Abstract PS14-03: Delayed and post-treatment immune toxicities in patients with breast cancer receiving immune checkpoint inhibitors

Abstract Background: Immune checkpoint inhibitors (ICI) have improved outcomes in patients (pts) with breast cancer, however identification and management of immune-related adverse events (irAE) remains challenging. Delayed irAE have been observed, occurring even after ICI cessation. Prior analyses of delayed irAE have focused on patients with melanoma, lung, and head 90 days after ICI start; post-treatment irAE were defined as occurring 60 days after ICI discontinuation. Events were categorized as irAE if provider notes indicated that the toxicity was related to or possibly related to ICI. Results: 320 consecutive pts with breast cancer (219 metastatic, 96 localized disease) underwent treatment with ICI, of which 79 (25%) developed delayed irAE (40 with metastatic disease, 39 with localized disease). 54 pts had triple negative breast cancer (TNBC), 22 had hormone receptor positive (HR+) disease, and 4 had HER2+ disease. 44 pts received ICI on a clinical trial. The median number of ICI doses was 8, and the median length of follow-up after the first ICI infusion with 29 months. For delayed iRAE, median time to onset was 183 days (range 91-1800 days) with a total of 103 delayed irAE events (see table). 24 pts (7.5%) developed a post-treatment irAE with a total of 27 events. For post-treatment irAE, median time to onset was 112 days after ICI cessation (range 69-1100 days) and median time on ICI was 63 days (range 22-1724 days). 11 of 24 patients who experienced a post-treatment irAE also experienced an irAE while on treatment. Hypothyroidism, colitis, adrenal insufficiency, and hepatitis were the most common irAE at all time points. Rash primarily occurred while still on ICI. All irAE at any time point either self-resolved, resolved with systemic steroids, or continued with indefinite hormone replacement. 45 irAE required systemic steroids, not including hydrocortisone replacement for adrenal insufficiency. There were no grade 4 or 5 irAEs. Conclusions: This is the first evaluation of delayed/post-treatment irAE in a large cohort of patients with breast cancer. 79 pts (25%) experienced delayed irAEs occurring ≥ 90 days after ICI start, with median time to onset of 183 days. 89% of patients with delayed irAE also experienced an irAE at an earlier time point, indicating that early irAE may be a risk for delayed irAE. 24 pts (7.5%) experienced post-treatment irAE with median onset of 121 days after ICI cessation. Diagnosis of delayed irAE requires heightened awareness and prompt treatment. Future work is needed to identify key clinical and molecular markers that predict risk for delayed irAE to better inform treatment 1 delayed irAE 21 (27% of pts with delayed irAE) Patients with delayed irAEs who also had irAE occurring 90 days after ICI start 70 (89% of pts with delayed irAE) Post-treatment irAE 24 (7.5) Types of delayed irAE (90 days after ICI start) Any Grade Grade 3 Hypothyroidism 24 (23) 1 (1) Colitis/Diarrhea 19 (18) 3 (3) Rash 14 (14) 0 Adrenal Insufficiency/Hypophysitis 11 (11) 4 (4) Hepatitis 10 (10) 3 (3) Pneumonitis 4 (4) 1 (1) Arthralgia 4 (4) 0 Diabetes mellitus 3 (3) 0 Mucositis 2 (2) 1 (1) Neuropathy 2 (2) 0 Neutropenia 2 (2) 1 (1) Bronchitis 1 (1) 0 Fever 1 (1) 0 Neuromuscular Weakness 1 (1) 0 Autoimmune Hemolytic Anemia 1 (1) 1 (1) Myocarditis 1 (1) 1 (1) Acid Reflux 1 (1) 0 Sarcoidosis 1 (1) 0 Vitiligo 1 (1) 0 Types of post-treatment irAE (60 days post ICI cessation) Any Grade Grade 3 Hypothyroidism 7 (26) 2 (7) Colitis 7 (26) 3 11) Adrenal Insufficiency/Hypophysitis 6 (22) 1 (4) Hepatitis 4 (15) 3 (11) Arthralgia 1 (4) 0 Neuropathy 1 (4) 0 Neutropenia 1 (4) 1 (4) Rash 1 (4) 0 Citation Format: Saya Jacob, Samantha Fisch, Carolyn Face, Laura Huppert, Zoe Quandt, Laura Quintal, Michelle Melisko, Melanie Majure, Jo Chien, Anne Blaes, Hope Rugo. Delayed and post-treatment immune toxicities in patients with breast cancer receiving immune checkpoint inhibitors abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS14-03.