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The use of 5-aminolevulinic acid (ALA) as a protoporphyrin IX (PpIX) precursor for photodynamic therapy (PDT) became very popular in a short time. However, despite its advantages, ALA also has a drawback; it shows a poor ability to diffuse through biological membranes because of its low lipophilicity. As a consequence, a high dose of ALA must be administered in order to increase PpIX in the afflicted tissue at a level sufficient for PDT. A possible solution to this problem is the use of derivatives of ALA. ALA prodrugs are expected to have better diffusing properties as a result of their enhanced lipophilicity and are converted into the parent ALA after enzymatic hydrolysis. In this report, results are presented of the synthesis of a number of ALA derivatives. The ALA prodrugs were investigated regarding the optimum conditions for cell penetration and PpIX formation in an in vitro cellular test system. It is shown that several prodrugs do indeed enhance the amount of accumulated PpIX considerably as compared to ALA. Finally, the most promising prodrugs were tested in an animal model and showed increased PpIX formation under these conditions as well.
Kloek et al. (1996) studied this question.