Increase in protoporphyrin IX after 5-aminolevulinic acid based photodynamic therapy is due to local re-synthesis

Protoporphyrin IX (PpIX) fluorescence that is bleached during aminolevulinic acid (ALA) mediated photodynamic therapy (PDT) increases again in time after treatment. In the present study we investigated if this increase in PpIX fluorescence after illumination is the result of local re-synthesis or of systemic redistribution of PpIX. We studied the spatial distribution of PpIX after PDT with and without cooling using the skin-fold observation chamber model. We were unable to show a correlation between the local PpIX fluorescence increase and the distance from a blood vessel. The spatial distribution of PpIX fluorescence within normal tissue or tumour is not changed in response to the illumination. These observations suggest that there is no diffusion of PpIX into the treated tissue. Cooling the tissue to 12 degrees C, a temperature at which PpIX synthesis is inhibited, inhibited the PpIX fluorescence increase normally observed after illumination. We also found a strong correlation between local PpIX photobleaching during illumination and the fluorescence intensity 1 h after illumination similar to what we have observed in patients treated with ALA-PDT. Therefore we conclude that the increase in PpIX fluorescence after illumination is due to local cellular re-synthesis.