Anaplastic lymphoma kinase (ALK) fusion gene positive advanced non-small cell lung cancer
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 [1]. 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 [2]. 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 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 [3]. 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) [3-8]. 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 [7,9].
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 [3]. In transgenic mouse models, lung-specific expression of EML4-ALK leads to the development of numerous lung adenocarcinomas [10].
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 [6]. In vivo, treatment of EML4-ALK transgenic mice with ALK inhibitors results in tumor regression [10], supporting the notion that ALK-driven lung cancers are "addicted" to the fusion oncogene.
DIAGNOSIS — ALK gene rearrangements or the resulting fusion proteins may be detected in tumor specimens using immunohistochemistry (IHC), reverse transcription polymerase chain reaction of cDNA (RT-PCR), and fluorescence in situ hybridization (FISH).
- FISH — The gold standard assay for diagnosing ALK-positive NSCLC is FISH [11-14]. The commercial break-apart probes include two differently colored (red and green) probes that flank the highly conserved translocation breakpoint within ALK. In non-rearranged cells, the overlying red and green probes result in a yellow (fused) signal; in the setting of an ALK rearrangement, these probes are separated and splitting of the red and green signals is observed (figure 2).There are atypical patterns of rearrangement that also respond to crizotinib.
- IHC — The ALK1 antibody, which has proven useful in anaplastic large cell lymphomas (ALCL) and inflammatory myofibroblastic tumors (IMT), has not been as reliable in lung carcinomas [11]. Strategies to improve the accuracy of the IHC ALK assays in lung cancer include amplification of the signal with a tyramide cascade [15] and intercalation of an antibody-enhanced polymer [7]. An IHC assay using a different, highly sensitive ALK antibody has shown excellent sensitivity and specificity (100 and 99 percent, respectively) for the detection of ALK rearrangements in NSCLC [16]. This assay is undergoing further validation to determine whether it will be useful in routine clinical practice.
- RT-PCR — RT-PCR of cDNA is another commonly used screening strategy for detecting ALK gene rearrangements in NSCLC. A number of multiplex assays have been developed to simultaneously capture all possible in-frame fusions between EML4 and ALK in which the kinase domain of ALK would be preserved [3,4,6]. While different breakpoints in EML4 or ALK may be detected, novel ALK fusion partners will not. Furthermore, this method is frequently limited by the quality of the RNA that can be isolated from archival tissue.
CLINICOPATHOLOGIC FEATURES — With increasing identification of this molecular abnormality, the key pathologic, demographic, and epidemiologic features associated with ALK fusion oncogenes have been identified.
Age of onset — Patients with ALK fusion oncogene-positive lung cancer are relatively younger at onset than those without this abnormality [13,17]. 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) [17]. The estimated median age for other patients with lung cancer is approximately 66 years [13].
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 [1,13,17]. In the crizotinib study database of 255 patients, never smokers and former smokers comprised 70 and 28 percent of cases, respectively [1,13,17].
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 [17]. ALK rearrangement has also been seen in squamous cell carcinomas but is less likely.
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 [15]. 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.
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 [3,4,6-9,11-14,18,19]. Except in rare cases, the presence of ALK gene rearrangements in NSCLC tumors is mutually exclusive to 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 "Small molecule epidermal growth factor receptor inhibitors for advanced non-small cell lung cancer", section on 'Predictors of responsiveness'.)
CHEMOTHERAPY VERSUS TARGETED THERAPY — Whenever possible, therapy for patients with advanced or metastatic NSCLC should be individualized based upon the molecular and histologic features of the tumor.
If feasible, patients should have tumor tissue assessed for the presence of ALK rearrangement, as well as for a somatic mutation in the epidermal growth factor receptor (EGFR). (See "Small molecule epidermal growth factor receptor inhibitors for advanced non-small cell lung cancer".)
Treatment with crizotinib, the inhibitor of ALK fusion kinases, should be limited to patients whose tumors contain this abnormality as demonstrated by FISH. (See 'Diagnosis' above.)
CRIZOTINIB — The identification of a specific molecular abnormality in this subset of patients with NSCLC led to the development of crizotinib, an ALK inhibitor [20]. In preclinical studies, several ALK inhibitors have shown activity against NPM-ALK and EML4-ALK containing cell lines [3,6,10,21,22]. 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 ALK phosphorylation and signal transduction [21]. This inhibition is associated with G1-S phase cell cycle arrest and induction of apoptosis in positive cells in vitro and in vivo [21].
Crizotinib was approved by the US Food and Drug administration in August 2011; this approval is independent of whether or not the patient has received previous treatment for advanced NSCLC [17]. This accelerated approval was based upon response rate rather than survival data. An ongoing, randomized phase III trial is studying the effect of treatment on progression-free survival.
Efficacy — The antitumor efficacy of crizotinib was demonstrated in two multicenter, single arm studies [17]. Dosage for these extended series was 250 mg, orally, given twice a day, based upon results from the initial phase I dose escalation study [23].
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. Overall, 94 percent of patients had received prior systemic therapy for advanced or metastatic disease, and 76 percent had received two or more treatment regimens.
The combined objective (complete plus partial) response rate 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 [24]. 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.
Toxicity — Treatment with crizotinib was generally well tolerated. The key toxicities observed in the phase II database included [17]:
- Visual disturbances, which were seen in 60 to 65 percent of cases, are primarily associated with the transition from light to dark. Uncommon visual manifestations including photophobia, decreased visual acuity, and blurred vision are uncommon. There were no grade 3 or 4 visual complications.
- Less severe side effects included gastrointestinal symptoms, which were seen in 25 percent or more of patients and included nausea, diarrhea, vomiting, and constipation. Less than 1 percent of these were grade 3 or 4. Edema and fatigue each occurred in 20 to 40 percent of cases, but were severe in 2 percent or less of cases.
- Abnormal liver function tests occurred in 10 to 15 percent of cases. In 4 to 7 percent these were grade 3 or 4 and required interruption of treatment. Treatment had to be permanently discontinued in 4 of 255 (1.6 percent) of cases.
- Severe, life-threatening, or fatal treatment-related pneumonitis occurred in four cases (1.6 percent). The development of pneumonitis due to crizotinib should result in discontinuation of treatment with this agent.
- Sinus bradycardia (heart rate ≤45) has been observed in several patients [25]. Patients with sinus bradycardia were asymptomatic, and the bradycardia was not explained by other comorbidity or medications. Caution should be used in the concomitant administration of beta blockers in patients treated with crizotinib.
- Crizotinib is metabolized by cytochrome P450 CYP3A. Thus caution should be used when it is given concomitantly with CYP3A inhibitors, and care is required when crizotinib is coadministered with other agents that are predominantly metabolized by this system (table 1).
QTc interval prolongation has been observed with crizotinib, and crizotinib should be avoided in patients with congenital long QT syndrome. Treatment should be temporarily discontinued if severe QTc prolongation develops and permanently discontinued if it recurs or is accompanied by an arrhythmia, heart failure, hypotension, shock, syncope, or torsade de pointes. (See "Cardiotoxicity of nonanthracycline cancer chemotherapy agents", section on 'Crizotinib'.)
Ongoing trials — Two ongoing phase III trials are exploring the role of crizotinib in the treatment of NSCLC:
- One trial is currently randomizing patients to either crizotinib or single-agent docetaxel or pemetrexed (NCT00932893) [26]. Enrollment is now limited to sites outside the United States. Patients must have progressive, metastatic NSCLC who have received one prior, platinum-based chemotherapy regimen and must be positive for the ALK fusion gene by FISH. This trial is also closed in Asia, in addition to the US.
- A second trial is randomly assigning patients to crizotinib or chemotherapy with pemetrexed plus a platinum compound (either cisplatin or carboplatin) (NCT01154140) [26]. Patients assigned to chemotherapy can crossover to crizotinib when progressive disease is documented. The primary endpoint for the trial is progression-free survival.
Resistance to ALK inhibitors can develop after an initial response to treatment. At least one report has documented the presence of mutations within the tyrosine kinase domain of ALK that can confer resistance to crizotinib [27].
OTHER AGENTS
Chemotherapy — Cytotoxic chemotherapy appears to have a similar level of activity in ALK-positive patients with NSCLC compared to those with ALK-negative NSCLC. (See "Overview of the treatment of advanced non-small cell lung cancer".)
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 [13]. 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".)
SUMMARY AND RECOMMENDATIONS
- The presence of an ALK fusion oncogene defines a molecular subset of non-small cell lung cancer (NSCLC) with distinct clinical and pathologic features. The patients most likely to harbor ALK rearrangement are relatively young, never or light smokers with adenocarcinoma. (See 'Molecular pathogenesis' above and 'Clinicopathologic features' above.)
- Whenever possible, therapy of patients with advanced NSCLC should be individualized based upon the molecular and histologic features of the tumor. If feasible, patients should have tumor tissue assessed for the presence of a somatic mutation in the epidermal growth factor receptor (EGFR), which confers sensitivity to EGFR tyrosine kinase inhibitors, and for the ALK fusion oncogene, which is associated with sensitivity to crizotinib. In accordance with the FDA label, ALK positivity must be demonstrated by FISH using the FDA-approved test (Vysis Probes). (See 'Chemotherapy versus targeted therapy' above.)
- For patients with advanced or metastatic non-small cell lung cancer whose tumors contain a characteristic ALK fusion oncogene, we recommend treatment with crizotinib (Grade 1B). Crizotinib should not be used in patients without ALK rearrangement. (See 'Crizotinib' above.)
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