Diagnosis and treatment of anaplastic lymphoma kinase (ALK) rearranged non−small cell lung cancer

Introduction

Historically, non–small cell lung cancer (NSCLC) was divided on the basis of histology (e.g., adenocarcinoma, squamous), but with the development of targeted therapies, NSCLC was further divided on the basis of the presence or absence of a molecular alteration. For example, epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) were developed in unselected patients. However, higher response rates were observed in patients with the clinical characteristics of a history of never smoking, adenocarcinoma, and Asian ancestry. This clinical observation led to the identification of the EGFR mutation. ^, A subsequent prospective phase 3 trial revealed the superiority of the EGFR TKI compared with chemotherapy in patients with EGFR -mutant NSCLC and that the EGR TKI had minimal activity in EGFR wild-type NSCLC. This established biomarker-driven care and represented a paradigm shift in the treatment of metastatic NSCLC. The research focus within thoracic oncology changed to identifying novel molecular alterations and developing targeted therapies for the specific alteration.

Anaplastic lymphoma kinase ( ALK ) rearrangements were identified in 2007. The alteration involved an inversion within chromosome 2p resulting in a fusion gene with portions of the echinoderm microtubule–associated protein-like 4 (EML4) gene and the ALK gene ( Fig. 11.1 ). This leads to activation of the Ras/Mek/Erk and PI3K/Akt pathways and increased cell survival and proliferation. Clinical characteristics associated with ALK- rearranged ( ALK +) NSCLC are younger age at diagnosis, adenocarcinoma histology, and a history of never smoking or light tobacco use. The rate of ALK rearrangements in patients with NSCLC with adenocarcinoma histology is approximately 5%–7%.

The early drug development of ALK TKIs fundamentally differed from the EGFR TKIs, in that patients were required to have an ALK rearrangement by central testing to enroll in the initial trials. The US Food and Drug Administration (FDA) approved crizotinib on the basis of a phase 1 study with expansion cohort, along with a companion diagnostic for the EML4-ALK rearrangement in 2011. The simultaneous development of the biomarker and the targeted agent established the drug development strategy for future targeted therapies.

Diagnosis

Since crizotinib was only available to the patient if the tumor demonstrated the ALK rearrangement, it became standard clinical practice to test all patients with adenocarcinoma for the ALK rearrangement regardless of age, smoking history, or clinical characteristics. The companion diagnostic test approved was a dual-probe “break-apart” florescence in situ hybridization assay (FISH). The green fluorophore binds to the 5’ region of ALK , and the red fluorophore binds to the 3’ region of the ALK kinase encoding region. In the absence of ALK rearrangement, these signals are close together, but an ALK rearrangement leads to separation of the signals. In order for this test to be considered positive, a split signal (i.e., green and red fluorophores are separated or loss of the green signal of 5’ probe with the remaining unpaired red signal of the 3’ probe (indicating an unbalanced rearrangement) must be observed in ≥15% of cells with at least 50 nuclei evaluated. The weaknesses of the test are its relative expense, the signal can be subtle, it requires specialized expertise, and it does not identify the specific translocation types. ^, The advantages are that it will detect the fusion regardless of the fusion partner.

Given the limitations of ALK FISH testing and the simplicity of immunohistochemistry (IHC) assays, IHC tests were developed to detect the ALK fusion protein. The specific antibody clone is relevant, and several antibodies were evaluated, but the D5F3 IHC assay is the one currently approved by the FDA as a companion diagnostic. Several studies have investigated this assay, and collectively they have revealed a sensitivity of 76%–100% and specificity of 76%–100%. The low sensitivity was associated with a focally and/or weakly ALK-positive test, and testing refinements improved the sensitivity and specificity. The concordance in interpretation between pathologists is >95%. ^, The 5A4 monoclonal antibody is also commercially available as well. A meta-analysis of both antibodies revealed a pooled sensitivity of 97% and a pooled specificity of 99%. The advantages of ALK IHC testing are the lower cost, wide availability of IHC testing, and shorter turnaround time.

Many centers have adopted next-generation sequencing (NGS) as a method of assessing a broad range of molecular alterations using a single testing platform. This method allows the detection of translocations, insertions or deletions, and copy number variants using a targeted gene panel. NGS has revealed similar sensitivity and specificity to ALK FISH testing. ^, There is acceptance that patients with a positive result by NGS may be treated with ALK TKI given the high specificity. The advantage of NGS testing is the ability to precisely define the breakpoint regions and detect unknown partner genes. The disadvantages are the variation in the NGS testing methods, the detection of alterations of uncertain significance, and concerns about false negatives due to variation in fusion partner.

The current recommendation is that ALK FISH or ALK IHC testing is acceptable, and there are insufficient data to recommend for or against the use of NGS as the sole determinant for selection ALK TKI. Clinically, the more challenging situations are when there is a concern about false-positive or false-negative results or an ambiguous test result. Some ambiguous results may be related to the quality or quantity of the tumor specimen, and if there is a high clinical suspicion, a repeat biopsy should be pursued if safe and feasible. An additional test (i.e., ALK FISH is ambiguous and testing with ALK IHC) may be helpful. If both tests are negative, then ALK TKI therapy would not be indicated and ALK TKI could be initiated if one of the tests are positive.

An emerging area of interest is the role of circulating tumor DNA (ctDNA) for detection of oncogenic mutations. The sensitivity of ctDNA testing for ALK rearrangements is approximately 80%, and the specificity is 100%. The sensitivity of these assays is higher in patients with extrathoracic metastases and multiple metastatic sites reflecting higher amounts of ctDNA. The advantages of this testing method are quicker turnaround time than tumor NGS testing, and the panel allows for testing of multiple alterations simultaneously. If ctDNA reveals an ALK rearrangement, ALK TKIs can be initiated. However, if the testing does not have detectable ctDNA or reveal another oncogenic driver, tumor testing should be performed given the concerns about the sensitivity of ctDNA.

Staging

Patients with ALK + NSCLC generally present with advanced stage, and the standard staging evaluation consists of imaging of the chest and abdomen with computed tomography and imaging of the brain, preferably with magnetic resonance imaging. The prevalence of brain metastases at time of diagnosis is approximately 15%−40% based on enrollment to recent first-line trials. The high prevalence of brain metastases necessitates assessment for central nervous system (CNS) disease at the start of therapy. Cystic brain metastases have been observed in patients with ALK + NSCLC. Bone scan or positron emission tomography scan to assess for bone metastases should be performed if clinically indicated.