Circulating tumor cell
[1] CTCs are carried around the body to other organs where they may leave the circulation and become the seeds for the subsequent growth of secondary tumors.
[6][7] For example, analysis of blood samples from cancer patients has found a propensity for increased CTC detection as the disease progresses.
The ability to monitor the disease progression over time could facilitate appropriate modification to a patient's therapy, potentially improving their prognosis and quality of life.
To this end, technologies with the requisite sensitivity and reproducibility to detect CTCs in patients with metastatic disease have recently been developed.
These clusters can consist of traditional, small, or cytokeratin-negative CTCs and carry cancer-specific biomarkers that distinguish them from other cells in circulation.
For example, research has demonstrated that patients with prostate cancer who have only single CTCs exhibit an eight-fold longer mean survival rate compared to those with CTC clusters.
[21] The cancer exodus hypothesis suggests that CTC clusters remain intact throughout the metastatic process, rather than dissociating into single cells, which was previously assumed.
According to this hypothesis, the clusters enter the bloodstream, travel as a cohesive unit, and exit circulation at distant metastatic sites without breaking apart.
The hypothesis posits that the survival advantage provided by intercellular support within clusters increases their metastatic potential compared to single CTCs.
[22][24] CTC clusters exhibit distinct gene expression profiles, which confer resistance to certain cancer therapies, making them more resilient than individual tumor cells.
Detecting and analyzing CTC clusters provides critical prognostic information and could help guide therapeutic decisions for cancer patients.
[28] Circulating tumor cells are found in frequencies on the order of 1-10 CTC per mL of whole blood in patients with metastatic disease.
This low frequency, associated to difficulty of identifying cancerous cells, means that a key component of understanding CTCs biological properties require technologies and approaches capable of isolating 1 CTC per mL of blood, either by enrichment, or better yet with enrichment-free assays that identify all CTC subtypes in sufficiently high definition to satisfy diagnostic pathology image-quantity requirements in patients with a variety of cancer types.
Moreover, since EpCAM and other proteins (e.g. cytokeratins) are not expressed in some tumors and can be down regulated during the epithelial to mesenchymal transition (EMT), new enrichment strategies are required.
[40] The only U.S. Food and Drug Administration (FDA) cleared methodology for enumeration of CTC in whole blood is the CellSearch system.
[41] Extensive clinical testing done using this method shows that presence of CTCs is a strong prognostic factor for overall survival in patients with metastatic breast, colorectal or prostate cancer.
[8][42][43][44][45][46][47] CTCs are pivotal to understanding the biology of metastasis and promise potential as a biomarker to noninvasively evaluate tumor progression and response to treatment.
Biological methods isolate cells based on highly specific antigen binding, most commonly by monoclonal antibodies for positive selection.
[59][60] An antibody coated metal wire is inserted into a peripheral vein and stays there for a defined period (30 min).
This method is based on the use of iron nanoparticles coated with a polymer layer carrying biotin analogues and conjugated with antibodies against EpCAM for the capture of CTCs.
If the total number of tumor cells found to meet the criteria cited above is 5 or more, a blood sample is positive.
[64] This automated method uses size filtration to enrich larger and less compressible circulating tumor cells from other blood components.
In May 2022, the Parsortix PC1 system was cleared by the FDA as a medical device for the capture and harvest of circulating tumor cells (CTCs) from metastatic breast cancer patient blood for subsequent analysis.
In addition to the IVD application, the PC1 may be used with the MBC-01 Metastatic Breast Cancer Kit for use in research studies or Lab Developed Tests (LDTs) that have been created and validated in a clinical laboratory.
[65] Epic's molecular assays measure protein expression and also interrogate genomic abnormalities in CTCs for more than 20 different cancer types.
[77] The principle of negative selection is based on the retrieval of all blood cells by using a panel of antibodies as well as traditional gradient centrifugation with Ficoll.
[81] Similarly, researchers at Massachusetts General Hospital have developed a negative selection method which employs inertial focusing on a microfluidic device.
Once the primary tumor is removed, biopsy of the current state of the cancer through traditional tissue typing is not possible anymore.
Modern cancer research has demonstrated that CTCs derive from clones in the primary tumor, validating Ashworth's remarks.
[94] The significant efforts put into understanding the CTCs biological properties have demonstrated the critical role circulating tumor cells play in the metastatic spread of carcinoma.
