• Assess eligibility for non–small cell lung cancer (NSCLC) targeted therapies

NSCLC accounts for about 85% of lung cancers and includes predominantly adenocarcinomas (the most common type in the United States) and squamous cell carcinomas.¹ Depending on the patient’s medical status and stage of disease, treatment options include surgery, radiation therapy, and chemotherapy. Although cytotoxic chemotherapy remains an important part of treatment, targeted therapies based on genetic alterations in the tumor are appropriate for selected cases. Identifying mutations in oncogenes associated with non-squamous NSCLC can help determine which patients are more likely to benefit from a targeted therapy. Such oncogenes include EGFR, KRAS, and ALK.

EGFR

Activation of the epidermal growth factor receptor (EGFR) protein stimulates protein tyrosine kinase, which leads to activation of signaling pathways associated with cell growth and survival. Both EGFR overexpression and activating mutations in the tyrosine kinase domain of the EGFR gene lead to tumor growth and progression. Consequently, EGFR has become a target for anti-cancer drug therapy. Erlotinib and gefitinib are examples of EGFR tyrosine kinase inhibitors (TKIs) that can prevent activation of the signaling pathways and improve response rates in selected NSCLC patients.

Activating EGFR mutations, which are associated with increased sensitivity to EGFR TKIs, predominate in never-smokers, females, and tumors with adenocarcinoma histology. However, these characteristics are not as effective as mutation testing for predicting which patients might benefit from targeted TKIs.² The most common mutations associated with sensitivity to EGFR TKIs include exon 19 deletions and the L858R point mutation.³ These mutations are associated with response rates of >70% in patients treated with either erlotinib or gefitinib.^2,4 Other EGFR mutations (eg, T790M and exon 20 insertion) have been associated with much lower response or acquired resistance to TKIs.³ 

KRAS

The KRAS protein stimulates signaling pathways downstream from EGFR. KRAS mutations lead to a constitutively activated KRAS protein that continually stimulates these downstream pathways. Although EGFR TKIs can block EGFR activation, they cannot block the activity of the mutated KRAS protein. Thus, patients with KRAS mutations tend to be resistant to erlotinib and gefitinib.^2,5-7 KRAS mutations are more likely found in adenocarcinomas, in patients who are smokers, and in Caucasian patients rather than East Asians. KRAS mutations are prognostic for poor survival, independent of therapy.^1,6,7

ALK

Rearrangements of the gene encoding anaplastic lymphoma kinase (ALK) have been linked to abnormal cell proliferation and NSCLC (most commonly adenocarcinomas). The most common ALK rearrangement in NSCLC is EML4-ALK, which arises from fusion between the 5′ end of the EML4 gene and the 3′ end of the ALK gene on chromosome 2p23. Patients with ALK rearrangements are younger than most patients with NSCLC.⁸ EML4-ALK rearrangements are also more common in adenocarcinomas of never or light smokers whose tumors lack EGFR and KRAS mutations.

Patients with ALK rearrangements do not benefit from EGFR-specific TKI therapy⁸ but may be considered for therapy targeting the constitutively activated receptor tyrosine kinase that results from EML4-ALK and other ALK fusions. Crizotinib (Xalkori^®, Pfizer) is the first FDA-approved ALK TKI. It is indicated for treatment of locally advanced or metastatic NSCLC in patients whose tumors are positive for ALK as determined using an FDA-approved test.⁹ Patients with ALK-positive advanced NSCLC have exhibited objective response rates of 50% to 61% in single-arm clinical studies.^9,10

Mutation Profiling

Although some clinical and histologic features correlate with particular genetic changes, only molecular testing can definitively identify the mutations associated with targeted therapy response or resistance. Additionally, EGFR, KRAS, and ALK mutations are almost always mutually exclusive (ie, mutations of only 1 of the 3 genes occur within any individual tumor). Therefore, Quest Diagnostics offers a Lung Cancer Mutation Panel that tests for mutations in all 3 oncogenes. Individual tests for mutation detection are also available for each gene.

The Table summarizes information relevant to patient management.

• Patients with NSCLC who are being considered for treatment with an EGFR or ALK TKI

• EGFR and KRAS: polymerase chain reaction (PCR) amplification and dye-terminator sequencing:

–   Covering the kinase domain of the EGFR gene (exons 18�21)

–   Covering codons 12 and 13 (exon 1) and codon 61 (exon 2) of the KRAS gene

–   Analytical sensitivity: 10% to 20% tumor cells in the background of normal cells. Testing is performed

on sample where tumor is enriched.

• ALK: fluorescence in-situ hybridization (FISH) testing with ALK break-apart probes to detect rearrangements¹¹

Activating EGFR mutations, including exon 19 deletions and the L858R point mutation, predict response to erlotinib or gefitinib therapy. T790M and exon 20 insertion mutations predict resistance (Table). KRAS mutations predict non-response to EFGR and ALK inhibitors, and thus alternative therapies should be considered.¹ Detection of ALK rearrangements in patients with advanced NSCLC suggests eligibility for treatment with the ALK inhibitor crizotinib¹⁰ but not with EGFR-directed inhibitors.

During therapy, acquisition of additional, secondary mutations can confer resistance to EGFR or ALK inhibitors. Such mutations include the T790M mutation in EGFR, mutations in KRAS, and ALK mutations that may not be detected in this assay (eg, somatic ALK kinase domain mutations and copy number gains of the fusion gene¹²).