ABSTRACT
Semisolids constitute a significant portion of pharmaceutical dosage forms. They serve as carriers for drugs that are topically delivered by way of skin, cornea, rectal tissue, nasal mucosa, vagina, buccal tissue, urethral membrane, and external ear lining (1). The primary advantage of topical delivery is the direct accessibility of the drug to affected tissues, with minimal, systemic side effects. In some cases, for systemic delivery, topical application enables delivery of therapeutic agents, avoiding first-pass gastrointestinal tract and hepatic metabolism and allowing maintenance of constant drug levels in the bloodstream. However, it is also generally recognized that the bioavailability of topically applied drugs is very low. The vehicle plays a key role in the appearance, feel, and successful application of a topical drug (2). Excipients, in large part, determine the physical properties of the vehicle as well as its ability to modify the stratum corneum or the mucosa to deliver the drug effectively. For example, it is possible to enhance the bioavailability by employment of an innocuous chemical means to reversibly improve the solubility of the drug in the barrier, e.g., stratum corneum, and facilitate diffusion of the drug through the barrier (3). Excipients, such as fatty acids, alcohols, amines, and amides, are absorbed into the barrier where they alter the overall solvent potential of the barrier. At the same time, the enhancers may disrupt the ordered lipid structure within the barrier, thereby lowering its viscosity. These physicochemical changes will facilitate drug partition from a topically applied formulation into the barrier as well as diffusion of drug molecules through the barrier. Thus, the understanding of excipients and proper selection is critical to successful formulation of semisolid dosage forms to meet the therapeutic needs.
