The purpose of this study is to learn more about the effectiveness and safety of VX-561 used in patients with cystic fibrosis. Patients who have cystic fibrosis and are currently taking a stable dose of ivacaftor (IVA) are invited to take part in this research study. VX-561 is a form of IVA, an approved medication for some types of CF, that lasts longer in the body, allowing the medicine to be taken once a day instead of twice a day. Participation could last for approximately 20 weeks. There will be approximately 88 people participating in this study.