It is well established that detecting and diagnosing cancer earlier significantly improves patient outcomes. Therefore, the identification of cancer biomarkers and development of liquid biopsy technologies are set to revolutionise cancer screening, treatment and surveillance. Initial studies using these methods have provided promising data, yet there are still important challenges that must be addressed before the widespread clinical implementation of liquid biopsy assays.
Joanna Janus, Research Programme Manager (Early Detection & Prevention) at Cancer Research UK, spoke at a recent Front Line Genomics webinar series about the applications of ctDNA in early cancer detection. To hear Joanna’s presentation in full, as well as other talks on ctDNA as an early detection biomarker for cancer, please check out the on-demand webinar here.
ctDNA – The prominent liquid biopsy biomarker
In healthy individuals, the release of DNA from cells into our circulation is a normal process that occurs when cells undergo programmed cell death (apoptosis). However, in cancer patients, tumour cells can also shed DNA and other cellular material into the blood via several mechanisms – including apoptosis, necrosis or active secretion. This tumour-derived DNA (known as circulating tumour DNA, ctDNA) has become an important diagnostic marker for cancers.
By analysing ctDNA from patient blood samples, oncologists can now identify key DNA changes present within tumour cells (see Figure 1). In particular, DNA mutations (point mutations, copy number alterations and chromosomal rearrangements) offer key insights into tumouriogenesis. Additional information, such as methylation status and ctDNA fragmentation size, may also provide clinically relevant details about tumours. Therefore, quantifying and analysing ctDNA is a promising liquid biopsy marker for oncological care.
Monitoring cancer patients from diagnosis to treatment
Analysis of ctDNA has multiple opportunities to guide decision making across the entire treatment pathway, from screening to post-treatment monitoring (see Figure 2). Using liquid biopsy techniques, it’s possible to detect cancer before symptoms are apparent using a simple blood test. If ctDNA is detected during routine screening, it can then be molecularly profiled to inform prognosis and treatment options.
ctDNA may also be used as a biomarker to track treatment efficiency. Should a tumour be successfully treated, tumour size (and therefore, detectable ctDNA levels) will decrease. Additionally, ctDNA is a useful tool in the later stages of disease, allowing disease monitoring, determining treatment response, identifying drug resistance and detecting minimal residual disease (MRD). Therefore, ctDNA-based liquid biopsy techniques stand to drastically alter many aspects of oncological care.
The next generation of cancer screening: Multi-cancer detection tests
One clear role of ctDNA biomarkers is in the development liquid biopsy assays for early cancer detection. Multiple-cancer early detection tests, or MCEDs, have been designed to detect multiple cancers simultaneously, making them a promising tool for widespread cancer screening programs. This comes as a welcome alternative to current cancer screening tests, which typically only identify one cancer type – although ctDNA can still be used to screen for specific cancers in high-risk populations.
There are several prominent examples of studies evaluating the use of targeted ctDNA tests used for lung cancer screening:
- iDx Lung: a study established in 2021 that aims to determine the value of adding blood or tissue biomarkers (such as ctDNA) to routine low-dose CT scanning. This approach is going to be adopted in the NHS soon for lung cancer screening in high-risk individuals, to evaluate whether biomarker analysis improves risk prediction compared to CT scans alone.
- SUMMIT: a study using the well-established Galleri MCED test, aiming to clinically validate the test in individuals who are at high-risk of lung cancer.
MCEDs are also a helpful tool for patients where the cause of their symptoms is uncertain. For cancer, early diagnosis is a key factor in better patient outcomes. By introducing MCEDs into routine healthcare assessment, cancer may be detected earlier leading to accelerated diagnosis and earlier treatment options. Recently, the SYMPLIFY study1 showed promising data of MCED validation in symptomatic patients.
Recognising reoccurrence and resistance
At the other end of the cancer treatment pathway, ctDNA can also be used to monitor patient progress during remission. As liquid biopsies are non-invasive, serial monitoring through repeated blood tests is a feasible method of surveillance that may enable the detection of disease reoccurrence or therapy resistance. Importantly, it may be possible to identify such events before they are clinically apparent by imaging. In one 2019 study of breast cancer patients, researchers detected ctDNA 2 years before relapse was clinically confirmed2, highlighting the utility of ctDNA in disease monitoring.
ctDNA tests are now successfully being used within the NHS for late-stage metastatic cancers. However, the use of ctDNA tests for early-stage cancers is still limited, due to low sensitivity and specificity. This may be explained by the fact that in early-stage cancers, tumours are smaller and shed less ctDNA (although this may not be true for more aggressive early-stage cancers). Overall, it still needs to be determined whether ctDNA tests are a successful strategy for improving patient survival.
The roadmap to clinical implementation
- Sufficient clinical performance
Most ctDNA screening trials have been conducted in case-control type study populations, where individuals with and without cancer have been selected. Often trials select patients with aggressive cancers and underrepresent patients with early-stage cancers, which is not reflective of the populations that these tests are intended for. Additionally, the case study controls for these trials may not reflect the health and diversity of the people in the general population. Therefore, prospective study designs, rather than case-control studies, would be preferable.
The Galleri test (an MCED) is a notable example of a test that is now being taken forward into a prospective study design. MCEDs are complicated tests to evaluate, as their performance varies across cancer types. So, although an MCED may claim to recognise over 50 different cancer types, it may do so with different success rates, making it harder to validate in a screening study.
Another important factor to consider for the evaluation of ctDNA tests is the timepoint at which they’re used. ctDNA is shed into the blood over time; its concentration can fluctuate during screening studies. Currently, it is unclear how frequently ctDNA tests should be used for screening or disease monitoring. Time points used also vary across studies, causing difficulties when comparing studies against each other.
- Evidence of clinical utility
When we have a ctDNA test that works, is sensitive enough, and has specificity in the right populations, the clinical utility of that test must then be determined. For screening, this would require randomised control trials to evaluate the impact of the test. Typically, these trials have very long timescales (sometimes 10+ years including patient follow-ups), which is a common problem for a lot of screening tools. Clinical utility evaluation could also involve determining how to avoid overdiagnosis, overtreatment or false reassurance. If a patient receives a negative screening result, they may be temped to go for less screening. Therefore, knowing the impact of tests on patients is essential.
Several studies on disease reoccurrence prediction using ctDNA have been carried out, but we’re not yet sure of how best to intervene based on a positive result. For example, if a ctDNA test shows a patient’s cancer is likely to recur, should you start them on a new therapy, or wait for clinical relapse? In the latter, what is the need for the ctDNA test in the first instance?
In one study using ctDNA to track MRD in high-risk patients3, they performed three-monthly blood sampling for 12 months after the cancer had been surgically removed. However, in early stage triple negative breast cancer patients, by the time the ctDNA test had been conducted, most patients had already developed metastatic disease and were ineligible for intervention. This study demonstrates that the time period and practical implementation of these types of tests is important.
- Evaluation of economic impact
There’s also a clear need to determine the economic impact of these tests on clinical pathways. Such analysis is already happening in the NHS Galleri trial, but multiple clinical applications for tests make this more difficult. It must also be considered whether the NHS has the necessary resources and infrastructure to deliver these tests, especially if doing so would put the system under more pressure in the short-term. To alleviate this, some tests may be partly or wholly delivered outside the NHS. Additionally, patient and clinician acceptability of new tests is another factor that may affect MCED’s clinical implementation.
- Test selection
If a healthcare service is considering providing a new ctDNA test, there are many considerations. Firstly, do they have the technology to perform the test in the current healthcare system? Does the test need to be outsourced instead? What are the minimum and desirable performance criteria, and which test is the best value for money? More standardised test validation would make it easier to answer these questions.
Next steps and future prospects
For most ctDNA tests, clinical implementation won’t be achieved any time soon. Simply, sufficiently powered randomised controlled trials to provide evidence of clinical utility takes time. One potential solution is to use surrogate endpoints for screening efficacy to try and reduce the timescales for screening studies – possibly getting tests into practice sooner.
The scientific community are working to drive progress forward. The MCED consortium, a UK-American partnership, have formed to generate an evidence strategy for MCEDs. BLOODPAC are another notable consortium, which is formed from industry collaborators to address the regulatory challenges in the field, amongst other issues. Although there are many issues that must be addressed before ctDNA tests enter screening programs, one thing is clear: the future of early cancer detection is looking brighter.
Webinar Q&A (Direct quotes, edited for brevity)
Q: “Is there a limit to the number of cancers an MCED could detect? Why are some cancers more difficult to detect?”
A: ““In theory, I suppose there’s no limit. Hence, why often these tests claim to do 50+ cancers. As long as they have a shared feature that that test is supposed to detect, in theory, it should be able to detect the cancer. I think [the difference in MCED detection] is because some cancers tend to shed a lot less ctDNA. It’s fair to say we don’t really understand the biology of cancer shedding ctDNA, and that will be quite important in the future in order to optimize these tests.”
1. Nicholson, B. D. et al. Multi-cancer early detection test in symptomatic patients referred for cancer investigation in England and Wales (SYMPLIFY): a large-scale, observational cohort study. Lancet Oncol. 24, 733–743 (2023).
2. Coombes, R. C. et al. Personalized Detection of Circulating Tumor DNA Antedates Breast Cancer Metastatic Recurrence. Clin. Cancer Res. 25, 4255–4263 (2019).
3. Turner, N. C. et al. Results of the c-TRAK TN trial: a clinical trial utilising ctDNA mutation tracking to detect molecular residual disease and trigger intervention in patients with moderate- and high-risk early-stage triple-negative breast cancer. Ann. Oncol. 34, 200–211 (2023).
4. Klein, E. A. et al. Clinical validation of a targeted methylation-based multi-cancer early detection test using an independent validation set. Ann. Oncol. 32, 1167–1177 (2021).