Background: Immune checkpoint inhibitors (ICIs) have revolutionized cancer treatment, significantly improving patient outcomes across various malignancies. However, ICIs exhibit unique adverse event profiles, distinct from those of conventional cancer therapies. In addition to de novo inflammatory side effects, ICIs frequently exacerbate pre-existing autoimmune diseases. Rheumatic immune-related adverse events (rh-irAEs), characterized by musculoskeletal involvement, have become increasingly relevant in rheumatology practice, often requiring prolonged immunosuppressive therapy beyond glucocorticoids. Despite their clinical significance, rh-irAEs are underreported, and their management remains largely empirical due to a lack of evidence-based guidelines. Concerns also exist regarding the impact of disease-modifying antirheumatic drugs (DMARDs) on cancer outcomes. In May 2024, the ERIN registry (Registry for the Documentation of Rheumatic Immunotherapy-Related Adverse Events) was established at LMU University Hospital as a national database under the auspices of the German Society for Rheumatology (DGRh) and the German Society of Internal Medicine (DGUM) to systematically document rheumatological side effects related to ICIs. The main objectives of the registry are to better understand the clinical presentations of rh-irAEs, their associations with specific ICIs, and the most effective treatment options, as well as to identify high-risk patients and biomarkers for specific outcomes. Ultimately, the data should help to develop evidence-based therapeutic guidelines.

Objectives: To describe the clinical and demographic characteristics of the first 30 patients included in the ERIN registry, documenting ICI-associated rheumatic adverse events, their treatment, and their clinical course.

Methods: The ERIN registry is a national, prospective, and retrospective multicenter observational registry designed for use by healthcare professionals. Data collection includes patients aged 18 years or older, with entries recorded either longitudinally or as one-time submissions. The collected data cover a variety of variables, including demographics, cancer type, checkpoint inhibitor regimen, rh-irAE characteristics, management approaches, and outcomes. The registry supports the classification of conditions such as arthritis, arthralgia, tenosynovitis/enthesitis, vasculitis, myositis, and conditions resembling collagenosis, polymyalgia rheumatica (PMR), spondyloarthritis (SpA), or sarcoidosis. The registry is approved by the ethics committee and the data protection officer of LMU University Hospital. The initial analysis of the data provides a demographic overview of the patients enrolled.

Results: Since its initiation, the ERIN registry has enrolled 30 patients (56.7% male, 17/30; 43.3% female, 13/30), with a median age of 63.5 years (IQR: 58–74 years). The most frequently reported primary malignancy was malignant melanoma (46.7%, 14/30), followed by non-small cell lung carcinoma (NSCLC), urothelial carcinoma, breast cancer, and squamous cell carcinoma, each accounting for 10.0% (3/30). Additional malignancies included renal cell carcinoma (6.7%, 2/30), pleural mesothelioma (3.3%, 1/30), and diffuse large B-cell lymphoma (3.3%, 1/30). Immunotherapeutic regimens consisted of nivolumab monotherapy in 30.0% (9/30), pembrolizumab monotherapy in 30.0% (9/30), and nivolumab/ipilimumab combination therapy in 23.3% (7/30). The remaining patients received other ICIs, such as atezolizumab, durvalumab, or cemiplimab. The most common rh-AE was arthritis, reported in 70.0% (21/30) of patients. Among these, 23.3% (7/30) had concurrent inflammatory conditions, including tenosynovitis (10.0%, 3/30), enthesitis (10.0%, 3/30), and giant cell arteritis (3.3%, 1/30). PMR-like syndromes and SpA-like conditions were observed in 10.0% (3/30) each. One case involved the exacerbation of a pre-existing inflammatory rheumatic disease. Interestingly, a sarcoidosis-like condition was documented in one case. 23.3% of patients (7/30) required temporary discontinuation of ICI therapy. Despite the relatively small sample size of the registry, preliminary observations have yielded important insights. Notably, patients with SpA were considerably younger, with a median age of 46 years, compared to the overall cohort.

Conclusion: The ERIN registry offers valuable insights into the diverse spectrum of rh-irAEs associated with ICIs, highlighting their clinical complexity and the need for tailored management strategies. Arthritis was the most frequently observed rh-irAE, with a significant proportion of cases involving concurrent inflammatory conditions like tenosynovitis and enthesitis. Younger patients with SpA-like manifestations suggest demographic distinctions within rh-irAE subtypes. The heterogeneity of rh-irAEs underscores the urgent need for evidence-based guidelines to optimize treatment while preserving oncologic outcomes. Cross-sector collaboration and integration with global initiatives such as the Side Effect Registry for Immuno-Oncology (SERIO) are essential to enhance understanding and improve outcomes for patients undergoing immunotherapy.

REFERENCES: NIL.

Acknowledgements: NIL.

Disclosure of Interests: None declared.

© The Authors 2025. This abstract is an open access article published in Annals of Rheumatic Diseases under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). Neither EULAR nor the publisher make any representation as to the accuracy of the content. The authors are solely responsible for the content in their abstract including accuracy of the facts, statements, results, conclusion, citing resources etc.