Drug delivery of aminolevulinic acid from topical formulations intended for photodynamic therapy

Topical photodynamic therapy is used for a variety of malignant and pre-malignant skin disorders, including Bowen's Disease and Superficial Basal Cell Carcinoma. A haem precursor, typically 5-aminolevulinic acid (ALA), acting as a prodrug, is absorbed and converted by the haem biosynthetic pathway to photoactive protoprophyrin IX (PpIX), which accumulates preferentially in rapidly dividing cells. Cell destruction occurs when PpIX is activated by an intense light source of appropriate wavelength. Topical delivery of ALA avoids the prolonged photosensitivity reactions associated with systemic administration of photo-sensitisers but its clinical utility is influenced by the tissue penetration characteristics of the drug, its ease of application and the stability of the active agent in the applied dose. This review, therefore, focuses on drug delivery applications for topical, ALA-based PDT. Issues considered in detail include physical and chemical enhancement strategies for tissue penetration of ALA and subsequent intracellular accumulation of PpIX, together with formulation strategies and drug delivery design solutions appropriate to various clinical applications. The fundamental aspects of drug diffusion in relation to the physicochemical properties of ALA are reviewed and specific consideration is given to the degradation pathways of ALA in formulated systems that, in turn, influence the design of stable topical formulations.