Concert Pharmaceuticals Inc. said in order to support dose selection for a future phase III trial of CTP-656 in cystic fibrosis (CF), the FDA will require its ongoing phase II trial to institute a washout period during which Kalydeco (ivacaftor, Vertex Pharmaceuticals Inc.) treatment would be withheld – potentially a hard ask for patients meeting the study's screening criteria of having been on at least three months of stable Kalydeco therapy.