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 < 5, as well as those of the rest of the drugs (inflection points of titration curves occurred at about 5.1, 3.8 and 2.4 for TPPS2a, mTHPP and mTHPC, respectively), are likely to be due to the protonation of imino nitrogens. The tumor localizing properties of mTHPP and mTHPC reported in the literature appear to be due to factors other than pH-dependent changes in the lipophilicity of the drugs.