Circulating tumor DNA refines risk stratification of neoadjuvant therapy-resistant breast tumors.

Early-stage breast cancers resistant to neoadjuvant therapy (NAT), characterized by high residual cancer burden (RCB) after treatment, have an increased risk of metastatic recurrence. Here, we show that circulating tumor DNA (ctDNA) detected using a tumor-informed test (1) can improve risk stratification of patients with NAT-resistant tumors (RCB-II/RCB-III) and (2) predict response to NAT. Stratification using ctDNA status at pretreatment or post-NAT and ctDNA dynamics identified NAT-resistant tumors with a significantly decreased risk of metastatic recurrence. ctDNA clearance as early as week 3 across receptor subtypes predicted favorable responses to NAT, including immunotherapies. Interestingly, less than a fifth of patients with NAT-resistant tumors were ctDNA-positive post-NAT. Serial mutation profiling of NAT-resistant tumors revealed that patient-specific ctDNA assay variants remained detectable over time, including in tumors of patients ctDNA-negative post-NAT. Refining risk stratification for NAT-resistant tumors using ctDNA and understanding ctDNA shedding in these tumors could guide treatment decisions to prevent or delay metastatic recurrence.