In vitro cellular uptake and dimerization of signal transducer and activator of transcription-3 (STAT3) identify the photosensitizing and imaging-potential of isomeric photosensitizers derived from chlorophyll-a and bacteriochlorophyll-a

Among the photosensitizers investigated, both ring-D and ring-B reduced chlorins containing the m-iodobenzyloxyethyl group at position-3 and a carboxylic acid functionality at position-17 ² showed the highest uptake by tumor cells and light-dependent photoreaction that correlated with maximal tumor-imaging [positron emission tomography (PET) and fluorescence] and long-term photodynamic therapy (PDT) efficacy in BALB/c mice bearing Colon26 tumors. However, among the ring-D reduced compounds, the isomer containing the 1′-m-iobenzyloxyethyl group at position-3 was more effective than the corresponding 8-(1′-m-iodobenzyloxyethyl) derivative. All photosensitizers showed maximum uptake by tumor tissue 24 h after injection, and the tumors exposed with light at low fluence and fluence rates (128 J/cm ², 14 mW/cm ²) produced significantly enhanced tumor eradication than those exposed at higher fluence and fluence rate (135 J/cm ², 75 mW/cm ²). Interestingly, dose-dependent cellular uptake of the compounds and light-dependent STAT3 dimerization have emerged as sensitive rapid indicators for PDT efficacy in vitro and in vivo and could be used as in vitro/in vivo biomarkers for evaluating and optimizing the in vivo treatment parameters of the existing and new PDT candidates.