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Depth Profile of Protoporphyrin IX Fluorescence in an Amelanotic Mouse Melanoma Model

Protoporphyrin IX (PpIX) fluorescence was measured at different depths in a subcutaneous amelanotic melanoma model (LOX) in mice. PpIX was induced by topical application of 5‐aminolevulinic acid (ALA) and two of its derivatives, the methylester (MAL) and hexylester (HAL) onto the normal skin covering the tumor. The PpIX fluorescence intensity on the surface of the tumors was the highest for HAL, followed by ALA and MAL. Using equimolar concentrations (0.5 mmol g−1), HAL induced nearly twice as much fluorescence as ALA did. The depth profile of PpIX fluorescence was measured at different layers of the tumor, which was carefully sliced and controlled in situ ex vivo. The PpIX fluorescence was mainly localized within the upper 2 mm of the tissue for ALA and within 1 mm for MAL and HAL. There were no significant differences in the shape of the fluorescence excitation spectra, but the long wavelength excitation peak (633 nm) was so weak that these results are unreliable for depth estimation. When considering the low fluorescence intensity (around 5% of the intensity at the tumor surface), the actual penetration depth of HAL was comparable to that of ALA. The fluorescence after topical application of ALA and HAL was significantly above the background level down to a depth of around 6 mm, and there were traces of PpIX fluorescence even at the tumor base (10 mm). The fluorescence after topical application of MAL was detectable down to 1 mm. In the depth of 2–6 mm, the fluorescence was slightly higher for HAL than for ALA. Using the estimated diffusion coefficients for topically applied ALA (0.16 ± 0.03 mm2 h−1), MAL (0.045 ± 0.005 mm2 h−1) and HAL (0.037 ± 0.003 mm2 h−1), the behavior of the drugs after different application times could be estimated in this tumor model.