Aided by regulators' buy-in on an interim endpoint that may hold the key to accelerated approvals in the U.S. and Europe, Retrophin Inc. is kicking off a pivotal phase III test of sparsentan, a potential treatment for the rare kidney disorder focal segmental glomerulosclerosis (FSGS). With the first patient enrolled, the company will pay licensor Ligand Pharmaceuticals Inc. a $4.6 million milestone.