Spectroscopic assessment of endogenous porphyrins in a rheumatoid arthritis rabbit model after the application of ALA and ALA-Me

The sensitization of inflamed knee tissues with endogenous porphyrins was studied by means of fluorescence spectroscopy in an experimental model of rabbit rheumatoid monoarthritis after intraarticular (i.a.) or intravenous (i.v.) injections of 5-aminolevulinic acid (ALA) or ALA methyl ester (ALA-Me). Fluorescence measurements in vivo on the skin of the inflamed knee joint showed the dominance of protoporphyrin IX (PpIX). The highest fluorescence intensity was registered 2 h after i.a. injection of ALA and ALA-Me. Comparative analysis of the PpIX fluorescence spectra ex vivo revealed that more PpIX accumulated in the tissues of the inflamed joint than in the tissues of the control joint, and that ALA-Me induced about five times more PpIX in the inflamed synovium than ALA. Meanwhile, the cartilages of the inflamed and control knee joints also accumulated water-soluble porphyrins. Thus, in vivo and ex vivo spectroscopic assessment of endogenous porphyrins in rabbit rheumatoid arthritis tissues implied that the injection of ALA is more appropriate for the diagnostics of inflammation due to the higher PpIX fluorescence intensity on the skin surface, while ALA-Me is more appropriate for the therapeutic applications due to the higher and more selective accumulation of PpIX in the inflamed synovium.

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► The induction of porphyrins after ALA and ALA-Me application was compared.
► Fluorescence of PpIX in vivo on the knee skin was higher in the case of ALA.
► Fluorescence of PpIX ex vivo in inflamed synovium was higher in the case of ALA-Me.
► Water-soluble porphyrins were detected in all knee cartilages of rabbits.