It has been speculated that topical application of 5-aminolevulinic acid (ALA) or methyl 5-aminolevulinate (MAL) may
be more painful during light exposure after longer application times of the compounds than after shorter times, even
though the same levels of protoporphyrin IX (PpIX) is produced in both cases.
The aim of our study was to investigate pain induction in the build-up and clearance phases of PpIX in the skin of
healthy volunteers. 0.6 mmol/g of ALA (10% wt/wt) and MAL (11% wt/wt) creams were applied on the volunteers. The
creams were maintained on the spots for 20- 24 hours and then wiped off. Subsequently, fresh creams were applied on
the other arm of the volunteers for 4- 6 hours. Fluorescence emission spectra for all spots were measured every hour until
the fluorescence levels were similar in both arms for ALA and MAL. Then the test areas were exposed to light until pain
occurred. Time for pain to occur was recorded. The fluorescence of PpIX was measured before and after light exposure.
PDT in the clearance phase seems to induce pain faster than in the build-up phase for ALA and MAL. Due to large
interpersonal variations between volunteers further investigation is needed.
5-aminolevulinic acid-mediated photodynamic therapy (ALA-PDT) is the most widely practiced form of PDT in
dermatology. One of the advantages of ALA-PDT is that undesirable photosensitization lasts only for 24-48 h. In order
to optimize ALA-PDT it is necessary to understand the mechanisms controlling intracellular PpIX clearance (efflux and
transformation into heme) in order to decrease protoporphyrin IX (PpIX) clearance rates in the early stages of its
production. The aim of this study was to investigate the factors controlling the clearance of intracellular PpIX.
Fluorescence spectroscopy was used to study PpIX kinetics in WiDr cells initially treated with ALA. The clearance rate
of PpIX in WiDr cells was faster after application of a low concentration of ALA (0.1 mM) than after application of high
concentration of ALA (1 mM). PpIX was cleared faster from cells which initially were seeded at low densities than cells
seeded at higher densities. The presence of the iron chelator deferoxamine reduced the clearance rate of PpIX, while the
presence of ferrous sulfate acted oppositely. The decay rate of PpIX in WiDr cells was faster at higher temperature than
at lower. The ferrochelatase activity at pH 7.2 was significantly greater than that at pH 6.7. ALA concentration,
application time, cell density, temperature, pH, intracellular iron content, intracellular amount and localization of PpIX
are factors controlling PpIX clearance.
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