Psoriasis is a lifelong disease which requires treatment adherence for successful management. Considering the complexity of this pathology, the combination of active pharmaceutical ingredients with a synergistic mechanism of action can improve the safety and efficacy of the treatment with respect to the conventional monotherapy. Moreover, a fixed dose of therapeutic agents in a topical formulation offers the possibility to simplify administration, reduce the doses of each active ingredient, and improve patient's compliance. Among the first-line treatments in mild to moderate psoriasis, the formulation of calcipotriol (Cal) and betamethasone dipropionate (BD) in a single vehicle is challenging due to their chemical incompatibility in an aqueous environment and the formation of degradation products. Based on these considerations, this review aims to provide an overview on the biopharmaceutical properties of Cal/BD fixed-dose combination products available on the market (namely ointment, oleogel, foam, and O/W cream), highlighting also the novel approaches under evaluation. The main differences among topical formulations are discussed considering the different features of the anatomic districts involved in psoriasis and the patient's adherence. Moreover, since in vitro experiments are fundamental to evaluate the skin permeation profile during the development of an efficacious medicinal product, special emphasis is given to models proposed to mimic psoriatic lesions.
Keywords: chemical compatibility; epidermal barrier; microneedles; nanovectors; polyaphron dispersion; psoriasis model; rheological properties; skin permeability; topical formulation; transepidermal water loss.