pH-Dependent Spectral Properties of HpIX, TPPS2a, mTHPP and mTHPC

Lower extracellular pH in tumors as compared to normal tissues has been proposed to be a factor contributing to the tumor selective uptake of several photosensitizers. Therefore, the pH dependence of absorption and fluorescence spectral properties of four different drugs relevant for photodynamic therapy (hematoporphyrin IX [HpIX], disulfonated meso-tetraphenylporphine [TPPS2a], meso-tetra(3-hydroxyphenyl)porphine [mTHPP] and meso-tetra(3-hydroxyphenyl)chlorin [mTHPC]) has been examined. Spectral analysis of the dyes dissolved in phosphate buffered saline (PBS) indicates pH-dependent modification in the physiologically important region (6.0–8.0) only in the case of HpIX. This modification is probably related to the protonation of carboxylic groups. Spectral changes of HpIX in PBS observed at acidic pH values