Used in the treatment of cystic fibrosis, VX809 (also known as lumacaftor) helps correct the folding and processing of the defective CFTR protein, particularly the F508del mutation. It works by stabilizing the misfolded protein, allowing it to reach the cell surface and function more effectively in chloride ion transport. Often combined with ivacaftor, this compound improves lung function and reduces the frequency of respiratory exacerbations in patients. It is part of a class of drugs known as CFTR modulators, which target the underlying cause of cystic fibrosis rather than just managing symptoms.