1. Zacchigna M. (a), Cateni F. (a), Drioli S. (a), Bonora GM. (a), Zorzet S. (b), Rapozzi V. (c), Xodo L.E. (c) (a) Department of Chemical and Pharmaceutical Sciences, via Giorgieri 1, 34100 Trieste, Italy; (b) Department of Life Science, Via Valerio 8, 34100 Trieste, Italy; (c) School of Medicine, University of Udine, P.le Kolbe 4, 33100 Udine, Italy; Photodynamic therapy (PDT) is a non-invasive therapeutic modality used in a various number of diseases and cancer. It involves the systemic or topic administration of a photosensitizer, followed by irradiation with light. The activated photosensitizer converts oxygen to singlet oxygen and/or reactive oxygen species (ROS) which lead to cell death and tissue necrosis. One aim of PDT research is the discovery of new photosensitizers possessing minimal dark cytotoxicity, high photodynamic properties, improved pharmacokinetics, preferential retention in diseased instead of healthy tissues, chemical stability and a good cellular uptake [1]. We recently focused our efforts on pheophorbide a (Pba), a chlorophyll derivative. Pba is characterized by a stronger absorption between 650-700 nm, in the tissue-penetrating wavelength range. For in vivo applications the capacity of the photosensitiser to reach in the diseased tissues becomes critical, in particular when a large peritoneal area is interested as occurring in carcinomatosis and sarcomatosis. Time-Domain Optical Imaging In vivo time domain optical images of mice treated with Pba or mPEG-Pba. Biodistribution of Pba (A) and mPEG-Pba (B) in mice after i.p. injection. Whole body fluorescence intensity was acquired at indicated times following post-injection. Ex vivo images of organs obtained from mice bearing a tumor, 48 h following i.p. injection of 250 μl of Pba or mPEG-Pba solution (50 μmol/Kg). (A) mouse treated with Pba; (B) mouse treated with mPEG-Pba. (C) Comparison of tumor uptake (large and small tumor mass) of Pba and mPEG-Pba. The analyses A–C are independent in terms of laser intensity and signal integration time PEG-Pba A) Means of 6 animals for groups; * =p<0.05, ** = p<0.01, ANOVA and Tukey Kramer post-test; B) PEG30 vs Controls p<0.001, Pba30 vs Controls p<0.01, PEG30 vs Pba 30 p<0.05, Kaplan-Meier, logrank test. Activity Tumour growth rate (A) and survival time (B) in C57BL/6 mice bearing a subcutaneously transplanted amelanotic melanoma (B78H1). The tumour lesion was subjected to PDT treatment (600–700 nm, 60 mW/cm 2, and 193 J/cm 2 ) at 4 h after i.p. injection of 30mg/kg (50µmol/kg or 100µmol/kg) Pba or PEG-Pba. The effectiveness of the treatment was evaluated by comparing the rate of tumour growth as a function of the postirradiation time for the photosensitised mice with that observed for control mice that had been exposed to light but not had been injected with the photosensitiser. The tumour size was measured at daily intervals by means of a calliper. Survival time was evaluated by Kaplan-Meier analysis. Conclusion The PDT performed at 4 h after injection of the PEG-Pba causes a tumour response which is markedly more extensive than that observed for PDT in the presence of the free Pba. Preliminary studies suggested that no significant difference in the rate of tumour growth was caused by irradiation of the mice in the absence of photosensitiser using the same irradiation regime adopted for the PDT experiments with the Pba or PEG-Pba injected animals. A B A) Means of 6 animals for groups; * =p<0.05, ** = p<0.01, ANOVA and Tukey Kramer post-test; B) PEG30 vs Controls p<0.001, Pba30 vs Controls p<0.01, PEG30 vs Pba 30 p<0.05, Kaplan-Meier, logrank test. Activity Tumour growth rate (A) and survival time (B) in C57BL/6 mice bearing a subcutaneously transplanted amelanotic melanoma (B78H1). The tumour lesion was subjected to PDT treatment (600–700 nm, 60 mW/cm 2, and 193 J/cm 2 ) at 4 h after i.p. injection of 30mg/kg (50µmol/kg or 100µmol/kg) Pba or PEG-Pba. The effectiveness of the treatment was evaluated by comparing the rate of tumour growth as a function of the postirradiation time for the photosensitised mice with that observed for control mice that had been exposed to light but not had been injected with the photosensitiser. The tumour size was measured at daily intervals by means of a calliper. Survival time was evaluated by Kaplan-Meier analysis. A B