Incidence of checkpoint inhibitor-associated inflammatory arthritis, immunomodulation and mortality in cancer patients on immunotherapy: a retrospective cohort study.

Objectives: Immune checkpoint inhibitor (ICI)-associated inflammatory arthritis (ICI-IA) occurs in 4-6% of ICI-treated patients based on one observational study. We identified cases of ICI-IA using administrative claims to study its incidence and characteristics at the population level.

Methods: We used the Medicare 5% sample to identify patients initiating ICIs. Cancer patients were identified by having ≥2 ICD-9/10-CM diagnosis codes from an oncologist for lung cancer, melanoma or renal/urothelial cancer. ICI-IA was defined as having two Medicare claims ≥30 days apart with combinations of ICD-9/10-CM diagnosis codes that favoured specificity. ICI-IA was identified in patients with a musculoskeletal diagnosis after ICI initiation, who had (i) no inflammatory arthritis or inflammatory rheumatic disease before ICI initiation ever, and (ii) no musculoskeletal complaint in the one year prior to ICI. We examined DMARD utilization and visits to rheumatology in patients with ICI-IA. Landmark analysis and a time varying Cox proportional hazards model for overall survival were constructed.

Results: The incidence of ICI-IA was 7.2 (6.1-8.4) per 100 patient years. Patients with ICI-IA had a mean (s.d.) age of 73.5 (7.0) years, 48% were women and 91% were white. Median (IQR) time from ICI initiation to first ICI-IA diagnosis was 124 (56, 252) days. Only 24 (16%) received care from a rheumatologist, and 24 (16%) were prescribed a DMARD (46% by a rheumatologist). The HR for mortality in patients with ICI-IA was 0.86 (95% CI 0.59-1.26, P = 0.45).

Conclusions: The incidence of ICI-IA identified in claims data is similar to that reported in observational studies; however, few patients are treated with a DMARD or are referred to rheumatologist. There was no difference in overall survival between ICI-treated patients with and without ICI-IA.