Solubility of sparingly-soluble ionizable drugs.

The experimental and computational basis of the pH-dependent measurement of solubility of sparingly-soluble ionizable drugs is reviewed. Recently described compound-sparing (but still accurate) approaches, suitable for application in preclinical development, and appropriate for the analysis of solubility of "problematic" molecules, are critically examined. A number of useful experimental methods are reviewed, including the miniaturized shake-flask microtitre plate, the micro solubility self-calibrating direct UV, potentiometric, and the micro dissolution methods. Several molecules were selected as case studies to illustrate important concepts, with re-analysis of literature data using recently established computational tools.