Neoadjuvant Dalpiciclib + AI → SHR-A1811 for HR+/HER2-Low Breast Cancer

This is a single-arm, open-label, phase II exploratory trial conducted at a single center (Second Affiliated Hospital of Air Force Medical University, China). The study aims to investigate the activity and safety of dalpiciclib plus an aromatase inhibitor (AI) followed by the HER2-directed antibody-drug conjugate (ADC) SHR-A1811 as neoadjuvant therapy for patients with intermediate-to-high risk, hormone receptor-positive (HR+), HER2-low breast cancer. Preclinical evidence suggests that CDK4/6 inhibition may enhance the immunogenic cell death induced by ADCs and overcome tumor heterogeneity. Dalpiciclib is a selective CDK4/6 inhibitor with a distinct piperidine structure associated with low hepatotoxicity and minimal gastrointestinal side effects. SHR-A1811 is a HER2-targeted ADC composed of a humanized anti-HER2 IgG1 monoclonal antibody (based on trastuzumab sequence), a cleavable maleimide tetrapeptide (GGFG) linker, and a DNA topoisomerase I inhibitor (SHR169265). It demonstrates bystander killing of HER2-low cells. The 4-cycle induction with dalpiciclib plus AI prior to SHR-A1811 is designed to reduce tumor proliferation and potentially prime the tumor microenvironment before ADC administration. Treatment administration * Dalpiciclib: 125 mg orally once daily on days 1-21 of each 28-day cycle (4 cycles total). Patients are instructed to take the tablet at approximately the same time each day, on an empty stomach (≥1 hour before or after food). Missed doses are not supplemented; vomiting within a short time after intake does not trigger a replacement dose. Interruption for \>21 consecutive days requires study discontinuation. Dose reductions follow a stepwise scheme: 125 mg → 100 mg → 75 mg. * Aromatase inhibitor (AI): Investigator's choice of anastrozole 1 mg daily, letrozole 2.5 mg daily, or exemestane 25 mg daily, taken continuously during the first 4 cycles. Premenopausal patients (mandatory) and perimenopausal patients (at investigator's discretion) receive concomitant LHRH agonists (e.g., goserelin or leuprolide). * SHR-A1811: 4.8 mg/kg intravenously over a standard infusion duration on day 1 of each 21-day cycle (4 cycles total). The dose is recalculated based on body weight measured immediately before each cycle. If weight change from baseline is \<10%, no dose adjustment is required. Dose reductions follow a 4.8 → 3.2 mg/kg step. Infusion-related reactions are managed according to a predefined algorithm (slow infusion, antihistamines, corticosteroids, or permanent discontinuation for grade ≥3 reactions).