e13078 Background: The CDK4/6 inhibitors (CDK4/6i) and aromatase inhibitors (AIs) regimen is the first-line standard for HR+/HER2- advanced breast cancer. Despite this, high-risk patients often receive combination chemotherapy for quicker disease control, a practice supported by anecdotal evidence of the efficacy of the CDK4/6i and AI combination but lacking rigorous research validation. Early tumor shrinkage (ETS), a prognostic marker in colorectal cancer, remains undefined in HR+ breast cancer. This study (NCT04858997) aims to investigate the early effectiveness of the CDK4/6i and AI combination through ETS, addressing a significant gap in the literature. Methods: This study enrolled HR+/HER2- advanced breast cancer patients diagnosed between April 2021 and October 2023, who were treatment-naïve for their advanced disease and consented to palbociclib and AI therapy. Participants underwent CT scans to evaluate treatment response at baseline, week 4, week 12, and every 12 weeks thereafter. The study primarily investigated ETS, defined herein as a ≥10% reduction in the sum of diameters of target lesions by week 4. Secondary endpoints encompassed TTR, ORR, and PFS. Results: The study enrolled 30 patients with a median age of 55 years (range 29-76); 30% were premenopausal, and 40% presented with ≥3 metastatic sites. Visceral metastases were observed in 53.3% of the cohort (N=16). ETS was achieved in 73.3% of patients at the first assessment 4 weeks post-treatment initiation, with 68.8% in patients with visceral metastases and 78.8% in those without. The mTTR=3 months, differing between patients with visceral metastases (6 months) and those without (3 months). The overall response rate (ORR) was 63.3%. The ETS outcomes predict the best response in patients, with 77.3% reaching ETS eventually achieving partial or complete response (PR/CR), compared to only 25% of those not reaching ETS. Conclusions: This study validates the CDK4/6i and AI combination's early effectiveness in HR+/HER2- advanced breast cancer, showing that 4-week treatment responses predict best of response outcomes. Achieving ETS is closely linked to a higher likelihood of therapeutic success, underscoring its value as a prognostic marker for informed therapy choices.