DENVER—Circulating tumor cells (CTCs) might be thought of as a “liquid biopsy,” because CTCs can be detected from a simple blood draw. That's what Siminder Kaur Atwal, PhD, said here as one of the speakers at AACR's International Conference on Molecular Diagnostics in Cancer Therapeutic Development.
And since their capture does not require surgical intervention, CTCs can be captured for a longitudinal analysis of marker expression at any time—from diagnosis to follow-up after treatment—for real-time information, explained Dr. Atwal, senior research associate at Genentech, Inc.
In a session titled “Minimally Invasive Access to Tumors,” Dr. Atwal said there is significant interest in CTCs as an enabling technology for biomarker evaluation, and that several Genentech clinical programs already incorporate CTC molecular characterization for exploratory analysis. She presented data from a study in which she and colleagues compared the CTC capture efficiency of the Food and Drug Administration-approved CellSearch assay with two biochip platforms.
The aim was to detect epidermal growth factor receptor (EGFR) protein expression in CTCs from patients with lung cancer, and HER2 expression or amplification in CTCs in patients with metastatic breast cancer.
Under the tested conditions, she reported, CellSearch and the newer biochip platforms offered similar efficiency.
The captured CTCs were amenable to biomarker analyses including HER2 status, qRT-PCR for breast cancer subtype markers, KRAS mutation detection, and EGFR staining by immunofluorescence. There was a complication in that capture efficiency is apparently dependent on EpCAM (epithelial cell adhesion molecule) expression, which limits CTC capture from certain tumor types, especially triple-negative breast cancers, Dr. Atwal said.
But using antibodies to mesenchymal markers in addition to EpCAM could overcome this and enable routine biomarker analysis from CTCs, she added.
Biomarkers Change as Disease Progresses
There was an 89% concordance in this study in HER2 status between the circulating cells and cells from the patient's tumor tissue, determined by immunofluorescence and fluorescence in situ hybridization, she said. But in the other 11% of patients, the HER2 status of CTCs was different from that of the primary tumor at diagnosis.
“In some cases, CTCs' real-time view of a patient's biomarker status is different from diagnostic tissue [from the primary tumor].”
In a news conference highlighting this and other noteworthy studies at the meeting, Dr. Atwal explained that HER2 can change through the progression of disease. It's possible, therefore, that HER2 status can change later in the disease process, so a patient could have an HER2-negative tumor with an HER2 positive CTC.
There could also be an error in testing from archival tissue, or an error in HER2 scoring because too few CTCs were available for analysis.
Also, HER2 scoring methods might be differ from one institution to the next.
Dr. Atwal admitted that this discordance in HER2 status can confound the clinician's decision about whether to prescribe trastuzumab to such a patient, if the patient's cancer cells are read as both HER2 positive and HER2 negative.
That conundrum has not been answered yet, she said, but one clinical study did show that trastuzumab had efficacy in patients with HER2-negative status.
Another Surprising Finding
Another surprising finding Dr. Atwal reported was that CTC counts were higher in ER+ patients compared with HER2- and patients with triple-negative breast cancer, despite their more aggressive phenotype.
Dr. Atwal said this happens because the basal-like molecular breast cancer subtype has low EpCAM expression and a more mesenchymal phenotype, so CTCs are not as efficiently captured using EpCAM alone in tumors of this subtype.
“Additional cocktails with mesenchymal markers in addition to EpCAM would further improve CTC capture to enable routine biomarker analysis,” she said.
Finding CTCs in a Haystack
The Chairperson of the session, Peter Kuhn, PhD, Professor of Cell Biology at Scripps Research Institute, called circulating tumor cells “the messenger and the message of metastasis,” saying, “If we can understand CTCs, we can understand metastasis.”
He characterized CTCs as the cells that escape from the primary tumor and settle down at a secondary site to cause metastasis—“And if we can interfere with metastasis early on, we have a better chance of successfully treating patients.”
Finding CTCs, though, is the first challenge. From a more humorous point of view, Dr. Kuhn compared finding a CTC to finding the Waldo of Where's Waldo fame. Waldo wears a red cap and a shirt with horizontal red and white stripes. But on a page overflowing with hundreds of cartoon people, there are characters wearing shirts with vertical red and white stripes, some with red caps, and street signs with red and white stripes, etc.
Dr. Kuhn said he believes CTC research will have the fastest impact on molecular diagnostics in the metastatic setting.
“Part of why I am so excited about our ability to store a large fraction of that [CTC] sample is that as new molecular diagnostic tools become available, I can test it on the fresh sample,” he said. “I can also go back to storage and look at that same patient, at that same patient's history and see how that has evolved.”