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Background: Poor body composition metrics (BCM) are associated with inferior cancer outcomes. Alpelisib, a PIK3CA inhibitor, is increasingly used in approximately 40% of hormone receptor positive HER2 negative (luminal) metastatic breast (HR-MBC) patients who carry a somatic mutation in the PIK3CA gene. In the SOLAR-1, the clinical trial that led to FDA approval, serious adverse reactions occurred in 35% patients who were treated with Alpelisib and severe hyperglycemia was reported in 65% of the patients. Alpelisib is given at a fixed dose (300 mg daily) regardless of variables such as high adiposity or low muscle mass also called sarcopenia. In this study we investigated the association of BCM with toxicity and response to Alpelisib. Methods: Eligible HR-MBC patients treated with Alpelisib at Tel Aviv Sourasky Medical Center were identified. Toxicity grading, dose reduction, treatment discontinuation and hospitalizations were recorded. Using computerized tomography (CT) images taken for the evaluation of disease burden, skeletal muscle area (SMA), and skeletal muscle density (SMD) were measured at the third lumbar (L3) vertebrae. Sarcopenia was defined as skeletal muscle index (SMI = SMA/height2) Data and statisticsSarcopenicNon-SarcopenicP-valueLow SMDHigh SMDP-valueHyperglicemia all grades100%80%0.294192.31%100%0.0475Hyperglicemia grade 3-433.33%40%138.46%25%1AEs all grades83.33%20%0.027669.23%50%0.584AEs grade 3-425%00.514715.38%25%1SAE including G3-4 AEs + hospitalization + dose reductions66.67%40%0.592869.23%25%0.25TTF (days)82114187991 Citation Format: Ari Raphael, Rivka Kessner, Rebeka Raphael, Amir Sonnenblick, Shlomit Strulov Shachar. Sarcopenia and skeletal muscle density as predictors of toxicity in patients with metastatic breast cancer receiving alpelisib [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-13-19.