Dissolution Mechanisms of Amorphous Solid Dispersions: A Close Look at the Dissolution Interface.

Area of Science:

• Pharmaceutical Sciences
• Materials Science
• Physical Chemistry

Background:

• Amorphous solid dispersions (ASDs) enhance solubility of poorly soluble drugs but are limited by low drug loading capacity.
• The 'limit of congruency' (LoC) signifies a critical drug loading threshold beyond which drug release significantly declines.
• Understanding the molecular mechanisms at the ASD/solution interface is crucial for overcoming LoC limitations.

Purpose of the Study:

• To investigate the molecular and phase behavior at the ASD/solution interface for copovidone-based ASDs.
• To elucidate the relationship between gel layer morphology and drug release kinetics.
• To establish a predictive model for ASD release behavior based on drug-polymer interactions.

Main Methods:

• Formulation of amorphous solid dispersions (ASDs) with two model compounds and copovidone.
• In situ observation of gel layer evolution and phase behavior using fluorescence confocal microscopy.
• Labeling of hydrophobic and hydrophilic phases with fluorescent probes to analyze morphology.

Main Results:

• Phase separation was observed within the gel layer of most ASDs studied.
• Hydrophobic phase morphology directly correlated with drug release: discrete phase yielded good release, continuous phase led to poor release.
• Stronger drug-polymer interactions promoted the formation of a continuous hydrophobic phase at lower drug loadings.

Conclusions:

• The study reveals complex molecular and phase dynamics at the ASD/solution interface involving copovidone-based ASDs.
• Hydrophobic phase morphology, influenced by drug-polymer interactions, is a key determinant of drug release.
• A thermodynamic approach can qualitatively predict ASD release behavior based on the strength of drug-polymer interactions.