The present invention describes peptide drugs that inhibit the interaction between CAL and CFTR, and other proteins in cystic fibrosis and other diseases. These invented drugs have been chemically optimized to impart solubility, stability, cell permeability, mucus penetration, intracellular targeting and sequestration, increased potency and non-immunogenicity, while conserving and imparting efficacy. This renders these compositions suitable for human use, which is exemplified by use in the treatment of cystic fibrosis.