Lack of selectivity of protoporphyrin IX fluorescence for basal cell carcinoma after topical application of 5-aminolevulinic acid: implications for photodynamic treatment

Clinical trials of topical ALAin photodynamic therapy (PDT) of basal cell carcinoma (BCC) show significant recurrence rates. Exogenous 5-aminolevulinicacid ( ALA) is converted by intracellular enzymes to photoactive protoporphyrin IX(PpIX) in human tissues. PpIX generates cytotoxic singlet oxygen when irradiated with visible light in the 400–640 nm range. To evaluate variability and heterogeneity in PpIX production by tumors in such trials, and to assess the usefulness of PpIX for marking skin tumors, we measured PpIX fluorescencedistribution in BCC after topical application of 20% ALAcream. ALAcream was applied under occlusion for periods ranging from 3 to 18 h (average 6.9 h, SD 4 h) to 16 BCCs. ALAconversion to PpIX in the BCCs was assessed by in vivo photography, ex vivo video fluorescenceimaging, and fluorescencemicroscopy. External macroscopic PpIX fluorescence, as assessed by in vivo and ex vivo imaging, correlated with the clinical presence of BCC. Examination by a digital imaging fluorescence microscoperevealed inter- and intratumor fluorescencevariability and heterogeneity. PpIX fluorescencecorresponding to full tomor thickness was found in six superficial and four nodular tumors, and partial-thickness fluorescencewas observed in five nodular tumors, but no PpIX fluorescencewas observed in some areas of superficial, nodular and infiltrating tumors. In a significant number of nodular and infiltrating BCCs, topical ALAappeared to provide little or no PpIX in deep tumor lobules. In addition, no selectivity for tumor tissue versus normal epidermis was seen. The grossly brighter external PpIX fluorescenceover tumors may be due, therefore, to enhanced penetration through tumor-reactive stratum corneum and to the tumor thickness. The absence of reproducible fluorescencemarking of nodular and infiltrating BCC suggests that topical ALA, at least under the present delivery protocols, may not be a reliable regimen for photodynamic treatment of these BCCs.