On the solubility advantage of a pharmaceutical’s glassy state over the crystal state, and of its crystal polymorphs

• Heat capacity data do not yield the solubility advantage of amorphous and metastable crystal pharmaceuticals.
• There is no reversible equilibrium of an amorphous solid with its saturated solution.
• Solubility advantage of an amorphous solid depends upon the solvent and other interactions.

At equilibrium, the saturation solubility and vapor pressure of a material in a state of high free energy are greater than in its state of low free energy. This knowledge from classical thermodynamics is currently used for increasing the solubility of crystalline pharmaceuticals by producing them in their glassy state, or in other solid states of high free energy. The ratio of the apparent saturation solubility of these solids to that of a crystal, calculated from the thermodynamic data of the pure solute, ϕcal, is called the solubility advantage, and it is used as a guide for increasing the solubility of a pharmaceutical. We argue that the ϕcal differs from the measured solubility ratio, ϕmeas, because, (i) ϕcal is independent of the solvent, but ϕmeas is not so, (ii) ϕcal would increase with the dissolution time monotonically to a constant value, but ϕmeas would first reach a maximum and then decrease, and (iii) approximations are made in estimating ϕcal and the effect of thermal history on high free energy solids is ignored. On the other hand, ϕmeas is affected by, (a) another chemical equilibrium in the solution, e.g., hydrogen-bond formation and ionic dissociation, (b) the production method and thermal history of a glass or an amorphous samples, and (c) mutarotation in the solution, isomerization or tautomeric conversion in the solid. We also discuss the effects of structural relaxation and crystallization on ϕmeas. The ϕmeas value of a (crystal) polymorph would be affected by all the three, and further if the polymorph is orientationally disordered. We provide evidence for these effects from analysis of the known data. The ϕmeas value is preferable over ϕcal.