With homodimerization, intracellular tyrosine residues are trans-autophosphorylated, signaling adaptors are recruited, and several signaling cascades are activated. RET-associated pathways include RAS/MAPK/ERK, PI3K/AKT, JAK/STAT, and C-gamma, all of which regulate cell proliferation (Figure).13—16With knowledge of associations between RET and downstream cell proliferation pathways, it follows logically that cancer might result from RET dysfunction. Such dysfunction can be retraced to various RET aberrations, which are divided into 2 types: chromosomal rearrangements (fusions) or mutations.1

Chromosomal rearrangements, such as the gene fusion originally described by Takahashi, most commonly lead to overexpression of RET or uncontrolled activation of the RET intracellular domain. Either mechanism can increase downstream activity, thereby contributing to malignancy via angiogenesis, cell survival, or invasion.17

Figure. RET Pathway Signaling Associated With Cancer13-16

Other rearrangements have also been documented, resulting in gain-of-function of RET or inhibition of tumor suppressor gene partners, but such instances are rare.18 In NSCLC, KIF5B—RET is the most common fusion, whereas CCDC6—RET and NCOA4—RET are most common in PTC.18,19 Each of these genes (KIF5B, CCDC6, NCOA4) is located on chromosome 10 with RET, and rearrangements occur via inversion.1 Rarely, interchromosomal RET rearrangements or translocations occur.17 At a population level, RET aberrations have been identified in less than 2% of diverse cancers, according to a molecular analysis of malignancies present in nearly 5000 patients. These include mutations (38.6%), fusions (30.7%), and amplifications (25%).20 RET rearrangements occur in 1% to 2% of patients with NSCLC (usually adenocarcinomas).19 These cases often share characteristics with patients who have ROS1 or ALK rearrangements; namely, that they tend to be nonsmokers less than 60 years of age.21

With a prevalence rate of 10% to 20%, RET rearrangements are more common in PTC compared with NSCLC, and they are still even more prevalent in patients who develop PTC secondary to radiation exposure, such as those involved in the Chernobyl disaster. In this population, approximately three-quarters exhibited RET rearrangements.2,22

In contrast with the rearrangements found in NSCLC and PTC, proto-oncogene activation in MTC is most commonly due to an RET mutation.23 These can be somatic or germline and are often substitution mutations of the intracellular or extracellular domains; in the latter case, cysteine residues may be replaced with other amino acids, ultimately resulting in ligand-independent dimerization and activation.24 RET germline mutations are responsible for MEN2, a syndrome that leads to about 25% of MTC cases.15,25

Specifically, certain mutations can be associated with subsets of this disease, such as substitutions at the V804 codon, which lead to familial MTC.17 In the other 75% of MTC cases, which are sporadic, 43% to 71% have a RET mutation.20 M918T mutations are most common, followed by Cys634 point mutations, deletions, or small insertions.17,26

Targeting RET

Somatic RET mutations also occur in about 20% of patients with sporadic pheochromocytoma.27 Recently, both similar and novel mutations have been found in a variety of other cancer types, including hepatoma, gastrointestinal stromal tumor, meningioma, atypical lung carcinoid, paraganglioma, and Merkel cell, endometrial, colorectal, and breast carcinoma.20 This list may continue to expand with increasing usage of next-generation sequencing.1Multikinase Inhibitors

Based on the prevalence of RET aberrations in NSCLC and thyroid cancer, the ever-expanding list of other RET-mutated cancers, and the upstream location of RET itself, RET targeting has potential for pan-cancer application. However, the orientation of RET and its roles in organ development and homeostasis have given rise to safety concerns for inhibition.1

Preclinical Ret-knockout models and diseases with inactivating RET mutations have provided some hints at safety issues, as documented by a plethora of studies, but the relevance of their findings is diminished by recent clinical trials with RET-specific inhibitors. In short, despite the potentially catastrophic consequences of RET inhibition in early development, resulting in tissue agenesis and fatal physiologic derangements, inhibition from late childhood onward appears relatively safe.1

A variety of RET inhibitors have been approved as multikinase agents, in which RET is not the primary target.1 These include sunitinib (Sutent), cabozantinib (Cabometyx), and lenvatinib (Lenvima), with various indications. Although some are labeled for thyroid cancer, none are specifically approved for RET-aberrant subtypes.

Efficacy is comparable across agents, but mechanisms of action vary.28 For example, sunitinib is a type I inhibitor that attaches to the adenosine triphosphate binding pocket of activated RET, cabozantinib is a type II inhibitor, binding the inactive form; others have unknown mechanisms.6,29 Several of these agents have shown activity against RET-rearranged models, resulting in suppression of downstream pathways and decreased cell proliferation.29,30,31 Similarly, treatment of mutant models results in decremental tumor growth, both through RET wild-type targeting and activity against specific mutant isoforms.1

Clinical trials have echoed these findings. For example, in the phase III EXAM trial evaluating cabozantinib, patients with RET M918T—mutated MTC had longer progression-free survival with the drug compared placebo than did those whose RET mutation status was not known.33 Cabozantinib has also shown activity in RET fusion-positive NSCLC, with response rates ranging from 20% to 50% in patients who have received 2 or more lines of therapy.28,34 These are, however, essentially proof-of-concept trials, because results do not match the efficacy of multikinase inhibitors against other major oncogenic drivers such as EGFR or ROS1.28 In part, this is because off-target activity results in dose-limiting toxicities before substantial RET inhibition can occur.1

LOXO-292

Other agents are required to achieve significant RET inhibition without toxicity. Combination therapies may be needed, since some studies suggest that RET inhibition is most effective with additional downstream targeting. Similarly, custom multikinase agents that are more selective for RET but still maintain some downstream inhibition could potentially accomplish the same goal. Although the possibilities of multitarget agents remain open, the research spotlight is currently on RET-selective inhibitors. Of these, three leading agents warrant a closer look:The foremost RET-selective inhibitor, LOX-292, was recently granted a breakthrough designation by the FDA for RET fusion-positive NSCLC, RET fusion-positive thyroid cancer, and RET-mutated MTC.7,8 These designations were given following the notable responses among patients treated with LOXO-292 during the phase I/II LIBRETTO-001 trial (NCT03157128). Patients were heavily pretreated, with a median 3 lines of prior therapy.

At the 2018 American Society of Clinical Oncology Annual Meeting, investigtors reported an objective response rate (ORR) of 77% (95% CI, 58%-90%) with LOXO-292 for patients with RET fusion—positive (NSCLC and 45% for patients with RET-mutated MTC. By the April 2018 data cutoff, 82 patients had been treated with LOXO-292 across 7 cohorts.

In the most recently reported data, treatment with LOXO-292 resulted in the following ORRs: RET-mutated MTC, 59%; RET fusion-positive NSCLC, 68%; and RET fusion-positive thyroid cancer, 78%.34,35, 36

Additionally, all patients (n = 4) with intracranial lesions responded to therapy. LOXO-292 appeared safe and well tolerated, with very few patients discontinuing therapy; most adverse events (AEs) were limited to grade 1 and unrelated to treatment. About 5% of patients had grade 3 treatment-related AEs, including elevated liver enzymes, diarrhea, thrombocytopenia, and tumor lysis syndrome, but all resolved with dose interruption.

BLU-667

In an interview with OncologyLive®, lead investigator Alexander E. Drilon, MD, a medical oncologist and clinical director of the Early Drug Development Service at Memorial Sloan Kettering Cancer Center in New York, New York, reflected on the LIBRETTO-001 findings. “Just to put the [results] in perspective: With the older and ‘dirtier’ multikinase inhibitors for RET fusion—positive cancers, we see a 30% response rate with much more toxicity. Here, we are seeing a drug that is achieving outcomes that are similar to what we would expect for targeted therapy in EGFR, ALK, and ROS1,” Drilon said.Results from the phase I ARROW trial (NCT03037385) demonstrated slightly lower response rates than LOXO292.37 During the dose-escalation study, 51 patients were treated. Those with RET-mutated MTC had a best ORR of 40%, and the RET fusion-positive NSCLC group had a best ORR of 50%. Treatment with BLU-667 resulted in 3 dose-limiting toxicities. Grade 3 AEs included hypertension and neutropenia.

RXDX-105

Reporting these findings at the American Association for Cancer Research Annual Meeting in April 2018, lead author Vivek Subbiah, MD, of The University of Texas MD Anderson Cancer Center in Houston, said, “ARROW dose escalation data validate BLU-667 as a promising precision therapy for RET-altered cancers.” He added that the responses were noteworthy considering the low doses given to most patients.In 2017, at the European Society for Medical Oncology Annual Congress, investigators reported results from a phase I/IB clinical trial (NCT01877811) in which the RET inhibitor RXDX-105 was given to 152 patients.16 The presenters highlighted a subgroup of 22 patients with RET fusion—positive NSCLC. Six out of 8 patients (75%) with non–KIF5B-RET fusions had partial responses. Of the 14 NSCLC patients with KIF5B-RET fusions, 3 achieved stable disease. Early data also pointed to some potential issues with toxicity. Grade 3 or higher rash occurred in 12% of patients, and nephrotoxicity was documented. Additionally, 1 patient had a serious AE that did not resolve upon dose interruption.

Summary

Presenting the findings, coauthor Lyudmila A. Bazhenova, MD, of the University of California, San Diego, Health Moores Cancer Center in La Jolla, said, “RXDX-105 has demonstrated a manageable safety profile in patients with advanced or metastatic solid tumors.”Over the past 30 years, from the discovery of a fusion rearrangement to the recent promise of LOXO-292, RET targeting has gone from distant concept toward clinical reality. RET rearrangements and mutations are represented across multiple tumor types, so emerging agents may serve as pan-cancer therapies. Whether RET-selective inhibitors are used alone or in combination with other agents remains to be seen, as does first-line efficacy. Regardless, the results from LIBRETTO-001 prompt optimism for the future of RET inhibition.

References

  1. Drilon A, Hu ZI, Lai GGY, Tan DSW. Targeting RET-driven cancers: lessons from evolving preclinical and clinical landscapes [erratum in Nat Rev Clin Oncol. 2018;15(3):150. doi: 10.1038/nrclinonc.2017.188]. Nat Rev Clin Oncol. 2018;15(3):151-167. doi: 10.1038/nrclinonc.2017.175.
  2. Nikiforov YE, Rowland JM, Bove KE, Monforte-Munoz H, Fagin JA. Distinct pattern of ret oncogene rearrangements in morphological variants of radiation-induced and sporadic thyroid papillary carcinomas in children. Cancer Res. 1997;57(9):1690-1694.
  3. Santoro M, Carlomagno F, Romano A, et al. Activation of RET as a dominant transforming gene by germline mutations of MEN2A and MEN2B. Science. 1995;267(5196):381-383.
  4. Viglietto G, Chiappetta G, Martinez-Tello FJ, et al. RET/PTC oncogene activation is an early event in thyroid carcinogenesis. Oncogene. 1995;11(6):1207-1210.
  5. Knowles PP, Murray-Rust J, Kjær S, et al. Structure and chemical inhibition of the RET tyrosine kinase domain. J Biol Chem. 2006;281(44):33577-33587. doi: 10.1074/jbc.M605604200.
  6. Plaza-Menacho I, Mologni L, McDonald NQ. Mechanisms of RET signaling in cancer: current and future implications for targeted therapy. Cell Signal. 2014;26(8):1743-1752. doi: 10.1016/j.cellsig.2014.03.032.
  7. Inman S. FDA Grants LOXO-292 breakthrough designation for NSCLC, MTC. OncLive website. onclive.com/web-exclusives/fda-grants-loxo292-breakthrough-designation-for-nsclc-mtc. Published September 5, 2018. Accessed October 23, 2018.
  8. Columbus G. FDA grants LOXO-292 breakthrough designation for RET fusion-positive thyroid cancer. OncLive website. onclive.com/web-exclusives/fda-grants-loxo292-breakthrough-designation-for-ret-fusionpositive-thyroid-cancer?p=2. Published October 15, 2018. Accessed October 23, 2018.
  9. Ishizaka Y, Itoh F, Tahira T, et al. Human ret proto-oncogene mapped to chromosome 10q11.2. Oncogene. 1989;4(12):1519-1521.
  10. De Graaff E, Srinivas S, Kilkenny C, et al. Differential activities of the RET tyrosine kinase receptor isoforms during mammalian embryogenesis. Genes Dev. 2001;15(18):2433-2444. doi: 10.1101/gad.205001.
  11. Airaksinen MS, Saarma M. The GDNF family: signalling, biological functions and therapeutic value. Nat Rev Neurosci. 2002;3(5):383-394. doi: 10.1038/nrn812.
  12. Wang X. Structural studies of GDNF family ligands with their receptors - insights into ligand recognition and activation of receptor tyrosine kinase RET. Biochim Biophys Acta. 2013;1834(10):2205-2212. doi: 10.1016/j.bbapap.2012.10.008.
  13. Maeda K, Murakami H, Yoshida R, et al. Biochemical and biological responses induced by coupling of Gab1 to phosphatidylinositol 3-kinase in RET-expressing cells. Biochem Biophys Res Commun. 2004;323(1):345-354. doi: 10.1016/j.bbrc.2004.08.095.
  14. Schuringa JJ, Wojtachnio K, Hagens W, et al. MEN2A-RET-induced cellular transformation by activation of STAT3. Oncogene. 2001;20(38):5350-5358. doi: 10.1038/sj.onc.1204715.
  15. Arighi E, Borrello MG, Sariola H. RET tyrosine kinase signaling in development and cancer. Cytokine Growth Factor Rev. 2005;16(4-5):441-467. doi: 10.1016/j.cytogfr.2005.05.010.
  16. AE Drilon, Liu S, Doebele R, et al. A phase IB study of RXDX-105, a VEGFR-sparing potent RET inhibitor, in RET inhibitor naïve patients with RET fusion-positive NSCLC. Proceedings from the European Society for Medical Oncology 2017 Congress; September 8-12, 2017; Madrid, Spain. Abstract LBA19. oncologypro.esmo.org/Meeting-Resources/ESMO-2017-Congress/A-Phase-1b-study-of-RXDX-105-a-VEGFR-sparing-potent-RET-inhibitor-in-RETi-naive-patients-with-RET-fusion-positive-NSCLC.
  17. Romei C, Ciampi R, Elisei R. A comprehensive overview of the role of the RET proto-oncogene in thyroid carcinoma. Nat Rev Endocrinol. 2016;12(4):192-202. doi: 10.1038/nrendo.2016.11.
  18. Santoro M, Carlomagno F. Central role of RET in thyroid cancer. Cold Spring Harb Perspect Biol. 2013;5(12):a009233. doi: 10.1101/cshperspect.a009233.
  19. Kohno T, Ichikawa H, Totoki Y, et al. KIF5B-RET fusions in lung adenocarcinoma. Nat Med. 2012;18(3):375-377. doi: 10.1038/nm.2644.
  20. Kato S, Subbiah V, Marchlik E, Elkin SK, Carter JL, Kurzrock R. RET aberrations in diverse cancers: next-generation sequencing of 4,871 patients. Clin Cancer Res. 2017;23(8):1988-1997. doi: 10.1158/1078-0432.CCR-16-1679.
  21. Wang R, Hu H, Pan Y, et al. RET fusions define a unique molecular and clinicopathologic subtype of non-small-cell lung cancer. J Clin Oncol. 2012;30(35):4352-4359. doi: 10.1200/JCO.2012.44.1477.
  22. Espinosa AV, Gilbert J. RET in Thyroid Cancer. My Cancer Genome. mycancergenome.org/content/disease/thyroid-cancer/ret/. Updated June 18, 2015. Accessed November 19, 2018.
  23. Eng C, Smith DP, Mulligan LM, et al. Point mutation within the tyrosine kinase domain of the RET proto-oncogene in multiple endocrine neoplasia type 2B and related sporadic tumours. Hum Mol Genet. 1994;3(2):237-241.
  24. Eng C, Clayton D, Schuffenecker I, et al. The relationship between specific RET proto-oncogene mutations and disease phenotype in multiple endocrine neoplasia type 2. International RET mutation consortium analysis. JAMA. 1996;276(19):1575-1579.
  25. Zbuk KM, Eng C. Cancer phenomics: RET and PTEN as illustrative models. Nat Rev Cancer. 2007;7(1):35-45. doi: 10.1038/nrc2037.
  26. Uchino S, Noguchi S, Yamashita H, et al. Somatic mutations in RET exons 12 and 15 in sporadic medullary thyroid carcinomas: different spectrum of mutations in sporadic type from hereditary type. Jpn J Cancer Res. 1999;90(11):1231-1237.
  27. Beldjord C, Desclaux-Arramond F, Raffin-Sanson M, et al. The RET protooncogene in sporadic pheochromocytomas: frequent MEN 2-like mutations and new molecular defects. J Clin Endocrinol Metab. 1995;80(7):2063-2068. doi: 10.1210/jcem.80.7.7608256.
  28. Iams W, Lovly C. RET rearrangements in non—small cell lung cancer. J Target Ther Cancer. 2018;7(3):33-38.
  29. Plenker D, Riedel M, Brägelmann J, et al. Drugging the catalytically inactive state of RET-arranged tumors. Sci Transl Med. 2017;9(394): eaah6144. doi: 10.1126/scitranslmed.aah6144.
  30. Patwardhan P, Ivy K, Musi E, de Stanchina E, Schwartz G. Significant blockade of multiple receptor tyrosine kinases by MGCD516 (Sitravatinib), a novel small molecule inhibitor, shows potent anti-tumor activity in preclinical models of sarcoma. Oncotarget. 2015;7(4):4093-4109. doi: 10.18632/oncotarget.6547.
  31. Lin C, Wang S, Xie W, Zheng R, Gan Y, Chang J. Apatinib inhibits cellular invasion and migration by fusion kinase KIF5B-RET via suppressing RET/Src signaling pathway. Oncotarget. 2016;7(37):59236-59244. doi: 10.18632/oncotarget.10985.
  32. Sherman SI, Clary DO, Elisei R, et al. Correlative analyses of RET and RAS mutations in a phase 3 trial of cabozantinib in patients with progressive, metastatic medullary thyroid cancer. Cancer. 2016;122(24):3856-3864. doi: 10.1002/cncr.30252.
  33. Drilon A, Wang L, Hasanovic A, et al. Response to cabozantinib in patients with RET fusion-positive lung adenocarcinomas. Cancer Discov. 2013;3(6):630-635. doi: 10.1158/2159-8290.CD-13-0035.
  34. Drilon AE, Subbiah V, Oxnard GR, et al. A phase 1 study of LOXO-292, a potent and highly selective RET inhibitor, in patients with RET-altered cancers. J Clin Oncol. 2018;36(suppl; abstr 102). meetinglibrary.asco.org/record/161573/abstract.
  35. Loxo Oncology announces LOXO-292 durability update in patients with RET-mutant medullary thyroid cancer and RET fusion-positive thyroid cancer from LIBRETTO-001 at the 88th annual meeting of the American Thyroid Association [press release]. Stamford, CT: Loxo Oncology, Inc; October 6, 2018. ir.loxooncology.com/press-releases/2370515-Loxo-oncology-announces-loxo-292-durability-update-in-patients-with-ret-mutant-medullary-thyroid-cancer-and-ret-fusion-positive-thyroid-cancer-from-libretto-001-at-the-88th-annual-meeting-of-the-american-thyroid-association. Accessed October 25, 2018.
  36. Loxo Oncology announces LOXO-292 durability update in patients with RET fusion-positive non-small cell lung cancer from LIBRETTO-001 at the IASLC 19th World Conference on Lung Cancer [press release]. Stamford, CT: Loxo Oncology, Inc; September 25, 2018. ir.loxooncology.com/press-releases/2368923-Loxo-oncology-announces-loxo-292-durability-update-in-patients-with-ret-fusion-positive-non-small-cell-lung-cancer-from-libretto-001-at-the-iaslc-19th-world-conference-on-lung-cancer. Accessed October 25, 2018.
  37. Subbiah V, Taylor M, Lin J, et al. Highly potent and selective RET inhibitor, BLU-667, achieves proof of concept in a phase I study of advanced, RET-altered solid tumors. presented at: American Association for Cancer Research 2018 Annual Meeting; April 14-18, 2018; Chicago, IL. CT043..abstractsonline.com/pp8/#!/4562/presentation/11125.