June 2015, Vol 4, No 3

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The RET Oncogene in Non–Small Cell Lung Cancer: Review of the Current Literature and Directions for the Future

Rebecca A. Shatsky, MD; Lyudmila Bazhenova, MD


Despite recent advances, lung cancer remains the leading cause of cancer-related death worldwide.1 In the past decade, the treatment of non–small cell lung cancer (NSCLC), which was once a disease with exceedingly few treatment options and historically poor outcomes, has begun to transform. The discovery of targetable cancer-driving mutations has added a promising new dimension to the field, which was once confined to the use of cytotoxic agents that often induced minimal responses that were frequently short-lived. The discovery of the EML4 fusion gene and the success of the ALK mutant–targeting tyrosine kinase inhibitor (TKI) crizotinib has proved that these mutations can have serious clinical impact, and that design of new drugs targeting the mutations can potentially save many lives.2

Importantly, the oncogenic driver mutations in lung adenocarcinoma are generally mutually exclusive, so the discovery of each new oncogenic driver mutation may be helpful to a population of patients who previously were not eligible for available targeted lung cancer therapies.3-6 The list of identifiable drug targetable genetic mutations in lung cancer is growing fast, and one of the newer targets that is just beginning to be explored in human clinical trials is the RET proto-oncogene.

Biology of the Native RET Oncogene

The RET oncogene, located in the pericentromeric region of chromosome 10q11.2, was first identified and mapped in 1985 using cells transfected with human lymphoma DNA.7 RET was first identified as a receptor for the glial cell–derived neurotropic factor family of ligands, which are important for neuron signaling.8,9 RET is expressed at its highest levels during embryogenesis and has been implicated in the process of nephrogenesis, neural crest migration, as well as the maintenance of normal neuron function. RET expression decreases to much lower levels in healthy adult tissues but is still routinely expressed in neurons, parasympathetic ganglia, thyroid C cells, urogenital tract cells, and testis germ cells.10-14

Like many oncogenes, RET encodes a receptor tyrosine kinase (RTK). The RET RTK transmembrane receptor consists of 4 cadherin-like domains in its extracellular region, a calcium binding site, a transmembrane region, and an intracellular region containing the active tyrosine kinase domain.15,16 Upon binding of its ligand and coreceptor, RET autophosphorylates and in turn activates multiple downstream targets, including the RAS-MAPK-ERK1/2 and the PI3K/AKT signaling pathway.17 This leads to activation of multiple essential cell cycle activities, including signaling to induce proliferation and to promote cell survival.18

Oncogenic RET Gene

There have been multiple previously identified mutations to the RET gene implicated in cancer development. For example, germline gain-of-function mutations in RET have long been implicated in the development of multiple endocrine neoplasia, which consists of medullary thyroid cancer, pheochromocytoma, and hyperparathyroidism.19 In addition, many irradiation-induced papillary thyroid cancers contain RET gene rearrangements.20 It is estimated that RET mutations can be found in 30% to 50% of both medullary and papillary thyroid cancers.21,22 Further, germline loss-of-function mutations are now known to be responsible for Hirschsprung disease, a congenital abnormality in which neuroblasts in the developing gut fail to migrate and neural cells do not mature in the gastrointestinal tract, resulting in chronic constipation.23,24

In lung cancer, in a mechanism very similar to that of ALK gene rearrangements, or ROS1, a somatic gain-of-function mutation in RET is responsible for creating a chimeric fusion oncogene, which can then lead to malignant transformation. When the intracellular kinase encoding domain of RET is fused to one of several recently identified gene partners, a fusion gene is created that encodes a constitutively active RTK. In 2011, Ju et al reported the first identified case of adenocarcinoma of the lung containing the RET fusion RTK, RET-KIF5B, in a 33-year-old nonsmoking patient.25 In 2012, several groups independently corroborated these results, identifying multiple different RET gene rearrangements and RET fusion proteins that are expressed at high levels in certain lung adenocarcinomas.6,25-28 The most frequent rearrangement is the fusion of RET to kinesin family member 5B (KIF5B), which is created by a pericentric inversion of chromosome 10.26 This particular gene fusion appears to be unique to adenocarcinoma of the lung. However, to date, 5 fusion partners of RET have been identified in NSCLC, including KIF5B-RET, CCDC6-RET, NCO4-RET, TRIM33-RET, and, most recently, RUFY2-RET.6,25-27,29 The function of these fusion transcripts appears to be similar to ALK gene fusions in that a coiled-coil domain of the N-terminal of the fusion partner is bound to the kinase domain of RET, which allows RET to dimerize and remain constitutively activated independent of ligand binding.26,30

Rearrangements in RET are reported to be found in approximately 1% to 2% of all adenocarcinomas of the lung, but these mutations may contribute to 6% to 19% of lung adenocarcinomas in never-smokers who are “pan-negative” for other similar oncogenic driver mutations, such as EML4-ALK and ROS1.3,30,31 RET mutations are found with higher frequency in light and never-smokers, younger patients, and in patients of European and Asian descent. The histology of lung cancers harboring RET rearrangements are most commonly moderately to well-differentiated adenocarcinomas; however, there are now rare examples of cancers with adenosquamous histology positive for these mutations as well.31

Preclinical Activity of RET Inhibitors

There are currently no drugs available that selectively inhibit only the RET tyrosine kinase. However, several small-molecule multikinase inhibitors that have already been developed for other targets have been found to have preclinical RET RTK inhibitory activity; this is true for sorafenib and sunitinib, the well-studied vascular endothelial growth factor (VEGF) inhibitors that are commonly used to treat renal cell carcinoma and hepatocellular carcinoma.32,33

Several drugs already approved for use in RET-mutated medullary thyroid cancer are currently being examined for their efficacy in RET-mutated NSCLC. Vandetanib is a potent multikinase inhibitor that was approved by the FDA for the treatment of patients with medullary thyroid carcinoma in 2011.34 In 2012, vandetanib was tested by Matsubara et al in preclinical trials using a patient-derived, RET-mutated lung adenocarcinoma cell line, LC-2/ad, which contained the CCDC6-RET fusion gene. Results from this experiment were the first of its kind to reveal specifically that lung cancer cells with RET mutations were sensitive to inhibition by vandeta­nib and insensitive to the epidermal growth factor receptor inhibitor gefitinib.35 Suzuki et al were able to corroborate these data in vivo using xenograft models with the same LC-2/ad cell line.36 Finally, Saito et al, using xenograft mice models harboring the most common RET fusion gene in NSCLC, KIF5B-RET, were recently able to prove that vandetanib was effective in significantly reducing tumor burden.37 A second drug with RET inhibitory activity, cabozantinib, was approved for the treatment of medullary thyroid carcinoma in 2012 and is just now beginning to be examined in the setting of NSCLC.38,39

Lenvatinib is an oral multi-TKI that targets VEGF receptors 1-3, fibroblast growth factor receptors 1-3, RET, mast/stem cell growth factor receptor, and platelet-derived growth factor beta. Cell-free kinase assays suggest that lenvatinib inhibits RET kinase-expressing cell lines. A recent preclinical study by Okamoto et al using LC-2/ad lung cancer cells revealed that in vitro lenvatinib inhibited cell growth with an inhibitory concentration of 50% of 48 nM. Lenvatinib was also found to decrease the percentage of cells in S phase, suggesting that the inhibition of signaling led to arrest of the growth phase.40

Finally ponatinib, a drug currently approved for the treatment of Philadelphia chromosome–positive leukemia, has been shown in vitro by Mologni et al to inhibit signaling of both wild-type and mutant forms of RET, with inhibition of RET signaling into the low nanomolar range in cell lines harboring RET-mutated thyroid cancer cells.41,42 At the 2013 Annual Meeting of the American Association for Cancer Research, Gozgit et al presented their data comparing the potency of ponatinib at inhibiting RET in the Ba/F3 cell line engineered to express the most common NSCLC RET fusion gene KIF5B-RET with 4 other TKIs known to inhibit RET. These data are presented in Table 1.43

Clinical Activity of RET Inhibitors in Lung Cancer In Vivo

The data surrounding use of the previously described RET inhibitors in patients with NSCLC positive for RET mutations are still extremely limited. Although several drugs with multikinase-inhibiting activity including RET are now in phase 2 clinical trials (Table 2), the majority of these trials are still enrolling patients, so formal reports of results are still very sparse. There are only 2 published reports describing the outcome of patients with NSCLC with RET mutations after treatment with one of the previously described RET-inhibiting drugs. The first, by Drilon et al, is a report of the first 3 patients in their trial testing the drug cabozantinib in patients with RET mutation–positive, advanced NSCLC. The first patient described was a 41-year-old female with a novel TRIM33-RET fusion. This patient had a durable partial response, with a 66% decrease in disease burden as measured by RECIST and was still on the trial with no reported dose reductions at 20 weeks. The second patient, who was significantly older than the first (75-year-old female) also had a confirmed partial response (33% disease reduction). However, this patient suffered several dose-limiting toxicities, including grade 3 fatigue and grade 3 proteinuria, which required 2 dose reductions. Despite dose reductions, she remained progression free and continued on the trial at 16 weeks. The third patient was a 68-year-old female who suffered from grade 3 hypertension requiring dose reduction, but she ultimately had disease stabilization with progression-free survival at 8 months.44 In 2013, Gautschi et al published a report of a 58-year-old male with RET-mutated NSCLC refractory to previous chemotherapy who responded to treatment with vandetanib at a dose of 300 mg per day.45


The field of thoracic oncology has enjoyed considerable growth during the past 5 years since the discovery of oncogenic driver mutations. For the first time in history, a subset of patients with NSCLC that once had a dismal prognosis are being afforded extended survival and improved quality of life on treatment because of the discovery of new “druggable” driver mutations that allow treatment with targeted therapies such as erlotinib or crizotinib, which have fewer toxicities than standard chemotherapy. The success of these therapies has proved that for patients who harbor one of these driver mutations or fusion proteins, treatment with cytotoxic chemotherapy may not be the safest or most effective option for treatment. Previously published trials examining drugs with RET inhibitory activity (sorafenib, sunitinib, and vandetanib) in patients with NSCLC were not performed in populations selected for RET mutations. So although these drugs were previously thought to have little activity in lung cancer, it is possible they were not being tested in the right subset of patients. Whereas it is true that RET mutations are only found in a small percentage of patients with NSCLC, the benefits that RET-inhibiting drugs may have in the right patient population could be, like crizotinib in ALK-mutated patients, fairly dramatic. The results from the various phase 2 clinical trials that are limited to patients predetermined to have RET mutations appear promising and will likely be extremely informative as to whether RET will be another useful drug target to help patients with NSCLC survive longer and with a better quality of life on therapy. What is clear is that the similarities between the RET oncogene and the ALK and ROS1 oncogenes are undeniable, and given the potential benefits to a patient population with too few treatment options, it would be a logical next step to add RET to the list of mutations examined in all new NSCLC cases.


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Dr Shatsky is an Oncology Fellow in the Division of Hematology/Oncology, Department of Medicine at the University of California, San Diego (UCSD).

Dr Bazhenova is Associate Clinical Professor of Medicine in the Division of Hematology/Oncology and is Medical Director of the UCSD Moores Cancer Center Infusion Center. Her clinical practice and research concentrate on lung cancer, particularly as it relates to females and nonsmokers. She actively participates in cooperative group trials and takes an active role in designing and implementing clinical investigations, including phase 2 studies and correlative science projects with several UCSD investigators.

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