This clinical trial is focused on studying the effects of a new treatment for cystic fibrosis, a genetic condition that affects the lungs and digestive system. The treatment being tested is called BI 3720931, which is an inhaled gene therapy. Gene therapy is a technique that uses genes to treat or prevent disease. In this study, the therapy is delivered through a nebulizer, a device that turns liquid medicine into a mist to be inhaled into the lungs.

The purpose of the study is to evaluate the safety, tolerability, and effectiveness of a single dose of BI 3720931 in adults with cystic fibrosis who cannot use other treatments known as CFTR modulators. The study will begin with an open-label phase, where all participants will receive the treatment to determine the best dose. This will be followed by a randomized, double-blind phase, where participants will receive either the treatment or a placebo without knowing which one they are getting. This helps to ensure that the results are not biased.

Participants will be monitored for changes in their lung function, specifically looking at a measure called FEV1, which stands for forced expiratory volume in one second. This is a common test to see how well the lungs are working. The study will also track any side effects that occur after receiving the treatment. The trial is expected to continue until 2027, with recruitment starting in late 2024.