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Circulating DNA an Appropriate Alternative for EGFR Testing When Tissue Unavailable, Study Finds

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NEW YORK (GenomeWeb) — Researchers from several European institutions working with AstraZeneca have published a study demonstrating that measuring EGFR mutations from circulating tumor DNA compares favorably to tissue-based EGFR mutation status.

The results, published in the September issue of theJournal of Thoracic Oncology, suggest that in cases where tissue biopsies are not possible, clinicians should potentially consider using blood-based EGFR testing to guide care.

Though interest is accelerating both clinically and commercially in using circulating tumor DNA for cancer diagnosis or disease and treatment monitoring, efforts to garner regulatory approval of such tests for clinical use are still nascent.

AstraZeneca announced earlier this year that it would work with Qiagen to advance a circulating tumor DNA-based companion diagnostic for its marketed non-small cell lung cancer drug Iressa (gefitinib), intending to seek a CE mark for the test in the European market. Iressa is indicated as a first-line treatment for EGFR mutation-positive advanced NSCLC patients in Europe, but is not approved in the US.

The JTO study, funded by AstraZeneca, compared ctDNA-based EGFR testing and tissue-based EGFR testing from the prior, Phase IV Iressa Follow Up Measure study, showing that circulating EGFR status compares favorably to tissue with over 65 percent sensitivity and almost 100 percent specificity.

As part of the study, both tumor tissue and plasma ctDNA samples were tested by laboratory Corporation of America, which was otherwise not involved in thei study, using the Qiagen QIAamp Ciruclating Nucleic Acid Kit and the Qiagen Therascreen EGFR RGQ PCR kit.

Jean-Yves Douillard, the study's first author, and a professor of oncology at the University of Nantes Medical School in France, told PGx Reporter in an email this week that while no ctDNA-based companion tests are currently approved to guide EGFR inhibitor therapy, there is a significant need for such tools because tissue biopsy availability is such an issue in lung cancer.

Data from published studies have suggested previously that tissue samples aren't available or don't yield test results for between 10 percent and 30 percent of lung cancer patients.

"In the study we published [we] showed that ctDNA could be an alternative to tissue material," Douillard wrote. "In addition, [other research has suggested that] repeated blood sampling along treatment can allow patient monitoring — for the patients in whom the mutation becomes undetectable in ctDNA, the outcome is better than if the mutation remains detectable."

Also, "it has already been reported that resistance mutations like the T790M may appear during treatment and can be detected in ctDNA well ahead of clinical progression," he added.

In the study, Douillard and his colleagues reported that tumor tissue was available for 1,033 patients and two plasma samples were available for 803 of the total 1,060 participants in the earlier trial. Among these samples, the group was able to analyze 652 matched tumor and plasma samples, and 224 cases in which a first and second plasma sample could be compared to one another.

Concordance between the 652 tumor and plasma samples in the study was 94.3 percent. The sensitivity of plasma in relation to tumor testing was 65.7 percent, and the specificity was 99.8 percent. The group also found that 12 patients of unknown tumor tissue mutation status were EGFR mutation-positive according to their circulating tumor DNA.

The concordance Douillard and his colleagues demonstrated was higher than that seen in other studies, such as a comparison of circulating DNA and tumor tissue EGFR status in the Iressa Pan Asian Study, or IPASS.

Comparing a first and second plasma sample in 224 of the study subjects, concordance between the two was 96.9 percent.

Looking at patients' response rate in the study, the researchers also calculated that overall response rate among those with EGFR mutation-positive ctDNA, regardless of the type of mutation, was similar — 69.8 percent — to the 72.7 percent response rate of those with EGFR mutation-positive tumor tissue.

Though the authors stressed that tumor issue should remain the sample of choice when available, they wrote that the concordance and sensitivity demonstrated in the study are high enough for clinicians to consider using ctDNA testing to help establish the EGFR status of patients in the future and guide treatment selection in the absence of tumor tissue samples.

"A lack of formal direction [from professional guidelines] has necessitated clinicians to either limit the use of this sample type to the research setting or form their own clinical perspectives on the use of surrogate samples," the authors wrote.

Douillard said in his email that other lung cancer trials are also collecting plasma samples in parallel to tissue to compare analysis of EGFR status. Results of these trials, coupled with his team's results, will may help inform the lung cancer community about the utility of ctDNA as a surrogate where tumor tissue is not available.

Qiagen told PGx Reporter earlier this year that its liquid biopsy ctDNA-based companion test to Iressa is slated to become available on the market in Europe by next year.

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