Monitoring Pc 4-mediated Photodynamic Therapy of U87 Tumors with 18F-fluorodeoxy-glucose PET Imaging in the Athymic Nude Rat

Introduction: We have previously demonstrated the use of phthalocyanine Pc 4 for the photodynamic therapy (PDT) of ectopic human glial tumors in the athymic nude rat brain. We wish to determine whether 18F-fluorodeoxy-glucose ( 18 F-FDG) Positron Emission Tomography (PET) imaging can detect the reduction in tumor metabolism that must occur after Pc 4-PDT-induced necrosis. Methods: 2.5 x 10 5 U87 cells were injected into the brains of 12 athymic nude rats. After 7 days of tumor growth, all 12 animals were imaged functionally by 18 F-FDG micro-PET (mPET) and structurally by micro-CT and/or micro-MR. These animals received 0.5 mg/kg b.w. Pc 4 via tail-vein injection. One day later the scalp was re-incised and the tumor illuminated with 30 J/cm 2 of 672-nm light from a diode laser. The next day these animals were again 18 F-FDG mPET imaged. Next, the animals were euthanized and their brains were explanted for H&E histology. Results: Histology showed that tumors in the 6 Pc 4-PDT-treated animals demonstrated necrosis ranging from full to frank (severe). Preliminary analysis showed that 18F-FDG mPET activity in 3 of the 6 non-PDT group (i.e., no tumor necrosis observed) animals was seen to increase 2.28 times following tumor photoirradiation, whereas 18 F-FDG mPET activity in 5 of the 6 PDT group ( i.e. , tumor necrosis observed) animals was seen to increase 1.15 times following tumor photoirradiation. Discussion: The increased 18 F-FDG mPET activity in the PDT group was unexpected. We had expected this activity to decrease and are presently investigating the cause of this observation.