Solubilization of meso -Tetraphenylporphyrin Photosensitizers by Substitution with Fluorine and with 2,3-Dihydroxy-1-propyloxy Groups

Abstract

The tetra(hydroxyphenyl)porphyrins (5,10,15,20-tetrakis(2-hydroxyphenyl)porphyrin (3), 5,10,15,20-tetrakis(3-hydroxyphenyl)porphyrin (4), and 5,10,15,20-tetrakis(4-hydroxy-phenyl)porphyrin (5) and the tetrahydroporphyrins (5,10,15,20-tetrakis(2-fluoro-3-hydroxy-phenyl)porphyrin (6), 5,10,15,20-tetrakis(2,4-difluoro-3-hydroxyphenyl)porphyrin (7), and 5,10,15,20-tetrakis(3,5-difluoro-4-hydroxyphenyl)porphyrin (8)) have been reported as potential photosensitizers for photodynamic therapy. In particular, the tetrahydropor-phyrin 7, which has been in Phase III clinical trials for three years in the USA and Canada, has shown great promise for primary and recurrent head and neck cancer. It is expected to be on the market soon. A potentially limiting property of compounds 3–8 is their hydrophobicity, which renders them insoluble in aqueous media. We therefore set out to develop an approach for their solubilization in aqueous media.The first attempt was to prepare analogues with fluorine substituents at the positions ortho to each of the four hydroxy groups on the meso-phenyl rings. This was expected to increase the acidity of the ortho hydroxy groups, and therefore the solubility of the compounds in hydroxylic media. This structural alteration resulted in a slight but significant decrease in the partition coefficients.The second attempt was substitution of each of the hydroxy groups on the phenyl rings with 2,3-dihydroxy-l-propyloxy groups. This structural alteration resulted in a substantial decrease in the partition coefficients.To harness the combined effect of these two structural variations porphyrins were prepared containing both 2,3-dihydroxy-l-propyloxy and ortho fluorine groups on the meso-phenyl rings. The partition coefficients decreased by more than two orders of magnitude. The most significant decrease was due to the substitution with 2,3-dihydroxy-l-propyloxy groups, rather than the substitution with fluorine, although fluorination alone contributed significantly. The effect of this method was demonstrated most clearly by comparison of the partition coefficients of meta and para isomers of these porphyrins.