intrinsic solubility

For a drug that can carry a charge, its solubility changes with pH — the ionized form usually dissolves much more readily than the neutral form. Intrinsic solubility, often written S0, strips that away: it is the solubility of the drug's fully unionized (neutral) form alone, measured at a defined temperature. It is the floor of solubility, the value you reach when ionization is suppressed.

Because the neutral species is the same regardless of which salt or buffer is around, intrinsic solubility is a fundamental, comparable property of the molecule itself. Combined with the pKa through the Henderson-Hasselbalch relationship, it lets you predict solubility at any pH — for example how a weak base will dissolve well in acidic stomach fluid but may precipitate in the more neutral small intestine.

In preformulation, intrinsic solubility flags whether solubility will be a development problem and guides strategy: a very low S0 points toward salt selection, cocrystals, particle size reduction, or an amorphous form to boost dissolution. The honest caveat is that S0 must be measured on a known, stable solid form at equilibrium, because polymorph conversion or a metastable form during the measurement can give a misleadingly high or unstable value.