A recent review outlined advances and challenges in utilizing liquid biopsy to detect measures such as circulating tumor DNA in patients with solid tumors.
Minimal residual disease (MRD) is a known indicator of possible short- or long-term relapse in patients with hematological cancers and solid tumors. Recent research has focused on the most reliable ways to detect MRD in non-hematological cancers, including the potential of accurate, minimally invasive liquid biopsies for solid tumors. A review published in Cancers discussed the impact MRD detection may have on cancer patient management and the various methodologies used to detect MRD though liquid biopsy in solid tumor cancers.
MRD refers to the presence of residual cancer cells in the body during or after treatment, even after tumors become undetectable in imaging or clinical exams. If these cells remain in the body, they can cause local or metastatic relapse. In blood cancers, MRD can be used to evaluate treatment efficacy and predict relapse; in solid tumor cancers, it can help identify whether a patient is at high or low risk of relapse.
“After completion of standard therapy with curative intent, treatment could theoretically be adapted based on liquid biopsy results. Treatment intensification in MRD-positive patients has the potential to improve disease-free survival and overall survival if MRD is efficiently treated,” the authors wrote. They added that the ability to de-escalate treatment in MRD-negative patients could also have a significant impact on patients by decreasing costs for the individual patient and payers, reducing treatment-related side effects, and improving quality of life without impacting survival.
Detecting MRD in solid tumor cancers has proven more challenging than in hematological disease, but liquid biopsy might present a solution if a standard method can be ironed out, the authors suggested. The review focused mainly on circulating tumor DNA (ctDNA), which is a common measure of MRD based on tumor-specific genomic alterations.
This has already been implemented in daily clinical practice with next generation sequencing (NGS), which includes several methods of high-throughput nucleotide sequencing. Large quantities of sequencing data can become available within a few hours, but there are significant error rates depending on the test. Some polymerase chain reaction (PCR)-based assays allow for more accurate variant detection but only identify limited, pre-determined variants.
Utilizing gene panels customized based on individual patients’ tumors is currently one of the most validated ways to detect ctDNA. Once a tumor tissue sample is biopsied and sequenced with whole-exome sequencing (WES), a custom panel can be created to highlight regions of interest in NGS in later samples. Using a custom panel with specific targets rather than WES each time can increase the detection sensitivity and depth of sequencing, meaning even variants present in low quantities can be found.
Limitations of custom-based NGS include the subjectivity of tumor variant selections for the custom panel and the lack of a standard threshold for ctDNA positivity. Obtaining a tissue biopsy for initial sequencing can also be challenging depending on the tumor, and samples from one tumor may not capture alterations on other parts of the same tumor or in distant metastases. The tumor makeup can also change over time, making the initial custom panel less accurate.
Droplet digital PCR (ddPCR) is another technique that can detect pre-defined genomic variants even with a very low quantity of DNA. It also detects alterations based on tumor biopsy, so the pros and cons of ddPCR largely align with those of custom-based NGS. Its sensitivity in MRD detection specifically is yet to be determined.
NGS panels and PCR assays also have potential when used with gene panels customized not to the patient, but to specific tumor subtypes. NGS gene panels for this implementation are typically either comprehensive or specific to a single tumor type, while PCR is generally used to identify mutations in highly specific tumor types.
Pan-cancer and cancer-specific NGS panels have been developed, but generally these custom panels follow the same general rule with regard to accuracy: the larger the panel size, the lower the detection rate. Accuracy has also varied depending on cancer type, and the negative predictive value of custom panels wavers, particularly post-treatment. Both NGS and PCR are costly and the personalized methods are more time-consuming.
The main goal of MRD detection from the clinician’s perspective is to be able to escalate or de-escalate treatment for patients appropriately based on their particular cancer. Studies have shown promising results, but challenges still arise. Aside from the aforementioned technical issues, there are currently no standardized methods for implementation in daily clinical practice.
The timing of post-treatment sampling is also important because ctDNA has an approximate 30-minute half-life and is secreted by tumor cells and cell death. Thus, there is no precise wash-out time for ctDNA decrease post-treatment. “Assessing for MRD within one to two weeks of curative treatment is, at least for the moment, not technically feasible, and delaying adjuvant therapy until results are available may lower patient prognoses,” the authors note.
Ongoing trials relying on ctDNA as an MRD marker mainly focus on showing the value of ctDNA as an indictor of MRD, and authors expect it to be a regular biomarker in clinical practice in the coming years.
“We now need to perform carefully designed randomized interventional clinical trials specifically powered to evaluate if treatment escalation improves cancer outcome in MRD-positive patients and if treatment de-escalation is safe in MRD-negative patients,” they wrote.
Honoré N, Galot R, van Marcke C, Limaye N, and Machiels J. Liquid biopsy to detect minimal residual disease: methodology and impact. Cancers (Basel). Published online October 26, 2021. doi:10.3390/cancers13215364