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Liquid biopsy in metastatic breast cancer management: Where does it stand in clinical practice?


Identifying the molecular features of metastatic breast cancer (MBC) offers a real-time window into a patient’s treatment options as well as the potential to follow the disease as it evolves over time.

Tissue biopsy remains the gold standard for characterizing tumor biology and guiding therapeutic decisions, but liquid biopsies — blood analyses that allow oncologists to detect circulating tumor cells (CTCs) and circulating tumor DNA (ctDNA) in the blood — are increasingly demonstrating their value. Last year, the U.S. Food and Drug Administration (FDA) approved two liquid biopsy tests, Guardant360 CDx and FoundationOne Liquid CDx, that can identify more than 300 cancer-related genes in the blood. In 2019, the FDA also approved the first companion diagnostic test, therascreen, to pinpoint PIK3CA gene mutations in patients’ ctDNA and determine whether patients should receive the PI3K inhibitor alpelisib along with fulvestrant.

Here’s an overview of how liquid biopsy is being used in monitoring MBC progression and treatment — and what some oncologists think of it.

What we do and don’t know

“Identifying a patient’s targetable mutations, most notably PIK3CA mutations, is currently the main use of liquid biopsy,” said Pedram Razavi, MD, PhD, a medical oncologist who leads the liquid biopsy program for breast cancer at Memorial Sloan Kettering (MSK) Cancer Center in New York City. “Patients who come to MSK are offered a tumor and liquid biopsy at the time of metastatic diagnosis as part of the standard of care.”

Liquid and tissue biopsy analyses can provide a more complete picture of a patient’s condition. Whereas tissue biopsy allows oncologists to target a more saturated sample of the cancer ecosystem and a wider array of biomarkers, liquid biopsy offers important advantages as well, including a less invasive way to sequence a sample, monitor patients’ treatment response, or track tumor evolution. Liquid biopsy also provides a bigger picture view of tumor heterogeneity by pooling information from many tumor locations as opposed to one.

But, cautioned Yuan Yuan, MD, PhD, liquid biopsy technology is not always sensitive enough to detect CTCs, ctDNA, or all relevant mutations. “When you collect a small tube of blood, you’re essentially trying to catch a small fish in a big sea and wading through a lot of background noise,” said Dr. Yuan, medical oncologist at City of Hope, a comprehensive cancer center in Los Angeles County. “The results may be hard to interpret or come back inconclusive.”

And although emerging data suggest that liquid biopsy provides important insights about tumor dynamics — including mapping disease progression, predicting survival, and even detecting signs of cancer recurrence before metastasis develops — the tool has limited utility in clinical practice outside of identifying sensitivity to various therapies or drugs.

“Right now, a lot of research is being done to understand how to use CTC and ctDNA in particular as a means of surveillance in breast cancer, but we’re still in the beginning stages of applying that outside of clinical trials,” said Joseph A. Sparano, MD, deputy director of the Tisch Cancer Institute and chief of the division of hematology and medical oncology, Icahn School of Medicine at Mount Sinai, New York City.


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