In cancer patients, “metastasis” occurs as tumour cells break-away from the site at which they are formed (a primary tumour), and spread to form new secondary tumours. This process has been referred to as the “seed and soil” phenomenon, as cancerous “seeds” invade the “soil” of healthy organs – and it is important. Estimates suggest that metastasis contributes to around 90% of all cancer deaths.
In order to reach new soil, metastatic cancer cells must first enter the systemic circulation (carrying blood) or the lymphatic system (a specialised highway for immune cells), and travel throughout the body to reach their destination. A wealth of ongoing research seeks to understand how cancer cells navigate these processes. But could research which focusses on capturing and characterising these “circulating tumour cells” (CTCs) revolutionise the diagnosis and treatment of cancer?
What are liquid biopsies?
The term “liquid biopsy” refers to analysing bodily fluids, such as blood, urine or saliva, to assess the health of a patient or to measure disease progression.
In the field of oncology, liquid biopsies often rely on the identification of circulating DNA molecules (called “cell free DNA” or “cfDNA”), DNA-protein complexes and “exosomes” (small structures secreted from a cell) in the blood of cancer patients. Assessing these “markers” in the blood of cancer patients is a highly attractive alternative to invasive tissue biopsies (that are traditionally used for to diagnose cancer and monitor its progression), because blood sampling is less costly, quicker, and less risky to the patient. In addition, because it is possible to take multiple blood samples from the same patient over time, the use of liquid biopsies could also allow for real-time monitoring of cancer progression (see our blog Minimally invasive, maximum potential: liquid biopsy is redefining diagnosis).
However, these liquid biopsies for cancer suffer the same fundamental weakness – cfDNA, DNA-protein complexes and exosomes are found in the blood of healthy people too. This means that liquid biopsies for cancer rely upon the use of complex molecular fingerprinting techniques (such as nucleotide sequencing or proteomics), which help to identify cancer-specific patterns in patients when compared to healthy individuals.
In this respect, CTCs are different. CTCs are not found in the blood of healthy individuals. This means that simply identifying and counting the number of CTCs in a patient (without the use of complex fingerprinting) is sufficient to diagnose and monitor cancer progression.
For example, Rugo and colleagues (2021) counted the number of CTCs present in the blood of 469 breast cancer patients, and found that patients with a high number of CTCs suffered with a more severe disease (a higher number of secondary tumours, requiring more aggressive treatment) when compared to patients with fewer CTCs. Importantly, the team also found that the number of CTCs decreased in 162 patients treated with chemotherapeutic drugs, and that this was statistically associated with improved survival of those patients. This evidence suggests that simply counting the number of CTCs in the blood of a cancer patient is a great predictive tool for assessing cancer progression and responsiveness to treatment.
The usefulness of CTCs also extends to predicting the survival of cancer patients. It is notoriously difficult for clinicians working in a palliative care setting to predict how long a terminally ill patient has left to live. However, Krell and colleagues (2014) used CTCs to assess the lifespan of cancer patients admitted to hospital with low life expectancy. The team found that patients exhibiting more than 100 CTCs in 7.5 mL of blood had an average survival of only 17 days, compared to an average survival of 182 days in patients exhibiting less than 100 CTCs. This evidence suggests that assessing the number of CTCs may also be clinically useful in the management of terminal cancer, providing security and some element of certainty to patients and their families.
Advances in technology
The CELLSEARCH® tool used by both the Rugo and Krell teams is a first-generation CTC counter, that is FDA approved for use in the assessment of metastatic breast, prostate and colorectal cancer. This system identifies CTCs by exposing live cells to a panel of biological or chemical “stains”. However, these stains are toxic, meaning that captured CTCs are killed during the process of counting.
In contrast, recent developments in the liquid biopsy field mean that “next generation” technologies are able to isolate and count CTCs whilst the cells are alive. The Parsortix® system (developed by Angle PLC) is one example of a next-generation liquid biopsy technology, which identifies CTCs based on their size and biophysical properties alone, without using toxic stains.
This means that CTCs isolated using the Parsortix® system may be subsequently cultured in vitro (i.e., grown in a lab), and used in a series of live-cell experiments to assess the behaviour or function of CTCs. The characterisation of CTCs is unprecedented, and allows clinicians and researchers to assess cancer progression in individual cancer patients. This is important to allow cancer treatment to be “personalised” – i.e., tailored to each individual patient.
For example, live CTCs may be assessed to measure how aggressively they grow in the lab (“tumourigenicity”), which may be helpful to predict tumour invasiveness in the donor patient. By exposing cultured CTCs to a panel of different chemotherapeutic drugs in vitro, it is also possible to assess that patient’s responsiveness to treatment. This avoids the trail-and-error process of administering drugs to a patient which may be ineffective, but which may still cause uncomfortable and dangerous side effects.
Personalising cancer treatment is increasingly important, as clinical trials of many newly developed cancer drugs, such as ‘’CAR T cell’’ immunotherapies, are not effective in all patients. Therefore, assessing tumour characteristics in different cancer patients is important to help researchers understand the differences between responsive and non-responsive patients. In this respect, characterising CTCs using live-cell experiments may also be critical for the design of new cancer treatments.
Liquid biopsies are an area of great innovation, where entrepreneurs, start-up businesses and their ‘disruptive technologies’ have the potential to make great progress to market. By combining the counting of CTCs with complex fingerprinting technologies already used in the analysis of cfDNA, DNA-protein complexes and exosomes, future innovations in the liquid biopsy space could hold the potential to overhaul our understanding of cancer, and how to treat it. Next-generation liquid biopsies will allow researchers and clinicians to generate patient-specific high-resolution tumour maps, which may be used to develop of new anti-cancer treatments, and to personalise these treatments to individual cancer patients.
