Evaluation of human erythrocytes as model cells in photodynamic therapy

The role of erythrocytes as targets in photodynamic therapy is a controversially discussed topic in the literature. Therefore five different, but well known photosensitisers (three zinc phthalocyanines, tetrabenzoporphine and pheophorbide a delivered in liposomes were used for photodynamic treatment of human erythrocytes. The phototoxic effect on these cells showed pronounced differences. It was in the range: zinc phthalocyanine = pheophorbide a > tetrabenzoporphine >> zinc octa-n-alkyl phthalocyanines. Data from the zinc octa-n-alkyl phthalocyanines were compared with photodynamic effects within cutaneous cell lines, treated under the same experimental conditions. The results show that erythrocytes are unlikely to make good models for predicting the efficiency of the photosensitiser in general, and the same applies to cells other than erythrocytes and in vivo. Possible reasons could be differences in dye accumulation. However, erythrocytes may well serve as model cells to explore the cellular and molecular mechanisms of photodynamic treatment.