Water soluble, core-modified porphyrins 1-5 bearing 1-4 carboxylic acid groups were prepared and evaluated in vitro as photosensitizers for photodynamic therapy. The 21,23-core-modified porphyrins 1-5 gave band I absorption maxima with lambda(max) of 695-701 nm. The number of carboxylic acid groups in the dithiaporphyrins 1-4 had little effect on either absorption maxima (lambda(max) of 696-701 nm for band I) or quantum yields of singlet oxygen generation [phi((1)O(2)) of 0.74-0.80]. Substituting two Se atoms for S gave a shorter band I absorption maximum (lambda(max) of 695 nm) and a smaller value for the quantum yield for generation of singlet oxygen [phi((1)O(2)) of 0.30]. The phototoxicity of 1-5 was evaluated against R3230AC cells. The phototoxicities of dithiaporphyrin 2, sulfonated thiaporphyrin 30, HPPH, and Photofrin were also evaluated against Colo-26 cells in culture using 4 J cm(-2) of 570-800 nm light. Compound 2 was significantly more phototoxic than sulfonated dithiaporphyrin 30, HPPH, or Photofrin. Cellular uptake was much greater for compounds 1, 2, and 5 relative to compounds 3 and 4. Confocal scanning laser microscopy and double labeling experiments with rhodamine 123 suggested that the mitochondria were an important target for dithiaporphyrins 1 and 2. Inhibition of mitochondrial cytochrome c oxidase activity in whole R3230AC cells was observed in the dark with compounds 1 and 30 and both in the dark and in the light with core-modified porphyrin 2.