Why do drug treatments fail in Sjogren's disease? Considerations for treatment, trial design and interpretation of clinical efficacy.

INTRODUCTION: Despite ongoing efforts to develop effective therapeutics, no disease-modifying drugs have been officially licensed for the indication of Sjogren's disease (SjD). This is partly due to heterogeneity in disease manifestations, which complicates drug target selection, trial design and interpretation of clinical efficacy in SjD. AREAS COVERED: Here, we summarize developments and comment on challenges in 1) identifying the right target for treatment, 2) selection of the primary study endpoint for trials and definition of clinically relevant response to treatment, 3) inclusion criteria and patient stratification, 4) distinguishing between disease activity and damage and 5) establishing the effect of treatment considering measurement error, natural variation, and placebo or nocebo responses. EXPERT OPINION: Targets that are involved in both the immune cell response and dysregulation of glandular epithelial cells (e.g. B-lymphocytes, type-I interferon) are of particular interest to treat both glandular and extra-glandular manifestations of SjD. The recent development of composite study endpoints (CRESS and STAR) may be a crucial step forward in the search for clinically effective systemic treatment of patients with SjD. Important additional areas for future research are symptom-based and/or molecular pathway-based patient stratification, prevention of irreversible damage, and establishing the effect of treatment.