ctDNA-Guided MRD Assessment Enters New Horizon in Colon Cancer

Sakti Chakrabarti, MD

Although minimal residual disease (MRD) assessment may be effective in detecting cure in patients with colon cancer following treatment, more reliable biomarkers are needed. Circulating tumor DNA (ctDNA) may offer a potential solution to address this need.

Early-stage colon cancer (ESCC), defined as a tumor confined to the bowel with or without involvement of the locoregional lymph nodes, is a curable malignancy. The current treatment strategy for patients with ESCC consists of up-front surgical resection in all patients and administration of adjuvant chemotherapy in a select group deemed to be at risk of cancer recurrence. The goal of adjuvant chemotherapy is to eradicate micrometastatic disease, or the MRD, to achieve cure. The decision of whether to administer adjuvant chemotherapy has been guided by clinicopathologic risk factors that have been considered to be a surrogate marker of MRD for several decades.

However, accumulating data indicate that patient selection for adjuvant chemotherapy based on clinicopathologic risk factors is flawed and leads to both overtreatment and undertreatment in a large number of patients.¹ For example, surgery alone cures about 50% of patients with stage III colon cancer,² although the current guidelines advocate for all patients with resected stage III colon cancer to receive adjuvant chemotherapy. Moreover, only 1 of 5 patients with stage III disease have been found to derive a benefit from adjuvant chemotherapy.¹ These data underscore the value of finding a reliable biomarker for MRD assessment. In this context, ctDNA has shown incredible promise.

Available ctDNA Assays

Emerging data have suggested that ctDNA can serve as a reliable biomarker for MRD in patients with resected ESCC. Several techniques have emerged to detect ctDNA in the plasma, typically constituting less than 1% of the total cell-free DNA.³ Currently available ctDNA assays for MRD assessment in colorectal cancer (CRC) can be broadly separated into 2 distinct categories: tumor-informed assays, such as Signatera^{TM 4} and the safe-sequencing system (Safe-SeqS)⁵ and tumor-agnostic assays, such as LUNAR-1.⁶

In tumor-informed assays, the primary tumor is sequenced to identify the patient-specific genomic alterations, which the primers for ctDNA testing in the plasma are designed to be based on. Tumor-agnostic assays rely on a panel of preselected primers designed to detect known genomic alterations and epigenetic signatures relevant to CRC.

Clinical Data Supporting Role of ctDNA in MRD Assessment

Jeanne Tie, MBChB, MD, FRACP, of the Walter and Eliza Hall Institute of Medical Research, and colleagues, provided the first compelling evidence that postoperative ctDNA testing could identify MRD in patients who are at a very high risk of cancer recurrence following definitive surgery.⁵ In this prospective cohort study, the tumor-informed Safe-SeqS assay was used for ctDNA testing in patients with resected, stage II colon cancer.

Among the patients who did not receive adjuvant chemotherapy (n = 178), postoperative (4-10 weeks after surgery) ctDNA was detected in 7.9% of patients (n = 14/178). Seventy-nine percent of these patients experienced disease recurrence at a median follow-up of 27 months. Conversely, only 9.8% (n = 16/164) of the patients with negative ctDNA had cancer recurrence (HR, 18; 95% CI, 7.9-40; P <.001). Postoperative ctDNA status was the strongest independent predictor of relapse-free survival (RFS) and outperformed any individual clinicopathologic risk factor or any combination of clinicopathologic risk factors in predicting cancer recurrence, according to a multivariable analysis. This was found to be the case for patients who did not receive chemotherapy (HR, 28; 95% CI, 11-68) and in the general patient population (HR, 14; 95% CI, 6.8-28). A plethora of subsequent studies reported similar results and further supported the utility of ctDNA in detecting MRD with a high degree of precision in patients with resected colon cancer.^4,6-10

Studies reporting cancer recurrence in almost all patients with detectable ctDNA in postoperative plasma samples triggered widespread interest in further developing this tool. For example, a prospective study, which included 125 patients who had been diagnosed with stage I-III CRC and utilized the personalized tumor-informed ctDNA assay Signatera^TM, is of particular importance.⁴ In this study, patients with detectable ctDNA 30 days post surgery were 7 times more likely to develop cancer recurrence compared with thoise who had undetectable postoperative ctDNA (HR, 7.2; 95% CI, 2.7-19.0; P <.001). Additionally, in a multivariable analysis, ctDNA status was the only significant prognostic factor associated with RFS.

Using the same SignateraTM ctDNA assay platform, Tenna Vesterman Henriksen, MSc, of AARHUS University, and colleagues, presented findings from a prospective study during the 2021 Gastrointestinal Cancers Symposium. The study included 218 patients with stages I-III colon cancer and measured postoperative ctDNA to assess whether it could be used to successfully stratify patients into high- and low-risk groups.⁷ Investigators were able to conclude that patients who had ctDNA detected immediately following surgery were at a higher risk of recurrence. Moreover, ctDNA was detected a median of 8 months prior to radiological detection of recurrence. Longitudinal monitoring was found to increase the predictive power of ctDNA, which outperformed carcinoembryonic antigen in predicting RFS in the study.

When investigators assessed postoperative ctDNA status in patients prior to adjuvant chemotherapy, 9.17% of patients were MRD positive; 75% of these patients relapsed. Conversely, 13.6% of patients with MRD negativity relapsed. Based on this, investigators concluded that a positive postoperative ctDNA test was associated with a recurrence (HR, 11.0; 95% CI, 5.9-21, P <.0001). Among the patients who received adjuvant chemotherapy, a positive ctDNA test after completion of treatment was associated with a recurrence rate of 83.3% compared with 12.5% in those who had a negative ctDNA test post treatment (HR 12; 95% CI, 4.9-27; P <.0001).

Overcoming Barriers to Testing

Despite compelling preliminary data, several barriers inhibit the broad implementation of ctDNA-based MRD assessment to help guide adjuvant therapy decisions. Numerous clinical trials are currently underway to validate the role of ctDNA in selecting patients for adjuvant chemotherapy by addressing several questions:

1. Can adjuvant chemotherapy be omitted in patients who have no detectable ctDNA after surgery (treatment de-escalation)?
2. Could treatment escalation help in patients with detectable, post-operative ctDNA?
3. Does the clearance of ctDNA with adjuvant chemotherapy result in a cure? Is ctDNA a predictive biomarker for treatment efficacy?
4. What is the best strategy to treat patients who continue to have detectable ctDNA after adjuvant chemotherapy?

Currently, the sensitivity of a single postoperative ctDNA test is less than 50%. As a result, treatment de-escalation based on a negative ctDNA result remains within the boundary of clinical trials at this time (NCT04068103, NCT03748680). Additionally, several trials are examining treatment escalation strategy in patients with ctDNA positivity, such as DYNAMIC-III.

The data published thus far have established ctDNA as a robust prognostic biomarker, but data to support the role of ctDNA as a predictive biomarker are lacking. However, several prospective cohort studies have described a few patients whose positive post-operative ctDNA turned negative following adjuvant chemotherapy; these patients achieved long-term DFS, providing preliminary evidence to support the predictive ability of ctDNA testing.^7,10

Currently, 3 well-known ctDNA assay platforms are used for MRD assessment in patients with resected colon cancer: SignateraTM,4 Safe-SeqS,¹⁰ and LUNAR-1.⁶ These assay platforms utilize different methodologies and have variable analytical sensitivity. Consequently, the lack of standardization among these assay platforms limits the interpretation of reported data. Future studies must focus on the standardization of ctDNA testing procedures to exploit the full potential of ctDNA technology.

The Future of ctDNA Technology

The overarching promise of ctDNA technology lies in its potential to reliably detect MRD after resection of the primary tumor, allowing for precise patient selection for adjuvant chemotherapy.

Once ctDNA technology evolves to fulfill this promise, several important objectives will be achieved: First, adjuvant chemotherapy could be withheld in patients with undetectable, post-operative ctDNA, who would ordinarily be considered high risk and receive treatment with adjuvant chemotherapy according to current guidelines.

Second, if the clearance of ctDNA with adjuvant chemotherapy demonstrates a high degree of correlation with survival and cure, ctDNA clearance could be used as an end point in adjuvant trials; this would foster prompt evaluation of novel adjuvant therapies.

Finally, novel therapies could be tested in populations who continue to have detectable ctDNA after the completion of adjuvant therapy, long before cancer recurrence is detected radiologically; this would increase the probability of long-term survival. In light of these findings, theories, and ongoing research, ctDNA technology holds the potential to transform the treatment paradigm of CRC.

References

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