PDT with ALA and MAL is established as a relatively effective treatment for non-melanoma skin cancer and premalignancies.
PDT is often repeated, because a single treatment gives poor long term results. Preclinical studies showed
that ALA-PDT applying a fractionated illumination scheme with a small first light fraction and a second larger light
fraction separated by a dark interval of two hours resulted in a significant increase in efficacy. Whereas the efficacy was
not enhanced by fractionating MAL-PDT, indicating that ALA-PDT mechanism is not the same as MAL-PDT
mechanism. The increase in efficacy using fractionated PDT was confirmed clinically. A randomized comparative
clinical study comparing fractionated ALA-PDT versus non-fractionated ALA-PDT in the treatment of superficial basal
cell carcinoma showed a significant higher response rate in the lesions treated with fractionated ALA-PDT after a
follow-up of one year ( p<0.002, log-rank test). The five year follow-up is studied at moment. So far the complete
response in the group treated with fractionated ALA-PDT seems to be only a few percentages lower compared to the one
year follow-up. Besides the gain in response rate, fractionated ALA PDT is cost effective. ALA gel is less expensive
than the commercially available MAL (Metvix) and moreover fractionated ALA-PDT takes one treatment day, instead
of two treatment days using the Metvix treatment protocol (two MAL-PDT treatments separated by one week), both
reducing direct and indirect costs and the burden to the patient.
Photodynamic therapy (PDT) using protoporpyrin IX (PpIX) precursors like 5-aminolevulinic acid (ALA) or
methyl-aminolevulinate (MAL) has shown to be effective in the treatment of various skin diseases. Using ALA
we have shown in numerous studies a significantly improved efficacy by applying light fractionation with a
long dark interval. In contrast, in the hairless mouse model, the PDT efficacy using MAL is unaffected by
adopting this approach. More acute edema is found after ALA-PDT suggesting a difference in response of
endothelial cells to PDT.
To investigate the role of endothelial cells, cryo-sections of hairless mouse skin after 4 hours of topical MAL
or ALA application were stained with a fluorescent endothelial cell marker (CD31). Co-localization of this
marker with the PpIX fluorescence was performed using the spectral imaging function of the confocal
microscope. We have also used intra-vital confocal microscopy to image the PpIX fluorescence distribution in
correlation with the vasculature of live mouse skin.
Our results show PpIX fluorescence at depth in cryo-sections of mouse skin after 4 hours of topical
application. Co-localization has shown to be difficult due to the changes in tissue organization caused by the
staining procedure. As expected we found high PpIX fluorescence levels in the epidermis after both MAL and
ALA application using intra-vital microscopy. After ALA application more PpIX fluorescence was found deep in
the dermal layer of the skin than after MAL. Furthermore we detected localized fluorescence in unidentified
structures that could not be correlated to blood vessels or nerves.
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