Validating ctDNA as a prognostic marker in residual triple negative breast cancer following neoadjuvant chemotherapy
Triple-negative breast cancer (TNBC) patients who have residual tumor at surgery (non-pathological complete response or non-pCR) after neoadjuvant chemotherapy (NAC) have a poor prognosis. In these cases, adjuvant chemotherapy with capecitabine improves disease-free survival in ~15% of patients. Identifying those who would benefit or not from such additional therapy remains a critical need. Circulating tumor DNA (ctDNA), a plasma-based biomarker, provides real-time insights into disease and treatment progression. We previously demonstrated that ctDNA detection after NAC and before surgery signals poor prognosis. In the TRICIA trial, 92 patients with non-pCR provided plasma before surgery and after NAC (T1), after surgery (T2), during adjuvant capecitabine therapy (T3) and late after surgery following completion of adjuvant treatment (T4). The sensitivity, specificity, and predictive values of a tumor-informed digital-droplet-based ctDNA detection assay were measured with a median follow-up of 38 months. ctDNA was detected in 97% of patients before clinical relapse. We confirmed that the lack of detection of ctDNA at the post-NAC pre-operative (T1) time point is highly prognostic, with 95% distant-disease relapse free survival. The other time points were not as strongly prognostic. The detection of ctDNA in patients with significant residual tumor (Residual Cancer Burden 2 or 3) was also highly prognostic and our test performed with 100% sensitivity and 100% specificity in RCB 3 patients. Although the extent of residual disease was correlated with the Fractional Abundance of individual variants, the effect of surgery on ctDNA detectability was not significant except for RCB 3 cases, in which large amounts of residual disease were removed. We measured 3 time points before, during and after capecitabine treatment and found that capecitabine treatment was associated with clearance of ctDNA in 41% of cases, and clearance (from detection to non-detection) was associated with good prognosis. These findings suggest that ctDNA testing using ddPCR assays in an academic hospital-based context can reliably identify a very low-risk group of non-pCR TNBC patients, and this personalized approach is ready for prospective testing for clinical utility in TNBC patients who have undergone NAC and require additional chemotherapy.