This is a single-arm, multicenter, open-label Phase II, proof-of-mechanism study to evaluate the efficacy, safety, tolerability and pharmacokinetics of losmapimod in approximately 21 subjects with primary (idiopathic) focal segmental glomerulosclerosis (FSGS) and substantive proteinuria as indicated by a Urinary protein/creatinine Up/c ratio >=2 gram/gram (g/g) or 24 hr urine protein >=2 g/day. Losmapimod will be orally administered twice daily over a 24-week treatment phase followed by a 12-week follow-up for safety and relapse assessments.