Suspension Quality by Design
Suspensions are dispersions of a drug in a liquid, usually an aqueous vehicle. Suspensions are the liquid presentation of choice for poorly soluble biopharmaceutical classification system (BCS) Class II/IV drugs, either as commercial products, or as a reference formulation against which to validate release from immediate-release solid dose formulations. More soluble BCS Class I/III drugs can also be presented as suspensions with the advantage of taste masking compared with a simple solution or syrup. Suspensions are often thought to be complicated and thermodynamically unstable, possibly because most references in the literature focus on the complexities of drug-specific flocculation in sedimenting dispersions. The structured vehicle approach to suspension development is less reported in the literature despite simplicity of formulation, general (as opposed to drug-specific) applicability, low viscosity, and absence of sedimentation throughout the product lifecycle. This chapter focuses on the development of aqueous structured vehicle suspensions. Relevant rheological concepts are presented to the extent necessary to avoid inappropriate design approaches based on viscosity. Guidance is given on formulation and manufacture to ensure simple robust products with minimal scale-up problems.