Progress in Discovery of KIF5B-RET Kinase Inhibitors for the

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Progress in Discovery of KIF5B-RET Kinase Inhibitors for the Treatment of Non-Small-Cell Lung Cancer Miniperspective Minsoo Song* New Drug Development Center (NDDC), Daegu-Gyeongbuk Medical Innovation Foundation (DGMIF), 80 Cheombok-ro, Dong-gu, Daegu 701-310, Korea ABSTRACT: A new chimeric fusion transcript of KIF5B (the kinesin family 5B gene) and the RET (Rearranged during Transcription) oncogene, KIF5B-RET, was found in 1−2% of lung adenocarcinomas (LADCs) in late 2011. Several related clinical trials for non-small-cell lung cancer (NSCLC) with KIF5BRET rearrangements using existing RET inhibitors, such as lenvatinib, vandetanib, sunitinib, ponatinib, cabozantinib, and AUY922, have been swiftly initiated by the discovery of the KIF5B-RET fusion gene. Anti-RET activity and the status of clinical development of these known RET tyrosine kinase inhibitors (TKIs) for KIF5B-RET fusion-positive NSCLC are discussed. A kinase inhibitor that can target a driver mutation specifically may lead to a superior clinical benefit compared with broad-spectrum kinase inhibitors. In this regard, an analysis of the structure of RET kinase and its complex with known RET inhibitors are also briefly discussed.

Monoclonal antibodies and small-molecule tyrosine kinase inhibitors (TKI) are two types of targeted therapy being used in the treatment of NSCLC. Monoclonal antibody therapies include bevacizumab (Avastin), a vascular endothelial growth factor (VEGF) inhibitor, and cetuximab (Erbitux), an epidermal growth factor receptor (EGFR) inhibitor.4 Bevacizumab binds to vascular endothelial growth factor (VEGF) and prevents angiogenesis, the growth of new blood vessels, which tumors need for growth. Cetuximab binds to the epidermal growth factor receptor (EGFR) and works by stopping cancer cells from growing and dividing. Over the past 20 years, numerous small-molecule tyrosine kinase inhibitors (TKI) have been rigorously developed to treat NSCLC (Figure 1). Two major categories of targeted small-molecule TKIs include vascular endothelial growth factor (VEGF) directed and epidermal growth factor receptor (EGFR) directed therapies, as summarized in Table 1.5 Most of these are reversible smallmolecule TKIs that block autophosphorylation and substrate phosphorylation in an ATP-competitive manner in general, thereby preventing oncogenic activity. In the case of the irreversible inhibitor afatinib, it is known that the thiol of Cys797 makes a covalent bond by addition to the α,β-unsaturated ketone of afatinib in a 1,4-addition fashion (Michael addition) at the ATP binding site and inhibits EGFR more potently than the reversible inhibitors do.6 Among these small-molecule

Lung cancer is the leading cause of cancer-related mortality in the world. More than 1.8 million patients were diagnosed with lung cancer worldwide, and it comprised 13% of all new cancer cases diagnosed in 2012. This number is expected to increase and produce a higher cancer burden worldwide as a result.1 Lung cancer is classified as small-cell lung cancer (SCLC) or non-small-cell lung cancer (NSCLC). NSCLC is any type of epithelial lung cancer other than SCLC, and it accounts for 85% of lung cancers in western countries.2 The most common types of NSCLC include squamous cell carcinoma, large cell carcinoma, and adenocarcinoma, based on histological types. Although NSCLCs are associated with cigarette smoke, adenocarcinomas may be found in those who never smoked. In the United States alone, the 5-year relative survival rate from 2003 to 2009 for patients with lung cancer was 17%, and for the subclasses of lung cancer, the 5-year survival rate was even lower: 6% for small-cell lung cancer and 18% for non-small-cell lung cancer.3 In 2014, 224 210 of the estimated new cases and 159 260 of the deaths from lung cancer (NSCLC and SCLC combined), accounting for approximately 27% of all cancer deaths, are expected to occur in the United States.3 Treatments for lung cancer include surgery, radiation therapy, chemotherapy, and targeted therapies. NSCLCs are relatively insensitive to chemotherapy and radiation therapy compared with SCLCs. For early stage NSCLCs, surgery is usually the treatment of choice. For patients with unresectable disease, local control can be achieved with radiation therapy combined with chemotherapy, targeted drugs, or some combination, but cure is limited only to a small number of patients. © XXXX American Chemical Society

Received: September 24, 2014

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DOI: 10.1021/jm501464c J. Med. Chem. XXXX, XXX, XXX−XXX

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Journal of Medicinal Chemistry

Figure 1. Small-molecule TKIs for NSCLC approved by the U.S. FDA.

Table 1. Representative Targeted Therapies for the Treatment of NSCLC Approved by the U.S. FDA VEGF-targeted EGFR-targeted others

monoclonal antibody

small-molecule TKIs

bevacizumab cetuximab

sorafenib, sunitinib, vandetanib, motesanib erlotinib, gefitinib, afatinib crizotinib (EML4-ALK), ceritinib (ALK+, crizotinib resistant)

independent research groups simultaneously.9−12 For example, Kohno et al. identified the four fusion variants of KIF5B-RET that contained the KIF5B coiled-coil domain in various lengths and the fully retained RET kinase domain as a major component using whole transcriptome sequencing. It was likely that aberrant activation of the kinase function of RET was initiated by homodimerization by the KIF5B coiled-coil domain.9 A simplified schematic representation of KIF5B, RET proteins, and KIF5B-RET fusions is depicted in Figure 2. Rearranged during Transcription (RET) is a transmembrane

inhibitors, the EGFR TKIs and crizotinib have shown better efficacy in tumors harboring sensitizing EGFR mutations or ALK translocations, whereas VEGF TKIs showed much less efficacy in NSCLC. It is noteworthy that ceritinib was approved by the FDA in early 2014 for patients with crizotinib-resistant NSCLC, just 2 years after crizotinib’s fast track approval by the FDA. 7 It is reported that more than 50% of lung adenocarcinomas (LADC) have critical driver mutations in oncogenes which potentiate tumor proliferation and spread.8 Such driver mutations commonly occur in KRAS, EGFR, ALK, HER2, and others. Recent achievements in the improvement of tumor regression and prolonged survival with small-molecule EGFR TKIs and crizotinib clearly suggest that the development of novel agents for newly evolving driver mutations may achieve successful targeting and lead to better outcomes in treating NSCLCs.

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KIF5B-RET Between late 2011 and early 2012, a new chimeric fusion transcript of KIF5B (the kinesin family 5B gene) and the RET (Rearranged during Transcription) oncogene, KIF5B-RET, was first identified as a new driver mutation in NSCLC by four

Figure 2. Schematic representations of wild-type KIF5B, RET proteins, and KIF5B-RET fusions in LADC. B


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Figure 3. Small-molecule RET inhibitors under clinical development for NSCLC with KIF5B-RET rearrangements.

Table 2. Selected FDA-Approved Tyrosine Kinase Inhibitors with Anti-RET Activitya compd (compd class)

anti-RET activity (IC50, nM)

other major targets

vandetanib (4-anilinoquinazoline) cabozantinib (dicarboxamide) sorafenib (urea) sunitinib (2-indolinone) lenvatinib (urea) motesanib (nicotinamide) ponatinib (benzamide)

100 5−10 6−47 220−1300 1.5 (Ki) 90 25.8

(V)EGFR VEGFR, MET, TIE2, AXL, FLT3, KIT Raf1, B-Raf, VEGFR1-3, PDGFR, FLT3, KIT VEGFR1-2, PDGFRβ, FLT3, KIT VEGFR1-3, FGFR1-4, PDGFR, KIT VEGFR1-3, PDGFR, KIT BCR-ABL, SCR, FLT3, KIT, FGFR, VEGFR, PDFGR

a

EGFR, epidermal growth factor receptor; VEGF, vascular endothelial growth factor; TIE2, tyrosine kinase with immunoglobulin-like and EGF-like domains 2; FLT3, FMS-like tyrosine kinase 3; Raf, rapidly accelerated fibrosarcoma; PDGFR, platelet-derived growth factor receptor; MTC, medullary thyroid cancer; DTC, differentiated thyroid cancer; NSCLC, non-small-cell lung cancer; AML, acute myeloid leukemia.

per year worldwide. Furthermore, considering RET mutations to be traditionally known thyroid cancer drivers, it is noteworthy that none of the subjects with KIF5B-RET-positive NSCLCs had a history of thyroid cancer or showed abnormal findings in their thyroid tissues but showed 2- to 30-fold higher levels of RET expression in noncancerous lung tissues. All subjects with LADC having the KIF5B-RET fusion were negative to the known cancer driver mutations, including EGFR, KRAS, ALK, and HER2, suggesting that KIF5B-RET is a new cancer driver mutation in LADCs. It was also shown that cells expressing oncogenic KIF5B-RET are sensitive to multikinase RET inhibitors, including sorafenib, sunitinib, and vandetanib, by decreasing the growth and signaling properties mediated by KIF5B-RET; however, an EGFR kinase inhibitor, gefitinib, showed no inhibitory activity against RET phosphorylation in Ba/F3 cells with the KIF5B-RET fusion protein.9,11 Triggered by these findings based on preclinical activity, several related clinical trials for NSCLC patients with KIF5B-RET rearrangements are currently ongoing using previously known RET inhibitors, such as lenvatinib, vandetanib, sunitinib, ponatinib, cabozantinib, and AUY922 (Figure 3), as summarized in Table 3.15 The anti-RET activity and the clinical development status of the known RET TKIs for both thyroid cancer and KIF5B-RET fusion-positive NSCLC are discussed below.

receptor tyrosine kinase (RTK). The extracellular portion contains four cadherin-like domains (CLD), a single Ca2+ binding site between CLD2 and CLD3, and a cysteine-rich domain (CRD). The intracellular portion contains the juxtamembrane domain and the core kinase domain. RET activation involves glial-derived neurotrophic factor (GDNF) family ligands and a co-receptor, glial-derived neurotrophic factor family receptor α1 (GFR-α), to trigger autophosphorylation on intracellular tyrosine residues and stimulate multiple downstream signaling pathways, including RAS-MAPK, PI3KAKT, PKC, and JAK-STAT.13 Traditionally, it has been thought that point mutations in the RET gene are associated with thyroid cancers, including multiple endocrine neoplasia 2 syndromes (MEN2A and MEN2B), familial medullary thyroid carcinoma (FMTC), and RET fusions, such as CCDC6 and others related to papillary thyroid carcinoma (PTC).14 Three RET kinase inhibitors, vandetanib, cabozantinib, and sorafenib, have been approved by the U.S. Food and Drug Administration (FDA) for the treatment of thyroid cancer. However, the clinical activity of RET kinase inhibitors in RET fusion-positive thyroid cancers has not been fully investigated. In this regard, discovery of the KIF5B-RET fusion gene is particularly important. The RET fusion gene was only found in 1−2% (6−19% for tumors without other genetic variants) of lung adenocarcinomas (LADCs) from both Asians and nonAsians, comprising approximately 12 000 lung cancer patients C


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Journal of Medicinal Chemistry Table 3. TKIs in Clinical Trials for KIF5B-RET-Associated NSCLC15 compd [manufacturer] lenvatinib [Eisai] vandetanib [AstraZeneca] sunitinib [Pfizer] ponatinib [ARIAD] cabozantinib [Exelixis] NVP-AUY922 (AUY922) [Novartis]

selected clinical studies (sponsor, clinical trial identifier code, estimated primary completion date) Ph II (2013−, recruiting): For patients with KIF5B-RET-positive LADCs and other confirmed RET translocations. (Eisai, NCT01877083, Sep 2014) Ph II (2013−, recruiting): For patients with advanced NSCLC harboring a RET gene rearrangement. (Seoul National University Hospital, collaborator: AstraZeneca, NCT01823068, Sep 2015) Ph II (2013−, recruiting): For never-smoker LADCs w/o known cancer genes other than RET mutations. (Dana-Farber Cancer Institute, NCT01829217, June 2015) Ph II (2013−, suspended): For advanced NSCLC w/RET translocation. Suspended because of a steady increase in the number of serious vascular occlusion events, such as blood clots and severe narrowing of blood vessels (Massachusetts General Hospital, NCT01813734, suspended) Ph II (2013−, recruiting): For patients whose tumors have a gene called KIF5B/RET. Cabozantinib in patients with RET fusion-positive advanced non-small-cell lung cancer. (Memorial Sloan-Kettering Cancer Center, collaborator: Exelixis, NCT01639508, July 2015) Ph II (2013−, recruiting in plan): For patients with stage IV EGFR T790M, EGFR exon 20, and other uncommon, HER2, or BRAF mutated and ALK-, ROS1-, or RET-rearranged NSCLCs. (National Taiwan University Hospital, NCT01922583, Aug 2016)

vandetanib showed a favorable response after 1 week of treatment in a patient with KIF5B-RET-positive NSCLC.23 Sunitinib (SU11284, Pfizer) is a multikinase inhibitor that belongs to the indolinone class of compounds.17 In 2006, sunitinib was the first cancer drug to be approved by the U.S. FDA simultaneously for the treatment of imatinib-resistant gastrointestinal stromal tumors (GIST) and advanced renal cell carcinoma (RCC, kidney cancer), and it was further approved for the treatment of advanced pancreatic neuroendocrine tumors in 2011. Kim et al. reported that sunitinib is a potent inhibitor of the oncogenic RET/PTC tyrosine kinase by showing decreased autophosphorylation of RET/PTC with an IC50 of 224 nM, and RET/PTC-mediated Y705 phosphorylation of STAT3 was also inhibited by sunitinib.24 Because sunitinib works by blocking certain protein signals within the cell, it is proposed that sunitinib may be useful for targeted therapy of PTC. To investigate the efficacy of sunitinib, an investigational phase II clinical study is underway by the DanaFarber Cancer Institute. The purpose is aimed at verifying sunitinib’s efficacy in certain types of adenocarcinoma tumors (1) that do not carry a mutation in a known cancer gene (EGFR, KRAS, or ALK) and occur in nonsmokers (less than 100 cigarettes in a lifetime) or (2) that have a mutation in the RET gene.15 Cabozantinib (XL184, Exelixis), a dicarboxamide-containing compound, inhibits a broad range of tyrosine kinases, including VEGFR, MET, and RET as the major target kinases. Originally developed as a dual inhibitor of VEGFR2 and MET for MTC, it was reported at the ASCO 2012 meeting that in the phase III EXAM (Efficacy of XM184 in Advanced Medullary Thyroid Cancer) trial, cabozantinib showed progression-free survival (PFS) of 11.2 months, 1 year progression-free survival of 47.3%, and an overall response rate of 28%, regardless of RET mutation status.25 On the basis of these results, the drug was approved by the FDA for the treatment of medullary thyroid cancer (MTC) in late 2012. Comparing cabozantinib with vandetanib and sunitinib, it is noteworthy that cabozantinib is the most potent against RET, as shown in Table 2. Preliminary clinical efficacy of cabozantinib in advanced, KIF5B-RETpositive NSCLC was reported by Drilon et al. Among three patients treated with cabozantinib in the phase II trial for patients with RET fusion-positive NSCLCs, confirmed partial responses were observed in two patients, and a third patient had prolonged stable disease for more than 8 months (31 weeks). All three patients remained progression-free on treatment.26 Currently, it is undergoing a phase II clinical study for the treatment of non-small-cell lung cancer (NSCLC)

Among 23 TKIs approved for cancer treatment by the U.S. FDA as of April 2014,16 about nine small-molecule tyrosine kinase inhibitors are known to have anti-RET activity.17,52 The anti-RET activities and the other major targets for some selected small-molecule kinase inhibitors are listed in Table 2. Among these, five TKIs are presently under clinical development for the treatment of NSCLC with KIF5B-RET fusion genes (Table 3).

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SMALL-MOLECULE RET INHIBITORS APPROVED FOR CLINICAL STUDY Lenvatinib (E7080, Eisai) is a multikinase inhibitor showing inhibitory activity against VEGFR1-3, FGFR1-4, PDGFR, KIT, and RET.18 Recently, the results from the phase III SELECT (Study of (E7080) LEnvatinib in differentiated Cancer of the Thyroid) trial for radioiodine-refractory differentiated thyroid cancer (RR-DTC) have been released showing significantly improved progression-free survival (PFS), although the overall survival (OS) data were somewhat troublesome.19 Okamoto et al. showed that lenvatinib inhibited autophosphorylation of RET fusion genes, including KIF5B-RET, CCDC6-RET, and NcoA4-RET, with IC50 values in the 10 nM range. Additionally, lenvatinib exhibited antitumor activity in vivo by suppressing tumor growth driven by RET fusion-transformed NIH3T3 cells.20 A phase II clinical trial of lenvatinib is currently ongoing for KIF5B-RET-positive LADCs and other confirmed RET translocations. Vandetanib (ZD6474, AstraZeneca) belongs to the quinazoline class of compounds and is a broad-spectrum kinase inhibitor. It inhibits VEGFR2-3, EGFR, and RET as the major targets.17 Vandetanib suppressed RET autophosphorylation and downstream signaling in cells transformed by both MEN2- and PTCassociated RET mutants. As a result, vandetanib blocked growth of NIH-RET/PTC3, showing specific anti-RET activity.21 In April 2011, vandetanib became the first drug approved by the FDA for the treatment of late-stage (metastatic) medullary thyroid cancer (MTC). This therapeutic activity is likely caused by inhibition of RET.22 Currently, vandetanib is undergoing a phase II investigational clinical trial for advanced NSCLC with RET rearrangements sponsored by Seoul National University Hospital, Korea. Because vandetanib has diminished growth and signaling properties mediated by KIF5B-RET, as mentioned earlier, the purpose of this investigational study is to examine the efficacy and safety of this drug.15 In a preliminary efficacy report by Gautschi et al., D


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AUY922 decreased cell viability in RET mutant medullary thyroid cancer cell lines and impaired signaling through the MAPK and mTOR pathways.32 Because AUY922 is intended to treat NSCLC patients with RET translocations resulting in KIF5B-RET fusions, as well as patients with other NSCLC drivers, it is of interest whether indirect regulation of AUY922 would result in a decrease of KIF5B-RET-associated tumor growth. Although these RET-inhibiting TKIs will provide valuable new therapeutic tools for use in RET-associated tumors, including those of the lung, the thyroid, and others, unwanted adverse side effects are of high concern because these are all broad-spectrum kinase inhibitors. It has been reported that an objective response rate from vandetanib and cabozantinib in treating thyroid cancer is presumably increased by the inhibition of VEGFR or EGFR, in addition to RET, and that adverse side effects are associated with VEGFR or EGFR inhibition.13 It is also known that the high TKI response rates in RET-associated tumors have been partial responses, leading to stable disease or progression-free survival,33 which suggests that longer term exposure to the currently available TKIs in patients with RET-associated tumors is necessary and will most likely lead to acquired drug resistance. Additionally, most of these TKIs show multikinase inhibitory activity and are known to work in an ATP-competitive manner. ATP-competitive inhibitors are highly potent but can inhibit other receptor tyrosine kinases because of their structural similarities; therefore, they can be less specific and may cause unintended adverse effects. Hence, development of more potent and RETspecific inhibitors are in high demand to limit undesired adverse effects. As an example of new drug discovery strategies, Dar et al. used a RET-driven drosophila model of MEN2 and kinome-wide drug profiling as a novel phenotypic screening strategy to identify potential RET inhibitors with maximal therapeutic indices.34 It is expected that a kinase inhibitor that can target a driver mutation specifically may lead to a superior clinical benefit compared to broad-spectrum kinase inhibitors.

with KIF5B-RET mutations, with the aim of determining the good and/or bad effects of cabozantinib. Ponatinib (AP24534, ARIAD), a member of a new class of compounds, is also a multitargeted, broad-spectrum tyrosine kinase inhibitor which has shown inhibitory activities against BCR-ABL, SCR, FLT3, RET, KIT, FGFR, VEGFR, PDFGR, and others. Ponatinib was approved in late 2012 for patients with resistant or intolerant chronic myeloid leukemia (CML) and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL). Recently, De Falco et al. reported that ponatinib inhibited RET with an IC50 of 25.8 nM and RET/ V804 with an IC50 of 33.9 nM and showed almost complete reduction of tumors in TT cell (MTC cells harboring the RET/ C634W mutation) xenografts after 3 weeks of ponatinib treatment (30 mg/kg/d).27 Additionally, unlike vandetanib, which is inactive against RET mutants carrying side chains bulkier than the isopropyl group of Val804, Mologni et al. showed that ponatinib exhibited potent inhibition against both RET and vandetanib-resistant V804M/L mutations in cellular assays.28 In early 2013, an investigational phase II clinical trial of ponatinib for NSCLC was designed to examine only patients with a RET mutation to study the efficacy of the drug in inhibiting or shutting off growth signals. However, in late 2013, the FDA requested the suspension of sales of the drug because of safety concerns caused by a steady increase in the number of serious vascular occlusion events, such as blood clots and severe narrowing of blood vessels; consequently, the phase II study is currently on hold. Sorafenib (Bayer & Onyx), a urea-containing compound, is a multityrosine kinase inhibitor targeting RAF, MEK, ERK, VEGFR1-3, PDGFR, KIT, RET, CRAF, and BRAF.29 Sorafenib was approved by the U.S. FDA for kidney cancer in 2005 and for liver cancer in 2007, and in late 2013, it was approved for radioiodine refractory differentiated thyroid cancer (RR-DTC). Plaza-Menacho et al. previously reported that sorafenib showed anti-RET activity by blocking enzymatic activity with an IC50 of 5.9 nM in vitro and inhibited RET phosphorylation, downstream signaling, and cell proliferation in human tumor cells expressing both wild-type and different oncogenic RET variants at concentrations of 15−150 nM.30 Motesanib (AMG706, Amgen), in the nicotinamide class of compounds, is another multikinase inhibitor that blocks VEGFR, PDGFR, Kit, and RET as the major targets. Motesanib is an investigational drug that is in several clinical studies. Coxon et al. has demonstrated that motesanib inhibited the activity of wild-type RET with an IC50 of 66 nM in a cellular phosphorylation assay, and in vivo studies resulted in a substantial decrease in the growth of TT tumor cell xenografts.31 Although nothing is known about both compounds’ activities against the KIF5B-RET fusion gene, if one considers the anti-RET activity of sorafenib and motesanib both in vitro and in vivo, it will be interesting if these results can translate into activity against KIF5B-RET driven NSCLC, as other anti-RET TKIs are employed in related clinical studies due to their anti-RET activities. AUY922 (5-(2,4-dihydroxy-5-isopropylphenyl)-N-ethyl-4-[4(morpholinomethyl)phenyl]isoxazole-3-carboxamide, NVPAUY922, VER-52296, Vernalis/Novartis), a diaryl isoxazole compound and HSP90 inhibitor, is in a phase II clinical trial for a rather broad range of NSCLC patients, including stage IV EGFR T790M, EGFR exon 20, and other uncommon, HER2, or BRAF-mutated and ALK, ROS1, or RET-rearranged NSCLCs. In a related study, Gild et al. has reported that

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X-RAY CRYSTAL STRUCTURE A more effective and even a target-specific inhibitor may be derived by deciphering the structure of RET kinase and its complex with known RET inhibitors and understanding the binding mode of those inhibitors. According to Kohno et al., vandetanib suppressed both the phosphorylation of Tyr905, which is located in the activation loop of the RET kinase site of KIF5B-RET, and anchorage-independent growth of NIH3T3 cells expressing KIF5B-RET, suggesting that the RET fusions are potential targets for existing small-molecule TKIs.9 Lipson et al. also independently demonstrated that Ba/F3 cells expressing KIF5B-RET were sensitive to anti-RET-active small-molecule TKIs, including vandetanib, sorafenib, and sunitinib, but not to an EGFR inhibitor, gefitinib. Additionally, the phosphorylation of Ba/F3 cells was inhibited by sunitinib.11 Both of these findings clearly suggest that the KIF5B-RET fusions are potential targets for existing anti-RET TKIs. As mentioned previously, two small-molecule broad-spectrum TKIs, vandetanib and cabozantinib, are approved by the U.S. FDA for MTC, and despite the presence of RET fusions in papillary thyroid cancers (PTC), no RET kinase inhibitors have been clinically evaluated thoroughly for RET-fusion positive thyroid cancers. Furthermore, there are no RET-specific inhibitors known to date. Because all current RET targeted TKIs are multikinase inhibitors, the increased response rate E


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Figure 4. Ligand binding sites. Electron density maps around ZD6474 (A) and PP1 (B) show REFMAC-calculated electron density maps with 2mFo −DFc contoured at 1σ in green and mFo − DFc contoured at 3σ in blue and (in part A) contoured at 8σ in red. Parts C and D are Ligplot schematic diagrams of vandetanib and PP1 contacts with RET. Water molecules (W) are shown in cyan. Parts E−G show molecular surfaces of the ligand binding pockets in RET-KD-P, with the solvent side of the surface in white and the inside of the surface in blue-green. Ligands are shown in stick form, with carbon atoms in magenta for vandetanib (E), green for AMP (F), and brown for PP1 (G). The Val804 side chain is highlighted in yellow. The pocket with access that is controlled by Val804 is in the center of each diagram; a second apparent pocket (∗) in parts E and G is the result of the Phe735 side chain being disordered in these two complexes. This research was originally published in J. Biol. Chem. 2006, 281, 33577−33587.22 ©American Society for Biochemistry and Molecular Biology.

Glu805, and vandetanib formed only one hydrogen bond with Ala807. The tolyl group of PP1 and the bromofluorophenyl group of vandetanib each occupy a pocket located in the back of the ATP binding site (hydrophobic back pocket).40,41 The hydrophobic back pocket has Val804 in the gatekeeper position. The gatekeeper residue Val804 controls access to the pocket and kinase sensitivity to the corresponding small-molecule TKIs.42 For example, vandetanib is able to selectively inhibit EGFR, VEGFR2, and RET but not IRK,43 in which the larger methionine side chain occupies the gatekeeper position. Additionally, although Val804 of RET cannot form hydrogen bonds with the inhibitors, the size of the isopropyl group of Val804 controls access to the pocket, which explains why Val804 mutants, such as MTC and MEN2 with the bulkier leucine or methionine groups, respectively, are not inhibited by vandetanib.44,45 Upon the basis of these findings, Grøtli et al. have utilized an extended hydrophobic arm, such as a phenylacetylene, on the pyrazolopyrimidine scaffold of PP1 to fit into the hydrophobic back pocket deeper than the tolyl group of PP1 and the bromofluorophenyl group of vandetanib, resulting in improved in vitro inhibitory activity against RET

from inhibition of other receptor tyrosine kinases (RKTs), such as VEGFRs, is also presumed.35−37 As mentioned above, adverse effects that are linked to VEGFR or EGFR inhibition can be significant, and how such “targeted” agents actually function remains unclear.36,38,39 Therefore, more potent and RET-specific inhibitors are clearly needed, and they would provide an important new stepping stone in curing RET-related diseases. In this regard, understanding the binding mode of anti-RET TKIs targeting the RET kinase domain could provide valuable information in deriving a highly selective KIF5B-RET inhibitor. Although the crystal structure of KIF5B-RET is unknown, it is known that the KIF5B-RET fusion gene has a kinase domain identical to RET kinase. Furthermore, the crystal structure of the phosphorylated RET kinase domain in complex with chemical inhibitors, such as vandetanib and PP1, in the ATP pocket has been elucidated by Knowles et al., as shown in Figures 4 and 5.22 Both inhibitors block the autophosphorylation and substrate phosphorylation of RET in an ATPcompetitive fashion and prevent its oncogenic activity. From the X-ray crystallography structures, PP1 formed two hydrogen bonds with the linker (hinge) region, one each with Ala807 and F



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CONCLUSION Since its first discovery as a proto-oncogene in human thyroid cancer in 1990,47 RET has become an important target for diverse tumor types, and numerous tyrosine kinase inhibitors have been developed that target the RET receptor, in addition to targeting other receptors, such as VEGFR. These RETinhibiting TKIs include cabozantinib, vandetanib, sunitinib, and axitinib. However, for each agent, the degree of RET inhibition relative to inhibition of other receptors varies greatly, and similarly, the potency against RET differs between agents. Furthermore, potency may vary according to the particular type of RET mutation.44 After the discovery of the EML4-ALK fusion gene in 2007, the ALK TKI crizotinib was swiftly developed, and in clinical trials crizotinib showed a dramatic therapeutic effect against NSCLCs harboring ALK fusions.48−50 On the basis of these results, crizotinib was approved by the U.S. FDA in 2011, 4 years after the discovery of the EML4-ALK fusion gene in 2007. In the less than 2 years that have elapsed since the discovery of KIF5B-RET in late 2012, several phase II clinical trials of RET TKIs for NSCLCs with KIF5B-RET fusions are currently ongoing, and all are open-label, single-arm trials with response rate as the primary end point.51 Among six phase II clinical studies, five studies are sponsored by nonprofit organizations as an investigational clinical trial which directly indicates the urgency and the high demand of a new targeted therapy for KIF5B-RET positive NSCLC. To date, one positive report about the clinical efficacy of cabozantinib has been published as

Figure 5. Mapping of disease-linked mutations in the RET kinase structure. Activating mutation sites in RET-KD identified in MTC, MEN2A, and MEN2B. Carbon atoms of the side chains are colored magenta for M918T, the predominant MEN2B mutant; pink for E768D/A919P; white for V804M/Y806C, two paired mutations where there is synergy; and cyan for L790F, Y791F, S891A, and R844L. The backbone schematic of RET-KD is shown in green, with the link to the N-terminal helix and the kinase insert domain indicated by dashed lines. The side chains of the wild-type RET-KD are shown, and the bound nucleotide is shown in stick form. This research was originally published in J. Biol. Chem. 2006, 281, 33577−33587.22 ©American Society for Biochemistry and Molecular Biology.

(IC50 = 8 nM) and selectivity against other kinases (Figure 6).46

Figure 6. Binding modes of small-molecule RET kinase inhibitors. Kinase inhibitor−protein interactions are depicted in the chemical structures. G


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fibroblast growth factor receptor; FMTC, familial medullary thyroid carcinoma; GDNF, glial-derived neurotrophic factor; GIST, gastrointestinal stromal tumor; HER2, human epidermal growth factor receptor 2; HSP90, heat shock protein 90; IRK, insulin receptor kinase; JAK, Janus kinase; KIF5B, kinesin family 5B gene; KRAS, GTPase KRas, also known as V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog; LADC, lung adenocarcinoma; MAPK, mitogen-activated protein kinase; MEN2A and MEN2B, multiple endocrine neoplasia 2 syndromes; MTC, medullary thyroid cancer; NSCLC, nonsmall-cell lung cancer; OS, overall survivla; PDGFR, plateletderived growth factor receptor; PFS, progression free survival; PI3K, phosphoinositide 3-kinase; PKC, protein kinase C; Raf, rapidly accelerated fibrosarcoma; RCC, renal cell carcinoma; RET, Rearranged during Transcription; RR-DTC, radioiodinerefractory differentiated thyroid cancer; RTK, receptor tyrosine kinase; SCLC, small cell lung cancer; STAT, signal transducer and activator of transcription; TKI, tyrosine kinase inhibitor; TIE2, tyrosine kinase with immunoglobulin-like and EGF-like domains 2; VEGF, vascular endothelial growth factor

a preliminary result.26 Since most of these RET TKIs are broad spectrum inhibitors, adverse side effects associated with VEGFR or EGFR inhibition are still of concern. Because the potency and the selectivity against RET differ between agents, the degree of RET inhibition in KIF5B/RET positive NSCLCs will vary. Acquired drug resistance by long-term exposure to these RET TKIs in patients can be a potential problem as well. The clinical results from current phase II studies will provide invaluable answers to these questions also with important biological information associated with KIF5B-RET positive NSCLC. Because these phase II clinical trials are expected to be completed by at least 2015, it is anticipated that a new RET TKI will follow crizotinib’s path and become available as an additional personalized targeted therapy for NSCLC patients in a couple of years. Additionally, other potent small-molecule anti-RET TKIs, such as sorafenib and motesanib, are known to be effective for thyroid cancer. Thus, an additional therapy for KIF5B-RET-positive adenocarcinomas is expected to be just around the corner.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82 53 790 5213. Fax: +82 53 790 5219. E-mail: [email protected].

REFERENCES

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Notes

The authors declare no competing financial interest. Biography Minsoo Song acquired his Ph.D. degree in Synthetic Organic Chemistry under the direction of Prof. John Montgomery at Wayne State University, MI, in 2004, and pursued postdoctoral training under the mentoring of Prof. Franklin A. Davis at Temple University, PA. He began his medicinal chemistry career at Cumbre in Dallas, TX, working on infectious disease projects. Then he worked at PsychoGenics in Tarrytown, NY, to pusue CNS disorder projects including NAAG peptidase inhibitor for TBI. In Korea, he pursued several kinase-related projects for arthritis and cancer at Oscotec, Inc. He is now a principal research scientist at NDDC at DGMIF, Daegu, Korea. His research interests lie in medicinal chemistry and drug discovery in the area of CNS disorders, oncology, and infectious diseases.

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ACKNOWLEDGMENTS The author is grateful to DGMIF colleagues Drs. Suk Kyoon Yoon, Seongheon Kim, Kyung-Hee Kim, Soosung Kang, and Hwan Geun Choi for their critiques; to Dr. Jong Sung (John) Koh at Genosco (Oscotec) for his inspirational leadership during the author’s stay at Oscotec; and to the editorial office and the reviewers for their careful review and critical suggestions.

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DEDICATION Dedicated to Professor Franklin A. Davis on the occasion of his 75th birthday. ABBREVIATIONS USED ABL, Abelson murine leukemia viral oncogene homolog; AKT, protein kinase B (PKB); ANLK, anaplastic lymphoma kinase; AML, acute myeloid leukemia; BCR, breakpoint cluster region protein; CLD, cadherin-like domain; CRD, cystein-rich domain; DTC, differentiated thyroid cancer; EML, echinoderm microtubule-associated protein-like 4; EGFR, epidermal growth factor receptor; FLT3, FMS-like tyrosine kinase 3; FGFR, H


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