Multicenter phase 2 trial of prognostic relevance of baseline CTC levels in HR-positive, HER2-negative metastatic breast cancer

The multicenter phase 2 PACE trial in the USA evaluated the prognostic role of baseline CTC enumeration in patient with HR+/HER2- mBC with progression on aromatase inhibitors and CDK4/6 inhibitors. Patients were randomized to fulvestrant (F), F plus palbociclib (P), or F plus P plus avelumab (A). Baseline CTCs were prognostic, with median PFS of 5.7 months for patients with stadium IV indolent disease (<5 CTCs/7.5 mL) and 3.5 months for patients with stadium IV aggressive disease (≥5 CTCs/7.5 mL). In patients with stage IV aggressive, but not indolent, disease F+P and F+P+A improved PFS over F alone. The authors conclude that these results highlight the potential of CTC enumeration to guide treatment decision-making.