The current use of targeted therapies for a number of cancers has made necessary the genetic testing of those patients to ensure the optimal treatment can be found for their particular disease state. One such targeted therapy is for those non-small cell lung cancer (NSCLC) patients who have epidermal growth factor receptor (EGFR) mutations that serve as a driver for their disease. For these patients, the use of a tyrosine kinase inhibitor (TKI) such as erlotinib or osimertinib is indicated. The use of diagnostics that identify the mutations which serve as oncogenic drivers for an individual's cancer has become an indispensable tool for fighting that disease.
Most recently, the use of tumor DNA obtained from blood or plasma samples has greatly enhanced the effectiveness of these methodologies. In this scheme, tumor DNA can be obtained in a non-invasive manner from patients when blood is being drawn for routine analyses. This should increase the amount of genetic screening, as many patients reject the invasive and quite often painful tissue collection.
Additionally, a more thorough representation of the tumor DNA within the individual can be obtained using this, as the genetic material obtained is shed from tumors all over the body and not taken from a single remote location.
When asked about the ramifications of liquid tumor biopsy genetic diagnostics for NSCLC patients, Ramaswamy Govindan, MD, Co-Director, Section of Medical Oncology, and Professor of Medicine, at Washington University School of Medicine, Alvin J. Siteman Cancer Center noted, “Cell-free DNA analyses are here to stay and have contributed significantly to identify appropriate patients for targeted therapies.”
EGFR Mutations in NSCLC
Each year, more than 200,000 Americans are diagnosed with lung cancer, and of these, roughly 80-85 percent have NSCLC. Approximately 10-20 percent of these NSCLC patients have EGFR mutations which serve as oncogenic drivers. In this patient subpopulation, the use of an EGFR TKI is indicated.
The first EGFR-TKI to receive FDA approval was erlotinib, which was approved in November 2004 for use in NSCLC patients who had undergone disease progression after previous treatments, including at least one round of chemotherapy. In the study which was cited by the FDA in its approval letter, approximately one-third of the patients had their tumor cells analyzed to determine EGFR expression levels. In that study, the overall survival tended to be greater in the 55 percent of the patients who had higher levels of EGFR expression in their tumors. In April 2010, FDA approval was granted to erlotinib as maintenance therapy for those NSCLC patients who had not undergone disease progression after 4 cycles of first-line platinum-based chemotherapy.
In May 2013, the FDA approved the use of erlotinib as a first-line therapy in those metastatic NSCLC patients who had EGFR L858R exon 21 substitution or exon 19 deletion mutations. Approval for this indication was concurrent with that of the EGFR mutation test (v1), a polymerase chain reaction (PCR)-based companion diagnostic which is utilized for patient selection.
EGFR Mutation Test v1
In May 2013, FDA approval was granted to the EGFR mutation test, which evaluated patients' EGFR genes for the presence of the relevant mutations (exon 19 deletion or exon 21 L858R substitution). This protocol utilized nucleic acid material derived from formalin-fixed paraffin-embedded tissue (FFPET) samples taken from patients' tumors using conventional biopsy techniques. In that protocol, genomic DNA, once obtained from the FFPET sample, is then measured spectrophotometrically and adjusted to a standardized concentration. At that point, the genomic material is subjected to PCR amplification and detection.
Emergence of EGFR-TKI Resistance
With the use of erlotinib, the emergence of disease resistance to EGFR-TKI therapy has been noted. The resistance which has been noted has been attributed to one or more of the following mechanisms: aberrant downstream pathways (e.g., PTEN loss, AKT mutations); alternative signaling activation (e.g., AXL, HGF, Met); impaired EGFR-TKI-mediated apoptosis pathway (e.g., BCL2-like 11 (BIM) deletion polymorphism); secondary mutations (e.g., C797S, T790M).
Of these mechanisms, the exon 20 T790M mutation of the EGFR gene is most frequently associated with acquired resistance to first generation TKIs, being present in about 50-60 percent of EGFR-TKI resistant tumors. Drug discovery efforts to address this resistance yielded osimertinib, a third-generation EGFR-TKI which was able to show some success in treating erlotinib-resistant disease having the EGFR exon 20 T790M mutation.
Osimertinib is a third-generation EGFR-TKI that has shown efficacy against cell lines having the most frequent sensitizing (exon 21 L858R) and resistance-conferring (exon 20 T790M) EGFR mutations. This compound showed greater than 200-fold greater selectivity for the L858R/T790M mutants over wild-type EGFR.
As a result of positive results obtained in both preclinical studies and clinical trials, in November 2015, osimertinib received FDA approval for the treatment of NSCLC patients who had the specific T790M EGFR mutation that had undergone disease progression while on previous EGFR-TKI therapy. Concurrent with this announcement was the approval of the companion diagnostic, the EGFR Mutation Test v2, which is used to confirm the presence of the T790M resistance mutation.
EGFR Mutation Test v2
The EGFR Mutation Test v2 is a real-time PCR protocol that was initially approved in November 2015 for the analysis of genetic material obtained from FFPET samples obtained from traditional biopsy samples. In addition to the detection of the earlier sensitizing mutations (exon 19 deletion, L858R substitution), the new test was also approved for the most frequently encountered resistance mutation—T790M. In June 2016, FDA approval was granted for the use of liquid biopsy techniques for this test, marking the first of its kind for use in clinical decisions.
In liquid biopsies, DNA which is shed from tumors all over the body into the blood, are collected via a simple blood drawing, often at the same time as routine blood analyses are performed, thereby minimizing the impact on the patients' time and discomfort. Additionally, it is thought that ultimately, as a result of this simple and relatively painless method of genetic material isolation, the genetic analysis of some patients will be done who would have been unwilling to be subjected to the often painful and invasive traditional biopsy tissue sampling.
Another potential advantage to this methodology could be the production of a more representative genetic picture of the disease within the patient. As has been amply documented, it is quite probable to have tumors with varying genetic makeups present at different locations in a patient. If a traditional biopsy obtains tissue from only one tumor within the patient, then clearly, the physician may be obtaining an incomplete picture of the disease within the individual. With blood-derived DNA isolation, in theory, DNA can be obtained from tumors all over the body.
When asked about the implications for this expanded isolation techniques, Govindan noted, “With improved technology, we will be able to identify complex alterations and perhaps even serially monitor patients after potentially curative therapy in early and locally advanced NSCLC.”
Richard Simoneaux is a contributing writer.