Evaluation of physicochemical properties and aggregation of the photosensitizers TPCS2a and TPPS2a in aqueous media.

Physicochemical properties of the novel photosensitizer meso-tetraphenyl chlorin disulphonate (TPCS2a) and the chemically related meso-tetraphenyl porphyrin disulphonate (TPPS2a) were investigated in aqueous solutions as part of pharmaceutical preformulation. Inflection points were calculated to be 3.9 for both compounds based on spectral shifts of aqueous solutions in the pH range 2-12, which likely correlate with indistinguishable pKa values of the imino nitrogens of the molecular cores. Accordingly, the fluorescence emission spectra showed pH dependent spectral shifts. Porphyrin-like compounds are known for aggregation in aqueous environments, and a small percentage of Tween 80 (0.006 % v/v = 4 x cmc) seemed to stabilize the aqueous samples of the two photosensitizers through hindrance of aggregation. The distribution coeffient of TPCS2a determined spectrophotometrically in 1-octanol/water is 0.4 (- 0.4 SD) and 1.5 (- 0.5 SD) for the reference TPPS2a. This confirms amphiphilicity which indicates preferred distribution and further restrain of diffusion in membranes, which is relevant for the use of TPCS2a as a photosensitizer in the process of photochemical internalization in vivo.