Epidemiology and treatment patterns of idiopathic multicentric Castleman disease in the era of IL-6–directed therapy

The epidemiology of human herpesvirus-8–negative/idiopathic multicentric Castleman disease (iMCD) remains incompletely understood. Prior epidemiologic studies of CD and iMCD have been hampered by difficulties in accurate case ascertainment resulting from a lack of uniform diagnostic criteria and a disease-specific International Classification of Diseases (ICD) code. In this study, we provide reliable estimates of CD and iMCD in the United States using a novel claims-based algorithm that includes a CD-specific ICD (10th revision) diagnosis code (D47.Z2) supported by the presence of ≥2 claims codes corresponding to the minor criteria from the international evidence-based diagnostic criteria for iMCD. We additionally analyzed the treatment classes and patterns in the clinical course of patients with iMCD. Using an administrative claims database of 30.7 million individuals enrolled between 1 January 2017 and 31 December 2018, we identified 254 patients with iMCD, with an estimated annual incidence and prevalence of 3.4 (95% confidence interval [CI], 1.4-9.2) and 6.9 (95% CI, 3.7-13.3) cases per million, respectively. Among patients with iMCD, 39% received corticosteroid monotherapy, 33.1% received no iMCD-directed treatment, and 9.8% received interleukin-6 (IL-6)–targeted therapy with tocilizumab or siltuximab. Siltuximab, which is the only US Food and Drug Administration–approved treatment and established first-line treatment recommendation, was used in only 8.7% of patients with iMCD. This study provides the most up-to-date understanding of the iMCD disease burden in the United States and identifies a major unmet treatment need for IL-6–directed therapy in this vulnerable cohort.