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INTRODUCTION — A group of patients with non-small cell lung cancer (NSCLC) have tumors that contain an inversion in chromosome 2 that juxtaposes the 5' end of the echinoderm microtubule-associated protein-like 4 (EML4) gene with the 3' end of the anaplastic lymphoma kinase (ALK) gene, resulting in the novel fusion oncogene EML4-ALK . This fusion oncogene rearrangement is transforming both in vitro and in vivo and defines a distinct clinicopathologic subset of NSCLC.
Tumors that contain the EML4-ALK fusion oncogene or its variants are associated with specific clinical features, including never or light smoking history, younger age, and adenocarcinoma with signet ring or acinar histology. ALK gene arrangements are largely mutually exclusive with EGFR or KRAS mutations . Screening for this fusion gene in NSCLC is important, as "ALK-positive" tumors are highly sensitive to therapy with ALK-targeted inhibitors.
The molecular pathogenesis, clinical features, and treatment of NSCLC associated with the ALK fusion oncogene are discussed here.
An overview of the treatment of metastatic NSCLC is presented separately. (See "Overview of the treatment of advanced non-small cell lung cancer".)
MOLECULAR PATHOGENESIS — The echinoderm microtubule-associated protein-like 4-anaplastic lymphoma kinas (EML4-ALK) fusion oncogene arises from an inversion on the short arm of chromosome 2 (Inv(2)(p21p23)) that joins exons 1-13 of EML4 to exons 20-29 of ALK . The resulting chimeric protein, EML4-ALK, contains an N-terminus derived from EML4 and a C-terminus containing the entire intracellular tyrosine kinase domain of ALK.
Since the discovery of this fusion oncogene in 2007, multiple variants of EML-ALK have been reported, all of which encode the same cytoplasmic portion of ALK but contain different truncations of EML4 (figure 1) [3-5]. In addition, fusions of ALK with other partners including TRK-fused gene TFG and KIF5B have also been described in lung cancer patients, but appear to much less common than EML4-ALK [5,6].
For EML4-ALK, the EML4 fusion partner mediates ligand-independent dimerization and/or oligomerization of ALK, resulting in constitutive kinase activity. In cell culture systems, EML4-ALK possesses potent oncogenic activity . In transgenic mouse models, lung-specific expression of EML4-ALK leads to the development of numerous lung adenocarcinomas .
The oncogenic role of the ALK fusion oncogene provides a potential avenue for therapeutic intervention. Cancer cell lines harboring the EML4-ALK translocation are effectively inhibited by small molecule inhibitors that target the ALK tyrosine kinase (TK) . In vivo, treatment of EML4-ALK transgenic mice with ALK inhibitors results in tumor regression , supporting the notion that ALK-driven lung cancers are "addicted" to the fusion oncogene.
DIAGNOSIS — Anaplastic lymphoma kinase (ALK) gene rearrangements or the resulting fusion proteins may be detected in tumor specimens using fluorescence in situ hybridization (FISH), immunohistochemistry (IHC), and reverse transcription polymerase chain reaction of cDNA (RT-PCR) .
CLINICOPATHOLOGIC FEATURES — With increasing identification of this molecular abnormality, the key epidemiologic, demographic, and pathologic features associated with anaplastic lymphoma kinase (ALK) fusion oncogenes have been identified.
Epidemiology — In unselected NSCLC populations, the ALK rearrangement is a relatively rare event. In the initial report, 5 of 75 lung tumors (7 percent) demonstrated expression of the fusion transcript. The overall incidence of ALK gene rearrangements in subsequent series has been about 4 percent [3,8,11-13,15-17]. Except in rare cases, the presence of ALK gene rearrangements in NSCLC tumors tends to occur independent of EGFR or KRAS mutations.
While the overall frequency of ALK fusion oncogene in the general NSCLC population is low, knowledge of the clinicopathologic features enables enrichment for this genetically defined subset. In one study in which patients were selected for genetic screening based on clinical features commonly associated with EGFR mutation, including never/light smoking status and adenocarcinoma histology, 13 percent harbored the ALK fusion oncogene. Within the group of never or light smokers in this study, the frequency of ALK positivity was 22 percent, and among never or light smokers who did not have an EGFR mutation, the frequency was 33 percent. These findings suggest that in NSCLC patients with clinical characteristics associated with EGFR mutation but with negative EGFR testing, as many as one in three may harbor the ALK fusion oncogene. (See "Initial systemic therapy for advanced non-small cell lung cancer with a mutation in the epidermal growth factor receptor", section on 'Rationale'.)
Age of onset — Patients with ALK fusion oncogene-positive lung cancer are relatively younger at onset than those without this abnormality [12,18]. The two studies that were used to support the approval of crizotinib included 255 patients whose tumors contained an ALK fusion oncogene; in this database, the median age was 52 years (range 21 to 82 years) . The estimated median age for other patients with lung cancer is approximately 66 years .
Interestingly, other cancers known to harbor ALK rearrangements are also associated with younger age and are in fact most common in children and young adults.
Smoking history — The ALK fusion oncogene in patients with NSCLC is strongly associated with a history of never or light smoking [1,12,18]. In the crizotinib study database of 255 patients, never smokers and former smokers comprised 70 and 28 percent of cases, respectively [1,12,18].
Histology — The vast majority of lung tumors that harbor the ALK fusion oncogene are adenocarcinomas. In the 255 patients with the ALK fusion oncogene included in the crizotinib database, 97 percent were adenocarcinoma . ALK rearrangement has been reported in squamous cell carcinoma but is rare [11,17].
Adenocarcinomas in ALK fusion oncogene positive cases from Caucasian patients are significantly more likely to have abundant signet ring cells than those with an epidermal growth factor receptor (EGFR) mutation or wild type tumors . Signet ring cells are frequently found in gastric cancers and rarely in cancers of other organs such as the lung.
Several small case series suggest that signet ring cells may be associated with an aggressive clinical course and a poor prognosis. Whether the presence of signet ring cells in ALK fusion oncogene lung cancer has biological or clinical significance remains to be determined.
Other studies of ALK in NSCLC have not reported an association with signet ring cells but have noted a possible association with the acinar subtype of adenocarcinoma, at least in Asian patients. This discrepancy may reflect differences in pathologic interpretation rather than ethnic differences in patients with ALK fusion oncogene-positive lung cancer.
CHEMOTHERAPY VERSUS TARGETED THERAPY — Advanced non-small cell lung cancer (NSCLC) associated with the anaplastic lymphoma kinase (ALK) fusion oncogene are highly sensitive to the ALK TK inhibitor crizotinib. Crizotinib is thus preferred as the initial therapy for patients whose tumor contains this genetic abnormality in areas where it is approved for this indication. In other areas, its use may be restricted to those who have progressed following chemotherapy. Thus patients should have tumor tissue assessed for the presence of ALK rearrangement, as well as for other driver mutations (especially in the epidermal growth factor receptor [EGFR]). (See "Personalized, genotype-directed therapy for advanced non-small cell lung cancer".)
If systemic treatment is required before the results of genotype testing are available, systemic chemotherapy rather than targeted therapy is indicated . When the results of genotype testing become available, the treatment plan should be reassessed. There are no clinical trials that directly address the optimal timing of crizotinib in patients who have already started on chemotherapy (algorithm 1).
If an ALK fusion oncogene is identified after the initiation of treatment, we suggest continuing chemotherapy for four cycles if therapy is tolerated and there is no evidence of disease progression. There are no data directly comparing continuation using maintenance chemotherapy with a switch to crizotinib after completion of this initial chemotherapy. The author’s preference is to switch to crizotinib, rather than to use maintenance chemotherapy, because there are some patients who will deteriorate due to disease progression and thus miss the opportunity to be treated with crizotinib.
For patients who continue treatment with chemotherapy, crizotinib is indicated when there is evidence of disease progression. Factors that should be discussed with the patient in defining a treatment plan include the quality of the initial response to chemotherapy and the potential side effects of maintenance therapy.
CRIZOTINIB — Crizotinib is a multitargeted small molecule tyrosine kinase inhibitor, which was originally developed as an inhibitor of mesenchymal epithelial transition growth factor (c-MET); it is also a potent inhibitor of anaplastic lymphoma kinase (ALK) phosphorylation and signal transduction . This inhibition is associated with G1-S phase cell cycle arrest and induction of apoptosis in positive cells in vitro and in vivo .
Crizotinib also inhibits the related ROS1 receptor tyrosine kinase. (See "Personalized, genotype-directed therapy for advanced non-small cell lung cancer", section on 'ROS1 translocation'.)
Efficacy — The antitumor efficacy of crizotinib was initially demonstrated in two multicenter, single arm studies [22,23]. Dosage for these extended series was 250 mg, orally, given twice a day, based upon results from the initial phase I dose escalation study. In aggregate, these studies included 255 patients, all of whose tumors contained an ALK gene rearrangement as shown by FISH . Overall, 95 percent of patients had metastatic disease and 5 percent had locally advanced NSCLC; 94 percent had received prior systemic, and 76 percent had been treated with two or more regimens.
The combined objective (complete plus partial) response rate to crizotinib was 55 percent, the majority of which was achieved during the first eight weeks of treatment. The median durations of response at the time of analysis on the two studies were 42 and 48 weeks, respectively. Indirect evidence of the impact of crizotinib on survival of patients comes from a nonrandomized, retrospective analysis of the patients enrolled in the phase I study . The one and two-year survival rates for patients treated with crizotinib were 74 and 54 percent, respectively, with a median follow-up of 18 months. In a cohort of 36 patients with the ALK fusion oncogene who were not treated with crizotinib, the survival rates at one and two year rates were 44 and 12 percent, respectively. A comparison with a larger cohort of patients with wild-type tumors did not identify a difference compared with those with the ALK rearrangement not treated with crizotinib, suggesting that the presence of ALK rearrangement was not prognostically significant.
The activity of crizotinib in ALK rearrangement positive NSCLC was subsequently confirmed in a phase III trial in which 347 patients were randomly assigned to crizotinib or single agent chemotherapy with either pemetrexed or docetaxel . Patients with progressive disease on chemotherapy were allowed to cross over and receive treatment with crizotinib. All patients had received one prior platinum-based regimen.
Preliminary results were presented at the 2012 European Society of Medical Oncology (ESMO) meeting. Treatment with crizotinib significantly increased progression-free survival, the primary endpoint of the trial, compared with chemotherapy (median 7.7 versus 3.0 months, hazard ratio [HR] for progression 0.49, 95% CI 0.37-0.64). There was no significant difference in overall survival (median 20.3 versus 22.8 months, HR for death 1.02), but 64 percent of chemotherapy-treated patients had crossed over to crizotinib.
Brain metastases — Surgery and/or radiation therapy are the primary treatment modalities for patients with brain metastases from NSCLC, including those with the EML4-ALK fusion oncogene [26,27]. There are only limited data on the activity of crizotinib on brain metastases in this setting. (See "Overview of the clinical manifestations, diagnosis, and management of patients with brain metastases", section on 'Overview of management' and "Systemic therapy for brain metastases", section on 'Targeted agents'.)
Isolated brain metastases (a central nervous system (CNS) relapse without evidence of extracranial progression) may be a particular problem for patients treated with crizotinib. In this setting, continuation of treatment with crizotinib after treatment of the CNS relapse may be associated with a significant period of control of extracranial disease .
Other toxicities that have been observed with crizotinib include the following:
Ongoing trials — In two ongoing phase III trials, patients with ALK-positive lung cancer who have not received prior systemic therapy for advanced disease are being randomly assigned to either crizotinib or chemotherapy with pemetrexed plus a platinum compound (either cisplatin or carboplatin) (NCT01154140 and NCT01639001). Patients assigned to chemotherapy can crossover to crizotinib when progressive disease is documented. The primary endpoint in both trials is progression-free survival.
SECOND GENERATION ALK INHIBITORS — Several second generation anaplastic lymphoma kinase (ALK) inhibitors are being studied in the clinic for patients with ALK-rearranged cancers. Some of these agents are more potent and selective than crizotinib, and may offer advantages in terms of higher response rates or activity in patients who have acquired resistance to crizotinib.
Additional clinical experience with these and other novel ALK inhibitors will be required to establish the role of these agents .
Chemotherapy — Cytotoxic chemotherapy appears to have a similar level of activity in ALK-positive patients with NSCLC compared to those with ALK-negative disease. The choice of a specific chemotherapy agent or regimen is based upon the same criteria applied in other cases of advanced NSCLC. When patients with ALK-positive advanced NSCLC require chemotherapy, pemetrexed or a pemetrexed-based regimen is generally preferred, since almost all of these patients have adenocarcinoma histology. (See "Initial systemic chemotherapy for advanced non-small cell lung cancer without an epidermal growth factor receptor mutation or the ALK fusion oncogene", section on 'Effect of histology' and "Second-line therapy for patients with previously treated advanced non-small cell lung cancer", section on 'Chemotherapy agents'.)
Two small, retrospective studies suggested that pemetrexed was more effective in patients with the ALK fusion oncogene compared with those without this driver mutation [37,38]. However, a larger, multicenter retrospective series that compared 121 patients with an ALK-positive NSCLC with 266 patients with ALK-negative, EGFR-mutation negative disease found that a statistically significant improvement in progression free survival was limited to the first line setting when pemetrexed was combined with either cisplatin or carboplatin .
EGFR tyrosine kinase inhibitors — In one retrospective study, 141 patients with tumors harboring an ALK fusion oncogene, an EGFR mutation, or neither genetic alteration (wild type) were compared in terms of response rate, time to progression and overall survival . Among metastatic patients who received any platinum-based combination, the 19 patients who were positive for ALK had similar response rates and time to progression compared to the wild type patients.
In contrast to patients with EGFR mutations, patients with the ALK fusion oncogene do not appear to respond to EGFR tyrosine kinase inhibitors (TKIs) such as erlotinib or gefitinib. Within the ALK cohort in the retrospective study described above, there were no clinical responses to EGFR TKIs, and the median time to progression was only five months. These findings are consistent with preclinical studies showing that the EML4-ALK-containing NSCLC cell line H3122 is resistant to erlotinib. (See "Initial systemic therapy for advanced non-small cell lung cancer with a mutation in the epidermal growth factor receptor".)
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