Rearrangements in anaplastic lymphoma kinase (ALK) gene and echinoderm microtubule-associated protein-like 4 (EML4) gene were first described in a small portion of patients with non-small cell lung cancer (NSCLC) in 2007. Fluorescence in situ hybridization is used as the diagnostic test for detecting an EML4–ALK rearrangement. Crizotinib, an ALK inhibitor, is effective in treating advanced ALK-positive NSCLC, and the US Food and Drug Administration approved it for treating ALK-positive NSCLC in 2011. Several mechanisms of acquired resistance to crizotinib have recently been reported. Second-generation ALK inhibitors were designed to overcome these resistance mechanisms. Two of them, ceritinib and alectinib, were approved in 2014 for advanced ALK-positive NSCLC in the US and Japan, respectively. Heat shock protein 90 (Hsp90) inhibitors also showed activity against ALK-positive NSCLC. Here we review the recent development of crizotinib, ceritinib, alectinib and other second-generation ALK inhibitors as well as Hsp90 inhibitors. We also discuss management strategies for advanced ALK-positive NSCLC.
Introduction
In the era of molecular targeted therapy, epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) such as gefitinib, erlotinib and afatinib have been demonstrated to prolong progression-free survival (PFS) and preserve the quality of life when used as the first-line treatment for patients with EGFR-mutated advanced non-small cell lung cancer (NSCLC) [Mok et al. 2009; Maemondo et al. 2010; Sequist et al. 2013; Yang et al. 2013].
In 2007, Soda and colleagues first described the rearrangements in the anaplastic lymphoma kinase (ALK) gene and the echinoderm microtubule-associated protein-like 4 (EML4) gene in a small proportion of patients with NSCLC [Soda et al. 2007, 2008]. Around 2–5% of NSCLC patients harboured this gene rearrangement [Soda et al. 2007; Kwak et al. 2010]. EML4–ALK rearrangement was identified as an oncogene and EML4–ALK fusion protein was found to possess transforming activity and oncogenic potential [Soda et al. 2007]. ALK inhibitors were effective in vitro for cell lines and in vivo for mouse models of tumours harbouring the EML4–ALK rearrangement [Koivunen et al. 2008; Soda et al. 2008]. The clinicopathological features of these patients included younger age, never/light smokers and adenocarcinoma histology (predominantly signet-ring cell subtype) [Inamura et al. 2008, 2009; Rodig et al. 2009; Shaw et al. 2009]. EML4–ALK rearrangements are typically mutually exclusive with EGFR mutations or K-RAS mutations [Wong et al. 2009; Gainor et al. 2013b].
In a retrospective study, malignant pleural effusions of patients with EGFR wildtype lung adenocarcinoma were tested for ALK rearrangement. All patients were not treated with any ALK inhibitor. The survival of patients with EML4–ALK rearrangement was better than that of patients without EML4–ALK rearrangement, although this study enrolled only patients with malignant pleural effusion, which may potentially lead to biases [Wu et al. 2012]. There were several genes reported to be fused to ALK, but the incidence was quite low [Takeuchi et al. 2009; Togashi et al. 2012].
Crizotinib, a multitargeted tyrosine kinase (including ALK, MET and ROS1) inhibitor, is effective in treating patients with ALK-positive (referred to as EML4–ALK rearrangement) advanced NSCLC [Cui et al. 2011]. In 2011, this drug received approval from the US Food and Drug Administration (FDA) in treating advanced ALK-positive NSCLC using the Vysis ALK Break-Apart FISH Probe Kit (Abbott Molecular, Inc.) based on the results of early phase clinical trials. Phase III clinical trials had been conducted to compare crizotinib monotherapy with standard second-line and first-line cytotoxic chemotherapy for advanced ALK-positive NSCLC patients [Shaw et al. 2013; Solomon et al. 2014]. Because of the rarity of ALK-positive NSCLC, immunohistochemical staining is a reliable screening tool to identify ALK-positive NSCLC and to reduce the cost of general screening using standard methods [Jokoji et al. 2010; Yi et al. 2011; Park et al. 2012; Conklin et al. 2013]. Crizotinib is effective in treating some subsets of ALK-negative NSCLC, such as tumours harbouring ROS1 rearrangement and tumours with de novo MET amplification [Ou et al. 2011b; Bergethon et al. 2012; Yasuda et al. 2012]. Subjects without ALK-positive NSCLC are not covered in detail in this review.
Ceritinib, a second-generation ALK inhibitor, which is more potent against ALK than crizotinib but not active against MET, was approved by the FDA in 2014 for advanced ALK-positive NSCLC pretreated with crizotinib [Shaw et al. 2014]. Alectinib, another second-generation ALK inhibitor, was approved in Japan in 2014 for advanced ALK-positive NSCLC. Heat shock protein 90 (Hsp90) inhibitors were also effective in treating ALK-positive NSCLC [Normant et al. 2011]. However, drugs in this class have not yet been approved by any regulatory agencies. Here, we review the recent development of crizotinib, ceritinib, alectinib and other second-generation ALK inhibitors, as well as Hsp90 inhibitors. We also discuss management strategies for advanced ALK-positive NSCLC.
A literature review of clinical studies published between January 2007 and July 2014 was conducted using PubMed and MEDLINE, with keyword entry of ‘non-small cell lung cancer’, ‘EML4–ALK rearrangement’, ‘crizotinib’, ‘ceritinib’, ‘alectinib’ and ‘heat shock protein 90 inhibitor’. A manual search of abstracts presented at major oncology meetings was also performed.
First-generation ALK inhibitor: crizotinib
Overview of clinical development of crizotinib
Crizotinib was approved under the FDA’s accelerated approval programme in 2011 based on the results of two single-arm clinical trials mentioned below [Kwak et al. 2010; Kim et al. 2012]. In a phase I study (PROFILE 1001), 149 patients with advanced ALK-positive NSCLC underwent treatment with crizotinib at a dose of 250 mg twice daily. Most patients (71%) were never-smokers and 97% of the patients had adenocarcinoma histology. The overall response rate (ORR) was 60.8% and the median PFS was 9.7 months. The estimated overall survival (OS) rates at 6 months and 12 months were 87.9% and 74.8%, respectively [Camidge et al. 2012]. Visual effects, nausea and diarrhoea were the most commonly reported adverse effects (AEs). The global single-arm phase II study (PROFILE 1005) of crizotinib in treating advanced ALK-positive NSCLC after the progression of at least one line of cytotoxic chemotherapy revealed an ORR of 59.8% and a median PFS of 8.1 months [Kim et al. 2012].
A phase III study (PROFILE 1007) compared crizotinib with pemetrexed or docetaxel chemotherapy after failing one prior platinum-based chemotherapy. A total of 347 patients with advanced ALK-positive NSCLC were randomised, and it was observed that the median PFS (primary endpoint) was significantly longer in the crizotinib group (7.7 months) compared with the chemotherapy group (3.0 months) [hazard ratio (HR): 0.49, 95% confidence interval (CI) 0.37–0.64; p < 0.001). ORRs were 65% in the crizotinib group and 20% in the chemotherapy group (p < 0.001). Patients in the crizotinib group reported greater reduction of lung cancer related symptoms and improvement in the overall quality of life compared with the chemotherapy group [Shaw et al. 2013]. The common AEs are listed in Table 1. The most common grade 3 or 4 AE was elevated aminotransferase levels, which developed in 16% of patients who underwent crizotinib treatment. Grade 3 or 4 neutropenia developed in 13% of patients in the crizotinib group. Interestingly, ORRs and PFS were different between the pemetrexed (29% and 4.2 months, respectively) and docetaxel groups (7% and 2.6 months, respectively). ORR was higher than that of the general population who underwent second-line chemotherapy with pemetrexed [Hanna et al. 2004]. Other retrospective studies also demonstrated that ALK positivity was a predictive factor of pemetrexed efficacy [Camidge et al. 2011; Lee et al. 2011].
Another phase III study (PROFILE 1014) compared crizotinib monotherapy with pemetrexed plus cisplatin chemotherapy as the first-line of treatment for advanced ALK-positive nonsquamous NSCLC. A total of 343 patients were randomised and the PFS (primary endpoint) was observed to be significantly longer in the crizotinib group (median 10.9 versus 7.0 months; HR 0.45, 95% CI 0.35–0.60; p < .001). ORR was 74% in the crizotinib group and 45% in the chemotherapy group [Solomon et al. 2014].
The OS results were immature in the aforementioned two phase III studies. The common AEs of crizotinib therapy are listed in Table 1. Other less common AEs had been reported, such as QTc prolongation, bradycardia, hypogonadism, renal cysts and interstitial pneumonitis [Ou et al. 2011, 2013; Weickhardt et al. 2012, 2013; Tamiya et al. 2013; Lin et al. 2014]. The management of these AEs has been reviewed elsewhere [Rothenstein and Letarte, 2014].
Activity against central nervous system metastases
The activity of crizotinib in central nervous system (CNS) metastases is still debatable [Gainor et al. 2013a; Maillet et al. 2013; Costa et al. 2015]. Many treatment strategies, such as high-dose crizotinib and high-dose crizotinib in combination with high-dose pemetrexed, had been reported to treat CNS disease [Gandhi et al. 2013; Kim et al. 2013c]. Some experts suggested that isolated CNS disease progression after crizotinib therapy should be treated with radiotherapy while continuing crizotinib therapy [Takeda et al. 2013]. A retrospective analysis of patients who developed RECIST-defined progressive disease (PD) in PROFILE 1001 and PROFILE 1005 studies revealed that 62% (120/194) of the patients continued crizotinib therapy after PD. Most of them had good performance status (ECOG PS 0-1) and 51% of them had brain metastases as the sole site of PD. The authors concluded that this treatment strategy may provide survival benefit [Ou et al. 2014a].
Mechanisms of crizotinib resistance
Despite the initial treatment response of crizotinib, PD inevitably develops after a period of treatment. The mechanisms of crizotinib resistance had been studied, and one of these mechanisms is a secondary mutation in the kinase domain of ALK, for example, the ‘gatekeeper mutation’ of substitution of leucine with methionine at position 1196 (L1196M) [Choi et al. 2010]. Other resistance mutations had been reported (Table 2) [Choi et al. 2010; Sasaki et al. 2010, 2011; Zhang et al. 2011; Doebele et al. 2012; Katayama et al. 2012; Lovly and Pau, 2012; Huang et al. 2013; Kim et al. 2013b; Ou et al. 2014b]. In addition to the secondary mutation, activation of alternative pathway (EGFR and KIT), ALK amplification, epithelial–mesenchymal transition (EMT) and insulin-like growth factor 1 receptor (IGF-1R) pathway activation also resulted in crizotinib resistance [Katayama et al. 2012; Tanizaki et al. 2012; Yamada et al. 2012; Kim et al. 2013; Kobayashi et al. 2013a; Lovly et al. 2014; Yamaguchi et al. 2014]. In some patients, the mechanism of acquired resistance remains unknown [Costa and Kobayashi,. 2012].
Novel approaches to overcome crizotinib-acquired resistance had been reported. The most common strategy is to use second-generation ALK inhibitors to overcome resistance mediated by secondary ALK mutations (discussed in detail below). Other strategies, such as combination therapy with Hsp90 inhibitors, EGFR inhibitors, KIT inhibitors (e.g. imatinib) or IGF-1R inhibitors, had been reported [Sasaki et al. 2010, 2011; Katayama et al. 2012; Tanizaki et al. 2012; Yamada et al. 2012; Kim et al. 2013; Lovly et al. 2014; Yamaguchi et al. 2014]. Other strategies of crizotinib therapy are under development. A phase Ib trial of crizotinib in combination with immunotherapeutic agent ipilimumab (an anti-CTLA-4 monoclonal antibody) is ongoing [ClinicalTrials.gov identifier: NCT01998126]. Continuous treatment with crizotinib beyond disease progression is another strategy to extend the benefit of crizotinib therapy [Ou et al. 2014a]. A randomised phase II trial of crizotinib plus pemetrexed versus pemetrexed alone in patients with ALK-positive nonsquamous NSCLC who have progressed after previous benefit from crizotinib therapy is ongoing [ClinicalTrials.gov identifier: NCT02134912].
Second-generation ALK inhibitors
Second-generation ALK inhibitors were designed to have more potent activity against ALK, to overcome crizotinib-resistant mutations and to have better activity in CNS disease. Many novel agents are under development as mentioned below. The characteristics of second-generation ALK inhibitors are listed in Table 2. Those drugs that were in phase II/III development are listed in Table 1 and those in phase I/II development are listed in Table 3.
Ceritinib
Ceritinib (LDK378) is an orally administered potent ALK inhibitor derived from the compound NVP-TAE684 [Galkin et al. 2007; Marsilje et al. 2013]. In preclinical studies, this drug demonstrated greater antitumour potency than crizotinib, and displayed activity against some crizotinib-resistant mutations (Table 2) [Friboulet et al. 2014]. In a phase I study (ASCEND-1) [ClinicalTrials.gov identifier: NCT01283516] (Table 1), 750 mg once daily was determined as the maximum tolerated dose (MTD) with the dose-limiting toxicities (DLT) of diarrhoea, vomiting, dehydration, elevated aminotransferase levels and hypophosphataemia. Among the 114 patients who received ceritinib at least 400 mg/day, ORR was 58% and the PFS was 7.0 months. Among the 80 patients who had been previously treated with crizotinib, ORR was 56% and 19 patients with crizotinib-resistant disease underwent a tumour biopsy before ceritinib therapy. ALK mutation and ALK amplification were detected in some of the responders, but other responders had neither ALK mutation nor ALK amplification. Among the patients who were crizotinib-naïve and treated with ceritinib at least 400 mg/day, ORR was 62%. The common AEs are listed in Table 1. The most common grade 3 or 4 AEs were increased ALT level (21%), increased aspartate aminotransferase (AST) level (11%) and diarrhoea (7%), All of these AEs were reversible after discontinuation of ceritinib therapy [Shaw et al. 2014].
Ceritinib received accelerated approval from the FDA in April 2014 for patients with metastatic ALK-positive NSCLC who were previously treated with crizotinib. An updated report of ASCEND-1 study disclosed the efficacy data of the expansion cohort, in which all of the patients were treated with a starting dose of 750 mg/day. A total of 246 patients with ALK-positive NSCLC were enrolled, including 163 ALK inhibitor-pretreated (crizotinib or alectinib) and 83 ALK inhibitor-naïve patients. ORRs were 58.5%, 54.6% and 66.3% in the overall population, ALK inhibitor-pretreated group and ALK inhibitor-naïve group, respectively. The median PFS periods were 8.2 and 6.9 months in the overall population and ALK inhibitor-pretreated group, respectively. The median PFS in the ALK inhibitor-naïve group was not reached and the PFS rate at 12 months was 61.3%. In addition to the AEs mentioned above, around 4% of the patients developed interstitial lung disease/pneumonitis, and 9.4% (24/255, including 9 non-NSCLC patients) of the patients discontinued the study drug because of the AEs. However, regarding its activity in CNS disease, 124 patients had brain metastases at baseline, and 10 and 4 patients had measurable lesions in the ALK inhibitor-pretreated and ALK inhibitor-naïve groups, respectively. The intracranial ORRs were 40% and 75%, respectively. The authors concluded that ceritinib therapy had a high rate of durable responses and prolonged PFS in both ALK inhibitor-pretreated group and ALK inhibitor-naïve patients [Kim et al. 2014]. Ceritinib treatment showed activity in patients with brain metastases. Ongoing phase II and phase III clinical trials of ceritinib therapy for patients with advanced ALK-positive NSCLC are listed in Table 1.
Alectinib
Alectinib (RO5424802/CH5424802) is a highly selective and orally administered ALK inhibitor. Preclinical data revealed that alectinib had activity against some crizotinib-resistant mutations (Table 2) [Sakamoto et al. 2011; Kinoshita et al. 2012; Kodama et al. 2014a]. Alectinib showed potent efficacy against intracranial tumour in mouse models [Kodama et al. 2014b]. It had no activity against MET, ROS1 and some crizotinib-resistant mutations (Table 2) [Ou et al. 2014b].
In a phase I/II study (AF-001JP study) conducted in Japan (Table 1), patients with ALK-positive and ALK inhibitor-naïve NSCLC were treated with alectinib. In the phase I portion, there were no DLT or AEs of grade 4 up to the highest dose, and 300 mg twice daily was the recommended phase II dose. In the phase II portion, 43 out of 46 (93.5%) treated patients had objective responses. The common AEs are listed in Table 1. Grade 3 AEs were recorded in 26% of the patients, and the most common grade 3 AE was decreased neutrophil count and increased blood creatine phosphokinase (4%). No grade 4 AEs were recorded [Seto et al. 2013; Yang, 2013]. Based on this study, alectinib was approved in Japan in July 2014 for advanced ALK-positive NSCLC.
In another study conducted in Japan, 35 patients were enrolled and 29 patients (83%) had been pretreated with one or more than one ALK inhibitors. Among 28 patients pretreated with crizotinib, the ORR was 58.3% (95% CI 36.6–77.9). Two patients with measurable brain tumours were observed to have complete response at the first assessment [Nakagawa et al. 2014].
In the US, a phase I/II study (AF-002JG study) [ClinicalTrials.gov identifier: NCT01588028] of 600 mg twice a day was selected as the recommended phase II dose in patients with crizotinib-resistant ALK-positive NSCLC. Among the 47 enrolled patients, 44 patients could be assessed for drug activity and the ORR was 55%. Of 32 patients with CNS metastases, ORR was 52% [Gadgeel et al. 2014].
V1180L (gatekeeper) and I1171T mutations conferred resistance to alectinib, and ceritinib might overcome these alectinib-resistant mutations [Katayama et al. 2014]. Ongoing studies of alectinib therapy for patients with advanced ALK-positive NSCLC are listed in Table 1. One of these ongoing studies (ALEX) [ClinicalTrials.gov identifier: NCT02075840], directly compares alectinib with crizotinib in treatment-naïve advanced ALK-positive NSCLC patients.
AP26113
AP26113 is a potent and orally active inhibitor of ALK. In preclinical data, this drug demonstrated activity against ROS1 and most crizotinib-resistant mutations, including G1202R [Zhang et al. 2010; Squillace et al. 2013]. This drug has activity against activating EGFR mutations and EGFRT790M resistance mutation while sparing native EGFR (Table 2) [Rivera et al. 2012; Camidge et al. 2013]. In a phase I/II study [ClinicalTrials.gov identifier: NCT01449461], early onset pulmonary symptoms developed in 14% of patients with a starting dose of 180 mg/day. In 28 patients with a starting dose of 90 mg/day for 7 days followed by an escalation to 180 mg/day, none of them developed pulmonary symptoms during the first 7 days at 90 mg or the first 7 days at 180 mg. This dosing strategy was chosen as the phase II dosage. In the phase II part, 57 patients were enrolled and ORR was 69% among the 51 crizotinib-pretreated patients and 100% among the 6 ALK inhibitor-naïve patients. The median PFS of the 49 crizotinib-pretreated patients with follow-up scans was 10.9 months. A total of 9 out of 13 patients (69%) with brain metastases responded to AP26113 therapy. The common AEs are listed in Table 1 [Gettinger et al. 2014a, 2014b]. A phase II trial of AP26113 (ALTA) [ClinicalTrials.gov identifier: NCT02094573] is ongoing for advanced crizotinib-pretreated ALK-positive NSCLC.
ASP3026
ASP3026 is a potent ALK and ROS1 inhibitor. In mouse models, this drug showed potent antitumour activity against ALK-positive NSCLC cells expressing crizotinib-resistant L1196M mutation [Mori et al. 2014]. In a phase I study [ClinicalTrials.gov identifier: NCT01401504], a dose level of 525 mg/day was chosen as a recommended phase II dose. The common AEs are listed in Table 3. ORR was 50% among 16 patients and the median PFS was 5.5 months (95% CI 3.7–11) [Maitland et al. 2014].
PF-06463922
PF-06463922 is a potent, macrocyclic inhibitor of ALK and ROS1. This drug has low propensity for P-glycoprotein 1 mediated efflux and good passive permeability in order to facilitate its CNS penetration [Johnson et al. 2014]. In preclinical studies, this drug demonstrated activity in mice with tumour xenograft that expressed ALK and some crizotinib-resistant mutations, including G1202R (Table 2). This drug achieved brain exposure of 20–30% of its plasma level in mice and regressed the brain tumours [Johnson et al. 2013; Zou et al. 2013a, 2013b]. A phase I/II study of PF-06463922 in patients with advanced ALK- or ROS1-positive NSCLC is ongoing [ClinicalTrials.gov identifier: NCT01970865].
TSR-011
TSR-011 is a potent ALK and tropomyosin-related kinase (TRK) A, B and C (encoded by NTRK1, NTRK2 and NTRK3, respectively) inhibitor. In preclinical studies, this drug showed activity against crizotinib-resistant L1196M mutation (Table 2) [Wilcoxen et al. 2012]. In a phase I/IIa study [ClinicalTrials.gov identifier: NCT02048488], 23 patients were enrolled (including five ALK-positive NSCLC). The DLT included dysaesthesia and QTc prolongation [Weiss et al. 2014]. Rearrangement in NTRK1 had been reported in a small portion of NSCLC patients without known oncogenic alterations. Treatment with inhibitors of TRKA kinase inhibited cell growth [Vaishnavi et al. 2013]. TSR-011 might have activity in this patient group.
RXDX-101
RXDX-101 (former name: NMS-E628) is an inhibitor of ALK, ROS1 and TRK A, B and C. In preclinical studies, this drug demonstrated in vitro and in vivo activity against ALK-positive NSCLC and some crizotinib-resistant mutations (Table 2) [Ardini et al. 2009, 2011]. This drug passed through the blood–brain barrier in animal models and controlled intracranial tumours [Ardini et al. 2011]. In a phase I study, RXDX-101 was administered to patients with advanced solid tumours with relevant molecular alterations. One patient with ALK-positive NSCLC achieved PR and another patient with ALK-positive NSCLC had prolonged stable decease (SD). No DLT had been observed in all the tested dose levels. The common AEs (mainly grade 1–2) are listed in Table 3 [De Braud et al. 2014a, 2014b]. A global phase I/II study was initiated in patients with cancer confirmed to be positive for ALK, ROS1 and TRK A, B and C molecular alterations, especially NSCLC, colorectal cancer, prostate cancer, papillary thyroid cancer, pancreatic cancer and neuroblastoma [ClinicalTrials.gov identifier: NCT02097810].
X-396
X-396 is an ALK and MET inhibitor. It was more potent inhibitor of ALK and less potent inhibitor of MET compared with crizotinib. This drug also had activity against some crizotinib-resistant mutations (Table 2). In combination with the mTOR inhibitor, rapamycin, X-396 displayed synergistic growth inhibition [Lovly et al. 2011]. The efficacy results and the common AEs of a phase I/II study [ClinicalTrials.gov identifier: NCT01625234] are listed in Table 3. Doses up to 225 mg were well tolerated. The enrolment is ongoing in the expansion cohort [Horn et al. 2014].
CEP-37440
CEP-37440, an analogue of CEP-28122, is an ALK and FAK inhibitor [Shanthi et al. 2014]. Preclinical studies demonstrated activity of CEP-28122 against ALK-positive human cancer cells and tumour xenograft mouse models [Cheng et al. 2012]. The development of CEP-28122 was terminated because severe lung toxicity was developed in animal studies [Wang et al. 2014]. A phase I trial of CEP-37440 [ClinicalTrials.gov identifier: NCT01922752] for patients with advanced and metastatic solid tumours is ongoing.
Summary of ALK inhibitors
Crizotinib provides benefits to patients with advanced ALK-positive NSCLC in the first-line, second-line or heavily pretreated settings, with the PFS being around 7–10 months. Resistance disease inevitably developed after crizotinib therapy. Crizotinib-resistant ALK mutations (e.g. L1196M and G1269A) were one of the resistance mechanisms. Brain metastasis was another cause of PD. Novel ALK inhibitors were active against various crizotinib-resistant ALK mutations and brain metastases. Ceritinib is approved by the FDA for crizotinib-pretreated ALK-positive NSCLC. In Japan, alectinib is also available for ALK-positive NSCLC in the setting of either crizotinib-naïve or crizotinib-pretreated disease. These two novel ALK inhibitors gained accelerated approvals based on phase I/II studies, and confirmatory phase III studies are needed to determine the efficacy and AEs in different clinical setting. However, these aforementioned clinical trials enrolled crizotinib-pretreated patients, but the mechanisms of crizotinib resistance in these patients were not all the same. Some of these patients underwent cytotoxic chemotherapy before entering the clinical trials and clonal repopulation of crizotinib-sensitive cells might develop during chemotherapy. This situation cannot be classified as a mechanism of crizotinib resistance. Tumours that develop resistance under an ALK inhibitor therapy warrant repeat biopsies to identify the mechanism of drug resistance, although this procedure is not current standard of care and should be performed under clinical trial settings.
Even the resistance mechanisms of ceritinib (F1174C and G1202R) and alectinib (I1171T, V1180L and G1202R) had been identified (Table 2) [Friboulet et al. 2014; Ou et al. 2014b; Katayama et al. 2014]. Ceritinib was active against alectinib-resistant I1171T and V1180L mutations, but both ceritinib and alectinib were ineffective against G1202R mutation. Other novel ALK inhibitors, such as PF-06463922 and AP26113, have activity against G1202R and could be effective in treating patients who developed this mutation [Politi and Gettinger, 2014]. We encourage patients with ALK-positive NSCLC to participate in clinical trials to address the best sequence, combination, intercalation or novel therapeutic strategy for ALK inhibitors and to extend survival for patients [Gainor and Shaw 2013].
Hsp90 inhibitors in ALK-positive NSCLC
Hsp90 is a molecular chaperone that guides the normal folding, proteolytic turnover, intracellular disposition of regulators of cell growth and survival [Whitesell and Lindquist, 2005]. Hsp90 is also a chaperone involved in the stabilisation of many oncoproteins and has been implicated in tumourigenesis [Trepel et al. 2010]. In ALK-positive NSCLC, the tumour is addicted to the fusion protein resulting from chromosomal rearrangements and this protein is thought to be a client of Hsp90 [Neckers and Workman, 2012]. Hsp90 inhibitors are effective in treating these patients in early clinical studies and four Hsp90 inhibitors (IPI-504, ganetespib, AUY922 and AT13387) are currently under clinical investigation in ALK-positive NSCLC patients [Pillai and Ramalingam, 2014].
IPI-504
Retaspimycin hydrochloride (IPI-504) is an Hsp90 inhibitor. In preclinical studies, the degradation of EML4–ALK fusion protein was induced by IPI-504 therapy and it resulted in the inhibition of downstream signalling pathways, induction of growth arrest and apoptosis [Normant et al. 2011]. In a phase II trial of IPI-504 monotherapy for patients with molecularly defined NSCLC, two out of three patients with ALK-positive NSCLC responded to IPI-504 therapy and the remaining one patient had prolonged SD. The most common AEs were fatigue, nausea and diarrhoea (grades 1 and 2). Grade 3 or higher liver function abnormality was observed in 11.8% of the patients [Sequist et al. 2010]. A phase II study of IPI-504 for ALK-positive NSCLC [ClinicalTrials.gov identifier: NCT01228435] was terminated early because of slow patient recruitment and competing studies.
Ganetespib
Ganetespib (STA-9090) is a nongeldanamycin triazolone-containing Hsp90 inhibitor. In preclinical studies, ganetespib demonstrated activity against KRAS mutant, EGFR mutant (including EGFRdel 19 and EGFRL858R/T790M), ERBB2 mutant and c-MET amplification in NSCLC in animal models [Acquaviva et al. 2012; Shimamura et al. 2012; Ying et al. 2012]. In ALK-positive NSCLC cell lines, ganetespib induced loss of EML4–ALK expression and depletion of oncogenic signalling proteins [Sang et al. 2013]. Ganetespib overcame multiple forms of crizotinib resistance, including some secondary ALK mutations, and ganetespib in combination with novel ALK inhibitors other than crizotinib also led to increased activity [Sang et al. 2013]. In addition to ALK-positive NSCLC cell lines, cancer cells driven by oncogenic rearrangement of ROS1 and RET were sensitive to ganetespib [Sang et al. 2013].
In a phase II study of ganetespib monotherapy for genotypically defined advanced NSCLC, 99 patients were enrolled and ORR was reported to be 4%. All of these responders (n=4) were patients with crizotinib-naïve ALK-positive NSCLC. The treatment duration of the responders ranged from 7.4 to 21 months. Three out of the remaining four patients with ALK-positive NSCLC achieved SD as the best response. The median PFS of these 8 patients was 8.1 months, which was longer than the patients without ALK rearrangement (HR, 0.223; 95% CI 0.085–0.582) [Socinski et al. 2013]. The most common AEs were diarrhoea (81.8%), fatigue (57.6%), nausea (41.4%), decreased appetite (37.4%) and constipation (26.3%). Two patients experienced treatment-related death (one cardiac arrest and one renal failure) [Socinski et al. 2013].
A phase II study of ganetespib for ALK-positive NSCLC [ClinicalTrials.gov identifier: NCT01562015] and a phase I study of ganetespib plus crizotinib for crizotinib-naïve advanced ALK-positive NSCLC [ClinicalTrials.gov identifier: NCT01579994] are ongoing. Ganetespib is under clinical development in combination with docetaxel in patients with advanced NSCLC, which is not restricted to ALK-positive NSCLC (GALAXY-2) [ClinicalTrials.gov identifier: NCT01798485] [Proia et al. 2012; Ramalingam et al. 2013, 2014].
AUY922
AUY922 is a potent isoxazole-based nongeldanamycin Hsp90 inhibitor, which acts via cytostasis, apoptosis, invasion and angiogenesis to inhibit tumour growth and metastasis [Eccles et al. 2008]. A phase II study [ClinicalTrials.gov identifier: NCT01124864] of AUY922 for advanced NSCLC enrolled 121 patients. A total of 6 out of 21 patients (29%) with ALK-positive NSCLC achieved PR, and 4 of the 6 responders were crizotinib-naïve. The estimated PFS rate at 18 weeks was 42%. The most common AEs were eye disorder (77%), diarrhoea (74%) and nausea (46%) [Felip et al. 2012]. A phase II study of AUY922 for advanced NSCLC pretreated with crizotinib [ClinicalTrials.gov identifier: NCT01752400] and a phase Ib study of combination therapy of ceritinib plus AUY922 for crizotinib-pretreated ALK-positive NSCLC [ClinicalTrials.gov identifier: NCT01772797] are ongoing. In addition to ALK-positive population, investigators also tested the efficacy of AUY922 in HER2-mutant/amplification, EGFR mutant (including EGFRT790M resistance mutation, exon 20 mutation and other uncommon mutations), BRAF mutant and ROS1 or RET rearrangement in NSCLC [ClinicalTrials.gov identifier: NCT01922583, NCT01854034, NCT01646125] [Garon et al. 2013; Nogova et al. 2014].
AT13387
AT13387 is a high-affinity Hsp90 inhibitor [Woodhead et al. 2010]. Preclinical studies demonstrated its activity against EGFR mutant and c-MET-amplified NSCLC [Graham et al. 2012]. A phase I/II study of AT13387 alone or in combination with crizotinib for ALK-positive and crizotinib-pretreated patients [ClinicalTrials.gov identifier: NCT01712217] is ongoing.
Summary of Hsp90 inhibitors
Hsp90 inhibitors had shown activity against ALK-positive NSCLC in early phase studies and even overcame crizotinib-resistant mutations [Katayama et al. 2012; Sang et al. 2013]. However, Hsp90 inhibitors had limited activity against CNS metastatic tumours and their clinical benefits were restricted to patients without CNS metastases. However, the AEs of Hsp90 inhibitor therapy were higher than with second-generation ALK inhibitors. While there are many second-generation ALK inhibitors available in clinical practice or clinical trial settings, the development of Hsp90 inhibitors should be influenced. Novel approaches such as combination therapy with crizotinib or second-generation ALK inhibitors in either crizotinib-naïve or crizotinib-pretreated patients are under investigation. We encourage patients to participate in clinical trials to address the best combination or treatment strategy of Hsp90 inhibitors.
Conclusion
In patients with advanced ALK-positive NSCLC, crizotinib therapy was deemed to be indispensable. After disease progression, second-generation ALK inhibitors, ceritinib and alectinib, provided opportunities to overcome acquired resistance and achieve tumour control. Second-generation ALK inhibitors are not widely available around the world and cytotoxic chemotherapy is still the standard of care. We hope clinical trials are able to develop the next generation of ALK inhibitors to overcome resistance, be effective in treating CNS metastases, and extend survival in patients. Hsp90 inhibitors are currently not available in daily practice. We still rely on clinical trials to identify the best way to incorporate these drugs into clinical practice.
