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Due to the high rate of EGFR T790M mutations seen in NSCLC patients, the thoracic tumor board recommends genomic testing for this alteration; read our case of a 69-year-old woman with metastatic disease.
The patient is a 69-year-old woman with metastatic lung adenocarcinoma who is progressing despite treatment with erlotinib. She was diagnosed in September 2016 with widespread bilateral pulmonary nodules, sclerotic bony lesions, and brain metastases. An initial CT-guided biopsy of the left upper lobe mass showed primary lung adenocarcinoma, which was confirmed through standard polymerase chain reaction testing to have an EGFR exon 19 deletion mutation.
The patient was treated with palliative whole-brain radiation therapy and then started on first-line therapy with erlotinib on 11/16/16 as part of the control arm of the BMS-370 study (NCT02574078). Her disease remained stable for 14 months, with mild side effects, including nausea, vomiting, diarrhea, weight loss, and lower extremity edema. She eventually developed slow progression of several pulmonary lesions and had recurrence of symptomatic pleuritic chest pain, concerning for disease progression. Due to the high rate of EGFR T790M mutations seen in this population, the thoracic tumor board recommended genomic testing for this alteration. Tissue biopsy was deemed challenging due to the small size of the diffuse pulmonary nodules and the high risk for pneumothorax, so circulating cell–free DNA (cfDNA) testing was recommended.
A T790M mutation was suspected, as these occur in approximately 60% of patients treated with first-generation EGFR tyrosine kinase inhibitors (TKIs). While TKI resistance can develop through a number of different mechanisms-including transformation to small-cell lung cancer, MET amplification, and downstream PIK3CA mutations-T790M mutations are the most common mechanism of resistance. Patients with non−small-cell lung cancer (NSCLC) with activating EGFR mutations typically respond well to initial TKI therapy for the first 1 to 2 years of therapy.
After this period, resistance frequently develops, with identifiable disease progression. Patients may re-respond to TKI therapy after a hiatus from targeted therapy; however, most will eventually need to change to other systemic therapies.
Repeat biopsy at the time of progression detects acquired T790M resistance mutations in exon 20 in approximately half of all cases. However, tissue biopsies at the time of progression are often difficult to obtain in patients with advanced disease. This was the case with this patient: tissue biopsy was considered challenging due to poor lung function and technical difficulties with getting adequate tissue. This restricted our ability to identify the presence of the T790M mutation via tissue biopsy and direct DNA sequencing. However, recent studies show that T790M may be identifiable by cfDNA. In cases where an initial activating EGFR mutation was present, T790M mutations were identified by cfDNA in approximately 30% to 40% of cases.[2,5] Still, discordance rates of 20% to 30% when comparing cfDNA testing with tissue-based testing have been reported. The most recent American Society of Clinical Oncology guidelines for molecular testing concerning cfDNA testing state that cfDNA may be used to identify EGFR mutations in lung carcinoma cases with progression on TKIs; however, if plasma results are negative, then tissue sample testing should be pursued.
After reviewing the case at thoracic tumor board, the recommendation was to obtain cfDNA testing for T790M mutation analysis. This was performed and did not reveal the presence of an EGFR T790M mutation. Due to high suspicion for the presence of this mutation, the molecular tumor board also reviewed the case and strongly recommended tissue biopsy. Repeat biopsy was challenging due to progressive disease and the military pattern of the diffuse pulmonary nodules. However, successful CT-guided biopsy obtained a small tissue sample. Due to the limited tumor specimen available, restriction fragment length polymorphism (RFLP) testing was recommended over other assays to test for T790M.
RFLPs are molecular marker DNA sequences specific for certain restriction endonucleases. Differences in homologous DNA samples can be identified by the presence of DNA fragments of differing lengths following digestion. In the case of the T790M mutation in EGFR exon 20, restriction endonucleases are specific for the CATG sequence of the mutant T790M allele with a C to T base substitution at the third position, which is not present in the wild-type allele. These fragment products are then assessed via RFLP probes that are labeled DNA sequences that hybridize with the DNA fragments following separation via gel electrophoresis, which then correlates with identification of the mutant sequence. RFLP is beneficial in that it can generally be performed reliably in situations where direct DNA sequencing may have a low detection rate, as in when the tumor sample may be small and contaminated with normal tissue or fibrosis, or when < 30% of DNA in the sample is mutated.
Osimertinib is a third-generation TKI that is highly selective for EGFR T790M mutations vs wild-type EGFR. It binds covalently to the cysteine-797 residue of the EGFR binding site and has demonstrated clinical activity in EGFR T790M–mutated NSCLC. The drug received US Food and Drug Administration approval in November 2015 for patients with T790M-positive NSCLC who had progressed on other EGFR TKI therapy. National Comprehensive Cancer Network (NCCN) guidelines give a category 1 recommendation to osimertinib for patients with prior EGFR TKI therapy and a known T790M mutation with brain or other symptomatic disease progression. This was largely based on the results of the phase II AURA2 trial, which demonstrated a disease control rate of 92%, with median progression-free survival of 8.6 months, in patients who progressed on previous EGFR TKI therapy. The NCCN guidelines were recently updated to include osimertinib (category 1 recommendation) in previously untreated patients with metastatic NSCLC harboring any sensitizing EGFR mutation based on the FLAURA trial results. Despite activity in T790M–mutated disease, recent evidence suggests resistance to third-generation TKIs can develop after 6 to 17 months via C797S mutation, causing loss of TKI covalent binding site. Still, osimertinib remains the standard of care for NSCLC harboring a T790M mutation.
Once a T790M mutation was confirmed via RFLP testing, osimertinib was started on 05/13/18. Since that time, the patient is doing well and tolerating therapy, without any significant adverse effects. Repeat CT scans following completion of 4 months of therapy show that she is currently responding to therapy, with an overall reduction in tumor burden, consistent with stable disease.
Financial Disclosure:Ms. Wynja, Ms. Hove, and Dr. Powell have no significant financial interest in or other relationship with the manufacturers or providers mentioned in this article.
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