Assessing the impact of Losmapimod on proteinuria in idiopathic focal segmental glomerulosclerosis

ABSTRACT INTRODUCTION Idiopathic focal segmental glomerulosclerosis (FSGS) is a leading cause of nephrotic syndrome and end-stage renal disease. In preclinical models and biopsies of human FSGS kidneys, p38 mitogen-activated protein kinase (MAPK) has demonstrated enhanced activity; and p38 MAPK inhibition has improved disease markers. This proof-of-concept trial aimed to assess efficacy, safety, tolerability, and pharmacokinetics of losmapimod, an oral p38 MAPK inhibitor, in humans with FSGS. METHODS A single-arm, multicenter, open-label, Phase II trial (NCT02000440) was conducted in FSGS adults; proteinuria ≥2.0 g/day; eGFR ≥45 mL/min per 1.73 m2; blood pressure RESULTS Seventeen patients received ≥1 losmapimod dose. No patients achieved the primary endpoint; therefore, the study was terminated following a prespecified interim analysis. At Week 24, proteinuria reductions between 20% and 20% in 3 patients. One patient achieved a proteinuria response (≥50% reduction) at Week 2 but subsequently relapsed. Losmapimod pharmacokinetics were consistent with prior studies. No serious AEs were reported. CONCLUSION p38 MAPK inhibition with losmapimod did not result in ≥50% reduction of proteinuria in FSGS patients. However, study population heterogeneity may have contributed to our negative findings and therefore this does not eliminate the potential to demonstrate benefit in a population more sensitive to p38 MAPK inhibition if identifiable in the future by precision-medicine methods.