The measurement of circulating tumor DNA (ctDNA) is changing the way gastrointestinal cancers are managed, according to Bassel F. El-Rayes, MD, Professor and Vice Chair for Clinical Research in the Department of Hematology and Oncology, Emory University, and Associate Cancer Center Director, Director of the Gastrointestinal Oncology Program, and the John Kauffman Family Professor for Pancreatic Cancer Research at the Winship Cancer Institute, Atlanta.
Bassel F. El-Rayes, MD
Updating attendees of Emory’s Debates and Didactics in Hematology and Oncology annual conference, Dr. El-Rayes addressed what he called a “definite hot topic that is rapidly evolving”—the use of ctDNA.1 These so-called liquid biopsies have greatly facilitated this approach to treating and monitoring colon cancer, he said.
“We want predictive biomarkers to tell us whether a treatment will work. We want prognostic biomarkers so we can tell patients how they will do. We want ways to monitor disease status and recognize early recurrence. All three of these goals can be achieved by using ctDNA,” he said.
Dr. El-Rayes has not embraced ctDNA as “the only test” but incorporates it into his clinical decision-making. The results often validate his inclination to treat some patients and forgo chemotherapy in others, and sometimes it nudges him one way or the other. “I find it very useful in complex cases, when I’m on the fence,” he said.
Advantages Over Solid Tumor Biopsy
Compared with tissue-based assays, liquid biopsies offer several advantages, starting with feasibility and rapid turnaround time. Its findings are representative of the disease as a whole and not just the biopsy site. Uniquely, ctDNA assays can quantitate mutations, which tissue-based assays cannot. It can stratify risk for recurrence and then molecularly detect it. It can better monitor for the emergence of resistance or sensitive clones, which guides targeted treatment selection, he said.
Its disadvantages, in contrast, are few. The main one is that the panels used for testing, though better than older panels, are still smaller than those used with tissue biopsies.
ctDNA as a Predictive Biomarker
“Of all the biomarkers, a predictive biomarker is the one we really want—to be able to tell which treatment will work in which patient,” Dr. El-Rayes said.
Clearly, in gastrointestinal malignancies, the large number of known mutations and alterations have impacted decision-making: amplification of HER2, translocations of NTRK and FGFR2; gain-of-function mutations in cKIT, IDH1, RAS, and BRAF; loss-of-function mutations related to DNA repair, BRCA1/BRCA2, and mismatch repair proteins; and tumor mutational burden.
As a case in point, Dr. El-Rayes recently used ctDNA information to guide the treatment of progressive disease in a 48-year-old man with stage IV colon cancer. He was found, after two disease progressions and no actionable mutations on tissue assay, to have ERBB2 overexpression. This qualified him for dual HER2-targeted therapy, and he was enrolled on Emory’s MOUNTAINEER trial of trastuzumab plus tucatinib.
However, it’s not just the presence of a mutation, but the number of alleles per mL of plasma that is most informative, and the ability to quantitate these mutations is unique to liquid biopsy. A seminal study from 2012, in metastatic colorectal cancer, clearly showed that progression-free survival and even overall survival were strongly correlated with plasma cell-free DNA (cfDNA) and plasma mutant KRAS levels (r = 0.85, P < 10-4).2 The disease control rate was 77% in patients with low cfDNA (< 25% quartile) and 30% in patients with high cfDNA (> 75%; P = .009). Patients with plasma mutant KRAS levels > 75% had a disease control rate of 0% compared with 42% in patients with lower levels (P = .048). Cox analysis confirmed the prognostic importance of both cfDNA and plasma mutant KRAS. High levels were clear indicators of a poor outcome.
The predictive value of liquid biopsy was similarly shown in a study of 53 patients with stage IV disease treated with oxaliplatin-based chemotherapy and followed with ctDNA.3 Both ctDNA and carcinoembryonic antigen (CEA) were assessed in plasma collected before treatment, 3 days after treatment, and before cycle 2 of chemotherapy. Computed tomography (CT) scans were done at baseline and at 8 to 10 weeks. Major reductions (≥ 10-fold) vs lesser reductions in ctDNA before cycle 2 were associated with a trend for increased progression-free survival (median = 14.7 vs 8.1 months; hazard ratio = 1.87; P = .266).
“This is much easier than a CT scan and probably more predictive,” he commented. “It’s another way to use ctDNA quantitively to predict long-term outcomes and to identify which patients will benefit from treatment.” Emory’s ACCRU COLOMATE trial will assign treatment to patients with progressive disease according to ctDNA findings.
Monitoring for Emergence of Clones, Rechallenging With EGFR Inhibitors
Dr. El-Rayes also emphasized the importance of understanding the patient’s disease as a whole and not just one biopsied part of the tumor. This makes it more likely to identify resistant clones and act on them.
Monitoring for the emergence of resistant (or sensitive) clones has become of practical interest, as studies are showing that rechallenge with epidermal growth factor receptor (EGFR) inhibitors can sometimes be effective. In an early trial of this approach, 28 patients with disease progression on an EGFR inhibitor plus chemotherapy were taken off the EGFR inhibitor and later rechallenged with it.4 A second response was achieved by 21% of patients, and 54% attained disease control, “just by rechallenging them with the drug that they had become resistant to,” he said.
Dr. El-Rayes explained how rechallenge may work: RAS wild-type tumors probably harbor a small “nest of cells” with RAS mutations; as patients are treated with an EGFR inhibitor (eg, cetuximab), this mutated clone expands and resistance occurs. Withdrawal of the EGFR inhibitor then allows the RAS wild-type cells to grow back and the RAS-mutated clone to disappear, resulting in sensitivity to cetuximab a second time.
Indeed, some studies have found that tumors identified at baseline as RAS wild-type by standard genomic sequencing are subsequently shown, by in-depth genomic sequencing, to harbor some RAS mutations. Such patients fare worse on EGFR inhibitors than patients lacking RAS mutations, again suggesting these clones are being selected out by EGFR inhibitors. When the EGFR inhibitors are withdrawn, the resistant clones disappear in 5 or 6 months, he added.
Such information “opens the door” to many trials examining EGFR inhibitor rechallenge and using ctDNA to select patients for this approach. The concept is to perform ctDNA testing and, if the patient remains RAS wild-type, rechallenge that patient; if the patient develops a RAS mutation, do not rechallenge. “This can be the path forward to help us select patients for this strategy,” he said.
Similarly, reactivation of the HER2 pathway (with emergence of HER2 expression in patients treated with EGFR inhibitors) is another mechanism of resistance to EGFR inhibitors, as HER2 is able to bypass EGFR signaling.
Future Approach to RAS/BRAF Wild-Type Colorectal Cancer
As more data emerge, these findings will potentially be put to use in the treatment of advanced colon cancer. Dr. El-Rayes envisions the following approach to the patient diagnosed with a left-sided extended RAS wild-type tumor:
- Start with an EGFR inhibitor and chemotherapy.
- Upon disease progression, obtain a liquid biopsy to identify mechanism of resistance. Switch the chemotherapy and add a vascular endothelial growth factor (VEGF) inhibitor, but drop the EGFR inhibitor.
- If the mechanism of resistance is, for example, emergence of RAS-mutant disease, rechallenge with an EGFR inhibitor can be attempted once the resistance mutation is no longer observed in the liquid biopsy.
- If the mechanism of resistance to an EGFR inhibitor is a new HER2 amplification, consider changing to HER2-directed therapy.
“You can see how liquid biopsy can potentially be used to tailor therapy each step of the way, deciding which patients will benefit from switching treatment, who will benefit from rechallenge with an EGFR inhibitor, and who will need a different approach,” explained Dr. El-Rayes. In the next few years, we will have more data from ongoing trials supporting this approach.”
Selecting Patients for Adjuvant Therapy
Finally, ctDNA has an evolving role in the molecular detection of measurable residual disease (MRD) after resection, which can guide the selection of patients for adjuvant therapy. In a study of 130 patients with stage I to III colon cancer treated with resection, longitudinal ctDNA analysis identified 14 of 16 relapses (87.5%).5 By postoperative day 30, ctDNA-positive patients were 7 times more likely to relapse than ctDNA-negative patients (P < .001) and, when positive shortly after adjuvant chemotherapy, they were 17 times more likely to relapse (P < .001). Relapse occurred, in fact, in all seven patients who were ctDNA-positive after adjuvant chemotherapy. During surveillance, ctDNA-positive patients were 43 times more likely to experience recurrence than ctDNA-negative patients (P < .001). Liquid biopsy was able to predict disease recurrence up to 16.5 months ahead of standard radiologic imaging.
“From this study, we learned that, with one test alone, patients who were ctDNA-positive had an extremely high risk of recurrence, and those who were ctDNA-negative had a lower chance. In the ctDNA-positive group, the patients who did not recur were probably those who shifted from positive to negative with adjuvant therapy,” Dr. El-Rayes said.
A larger study of 103 patients with 252 prospective serial plasma specimens clearly showed that the detection of persistent ctDNA after curative-intent surgery can identify patients with MRD who will ultimately have a disease recurrence.6 In samples drawn 1 month after surgery, ctDNA was detected in 15 patients, and all 15 (100%) had a recurrence. Of 49 patients without detectable ctDNA at that time point, 12 (24.5%) had a recurrence. Median recurrence-free survival was not reached for the ctDNA-negative group and was less than 6 months for ctDNA-positive patients. The ctDNA-positive patients had an 11-fold increased risk for recurrence (P < .0001), which was predicted by the assay 100% of the time. Notably, standard CEA levels did not predict recurrence.
“In all these studies, we are seeing that if you have a positive ctDNA test, no question—that’s bad, and you have to treat,” he said. “This is much more predictive when ctDNA is compared with CEA, CT scan, tumor staging, and so forth, and it can detect recurrence significantly earlier. It is a much better way of selecting patients at risk for recurrence who need adjuvant therapy and of monitoring the efficacy of adjuvant therapy.”
DISCLOSURE: Dr. El-Rayes has received honoraria from Bayer, Lexicon, and RTI Health Solutions; has served as a consultant or advisor to Bayer, BTG, Loxo, Merck, Merrimack, and RTI Health Solutions; has participated in a speakers bureau for Bristol Myers Squibb and Lexicon; has received institutional research funding from AstraZeneca/MedImmune, Aveo, Boston Biomedical, Bristol Myers Squibb, Cleave Biosciences, Five Prime Therapeutics, Genentech, Hoosier Cancer Research Network, ICON, Merck, Novartis, Pfizer, PPD, Taiho Pharmaceutical, and Xencor; and has held other relationships with Erytech Pharma and Exelixis.
REFERENCES
1. El-Rayes BF: Circulating tumor DNA in the management of GI cancers. 2021 Debates and Didactics in Hematology and Oncology. Presented July 31, 2021.
2. Spindler KL, Pallisgaard N, Vogelius I, et al: Quantitative cell-free DNA, KRAS, and BRAF mutations in plasma from patients with metastatic colorectal cancer during treatment with cetuximab and irinotecan. Clin Cancer Res 18:1177-1185, 2012.
3. Tie J, Kinde I, Wang Y, et al: Circulating tumor DNA as an early marker of therapeutic response in patients with metastatic colorectal cancer. Ann Oncol 26:1715-1722, 2015.
4. Cremolini C, Rossini D, Dell’Aquila E, et al: Rechallenge for patients with RAS and BRAF wild-type metastatic colorectal cancer with acquired resistance to first-line cetuximab and irinotecan: A phase 2 single-arm clinical trial. JAMA Oncol 5:343-350, 2019.
5. Teinert T, Henriksen TV, Christensen E, et al: Analysis of plasma cell-free DNA by ultradeep sequencing in patients with stages I to III colorectal cancer. JAMA Oncol 5:1124-1131, 2019.
6. Parikh AR, van Seventer EE, Siravegnal G, et al: Minimal residual disease detection using a plasma-only circulating tumor DNA assay in colorectal cancer patients. Clin Cancer Res. April 29, 2021 (early release online).
