Semi-Solid Dosage Forms

Semi-solid dosage forms — ointments, creams, gels, and pastes — are designed principally for topical, and occasionally rectal or vaginal, application, and...

Semi-solid dosage forms — ointments, creams, gels, and pastes — are designed principally for topical, and occasionally rectal or vaginal, application, and their formulation is governed largely by the composition of the base or vehicle in which the drug is incorporated. Ointments are typically formulated using an anhydrous hydrocarbon base such as white petrolatum, producing an occlusive film that reduces transepidermal water loss and is well suited to the treatment of dry, chronic dermatological conditions; their key quality attributes include consistency, homogeneity of drug distribution, particle size where the drug is suspended rather than dissolved, and compliance with microbial limits.

Creams are emulsified semi-solid systems and may be formulated as either oil-in-water systems, commonly employing cetyl alcohol and an emulsifying wax alongside an aqueous continuous phase, which are generally cosmetically preferred for their non-greasy feel and ease of washing, or as water-in-oil systems incorporating components such as lanolin and mineral oil, which provide a more occlusive, protective film better suited to very dry or damaged skin. Critical quality attributes for creams include viscosity, pH — generally targeted close to the physiological skin surface value — droplet size of the internal phase, drug content, and preservative efficacy, the last being particularly important given the aqueous phase present in these formulations.

Gels, formed through the incorporation of a gelling polymer such as carbomer, hydroxypropyl methylcellulose, or poloxamer 407 into an aqueous vehicle, are valued for their cosmetic elegance, ease of application, and, in many cases, favourable drug release characteristics suitable for evaluation by in-vitro release testing; critical attributes include viscosity, pH, typically controlled within a range of six to seven to ensure both polymer performance and skin compatibility, optical clarity, and drug content. Pastes, distinguished by a high concentration of insoluble powder, typically twenty to fifty per cent, dispersed within an ointment-type base, possess a notably stiff consistency well suited to protective applications requiring prolonged localisation at the site of application.

Across all semi-solid categories, formulation development must give particular attention to rheological behaviour, since the product must exhibit appropriate flow properties for ease of removal from its container and spreading on application, while simultaneously remaining sufficiently viscous to resist drainage from vertical or mobile application sites; this is frequently achieved through the selection of pseudoplastic or thixotropic rheology modifiers that reduce apparent viscosity under the shear of application while recovering higher viscosity at rest.