Compound 552-02 is a potent, selective inhibitor of epithelial sodium channels that is effective in enhancing mucociliary clearance and is well tolerated when administered as a single dose by inhalation aerosol in normal healthy adult volunteers. Parion Sciences was developing 552 02 for the treatment of xerostomia, cystic fibrosis, chronic bronchitis, chronic obstructive pulmonary disease, radiation injuries. Compound 552-02 was specifically designed for aerosol delivery to the pulmonary system as a more selective, potent, long-acting ENaC blocker to promote an expansion in ASL volume, with or without hypertonic saline. Compound 552-02 was selected from a series of novel ENaC blockers to produce a selective block on airway epithelial sodium channels, with a potency up to 2 orders of magnitude greater than amiloride. 552-02 blocked the majority (95%) of Isc, with a calculated IC50 value of 7 nM. Drug development was discontinued.