Perspectives in solubility measurement and interpretation

• Main Authors: Christel A.S. Bergström (Author), Alex Avdeef (Author)
• Format: Book
• Published: International Association of Physical Chemists (IAPC), 2019-04-01T00:00:00Z.
• Subjects: article
• Online Access: Connect to this object online.
• Notes: 1848-7718; 10.5599/admet.686
• Keywords: Solubility-pH; shake-flask solubility; intrinsic solubility; thermodynamic solubility; Henderson-Hasselbalch equation; supersaturation; pre-nucleation clusters, drug aggregates; drug salts; pharmaceutical cocrystals; Therapeutics. Pharmacology; RM1-950
• Source: ADMET and DMPK, Vol 7, Iss 2, Pp 88-105 (2019)

Several key topics in solubility measurement and interpretation are briefly summarized and illustrated with case studies drawing on published solubility determinations as a function of pH. Featured are examples of ionizable molecules that exhibit solubility-pH curve distortion from that predicted by the traditionally used Henderson-Hasselbalch equation and possible interpretations for these distortions are provided. The scope is not exhaustive; rather it is focused on detailed descriptions of a few cases. Topics discussed are limitations of kinetic solubility, 'brick-dust and grease-balls,' applications of simulated and human intestinal fluids, supersaturation and the relevance of pre-nucleation clusters and sub-micellar aggregates in the formation of solids, drug-buffer/excipient complexation, hydrotropic solubilization, acid-base 'supersolubilization,' cocrystal route to supersaturation, as well as data quality assessment and solubility prediction. The goal is to highlight principles of solution equilibria - graphically more than mathematically - that could invite better assay design, to result in improved quality of measurements, and to impart a deeper understanding of the underlying solution chemistry in suspensions of drug solids. The value of solid state characterizations is stressed but not covered explicitly in this mini-review.