Aggregation behavior of fosinopril sodium--a new angiotensin-converting enzyme inhibitor.

Fosinopril sodium is an effective new angiotensin-converting enzyme (ACE) inhibitor that is very useful for the clinical treatment of hypertension. After oral administration, fosinopril sodium is only partially absorbed (about one third of the drug). We studied the solution behavior of fosinopril sodium in aqueous media by a combination of high-resolution nuclear magnetic resonance (NMR) spectroscopy and laser light scattering (LLS). LLS characterizes the self-association properties of fosinopril sodium in solution, and NMR chemical shifts provide information on molecular conformation and interactions. The results revealed that fosinopril sodium has a micelle-like self-association behavior with a critical micelle concentration (cmc) approximately 1.5 mg/mL. Hydrophobic interactions could induce formation of micellar aggregates, which had a narrow hydrodynamic size distribution, with an average diameter of approximately 150 nm at concentrations above the cmc. The surface activity and self-association of fosinopril sodium may be responsible for its early observed concentration-dependent stability in aqueous solution, unexpected decrease in solubility in the presence of metal ions, as well as the limited absorption in clinical studies.