Novel pKa/DFT-Based Theoretical Model for Predicting the Drug Loading and Release of a pH-Responsive Drug Delivery System

Herein, we present a theoretical model that combines classical pKa theory with quantum mechanical calculations to predict the extent of interaction between acid-/base-dependent species over a full range of pH conditions. To demonstrate the theoretical model, we have predicted the drug loading and release of a pH-responsive drug delivery system consisting of sulfasalazine, an anionic anti-inflammatory drug molecule, loaded onto the positively charged trimethylammonium (TA)-functionalized mesoporous silica nanoparticle surface. The model relies on the possible combinations of pH-dependent states of the surface (S) and drug (D) molecules as neutral (0) and deprotonated (−1) states, whose relative probabilities depend on their pKa value and the desired pH. The four possible combinations were identified as S0D0, S0D–1, S–1D0, and S–1D–1, and periodic density functional theory calculations were performed for systems comprising drug fragments adsorbed onto a model TA-functionalized quartz surface to calculate th...