Semi-solid dosage forms

"Praeparationes semi solidae ad usum dermicum

Semi-solid preparations for cutaneous application are defined by the monograph (07/2021:0132) of European Pharmacopoeia supplement 10.7 as "preparations intended to be applied to the skin to deliver active substances for local or systemic action, or to exert an emollient or protective action. They are homogeneous in appearance. [...]. Semi-solid preparations for cutaneous application consist of a base, simple or compound, in which one or more active substances are usually dissolved or dispersed. Depending on its composition, this base can influence the activity of the preparation. The base used may be monophase or multiphase, and depending on its nature may impart hydrophilic or hydrophobic properties to the preparation. The preparation may contain preservatives and other suitable excipients such as antioxidants, stabilizers, emulsifiers, thickeners and penetrants. These excipients do not interfere with the desired medicinal action and, at the concentrations chosen, do not cause toxic effects or noticeable local irritation". There are several categories of semi-solid preparations for cutaneous application: ointments, creams, gels, pastes and poultices. Until supplement 10.5 of the European Pharmacopoeia (effective July 2021), medicated plasters were included in this monograph. They are now the subject of their own monograph (07/2021:3032) and are not covered in this document. Another form of the system-on-a-support type is the cutaneous device, which corresponds to transdermal systems (patches) which, unlike plasters, provide a systemic effect. These are also described in a separate monograph (07/2021:1011) and therefore not covered in this article. Paste systems are galenic forms in a physical state intermediate between a solid and a liquid. They have properties close to those of solids at rest, but can nevertheless be transformed into viscous liquids by the application of sufficient mechanical stress to enable them to be manipulated, deformed and spread during use.

A definition can be given in line with the common sense associated with practical applications: a pasty system is a viscous, self-supporting material. The term semi-solid sums up this combination of mechanical properties: the material is a solid that can undergo large-scale irreversible (viscous) deformation. In the field of pharmaceutical and cosmetic applications, pasty systems are the galenic forms of choice for the topical administration of pharmaceutical or cosmetic active substances. In short, the appeal of pasty forms lies in their two fundamental properties: these materials are deformable and adherent.

The range of possible physico-chemical properties is varied, adjusted by the type of formulation and the choice of excipients...