This is an experimental, single-blind, randomized, placebo-controlled, clinical research study of a study drug called brensocatib. The goal of the study is to find a safe and accepted brensocatib dose in people with cystic fibrosis. This study will also help to learn the pharmacokinetic (or "PK") and pharmacodynamic (or "PD") properties of brensocatib. As the Sponsor (Insmed Incorporated) wants to assess the effect of brensocatib in participants with cystic fibrosis, samples of sputum and blood will be collected in this study. About 36 to 48 adults will take part in this study and the duration of this study is expected to be 84 days or less.