Polymorphism
A polymorph is a solid material with at least two different
molecular arrangements that give distinct
crystal species. These differences disappear in the
liquid or the vapour state. Of concern are their relative
stabilities and solubility. The highest-melting
species is generally stable; other polymorphs are
metastable and convert to the stable form. There are
also potentially large differences in their physical
Fig. 8.3 Schematic differential scanning calorimeter thermogram.
properties so that they behave as distinct chemical
entities. Solubility (particularly important in suspensions
and biopharmaceutically), melting point,
density, crystal shape, optical and electrical properties
and vapour pressure are often very different for
each polymorph.
Polymorphism is remarkably common, particularly
within certain structural groups: 63% of barbiturates,
67% of steroids and 40% of sulphonamides
exhibit polymorphism.
The steroid progesterone has five polymorphs,
whereas the sulphonamide sulphabenzamide has
four polymorphs and three solvates. The importance
of polymorphism is illustrated by the biopharmaceutical
data for fluprednisolone (Fig. 8.4).
It is convention to number the polymorphs in
order of stability at room temperature, starting with
form I using Roman numerals. Form I usually has
the highest melting point and the lowest solubility; in
suspension formulation it is essential to use the least
soluble polymorph because of Ostwald ripening.
Accordingly, in preformulation the following
should be considered.
• How many polymorphs exist?
• How stable are the metastable forms?
• Is there an amorphous glass?
• Can the metastable forms be stabilized?
What is the solubility of each form?
Will a more soluble form survive processing and
storage?