Selection of study endpoints and patients for clinical trials in primary Sjögren's syndrome

In the last decade, many randomised controlled trials (RCTs) with biological DMARDs (bDMARDs) have been performed in patients with primary Sjögren's syndrome (pSS). Unfortunately, no bDMARD has yet been approved for systemic treatment of pSS. The heterogeneity of disease manifestations raises two essential questions: 1) which outcome measure is valid, reliable and responsive to demonstrate treatment efficacy and should be used as primary study endpoint? and 2) which pSS patients should be included in clinical trials? Both the selection of the primary study endpoint and the selection of patients are crucial and evolving issues in clinical trial design in pSS. This article summarises the history and comments the selection of primary study endpoints including the novel development of composite endpoints. Furthermore, this article gives an overview of inclusion criteria used for phase II and III trials, and illustrates by data-analysis based on two prospective observational cohorts that each additional selection criterion will (largely) decrease the number of eligible patients in daily clinical practice.