Optimizing photodynamic therapy by improving the design of the photosensitizer
Saenz, Courtney M.
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During the last several decades, scientists have investigated changes in the photosensitizers chemical structure on photosensitizing potencies. Quantitative structure-activity relationship (QSAR) studies have been reported with benzoporphyrin derivative analogues, sulfonated phthalocyanines, sulfonated tetraphenylporphine (TPPS), and other analogs. The amphiphilic compounds showed better photosensitizing potency than symmetrically hydrophobic or hydrophilic compounds. The overall lipophilicity and the addition of a metal to the core of the ring system may dictate the pharmacokinetics, tumor avidity and/ or increases the singlet oxygen yield. On the basis of structure-activity relationship (SAR) and QSAR studies of a series of alkyl ether analogs of pyropheophorbide- a, our laboratory selected an effective photodynamic therapy (PDT) agent 3-(1-hexyloxyethyl)-3-devinyl pyropheophorbide- a (HPPH), currently in Phase I/II clinical trials. Various novel analogs of HPPH were synthesized and characterized (Mass Spectrometry and NMR). Evaluation of these analogs for their biological efficacy, included screening in vivo in eradicating colon carcinoma (Colon26) and in vitro for intracellular localization and mediating photoreaction leading to cell killing (MTT Assay). In vivo PDT efficacy and pharmacokinetic properties of the PS were assessed in BALB/c mice bearing Colon26 tumors by 14 C biodistribution, fluorescence imaging, vascular damage and tumor response. The hypothesis of our present study was that structural modifications of HPPH should result in a significant difference in tumor uptake, tumor-specificity, pharmacokinetic properties, absorption/fluorescence characteristics and singlet oxygen producing ability. The approach was to modify HPPH by: (i) investigating the activity of the isolated stereoisomers (ii) determining the impact of reduced rings (ring-D versus ring- B) in the tetrapyrrolic system, (iii) examine the impact of variable number of anionic or cationic side groups, and (iv) investigate the presence of heavy metals (In, Ga or Pd) at the center of the PS in singlet oxygen producing efficacy, PDT efficacy, and toxicity. The results obtained from these studies should help identify more improved PDT agents, HPPH or other photosensitizers. In vitro and in vivo data documented that isolated stereoisomers of HPPH, as well as derivatives with either D- or ring-B reduced, have comparable PS activity and PDT efficacy. The PSs bearing the carboxylic acid functionality showed higher PDT efficacy over the methyl ester versions. Altering the overall lipophilicity or addition of charged side-groups significantly changed cellular uptake, subcellular site of accumulation and in vitro PDT efficacy. A prominent increase of in vitro PS function was achieved by addition of either an amino group to the 17 position or by inserting indium into the pyrrol ring. The enhanced cell uptake / binding and photoreaction of the latter HPPH derivatives, however, led to toxic reactions that precluded a further evaluation of the compounds for PDT application. The results from the present study in the thesis provide new information on the impact of structural isomers, stereoisomers, and metallation in PDT efficacy and toxicity. Both positive and negative biological results obtained from our study should help in designing agents with improved PDT efficacy and desired photophysical properties.