e13040 Background: Alpelisib-induced hyperglycemia (HG) is the most common adverse event leading to interruption, reduction or withdrawal of the treatment. HG is an on-target effect that depends on the baseline metabolic status of the patient. In SOLAR-1 90,6% patients with high-risk at baseline HG G3/G4 was reported. Prophylactic use of metformin may prevent HG in patients at risk. Methods: HR+ HER2-negative advanced breast cancer patients were stratified to low-, moderate- or high-risk groups of HG based on the algorithm published in [1] before starting alpelisib 300 mg + fulvestrant (Alp + Fulv). Moderate- and high-risk patients used prophylactic metformin at 500-1000 mg or 2000 mg BID with breakfast and dinner (Table). Metformin was started at least one-week prior Alp administration. Fasting plasma glucose (FPG) was monitored qw and bw in low- and moderate-risk patients, and daily with home glucometer in addition to bw FPG for high-risk patients. The primary endpoint of retrospective analysis was to assess the incidence of HG G3/G4 in different risk groups based on CTCAE V4.03 guidelines. Results: A cohort of 18 patients (median lines of previous treatment in advanced disease was five, all patients has received CDK4/6 inhibitors) treated with Alp+Fulv in the Loginov Moscow Clinical Scientific Centerin 2020-2021 included 7 low-, 3 moderate- and 8 high-risk patients. Any grade HG was reported in 13 out of 18 pts; no cases of HG G2 or G4 were detected. HG G3 was reported in 5 out of 18 patients including in 2 out of 3 moderate-risk patients, and in 3 out of 8 high-risk patients. Anti-HG drugs used included metformin (6/18); empagliflozin (4/18), and linagliptin (1/18); 1 patient required Alp dose reduction to 250 mg due to HG G3; none discontinued Alp due to AEs. Other AEs included rash (7/18; no grade 4; grade 3 in 4/18), and blood creatinine increase (8/18 – all grade 1-2).The median PFS on treatment with Alp+Fulv was 7 months. Conclusions: Prophylactic use of metformin in patients with high risk of hyperglycemia could markedly reduce frequency and severity of hyperglycemia on alpelisib.[Table: see text]