The treatment of patients with advanced non–small cell lung cancer (NSCLC) harboring chromosomal rearrangements of anaplastic lymphoma kinase (ALK) has been revolutionized by the development of crizotinib, a small molecule inhibitor of the tyrosine kinases ALK, ROS1, and MET. Resistance to crizotinib invariably develops, however, through a variety of mechanisms. In the last few years, a flurry of new and more potent ALK inhibitors has emerged for the treatment of ALK-positive NSCLC, including ceritinib (LDK378), alectinib (RO5424802/CH5424802), AP26113, ASP3026, TSR-011, PF-06463922, RXDX-101, X-396, and CEP-37440. Cancers harboring ALK rearrangements may also be susceptible to treatment with heat shock protein 90 inhibitors. This review focuses on the pharmacologic and clinical properties of these compounds, either as monotherapies or in combination with other drugs. With so many ALK inhibitors in development, the challenges of how these agents should be studied and ultimately prescribed are also discussed.
IntroductionNon–small cell lung cancer (NSCLC) is the leading cause of cancer-related deaths, both worldwide and in the United States. Most patients who have NSCLC present with advanced or incurable disease, and cytotoxic chemotherapy generally results in low response rates and only modest improvements in overall survival (OS). Groundbreaking research on the molecular drivers of NSCLC has led to major treatment advances over the past decade, starting with the discovery in 2004 that activating mutations in the epidermal growth factor receptor (EGFR) gene are the basis of dramatic responses to the EGFR tyrosine kinase inhibitors (TKIs) gefitinib (Iressa, AstraZeneca) and erlotinib (Tarceva, Genentech/Astellas).1–3
Just 3 years later, investigators in Japan identified anaplastic lymphoma kinase (ALK) as another potential target in NSCLC. In a small subset of NSCLC tumors, a chromosomal inversion event leads to fusion of a portion of the ALK gene with the echinoderm microtubule–associated protein-like 4 (EML4) gene. The resulting EML4-ALK fusion protein is constitutively activated and transforming, leading to a state of oncogene addiction. 4 EML4-ALK fusion and other ALK rearrangements occur in 3% to 7% of patients with NSCLC (herein referred to as “ALK-positive” lung cancer) and are associated with younger age, never smoking or light smoking history, and adenocarcinoma histology.4,5 Patients who have advanced ALK-positive NSCLC are highly responsive to the ALK inhibitor crizotinib (Xalkori, Pfizer), with an objective response rate (ORR) of approximately 60% and a median progression-free survival (PFS) of 8 to 10 months.6,7
Enthusiasm for crizotinib has been tempered, however, by the emergence of drug resistance. Most patients with ALK-positive lung cancer who respond to crizotinib undergo a relapse within a few years after starting therapy.8,9 In particular, the central nervous system (CNS) is one of the most common sites of relapse in patients with ALK-positive NSCLC, and CNS disease can prove refractory to standard therapies.10 In light of these limitations with crizotinib, many novel ALK inhibitors that have greater potency and different kinase selectivity compared with crizotinib are currently in development (Table 1). Additionally, heat shock protein 90 (Hsp90) inhibitors have emerged as potentially active agents in the treatment of ALK-positive lung cancers, and these are being tested alone and in combination with ALK TKIs. This review provides an update on each of the TKIs and Hsp90 inhibitors in clinical development for ALK-positive NSCLC (Table 2), focusing on drug potency, selectivity, and side effects (Table 3).
AlectinibAlectinib (RO5424802/CH5424802), which is being developed by Roche, was designed to be a more selective and potent ALK inhibitor than crizotinib; in addition to activity against ALK, it has activity against the kinases LTK and GAK, but it is not active against INSR, IGF-1R, MET, or ROS1.53–55 Preclinical studies have demonstrated that alectinib is active against the crizotinib-resistant ALK mutations L1196M, C1156Y, and F1174L,54 suggesting that alectinib may be effective in patients who have become resistant to crizotinib through these mechanisms. The activating mutations F1174L and R1275Q, which are found in neuroblastoma, are susceptible to inhibition by alectinib,54 indicating a potential clinical use for this compound in neuroblastoma.
The activity of alectinib was first studied in a multicenter Japanese phase 1/2 trial of patients with ALK-positive NSCLC who had not previously received crizotinib or other ALK inhibitors. Patients were identified as having ALK-positive NSCLC based on positive immunohistochemical staining for ALK expression, followed by ALK FISH for confirmation. Among 46 patients who were treated with the recommended phase 2 dose of 300 mg twice daily, the ORR was 93.5%. Median PFS has not yet been reached. The most common side effects of alectinib at this dose included predominantly grade 1 or 2 dysgeusia, elevated AST, increased bilirubin, decreased neutrophil count, increased creatinine, increased creatine phosphokinase (CPK), myalgias, gastrointestinal symptoms, and rash. The only grade 3 adverse event that occurred at this dose was decreased neutrophil count, and no grade 4 toxicities were observed.55,56
Like ceritinib, alectinib appears to be active in the majority of patients with crizotinib-resistant ALK-positive NSCLC. A phase 1 study of alectinib has been conducted in the United States, with doses ranging from 300 to 900 mg twice daily. This study enrolled 47 patients who had ALK-positive NSCLC previously treated with crizotinib (but not with other ALK inhibitors). Among 44 evaluable patients, the unconfirmed response rate to alectinib was 54.5%. Preliminary results also suggest that alectinib has antitumor activity in the CNS in patients with disease refractory to crizotinib.57 Common adverse events included fatigue (30%), myalgia, peripheral edema, CPK elevation, nausea, increased ALT, photosensitivity, constipation, and rash. Grade 3 or 4 adverse events were uncommon and included headache, neutropenia, fluid retention/peripheral edema, increased γ-glutamyltransferase, and hypophosphatemia.57 An ongoing phase 2 study of alectinib is open for patients who have ALK-positive NSCLC previously treated with crizotinib (NCT01871805).
AP26113AP26113 is in development by Ariad. In preclinical studies, AP26113 was found to be a potent inhibitor of wild-type ALK (IC50=21 nM in a Ba/F3 model of EML4-ALK) and maintains reasonable activity against several crizotinib-resistant ALK mutants (IC50=26–254 nM). This compound also inhibits ROS1 (IC50=16–41 nM in Ba/F3 models), with a potency similar to that for ALK regardless of the ROS1 upstream fusion partner (including CD74, FIG, SDC4, or EZR).58
In an ongoing phase 1/2 study of AP26113, among 16 patients with ALK-positive NSCLC resistant to crizotinib (but who did not receive any other ALK TKIs), 12 patients (75%) responded at doses of AP26113 between 60 and 240 mg daily, with response durations of up to more than 40 weeks. Of 4 TKI-naive patients with ALK-positive NSCLC, 2 patients responded to AP26113, and the other 2 patients had stable disease. Among 3 patients with NSCLC in the trial who had previously received 2 ALK inhibitors (crizotinib and ceritinib), one remains in the study with stable disease, one developed progressive disease, and the third discontinued treatment before follow-up scans. Additionally, 4 of 5 ALK-positive patients who had CNS lesions showed improvements with AP26113 treatment on follow-up magnetic resonance imaging of the brain. AP26113 has also shown clinical activity in a patient with an ALK-positive IMT. The most common adverse events in this trial (all grades) were fatigue (40%), nausea (36%), diarrhea (33%), and headache (18%).59 In this trial, early-onset pulmonary symptoms (dyspnea, hypoxemia, lung infection) were observed in 9% to 12% of patients, typically on day 1 or 2 after initiation of the medication at a dose of 180 mg daily. Because of this, the recommended phase 2 dosing is 90 mg daily for 1 week, and if no respiratory symptoms arise, the dose is increased to 180 mg daily.60
ASP3026ASP3026, which is being developed by Astellas Pharma, inhibits ALK with an IC50 of 3.5 nM in enzymatic assays and an IC50 of 64.8 nM in H2228 cells. This agent also displays activity against the neuroblastoma-activating ALK mutants F1174L (IC50=10 nM) and R1275Q (IC50=5.4 nM) and against the crizotinib-resistant gatekeeper mutation L1196M. Among a panel of 86 tyrosine kinases, ASP3026 showed the highest selectivity for ROS1 (IC50=8.9 nM) and ACK (IC50=5.8 nM). This compound has antitumor activity in both NCI-H2228 subcutaneous xenograft mouse models and EML4-ALK transgenic mice.61 Preliminary results of a phase 1 trial of patients with advanced solid tumors (ALK positivity not required) showed that ASP3026 has a favorable safety profile, with a maximum tolerated dose of 525 mg daily. Gastrointestinal symptoms were the most common side effects, but grade 3 rash and elevations of AST and ALT were also observed. The clinical activity of ASP3026 has not yet been reported.62
TSR-011TSR-011, which is being developed by Tesaro, is a potent inhibitor of ALK, with an IC50 of approximately 1 nM in various preclinical models.63 In a phase 1 trial, 65% of the 17 evaluable patients with advanced malignancies (including NSCLC, papillary thyroid cancer, pancreatic cancer, and colorectal cancer) achieved stable disease or a partial response at 8 weeks. Of 3 evaluable patients with ALK-positive NSCLC who had relapsed on crizotinib, 1 patient had a partial response and 2 had stable disease. Dose-limiting toxicities included QTc prolongation and dysesthesia.64
Of note, TSR-011 is also a potent inhibitor of TRK-A, TRK-B, and TRK-C (encoded by NTRK1, NTRK2,and NTRK3, respectively).64 Rearrangements in NTRK1 were recently described in a small fraction of patients with lung cancer who did not have other, more commonly found genomic alterations.65 BecauseTRK rearrangements have also been found in other tumor types, including colon cancer66 and papillary thyroid cancer,67 TSR-011 may be a useful targeted agent in a variety of cancer types.
PF-06463922PF-06463922, which is in development by Pfizer, is a novel, highly potent, selective inhibitor of both ALK and ROS1, and it has strong activity against all known ALK and ROS1 mutants identified in patients with crizotinib-resistant disease. This compound was designed to be a low-efflux substrate from cell lines overexpressing P-glycoprotein in order to increase its potential CNS penetration. Preclinical in vivo rodent models have demonstrated that levels of PF-06463922 in the brain are about 20% to 30% of the levels achieved in plasma. Furthermore, in mice harboring EML4-ALK–driven brain tumors, treatment with PF-06463922 caused brain tumor regression and increased OS, indicating that it may be an excellent therapeutic agent for patients with crizotinib-resistant disease and/or patients with CNS disease in either the crizotinib-naive or crizotinib-resistant setting.68–70 A phase 1/2 clinical study of PF-06463922 in ALK-positive and ROS1-positive NSCLC is currently under way.
RXDX-101RXDX-101 (formerly called NMS-E628, Nerviano) is an inhibitor of ALK, ROS1, TRK-A, TRK-B, and TRK-C. This compound, which is being developed by Ignyta, induces tumor regression in mouse models of NPM-ALK–driven lymphoma and EML4-ALK–driven NSCLC; it also has activity against the crizotinib-resistant ALK mutants L1196M and C1156Y. RXDX-101 also crosses the blood-brain barrier, inhibits tumor growth, and prolongs survival in mice with intracranially injected NCI-H2228 EML4-ALK cells.71,72In vitro and in vivo activity against ROS1-driven cancers has also been reported.73 Preliminary results of the RXDX-101 phase 1 trial show that the drug is well tolerated, with early evidence of antitumor activity.74
X-376 and X-396Compared with crizotinib, X-376 and X-396 inhibit ALK with approximately 10-fold greater potency in biochemical assays and 3- to 10-fold greater potency in cell-based assays. By contrast, crizotinib is a slightly more potent MET inhibitor than X-376 and X-396 in biochemical assays (IC50=0.51, 0.69, and 0.74 nM for crizotinib, X-376, and X-396, respectively) as well as in cell-based assays with the MKN-45 MET-driven cell line (IC50=51, 150, and 156 nM for crizotinib, X-376, and X-396, respectively). In Ba/F3 models of crizotinib resistance, X-396 was an approximately 10-fold more potent inhibitor than crizotinib of the ALK mutants L1196M and C1156Y.75 Initial results of a phase 1 study of X-396 showed responses in both crizotinib-naive and crizotinib-reistant ALK-positive NSCLC patients.76 Both X-376 and X-396 are being developed by Xcovery.
CEP-28122 and CEP-37440CEP-28122, which is being developed by Teva, is a potent and selective ALK inhibitor (IC50=1.9 nM in enzymatic assays). Against a panel of 259 protein kinases, CEP-28122 also displayed activity against the serine/threonine kinases RSK2, RSK3, and RSK4 (IC50 range, 7–19 nM); the degree of inhibition for all other kinases was at least 10-fold weaker than that for ALK. CEP-28122 displayed antitumor activity in the ALK-driven mouse xenograft models with the NCI-H2228 and NCI-H3122 cell lines. Complete tumor regression within 1 to 2 days after the initiation of CEP-28122 was observed in human primary NPM-ALK–positive anaplastic large cell lymphoma mouse grafts.77 Another Teva compound, CEP-37440, is an inhibitor of ALK as well as focal adhesion kinase (FAK); this drug is in phase 1 development in patients with advanced solid tumors.
Heat Shock Protein 90 InhibitorsHsp90 is a protein chaperone involved in regulating the stability of proteins involved in normal cellular functions as well as tumorigenesis.78 Fusion proteins formed as the result of chromosomal rearrangement are thought to be particularly dependent on Hsp90 for protein folding, transport, and stability,79 and at least 3 Hsp90 inhibitors have been tested in patients with ALK rearrangements: retaspimycin hydrochloride (IPI-504), ganetespib (STA-9090), and AUY922.
Preclinical studies have shown that crizotinib-resistant ALK-positive cell lines are highly sensitive to the Hsp90 inhibitor 17-allylamino-17-demthoxygeldanamycin (17-AAG).80,81 Retaspimycin hydrochloride (IPI-504), an analogue of 17-AAG, was tested in a phase 1/2 study of patients with NSCLC. Among 76 patients enrolled in the study, the ORR was 7%, but among the 3 patients in the study with ALK rearrangements, 2 had a partial response and a third had prolonged stable disease. Although the estimated median PFS for all patients in this study was 2.9 months, the 3 patients with ALK-positive NSCLC received IPI-504 for approximately 7 months.82 The most common side effects of IPI-504 were grade 1 or 2 fatigue, nausea, and diarrhea; grade 3 or 4 liver function test abnormalities occurred in 12% of patients.83
Preclinical work has demonstrated that NSCLC cell lines harboring oncogenic rearrangements of ALK,ROS1, or RET are all sensitive to the Hsp90 inhibitor ganetespib. Of the 99 patients enrolled in a phase 2 study of ganetespib, only 4 patients (4%) achieved a partial response, but all 4 of these patients were ALK-positive and crizotinib-naive, with response durations ranging from 7.4 to 21 months. The most common side effects of ganetespib were diarrhea (82%), fatigue (58%), nausea (41%), and anorexia (37%). Treatment-related deaths occurred in 2 patients (2%): one from cardiac arrest and the other from renal failure.84
Preliminary data from a phase 2 study of the Hsp90 inhibitor AUY922 in 121 patients with NSCLC showed partial responses in 6 of 21 patients (29%) with ALK rearrangements. Of these 6 responders who had ALK-positive NSCLC, 4 were crizotinib-naive and 2 had previously been treated with crizotinib. The estimated median PFS rate was 42% at 18 weeks in ALK-positive patients. The most frequent adverse events in this study included eye disorders (77%), diarrhea (74%), and nausea (46%).85
Hsp90 inhibitors may be challenging to develop in ALK-positive NSCLC, given the established efficacy and safety of crizotinib, ceritinib, and other ALK TKIs. Compared with TKIs, Hsp90 inhibitors appear to have lower response rates and side effects that are less tolerable. Their activity appears to be limited in the setting of crizotinib resistance, and they do not have CNS activity. One potential clinical use of Hsp90 inhibitors in ALK-positive NSCLC may be for those patients who cannot tolerate TKIs (eg, because of TKI-associated pneumonitis).
Because of the invariable emergence of resistance in patients who have ALK-positive NSCLC treated with crizotinib monotherapy, a number of trials are under way combining an ALK TKI with an Hsp90 inhibitor (see Table 2) in the hope that therapies with nonoverlapping mechanisms of action may be more effective than monotherapies in delaying or overcoming resistance. This approach is supported by preclinical evidence demonstrating synergistic antitumor effects when TKIs are combined with Hsp90 inhibitors in ALK- or MET-driven cancers.83,86 Crizotinib is being combined with ganetespib in patients who have crizotinib-naive ALK-positive disease (NCT01579994), and 2 clinical trials are open for patients who have crizotinib-resistant NSCLC; in one trial (NCT01712217), the Hsp90 inhibitor AT13387 is being administered alone or in combination with crizotinib, and in another trial (NCT01772797), AUY922 is being combined with ceritinib.
ConclusionsNew drugs are urgently needed in the treatment of NSCLC, a prevalent disease with a high mortality rate. Crizotinib has quickly become a promising treatment option for patients with ALK-positive NSCLC, IMT, and anaplastic large cell lymphoma, as well as for patients with cancers harboring aberrations in ROS1. However, the efficacy of crizotinib in these cancers is limited by poor activity in the CNS and the frequent emergence of drug resistance in a relatively short time. Several new TKIs are in various stages of clinical development for ALK-positive cancers, and many of these agents have activity both in the CNS and in cancers that have become resistant to crizotinib. Despite the increased potency and specificity of next-generation ALK TKIs, resistance to these compounds has also been described.49
These challenges raise a number of questions for the research community. In what sequence should ALK TKIs be prescribed? Should crizotinib still be prescribed as the first targeted therapy, with next-generation ALK inhibitors reserved for subsequent lines of treatment because they are still able to overcome crizotinib resistance in the majority of cases? On the other hand, would the administration of more potent ALK inhibitors in the first-line setting result in deeper responses and longer response durations, or would the earlier use of more potent ALK TKIs lead to the more rapid development of highly resistant disease? Ongoing clinical trials with ceritinib and alectinib in crizotinib-naive patients may help to address these questions once results are available.
Would the earlier use of brain-penetrable ALK TKIs like PF-06463922 delay or possibly prevent the development of CNS metastases, a common cause of morbidity and mortality in this patient population? Also, if patients become resistant to next-generation ALK TKIs, will they exhibit cross-resistance to all other ALK TKIs? Should patients be switched to a different ALK inhibitor before resistance to their prior treatment emerges? Will combinations of ALK TKIs with other agents (eg, cytotoxic chemotherapy, Hsp90 inhibitors, immunotherapy, other TKIs) be both tolerable and effective in patients with ALK-positive cancers? With so many ALK inhibitors in development, it will be difficult to sort out with randomized clinical trials which drugs should be used and at what times in a patient’s treatment course. Mathematical modeling of the growth kinetics of TKI-sensitive and TKI-resistant cancer cells may help us to determine how kinase inhibitors, cytotoxic agents, and other therapies should be dosed and intercalated.87–89
Despite the challenges that lie ahead in the treatment of ALK-positive cancers, OS for patients who have NSCLC with targetable mutations is increasing. Although it will be impossible to show OS benefits in clinical trials because of crossover effect, data presented at the 15th World Conference on Lung Cancer in October 2013 demonstrated that patients whose cancers had an oncogenic driver that could be treated with a genotype-directed therapy (eg, EGFR, ALK) had a median OS of 3.5 years, whereas patients whose cancers had an oncogenic driver that could not be treated with genotype-directed therapy (eg, KRAS) lived for 2.4 years (P<.0001).90 With many next-generation ALK inhibitors and combination strategies on the horizon, survival rates in patients with ALK-positive lung cancers will likely continue to improve. However, with these breakthroughs come increasing expenses that place significant financial burdens on health care systems.91,92 For example, the National Institute for Health and Care Excellence (NICE) denied approval of crizotinib in the United Kingdom because its use was not deemed cost-effective.93 Improving the efficiency of tumor genotyping and decreasing the costs of drug development and delivery will remain top priorities, so that all patients can have access to these life-prolonging cancer treatments.