Visualizing the Spatiotemporal Redistribution of Drugs in Destabilizing Nanoemulsions: A Quantitative Solution ¹H NMR...

While self-nanoemulsifying drug delivery systems (SNEDDS) spontaneously generate nanosized emulsions, they are susceptible to thermodynamic destabilization. Characterizing the spatiotemporal fate of drug molecules during this transition remains challenging due to the limited chemical resolution of conventional particle analysis. Herein, we employed solution-state ¹H NMR to elucidate the redistribution of three model drugs, fenofibrate (FFB), felodipine (FEL), and lopinavir (LPV), during the destabilization of a Labrafac PG-based nanoemulsion. While dynamic light scattering and cryo-transmission electron microscopy confirmed the progressive phase separation of the oil carrier into large oil droplets, NMR analysis revealed that drug redistribution is highly drug-specific rather than simple oil migration. FFB exhibited a redistribution profile proportional to the degree of oil phase separation, migrating into large droplets (∼38% redistribution), reflecting its higher solubility in the oil phase compared to the other drugs. In contrast, FEL and LPV were retained within the residual smaller droplets (∼89% and ∼99% retention, respectively), despite comparable separation of the oil phase. This retention suggests a higher affinity for the surfactant-rich interface relative to the oil core, rendering the drug redistribution independent of the oil phase separation. These findings demonstrate that monitoring the physical separation of the carrier is insufficient to determine the molecular state and localization of the drug. Consequently, quantitative ¹H NMR offers a powerful, noninvasive framework to overcome the analytical limitations of conventional methods, thereby enabling molecular-level design of robust lipid-based formulations.