The role of the carrier in the formulation of pharmaceutical solid dispersions. Part II: amorphous carriers.

INTRODUCTION: Amorphous solid dispersions are considered as one of the most powerful strategies to formulate poorly soluble drugs. They are made up of an active pharmaceutical ingredient (API) dispersed at the molecular level in an amorphous polymeric carrier. As the latter component constitutes the largest part of the formulation, its characteristics will contribute to a large extent to the properties and behavior of the solid dispersion. Areas covered: Amorphous polymers are most often used in modern solid dispersion formulations. This review discusses carrier properties like molecular weight, conformation, hygroscopicity, their stabilization effects, issues related to solid dispersion manufacturing technology, response to downstream processing, and potential to generate supersaturation, next to criteria to select a carrier to formulate stable amorphous solid dispersions. Expert opinion: Different amorphous carriers lead to solid dispersions with various properties in terms of physical stability, phase behavior and drug release rate and extent. Despite more than 50 years of intensive research in this field it remains difficult to predict what carrier is best suited for a given API, pointing to the complex nature of this formulation strategy. Sustained efforts to understand the link and complex interplay between material properties, processing parameters, physical stability and dissolution behavior are required from pharmaceutical scientists with a strong physicochemical background to shift the development from trial and error to science driven.