A real-time follow-up of photodynamic therapy during PET imaging

To monitor a real-time follow-up of tumor response to photodynamic therapy (PDT) by dynamic 2-deoxy-2-[(18)F]fluoro-d-glucose ((18)FDG) and positron emission tomography (PET) using two photosensitizing drugs in vivo, and to assess their mechanisms of action.

Two types of photosensitizers with different action mechanisms were used in rats implanted with two tumors: AlPcS4 mainly affecting the tumor vascular system, and ZnPcS2 largely inducing direct cell kill. Twenty-four hours after administration of either photosensitizer, one tumor served as control while the other was treated with red light during 30min within the 2h PET imaging by infusion of (18)FDG. The usual two-tissue compartment kinetic model was modified to take into account the perturbation of the treatment during imaging.

The illumination of the tumors during PET imaging provoked a net decrease of (18)FDG uptake in tumors treated with AlPcS4 and a near total absence of (18)FDG uptake in tumors treated with ZnPcS2. After the end of illumination, the tumors regained (18)FDG uptake with a more pronounced uptake in the tumors treated with ZnPcS2. The rate constant values of the new (18)FDG kinetic model reflected the response of the tumors to the treatment in both photosensitizers.

Dynamic PET imaging can be used to quantitatively assess in vivo and in real-time the response of tumors to treatments. It is demonstrated that the 30min of treatment was not sufficient to reduce the activity of the tumors. The technique could be extended to directly monitor the effects of drugs in vivo.