Controlled Sublimation Rate of Guest Drug from Polymorphic Forms of a Cyclodextrin-Based Polypseudorotaxane Complex and Its Correlation with Molecular Dynamics as Probed by Solid-State NMR

Encapsulation of active pharmaceutical ingredients (APIs) in confined spaces has been extensively explored as it dramatically alters the molecular dynamics and physical properties of the API. Herein, we explored the effect of encapsulation on the molecular dynamics and physical stability of a guest drug, salicylic acid (SA), confined in the intermolecular spaces of gamma-cyclodextrin (gamma-CD) and poly(ethylene glycol) (PEG)-based polypseudorotaxane (PPRX) structure. The sublimation tendency of SA encapsulated in three polymorphic forms of the gamma-CD/PEG-based PPRX complex, monoclinic columnar (MC), hexagonal columnar (HC), and tetragonal columnar (TC), was investigated. The SA sublimation rate was decreased by 3.0-6.6-fold and varied in the order of MC form > HC form > TC form complex. The C-13 and H-1 magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) spectra and C-13 spin-lattice relaxation time (T-1) indicated that the encapsulated SA molecules existed as the monomeric form, and its molecular mobility increased in the order of MC form > HC form > TC form complex. In the complexes, a rapid chemical exchange between two dynamic states of SA (free and bound) was suggested, with varying adsorption/desorption rates accounting for its distinct molecular mobility. This adsorption/desorption process was influenced by proton exchange at the interaction site and interaction strength of SA in the complexes, as evidenced by H-1 MAS spectra and temperature dependency of the C-13 carbonyl chemical shift. A positive correlation between the molecular mobility of SA and its sublimation rate was established. Moreover, the molecular mobility of gamma-CD and PEG in the complexes coincided with that of SA, which can be explained by fast guest-driven dynamics. This is the first report on the stability improvement of an API through complexation in polymorphic supramolecular host structures. The relationship between the molecular dynamics and physical properties of encapsulated API will aid in the rational design of drug delivery systems.