Essential Chemistry and Biochemistry Associated with Low Temperature and Freezing - Freeze-drying of Pharmaceuticals and Biopharmaceuticals - page 69

workers appears to be due mainly to the choice of water potential
functions and approximations in the permitted motional (rotation,
libration) degrees of freedom within the PHC molecules. It must also
be borne in mind that any agreement between computed and known
experimental properties is in itself no evidence for the correctness of the
simulation.
A characteristic property of ternary and multicomponent aqueous
solutions, especially where at least one of the components is a polymer,
is the occurrence of demixing, resulting in the appearance of two
aqueous liquid phases in equilibrium. This phenomenon finds practical
application as liquid/liquid extraction in the downstream processing of
proteins. Quite remarkable, however, is the coalescence of such coex-
isting phases into a homogeneous solution upon cooling. This phenom-
enon, known as ''lower critical demixing'', is related to that of cold
inactivation of proteins, already discussed earlier, see also Figure 1, 49
but similar behaviour is observed for mixtures of some small molecules
in aqueous solutions. By extrapolating from the mobile liquid to the
immobile liquid, the glass, it is not inconceivable that a similar process,
the separation of a homogeneous solution into two coexisting phases,
might occur during freezing, so that the separated phases would then
persist in the vitrified system. Indeed, the existence of two amorphous
phases in frozen and freeze-dried solutions has been reported; 47 the
phenomenon has been given the name of polyamorphism, but it is still
much debated in the literature 83 . Its possible impact on freeze-drying
operations is clear, especially because vial-to-vial differences in texture
and reconstitution behaviour cannot be ruled out where the possibility
of random generation of polyamorphism exists.
These examples of physical and chemical changes that may accom-
pany the removal of water by freezing will serve to illustrate that the
freezing of multicomponent solutions, such as are common in pharma-
ceutical formulations, is by no means a simple affair. The removal of
water should be thoroughly investigated and understood in terms of
phase changes, mutual miscibilities, non-equilibrium states and slow
kinetics that might help efforts to characterise the freeze-concentrate.
Only by such a better understanding can the product and process
development stages be refined to yield high-quality products by eco-
nomic processing.