Probing the free-state solution behavior of drugs and their tendencies to self-aggregate into nano-entities

The free-state solution behaviors of drugs profoundly affect their properties. Therefore, it is critical to properly evaluate a drug's unique multiphase equilibrium when in an aqueous enviroment, which can comprise lone molecules, self-associating aggregate states and solid phases. To date, the full range of nano-entities that drugs can adopt has been a largely unexplored phenomenon. This protocol describes how to monitor the solution behavior of drugs, revealing the nano-entities formed as a result of self-associations. The procedure begins with a simple NMR ¹H assay, and depending on the observations, subsequent NMR dilution, NMR T2-CPMG (spin-spin relaxation Carr-Purcell-Meiboom-Gill) and NMR detergent assays are used to distinguish between the existence of fast-tumbling lone drug molecules, small drug aggregates and slow-tumbling colloids. Three orthogonal techniques (dynamic light scattering, transmission electron microscopy and confocal laser scanning microscopy) are also described that can be used to further characterize any large colloids. The protocol can take a non-specialist between minutes to a few hours; thus, libraries of compounds can be evaluated within days.