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Stopping cancer in its path occurs when photosensitizers (PSs) induce apoptotic cell death after their exposure to light
and the subsequent formation of reactive oxygen species. In pursuit of our hypothesis that mitochondrial localizing PSs
will enhance the efficacy of the photosensitizing process in photodynamic therapy, since they provoke cell death by
inducing apoptosis, we synthesized and characterized tetraphenylporphyrins (TPPs) that are substituted at the paraphenyl
positions by two amino acids and two fluoro or hydroxyl groups, respectively. They were prepared according to
the Lindsey-modified Adler-Longo methodology using trifluoromethanesulfonylchloride (CF3SO2Cl) as a catalyst
instead of trifluoroacetic acid. The use of CF3SO2Cl yielded cleaner products in significantly higher yields. During the
synthesis, not only the yields and work-up procedure of the TPPs were improved by using CF3SO2Cl as a catalyst, but
also a better means of synthesizing the precursor dipyrromethanes was tested by using indium(III) chloride. Column
chromatography, HPLC, and NMR spectroscopy were used to separate and characterize the di-amino acid-dihydroxy, or
difluoro-substituted porphyrins and to ascertain their purity before subcellular localization studies were carried out.
Studies using androgen-sensitive human prostate adenocarcinoma cells LNCaP revealed that certain amino acid
substituted porphyrins that are positively charged in the slightly acidic medium of cancer cells are very useful in
shedding light on the targets of TPPs in subcellular organelles of cancer cells. Although some of these compounds have
properties of promising photosensitizers by revealing increased water solubility, acidic properties, and innate ability to
provoke cell death by apoptosis, the cell killing efficacy of these TPPs is low. This correlates with their subcellular
localization. The di-amino acid, di-hydroxy substituted TPPs localize mainly to the lysosomes, whereas the di-fluoro-substituted
TPPs are trapped in the plasma membrane. Only a pheophorbide derivative recently synthesized in our
laboratory localized to the mitochondria of LNCaP cells, which are at the center of cell death as is reflected in their key
role during apoptosis, thus reassuring our attempts toward rational drug design.
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