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Background and Importance Abemaciclib is a CDK4/6 inhibitor (CDKi) indicated for treatment of HER2 negative (HER2-) advanced luminal breast cancer (ALBC). Its safety profile is decisive in clinical practice.Aim and Objectives To analyse abemaciclib’s safety and its impact on treatment course of patients with HER2- ALBC.Material and Methods Design: multicentric, observational, descriptive and retrospective study.Inclusion criteria: Cáceres and Badajoz patients who started abemaciclib during 1/7/17–31/3/24.Exclusion criteria: early–stage patients starting adjuvant treatment.Follow–up period: from 1/7/17 to 31/3/25.Data: age at abemaciclib beginning, medical history, adverse events (AEs) classified per Common Terminology Criteria for AE (CTCAE), treatment suspensions, dose reductions and switches to another CDKi.Data collection and processing: electronic health record, FarmaTools and Excel .Results 64 patients met the inclusion criteria and finally 30 women resulted after exclusion criteria applied.Median age at beginning: 59 years (interquartile range IQR:55.25–71.5).Comorbidities included hypertension (10), dyslipidemia (10), smoking (10), cardiovascular events (4), diabetes (4), other neoplasms (6), and one immune–mediated disease.Recorded AEs by grade (table 1):Abstract 5PSQ-086 Table 1Recorded AEs by gradeAdverse Event Grade <3 (%) Grade ≥3 (%) Diarrhoea 46.7 10.0 Asthenia/Fatigue 30.0 6.7 Hyporexia/Anorexia 20.0 Anaemia 16.7 3.3 Skin Rash 16.7 6.7 Infections 16.7 6.7 Hypertransaminasaemia 13.3 6.7 Nausea 10.0 3.3 Thrombocytopenia 10.0 3.3 Musculoskeletal Pain 10.0 3.3 Onychopathy 10.0 Neutropenia 10.0 3.3 Vomiting <7 Constipation <7 3.3 Alopecia <7 Headache <7 Dysgeusia <7 Lymphoedema <7 Renal Damage <7 3.3 Pneumonitis <7 Epigastric Pain <7 Hyperbilirubinaemia 6.7 Pulmonary Thromboembolism (PTE) 3.3 Fever 3.3 Dyspnoea 3.3 Arterial Hypotension 3.3 Implications of these AEs during the course of therapy:o Dose reductions due to toxicity: 33.3% of cases.o Temporary suspensions due to toxicity: 43.3% of cases.o Permanent suspensions due to toxicity: 23.3% of cases, of which 85.7% switched to another CDKi.Conclusion and Relevance The most common AEs of any grade was diarrhoea, followed by asthenia/fatigue.Serious but uncommon AEs included PTE, neutropenia, pneumonitis.These AEs meant that more than 40% of patients had to temporarily suspend treatment, in more than 30% the dose was reduced, and more than 20% permanently suspended treatment to switch to another CDKi.Conflict of Interest No conflict of interest