The detailed structure of a pharmaceutical nanosuspension was investigated using three nuclear magnetic resonance (NMR) methods: solid-state, solution-state, and high resolution-magic angle spinning (HR-MAS) NMR. Carbamazepine (CBZ) and CBZ-saccharin (SAC) cocrystal nanosuspensions were prepared by wet-milling with hydroxypropyl methylcellulose (HPMC) and sodium dodecyl sulfate (SDS) as stabilizing agents. Solid-state 13C NMR indicated the presence of not only the crystalline drug substance but also solid-state HPMC, even though HPMC was used as an aqueous solution to prepare the nanosuspensions. Solution-state 1H NMR of the nanosuspensions with and without ultracentrifugation pretreatment indicated that a fraction of the CBZ, SAC, and SDS formed a solid or semisolid phase on the surface of the nanoparticles and was in equilibrium between the dissolved and undissolved states. 1H HR-MAS NMR was highly effective in detecting and quantifying the semisolid phase on the surface of the nanoparticles. From these comprehensive NMR studies, it was concluded that the nanosuspension was composed of crystalline drug core particles surrounded by a semisolid phase consisting of the drug and stabilizing agents. The semisolid phase on the nanoparticle surface was in equilibrium with the solution phase and contributed to the stabilization of the nanoparticle by steric hindrance and electrostatic repulsion.
Keywords: cocrystal; high-resolution NMR; interfacial structure; molecular mobility; nanococrystal; nanocrystal; solid-state NMR; solution-state NMR.