Optimizing EGFRT790M diagnostic testing in NSCLC: Experience from a local hospital
Third-generation EGFR tyrosine kinase inhibitors (TKIs) targeting the EGFRT790M mutation, such as osimertinib, have brought significant improvements to the management of EGFR-positive non-small-cell lung cancer (NSCLC). However, optimization of EGFRT790M mutation testing remains challenging in clinics. At a symposium organized by the Hong Kong Cancer Therapy Society, Dr Michael Tsz-Yeung Kam, Specialist in Clinical Oncology in Hong Kong, discussed real-world challenges in EGFRT790M testing and the current workflow at his centre.
EGFRT790M testing source, platform
EGFRT790M mutation is the most common acquired resistance mechanism and is detected in about 50 percent of EGFR-positive NSCLC patients who progressed on first- or second-generation EGFR TKIs. [ESMO Open 2016;1:e000060] Detection the T790M mutation is important because osimertinib, a third-generation EGFR TKI, has been showing promising efficacy in patients with T790M mutation (median progression-free survival [PFS] of 10.1 months vs 4.4 months with pemetrexed plus platinum in AURA3 trial). [N Engl J Med 2017;376;629-640]
“Currently, there are no clinical-pathological features that predict the development of T790M mutation. Therefore, it is important to improve the testing algorithm to maximize the chance of detecting T790M mutation,” Kam noted.
“Testing of plasma circulating tumour DNA [ctDNA], termed liquid biopsy, has been established as an alternative to conventional tissue biopsy for T790M detection. While plasma is the most validated medium, pleural fluid and cerebrospinal fluid have emerged as promising investigational liquid biopsy specimens for EGFR genotyping,” he continued. [BMC Cancer 2018;18:1236] “Compared with tissue biopsy, liquid biopsy is less invasive and has shorter turnaround time. Serial or repeated testing is feasible. Also, around 50 percent of patients may not have accessible sites for biopsy. However, it has lower sensitivity than tissue biopsy and is unable to determine the histology of the tumour.”
Results from the AURA1 study suggested that either tissue or liquid T790M positivity predicted response to osimertinib, while patient with negative liquid biopsy should consider a tissue biopsy, as false negativity from liquid biopsy may preclude patients from receiving osimertinib. [J Clin Oncol 2016;34:3375-3382]
“The real-time polymerase chain reaction [PCR] platform of the cobas® system and various digital PCR systems are commonly used to detect EGFRT790M mutation. Generally, real-time PCR is less sensitive for T790M detection, but it has the merit of detecting more EGFR mutations simultaneously” said Kam.
“In a real-world setting, clinicians are faced with various challenges regarding EGFRT790M mutation testing,” he pointed out.
Timing of testing
“Whether EGFRT790M mutation testing should be performed before or after radiological disease progression [PD] is yet to be answered,” noted Kam.
“Studies showed that T790M mutation could occur a few months prior to PD. However, a higher false-negative rate is also expected with earlier testing,” he said. [Cancer 2014;120:3896-3901; Sci Rep 2016;6:20913] “Meanwhile, not all radiological PD necessitates a change of systemic treatment. At our centre, EGFR-positive NSCLC patients with oligoprogression are usually treated with local ablative therapy plus current systemic therapy, while systemic treatment is continued beyond minimal systemic progression.”
“The timing and frequency of testing should be individualized based on factors such as availability of tests and tissue biopsy sites, follow-up interval, and whether a change of systemic treatment is warranted,” Kam suggested. “The ongoing APPLE trial may elucidate the optimal sequence and best timing of switching of treatment with gefitinib and osimertinib in EGFR-positive NSCLC patients.” [Clin Lung Cancer 2017;18:583-588]
Tackling negative or discordant results
“A T790M-negative result from liquid biopsy should be interpreted with caution. Clinicians should take EGFR TKI-sensitizing mutation status, diagnostic platform sensitivity and disease burden into consideration before concluding genuine T790M negativity,” said Kam.
“EGFR TKI-sensitizing mutations can act as internal control for T790M mutation testing. Results that lack both sensitizing mutations and T790M mutation are suggestive of insufficient ctDNA for analysis and are therefore labelled as ‘T790M-uninformative’,” he explained. “While a sensitizing mutation-positive and T790M-negative result can indicate inadequate T790M-positive subclones, it is more likely to be a true T790M-negative result.”
“The clinical status of the disease affects DNA shedding, which in turn influences plasma biopsy results. According to an exploratory analysis of the AURA3 study, a higher plasma T790M positivity was observed in patients with extrathoracic disease and larger tumour size,” Kam noted. (Table) [Cancer 2020;126:373-380]
“Discordance between plasma and tumour biopsy results can be explained by DNA shedding or intratumoural heterogeneity, which may have predictive significance. In the AURA1 trial, patients with plasma biopsy-negative and tissue biopsy-positive results had a longer median PFS than those with dual-positive results, probably due to a lower disease burden,” he said. “Meanwhile, plasma biopsy-positive and tissue biopsy-negative results might indicate intratumoural heterogeneity, and hence a shorter median PFS.” [J Clin Oncol 2016;34:3375-3382]
“At our centre, the cobas® system is used as initial diagnosis platform for patients who refuse or fail tissue biopsy. Digital PCR platforms, which have a higher sensitivity than cobas®, are reserved for those who experience PD on first- or second-generation EGFR TKI necessitating a switch of systemic treatment,” shared Kam. [Oncotarget 2017;8:100801-100818]
“Logical interpretation of negative results is important to guide subsequent steps. Tissue biopsy may be considered when liquid biopsy is negative. If tissue biopsy is not feasible, clinicians should consider repeating liquid biopsy at next PD,” he said. (Figure)
“Notably, rare mutations can coexist with the EGFRT790M mutation. Since there is no effective treatment indicated for these rare mutations, testing for T790M mutation should be continued when appropriate, even if another resistance mechanism has been identified,” said Professor Tony Mok of the Department of Clinical Oncology, Chinese University of Hong Kong.