How Molecular Mobility, Physical State, and Drug Distribution Influence the Naproxen Release Profile from Different Mesoporous Silica Matrices

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 γ-cyclodextrin (γ-CD) and poly(ethylene glycol) (PEG)-based polypseudorotaxane (PPRX) structure. The sublimation tendency of SA encapsulated in three polymorphic forms of the γ-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 13C and 1H magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) spectra and 13C spin-lattice relaxation time (T1) 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 1H MAS spectra and temperature dependency of the 13C carbonyl chemical shift. A positive correlation between the molecular mobility of SA and its sublimation rate was established. Moreover, the molecular mobility of γ-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.

Evidence of Strong Guest-Host Interactions in Simvastatin Loaded in Mesoporous Silica MCM-41

A rational design of drug delivery systems requires in-depth knowledge not only of the drug itself, in terms of physical state and molecular mobility, but also of how it is distributed among a carrier and its interactions with the host matrix. In this context, this work reports the behavior of simvastatin (SIM) loaded in mesoporous silica MCM-41 matrix (average pore diameter ~3.5 nm) accessed by a set of experimental techniques, evidencing that it exists in an amorphous state (X-ray diffraction, ssNMR, ATR-FTIR, and DSC). The most significant fraction of SIM molecules corresponds to a high thermal resistant population, as shown by thermogravimetry, and which interacts strongly with the MCM silanol groups, as revealed by ATR-FTIR analysis. These findings are supported by Molecular Dynamics (MD) simulations predicting that SIM molecules anchor to the inner pore wall through multiple hydrogen bonds. This anchored molecular fraction lacks a calorimetric and dielectric signature corresponding to a dynamically rigid population. Furthermore, differential scanning calorimetry showed a weak glass transition that is shifted to lower temperatures compared to bulk amorphous SIM. This accelerated molecular population is coherent with an in-pore fraction of molecules distinct from bulklike SIM, as highlighted by MD simulations. MCM-41 loading proved to be a suitable strategy for a long-term stabilization (at least three years) of simvastatin in the amorphous form, whose unanchored population releases at a much higher rate compared to the crystalline drug dissolution. Oppositely, the surface-attached molecules are kept entrapped inside pores even after long-term release assays.