Abstract P1-04-07: Incidence and Risk Factors of Immune-Related Adverse Events in Metastatic Breast Cancer Patients: Findings from a Multi-Institutional Study

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Abstract Background: For patients (pts) living with metastatic breast cancer (mBC), treatment with immune checkpoint inhibitors (ICI) offers the promise of improved outcomes. However, immune-related adverse events (irAE) are a potentially irreversible, dose-limiting toxicity of treatment and can significantly impact quality of life. There is a lack of real-world data on incidence of and risk factors for irAE among patients with mBC. Methods: This multi-institutional, retrospective study identified pts with mBC treated with ICI between 2014-2024. Pts on ICI therapy at time of study were excluded. Demographic, clinical, treatment, and irAE data were collected. Events were considered irAE if the treating team indicated toxicity was related to or likely related to ICI. Descriptive statistics were used to describe incidence of irAE. Pearson’s chi-squared, two-sample T tests, simple and multivariable logistic regression models were used to evaluate risk factors for irAE including demographics, body mass index (BMI), comorbidities, breast cancer subtype, menopausal status, Programmed Death-Ligand 1 (PD-L1) expression, baseline lab values, ICI type, number of ICI cycles, combination vs. monotherapy, and clinical trial participation. Results: 277 mBC pts were included. Mean age at start of ICI was 54 (SD 13.7, range 23-89). The study population was racially diverse (63.8% White, 8.7% Black, 12% Asian, 2.5% Latino), mostly pre-menopausal (52.3%), and more likely to have triple negative breast cancer (62.8%) than hormone receptor positive (31.0%) or human epidermal growth factor receptor 2 (HER2) positive (6.1%) disease. 18.1% had documented PD-L1 positivity. 15.9% had a history of autoimmune disease at time of ICI initiation. 128 (46.2%) experienced a total of 173 irAEs, most commonly thyroiditis (20.8%), rash (20.2%), and colitis (19.1%). 24.0% of irAEs were grade 3-4 at onset. IrAEs per patient ranged from 0-4; pts who experienced irAE had 1.35 (SD 0.63) on average. Pts were on ICI for an average of 138.4 (range 0-1380) days before their first irAE. Compared to those without irAE, mBC pts with irAE were more frequently post-menopausal (54.3% vs. 42.0%, p=0.04), more likely to have hyperlipidemia (HLD; 33.9% vs. 22.0%, p=0.03), had more comorbidities (1.0 vs. 0.7, p=0.01), and had higher baseline hemoglobin (Hb; 12.4 vs. 11.5, p<0.01). In unadjusted logistic models, being post-menopausal (OR 1.30, p=0.03) and having HLD (OR 1.78, p=0.03) were associated with greater likelihood of irAE. Similarly, for each additional baseline comorbidity (among type II diabetes, hypertension, lung disease, HLD, chronic kidney disease, and coronary artery disease) and for each increased point of baseline Hb, the odds of irAE increased by 30% (OR 1.30, p=0.02) and 40% (OR 1.40, p<0.01), respectively. Increasing cycles of ICI had a modest but statistically significant increased association with irAE (OR 1.04, p<0.01). Adjusted for demographics, clinical factors, comorbidities, baseline labs and treatment characteristics, higher baseline Hb (OR 1.30, p=0.02) and increasing number of ICI cycles (OR 1.03, p=0.03) were associated with increased likelihood of irAE. Conclusions: For a diverse population of women with mBC treated with ICI, real-world data demonstrates that irAEs are more common than previously reported in trials, with 46.2% of pts affected. The most common irAE, thyroiditis, often leads to irreversible physical effects. Pts that developed irAE were more frequently post-menopausal and had a greater number of baseline comorbidities. Adjusting for covariates, higher baseline Hb levels and increasing number of ICI cycles were associated with increased risk of developing irAE. This study identifies the frequency of and risk factors for irAEs among mBC pts, for whom balancing quality of life with prolonged courses of ICI therapy is paramount. This study may help mBC patients and their providers make informed decisions about ICI treatment. Citation Format: Nikita Baclig, Andrew Soliman, Saya Jacob, Alexis LeVee, Samantha Fisch, Carolyn Face, Madhuri Chengappa, Saliha Chaudhry, Dame Idossa, Laura Huppert, Laura Quintal, Michelle Melisko, Melanie Majure, Jo Chien, Joanne Mortimer, Anne Blaes, Hope S. Rugo, Melissa Lechner, Kelly E. McCann. Incidence and Risk Factors of Immune-Related Adverse Events in Metastatic Breast Cancer Patients: Findings from a Multi-Institutional Study [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P1-04-07.

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