The potential clinical value of liquid biopsy in oncology is huge. Its applications include detecting early stage disease, tracking response to therapy, prompt identification of disease relapse or recurrence, and catching sight of clonal evolution. In her presentation Professor Christina Lockwood (University of Washington, USA) referred to numerous small-scale studies that have demonstrated the feasibility of quantitation of mutations in circulating tumor DNA (ctDNA) from the blood of cancer patients.
Circulating tumor DNA (ctDNA) is the tiny proportion of cell-free DNA found in plasma that is derived from dying tumor cells. It is important to note that ctDNA is distinct from circulating tumor cells (CTC), which specifically refers to intact cells. Depending on the extent of disease ctDNA comprises <1% to over 10% of total cell-free DNA (cfDNA).
For diagnostic purpose ctDNA has been applied for early disease detection (lung, breast, gastric, colorectal cancer) and in case of disease recurrence or relapse (acute lymphoblastic leukemia, breast cancer, osteosarcoma, hepatocellular carcinoma). Furthermore this method has turned out to be of predictive value for the assessment of early treatment response (lung cancer, colorectal cancer, melanoma) and for the detection of clonal evolution (colorectal cancer, breast cancer, lung cancer). Last not least the ctDNA-assay can be deployed as a prognostic instrument for the evaluation of disease burden (CNS tumors like high-grade gliomas, breast cancer, colorectal cancer) and of recurrence risk (breast cancer, colorectal cancer, pancreatic cancer).
But, of course, there are also some obstacles impeding the broad adoption of liquid biopsy and ctDNA into clinical practice. Due to the noisy background milieu of non tumor-derived cell-free DNA it is quite challenging to detect the ctDNA fragments. Several highly sensitive methods have been developed for ctDNA quantitation, ranging from digital methods (non-sequencing) like BEAMing, Digital droplet PCR, Microfluidics digital PCR, and PAP to sequencing-based methods including Safe-SeqS, CAPP-Seq, Tam-Seq, Duplex Sequencing and others.
“As with all laboratory tests, the appropriate method depends on the clinical context” Prof. Lockwood added for consideration. If early detection and minimal residual disease detection of ctDNA is the aim, highly sensitive methods are to be used. If specificity is the issue genomic profiling for all types of alterations present (fusions, point mutations, gene amplification, etc) is best done by massively parallel sequencing. It is also called next-generation sequencing (NGS) and like liquid biopsy another term you couldn’t escape from noticing at the IAP/ESP congress.
With 2 ng the amount of needed input DNA is quite small. But a high variability depending on the tumor type and a high bias in quantification have to been taken into account. “In negative cases, where your oncologist is still suspicious, you shouldn’t rely on just one blood sample”, Lockwood recommended.
The detection of ctDNA still is a very new method. “We are doing it now for eight months”, Lockwood mentioned. Several challenges for its clinical implementation should be considered. The general lack of standardization of multiple aspects of ctDNA quantitation is one, the variability of preanalytic factors such as the appropriate specimen type, handling, and storage conditions another major issue. A third one is the lack of studies prospectively investigating the utility of ctDNA in cancer management.
Being another marker of liquid biopsy, intact circulating tumor cells (CTCs) are also not used routinely yet. According to Professor Paul Hofman (University of Nice) there are, for instance, a lot of clinical trials looking for the CTC number in breast cancer patients receiving chemotherapy. Meanwhile 45 techniques for isolating and characterizing CTC exist worldwide. As few as 5 to 10 CTCs are needed for the detection. “It is not a perfect method, it has its advantages and disadvantages “, Hofman said. While working with cfDNA, for example, is cheaper, the preanalytical conditions due to contamination by DNA of destroyed blood cells, are even more critical.
And there is another advantage of CTC over fragmented markers that is of value for the pathologic examination: The intact cells can be microscoped. In this regard the term “liquid biopsy” rather than “blood test” is really justified, Hofman pointed out.
As you might know, the first liquid biopsy test for use in cancer has been approved by the Food and Drug Administration (FDA) on 1 June 2016. Using plasma specimens, the cobus EGFR Mutation Test v2 detects EGFR exon 19 deletions or L858R mutations. These genetic alterations make patients with advanced non-small cell lung cancer (NSCLC) candidates for a targeted therapy with erlotinib, a specific EGFR-receptor tyrosine kinase inhibitor. In case of a negative test patients should undergo routine biopsy and testing for EGFR mutations with the paraffin-embeded (FFPE) tissue sample type.