4‑Amino-7,8-dihydro-1,6-naphthyridin-5(6H)‑ones as Inhaled Phosphodiesterase Type 4 (PDE4) Inhibitors: Structural Biology and Structure–Activity Relationships

Rational design of a novel template of naphthyridinones rapidly led to PDE4 inhibitors with subnanomolar enzymatic potencies. X-ray crystallography confirmed the binding mode of this novel template. We achieved compounds with double-digit picomolar enzymatic potencies through further structure-based design by targeting both the PDE4 enzyme metal-binding pocket and occupying the solvent-filled pocket. A strategy for lung retention and long duration of action based on low aqueous solubility was followed. In vivo efficacies were measured in a rat lung neutrophilia model by suspension microspray and dry powder administration. Suspension microspray of potent compounds showed in vivo efficacy with a clear dose–response. Despite sustained lung levels, dry powder administration performed much less well and without proper dose–response, highlighting clear differences between the two formulations. This indicates a deficiency in the low aqueous solubility strategy for long duration lung efficacy.