Innovative drug delivery strategies for topical photodynamic therapy using porphyrin precursors.

Photodynamic therapy (PDT) has been extensively investigated as a treatment for tumors and neoplasias of the skin, bladder, mouth, and female reproductive tract. The most convenient drug delivery route, when focusing on the photodynamic treatment of such tumors and neoplasia, is the transdermal. However, with the inherent "barrier function" of the stratum corneum of the skin, drugs with high molecular weight (> 500 Daltons) or extremes of polarity will find it difficult to successfully cross the skin. Therefore, preformed photosensitizers, which are generally large, highly conjugated molecules, are not commonly used in topical PDT. This has led to the idea of endogenous photosensitization using the small (167.8 Daltons), although hydrophilic, 5-aminolevulinic acid (ALA) being the most frequently employed agent in modern topical PDT. Although clinical application of ALA and its bimolecular effects within target cells remain as primary research themes, the design and evaluation of delivery systems required for effective photosensitizer administration have been less well addressed. This paper briefly reviews traditional approaches to topical delivery of ALA and its esters, and highlights several innovative strategies recently employed to increase the efficacy of ALA-PDT.