Circulating tumor cells

Circulating tumor cells (CTCs) are tumor cells that have shed or detached from the primary tumor and entered the bloodstream. CTCs reflect the molecular characteristics and heterogeneity of the primary and metastatic tumor. Counting and analyzing CTCs provides vital information about the presence and characteristics of cancer without invasive tissue biopsies.

CTCs can be separated from blood using both antibody-dependent and -independent isolation methods. Antibody-dependent methods isolate CTCs using antibodies targeting cell surface proteins that are differentially expressed on epithelial tumor cells compared to normal blood cells. Commonly targeted cell surface proteins include EpCAM and cytokeratins. Antibody-independent methods isolate CTCs through physical properties such as size, density or electric charge.

Detailed molecular analysis of isolated CTCs using techniques such as PCR, immunohistochemistry, fluorescence in situ hybridization and next-generation sequencing can reveal genomic alterations, expression of tumor markers and drug resistance mutations present in the primary tumor. CTC enumeration and characterization can help detect recurrence, monitor response to treatment and help guide selection of targeted therapies.

Circulating tumor DNA

Circulating tumor DNA (ctDNA) represents DNA fragments shed from dying tumor cells into the bloodstream. ctDNA carries tumor-specific molecular alterations that reflect the status of the primary tumor and metastatic lesions. Analyzing ctDNA provides a Liquid Biopsy of the tumor without an invasive tissue biopsy.

Sensitive techniques like digital PCR and next-generation sequencing can detect ctDNA even when circulating at very low levels, sometimes less than 0.01% of total cell-free DNA in blood. Analyzing mutations, methylation status, microsatellite instability and gene expression profiles in ctDNA can identify cancer-specific signatures to detect the presence of cancer, pinpoint the tissue of origin and track clonal evolution during treatment.

Monitoring ctDNA levels during and after treatment provides a real-time, non-invasive approach to detect residual disease, recurrence and resistance mutations earlier than imaging. This allows timely intervention and improved clinical outcomes for patients. The ability to repeatedly sample ctDNA longitudinally from blood overcomes limitations of tissue biopsy in assessing tumor heterogeneity and dynamic changes over time and treatment.

Exosomes

Exosomes are membrane-bound extracellular vesicles released by most cell types including tumor cells. Exosomes contain biomolecules derived from their parental cells including DNA, RNA, proteins and lipids that act as messengers communicating physiological and pathological information to other cells.

Analysis of proteins, mRNA and miRNA in tumor-derived exosomes provides insights into the molecular mechanisms and processes active in tumor progression. Exosomes from different cancer types display unique surface protein signatures that can potentially serve as non-invasive biomarkers for cancer detection and monitoring. Exosomal nucleic acids like DNA and non-coding RNA also carry cancer-specific mutations, methylation changes, and gene expression patterns reflective of the cancer.

Isolation of exosomes from liquid biopsies like blood, urine and other body fluids using techniques such as ultracentrifugation, immunoaffinity capture and microfluidics offers opportunities for non-invasive cancer screening, staging and response monitoring. Studies are ongoing to validate exosomal biomarkers and develop robust assays for clinical utility of analyzing tumor-derived exosomes from liquid biopsies.

Clinical applications

Liquid biopsy using CTCs, ctDNA and exosomes is poised to revolutionize cancer management through diverse clinical applications:

1. Early detection: Sensitive analysis of liquid biopsies could enable screening for cancer recurrence and detect early-stage cancers when curative treatments are most effective.

2. Disease monitoring: Sequential analysis of liquid biopsies provides a real-time view of tumor burden and molecular evolution during and after treatment. This helps optimize therapy management.

3. Recurrence monitoring: Changes in CTC counts, ctDNA levels and exosomal biomarkers indicate recurrence much before radiological confirmation, enabling early intervention.

4. Treatment guidance: Analysis of targets, mutations and resistance mechanisms in liquid autopsy specimens helps guide selection of targeted and immunotherapy agents.

5. Minimal residual disease detection: Close monitoring using liquid biopsies helps detect residual tumor foci left behind after initial treatment, improving outcomes.

6. Companion diagnostics: Liquid biopsies could serve as reliable substitutes for tumor biopsies for molecular profiling to select personalized therapies.

7. Screening high-risk populations: Sequential screening of at-risk individuals like cancer survivors using liquid autopsy blood tests could detect second cancers earlier.

The successful development and clinical validation of liquid autopsy technologies hold promise to revolutionize cancer management by enabling non-invasive, real-time monitoring of disease burden and molecular evolution with treatment. Longitudinal analysis of CTCs, ctDNA and exosomes from blood has the potential to transform cancer screening, early detection, treatment guidance and surveillance globally.

Get more insights on Liquid Biopsy