Circulating tumor DNA in neoadjuvant treated breast cancer reflects response and survival

Abstract Pathologic complete response (pCR) to neoadjuvant chemotherapy (NAC) is strongly associated with favorable outcome. We examined the utility of serial circulating tumor DNA (ctDNA) testing for predicting pCR and risk of metastatic recurrence in 84 high-risk early breast cancer patients treated in the neoadjuvant I-SPY 2 TRIAL. Cell-free DNA (cfDNA) was isolated from 291 plasma samples collected at pretreatment (T0), 3 weeks after initiation of paclitaxel (T1), between paclitaxel and anthracycline regimens (T2), or prior to surgery (T3). A personalized ctDNA test was designed to detect 16 patient-specific mutations (from whole exome sequencing of pretreatment tumor) in cfDNA by ultra-deep sequencing. At T0, 61 of 84 (73%) patients were ctDNA-positive, which decreased over time (T1-35%; T2-14%; T3-9%). Patients who remained ctDNA-positive at T1 were significantly more likely to have residual disease after NAC (83% non-pCR) compared to those who cleared ctDNA (52% non-pCR; OR 4.33, P=0.012). After NAC, all patients who achieved pCR were ctDNA-negative (n=17, 100%). For those who did not achieve pCR (n=43), ctDNA-positive patients (14%) had significantly increased risk of metastatic recurrence (HR 10.4; 95% CI, 2.3–46.6); interestingly, patients who did not achieve pCR but were ctDNA-negative (86%) had excellent outcome, similar to those who achieved pCR (HR 1.4; 95% CI, 0.15–13.5). Lack of ctDNA clearance was a significant predictor of poor response and metastatic recurrence, while clearance was associated with improved survival regardless of pCR status. Personalized monitoring of ctDNA during NAC may aid in real-time assessment of treatment response and help fine-tune pCR as a surrogate endpoint of survival.