Small Molecule Therapeutics The Use of Dipeptide Derivatives of 5-Aminolaevulinic Acid Promotes Their Entry to Tumor Cells and Improves Tumor Selectivity of Photodynamic Therapy

The use of endogenous protoporphyrin IX generated after administration of 5-aminolaevulinic acid (ALA) has led tomany applications in photodynamic therapy (PDT). However, the bioavailability of ALA is limited by its hydrophilic properties and limited cell uptake. A promising approach to optimize the efficacy of ALA-PDT is to deliver ALA in the form of prodrugs to mask its hydrophilic nature. The aimof this workwas to evaluate the potential of two ALA dipeptide derivatives, N-acetyl termi-nated leucinyl-ALA methyl ester (Ac-Leu-ALA-Me) and pheny-lalanyl-ALA methyl ester (Ac-Phe-ALA-Me), for their use in PDT of cancer, by investigating the generation of protoporphyrin IX in an oncogenic cell line (PAM212-Ras), and in a subcutaneous tumor model. In our in vitro studies, both derivatives were more effective than ALA in PDT treatment, at inducing the same protoporphyrin IX levels but at 50- to 100-fold lower concen-trations, with the phenylalanyl derivative being the most effec-tive. The efficient release of ALA from Ac-Phe-ALA-Me appears to be consistent with the reported substrate and inhibitor pre-ferences of acylpeptide hydrolase. In vivo studies revealed that topical application of the peptide prodrug Ac-Phe-ALA-Me gave greater selectivity than with ALA itself, and induced tumor photodamage, whereas systemic administration improved ALA-induced porphyrin generation in terms of equivalent doses administered, without induction of toxic effects. Our data sup-port the possibility of using particularly Ac-Phe-ALA-Meboth for topical treatment of basal cell carcinomas and for systemic administration. Further chemical fine-tuning of this prodrug template should yield additional compounds for enhanced ALA-PDT with potential for translation to the clinic. Mol Cancer Ther; 14(2); 440–51. 2014 AACR