73TiP SOLTI1710 PROMETEO II: Palbociclib in combination with letrozole in hormone receptor-positive (HR+)/HER2-negative residual disease after standard neoadjuvant chemotherapy (NAC)

The combination of cyclin-dependent kinase inhibitors (CDK4/6i) with first or second-line endocrine therapy are the standard of care for HR+/HER2-negative metastatic breast cancer (BC); its role in the early-setting is being evaluated in several studies with discordant results. In HR+/HER2-negative BC, pathologic complete response (pCR) rates after NAC are around 15%. Additional therapeutic strategies to eradicate these residual tumor cells are needed. To characterize the biological effect of CDK4/6i in residual disease (RD) after NAC, could help to better understand their role in early BC. SOLTI-1710 PROMETEO II is an open-label, multicenter window of opportunity trial of Palbociclib and letrozole tested in patients with HR+/HER-negative RD after completing anthracycline/taxane-based NAC. PROMETEO II will include 22 patients. Invasive RD must be confirmed by core-biopsy and have a diameter ≥ 10mm measured by ultrasound. Adequate organ function and ECOG PS 0-1 are required. A short course of Palbociclib is administered at a dose of 125 mg/day for 21 days and letrozole 2.5 mg/day continuously until surgery. BC surgery is carried out 1 week after the last dose of palbociclib. The primary objective of the trial is to evaluate the antitumor activity by the Complete Cell Cycle Arrest (CCCA) rate determined by Ki67 ≤2.7%, centrally assessed at surgery. Secondary endpoints include pCR rate, RCB and safety. To date, 12 patients have been included at 8 sites in Spain. NCT04130152. SOLTI. Pfizer.