Stability of the Metastable α-Polymorph in Solid Triglyceride Drug-Carrier Nanoparticles

Colloidal dispersions of crystalline nonpolar lipids are under intensive investigation as carrier systems in pharmaceutics and nutrition. In this context, the controlled preparation of particles in a metastable polymorphic state is of some interest for the delivery of active substances. In the present study, tristearin particles stabilized with three alpha-polymorph-preserving emulsifier regimes ((I) sodium glycocholate/saturated long-chain phospholipids, (II) sodium glycocholate, and (III) poly(vinyl alcohol) (PVA)) were investigated concerning the stability of the metastable alpha-polymorph after controlled crystallization of the particles from the melt. Upon long-term storage, the alpha-polymorph was preserved best in PVA-stabilized dispersions, followed by those stabilized with the glycocholate/phospholipid mixture and finally those stabilized solely with the bile salt. In particular for rapidly crystallized nanoparticles, the formation of an alpha-polymorph with highly reduced lamellarity was observed. According to time-/temperature-resolved synchrotron X-ray diffraction analysis with simultaneous DSC (differential scanning calorimetry) studies, this less-ordered alpha-polymorph transformed into the common, lamellar alpha-form upon heating. Although the presence of the less-ordered form is probably related to the extraordinarily high stability of the metastable alpha-polymorph observed in some of the dispersions, it could not completely prevent the transition into the stable beta-polymorph. The higher the transition temperature of the less-ordered alpha-form to the ordered one, the slower was the polymorphic transition to the stable beta-polymorph. To estimate the polymorphic stability of the differently stabilized particles upon isothermal long-term storage, standard DSC measurements on samples stored at 23 degrees C for 4 weeks seem to be of predictive value.