Circulating Codes: A New Era of Cancer Detection with Liquid Biopsy

For over a century, the tissue biopsy has been the gold standard for cancer diagnosis due to its high level of laboratory standardization, consistency, and accuracy. However, it is an invasive procedure, can be difficult to perform on inaccessible tumors, and is impractical for the continuous monitoring of disease progression. The emergence of liquid biopsy presents a promising alternative, providing new avenues for early diagnosis and prognosis. This minimally invasive approach analyzes biomarkers from bodily fluids, offering a real-time, comprehensive view of a patient's tumor landscape.

A Brief History of Liquid Biopsy:

The journey of liquid biopsy began long before its modern recognition, evolving through four distinct phases

• Scientific Exploration (Before the 1990s): The first glimpse of circulating tumor cells (CTCs) was in 1869 when physician Thomas Ashworth found "cells similar to tumor cells" in the blood of a deceased cancer patient. In 1948, the existence of cell-free nucleic acids in plasma was discovered. This was followed by a 1977 study showing that plasma-free DNA levels were significantly elevated in cancer patients compared to healthy individuals, leading to the hypothesis that free DNA is linked to tumors.

• Scientific Development (1990s): This period saw major technological advancements. In 1994, polymerase chain reaction (PCR) was used to identify the first KRAS mutation in blood cfDNA from a pancreatic cancer patient, with results matching those from tumor tissue. This proved the potential of using blood for genetic analysis of tumors.

• Industrial Growth (2000-2010): Liquid biopsy indicators began to be applied in clinical settings. A pivotal moment occurred in 2005 when CTC count was identified as an independent predictor of overall survival and progression-free survival in patients with metastatic breast cancer.

• Industrial Outbreak (2010-present): This era marked a rapid integration of liquid biopsy into clinical practice. The European Medicines Agency (EMA) approved the use of ctDNA to identify EGFR mutations in 2014, signaling the official clinical use of ctDNA. Today, guidelines from major oncology societies, such as the Chinese Society of Clinical Oncology (CSCO), now include CTC and ctDNA testing for prognostic assessment in various cancers.

The Molecular Markers of Liquid Biopsy: A Detailed View

Liquid biopsy is a multi-faceted approach that leverages several key biomarkers to provide a comprehensive understanding of cancer biology.

Circulating Tumor Cells (CTCs)

CTCs are cells shed from primary or metastatic tumors into the peripheral circulation. Although they are extremely rare, their presence is a powerful indicator of cancer development, particularly in the metastatic process. The CellSearch system is the only FDA-approved method for monitoring CTC counts in blood samples, relying on immunomagnetic enrichment. Studies have shown that higher CTC counts are associated with reduced progression-free survival and overall survival in patients with breast cancer.

Circulating Tumor DNA (ctDNA)

ctDNA is tumor-derived DNA fragments found in various body fluids like blood, urine, and cerebrospinal fluid. It typically constitutes only 0.1–1.0% of the total cell-free DNA (cfDNA). Compared to cfDNA from normal cells, ctDNA fragments are shorter, and its short half-life makes them ideal for real-time monitoring of tumor dynamics. Detection methods include droplet digital PCR (ddPCR) for known mutations and next-generation sequencing (NGS) for large amounts of genetic information.

Exosomes

Exosomes are small extracellular vesicles (40–150 nm) that act as intercellular messengers, transporting proteins, RNA, DNA, and other biomolecules. They are highly stable due to their protective phospholipid bilayer and are more representative of tumor cell information than other markers. Exosomal nucleic acids, such as non-coding RNAs (ncRNAs), have shown significant potential as biomarkers. For example, the upregulation of exosomal miR-1246, miR-4644, miR-3976, and miR-4306 can serve as highly sensitive biomarkers for prostate cancer.

Clinical Examples of Patient Benefit:

Liquid biopsy has been particularly useful for personalizing treatment strategies in various cancers. A compelling example is its use in guiding treatment for metastatic colorectal cancer (CRC).

• Clinical Scenario: A patient with metastatic CRC is undergoing chemotherapy. Traditional monitoring methods, such as imaging, might only show disease progression after a significant delay.

• Liquid Biopsy Application: Researchers used circulating tumor DNA (ctDNA) analysis to monitor the patient's tumor dynamics in real-time. They tracked the frequency of specific gene mutations, such as TP53, APC, KRAS, and PIK3CA, in the ctDNA before and after treatment.

• Patient Benefit: It was discovered that the rate of ctDNA mutations changed over the course of treatment, and this change was positively correlated with both tumor load and the effectiveness of the therapy. This dynamic monitoring allowed clinicians to see how the tumor was responding to the treatment in near real-time, helping them make timely decisions about continuing or changing the chemotherapy regimen based on the molecular response, well before changes would be visible on a CT scan. This ability to track tumor evolution and therapeutic response dynamically directly contributes to better patient outcomes by allowing for more responsive and personalized care.

The Future Horizon: Complementary Role and Technological Integration

While liquid biopsy has made remarkable progress, it cannot completely replace tissue biopsy. Its current limitations include a lack of standardized procedures, which weakens the consistency of results from different laboratories, and the need to increase accuracy. The most effective approach is to use both liquid biopsy and tissue biopsy in a complementary manner to achieve a more comprehensive understanding of a tumor's biology.  The future of liquid biopsy lies in the development of novel detection technologies and analytical platforms, potentially integrating with rapidly advancing artificial intelligence to further enhance accuracy and streamline clinical application. The research into liquid biopsy, especially with the power of next-generation sequencing, is transforming cancer care. This approach promises a future where cancer diagnosis is not only less invasive but also more dynamic, providing real-time insights into a patient's disease. The ability to monitor tumor progression and treatment response through a simple blood draw is moving us closer to truly personalized medicine. 

“Liquid biopsy, enhanced by the power of next-generation sequencing, is poised to revolutionize cancer care by offering a less invasive, more dynamic, and deeply personalized approach to diagnosis and treatment.”

References:

Ma, L., Guo, H., Zhao, Y. et al. Liquid biopsy in cancer: current status, challenges and future prospects. Sig Transduct Target Ther 9, 336 (2024). https://doi.org/10.1038/s41392-024-02021-w 

David Crosby et al., Early detection of cancer. Science375, eaay9040 (2022). DOI:10.1126/science.aay9040