Effects of Photodynamic Therapy on Peripheral Nerve: In Situ Compound-Action Potentials Study in a Canine Model

Objective: Our aim is to investigate the effects of photodynamic therapy (PDT) on peripheral nerve conductivity. Background Data: Interstitial PDT has been demonstrated as a promising treatment modality for prostate cancer. However, the sensitivity of nerves, in the immediate vicinity of the prostate gland, to PDT procedures has not been studied. This study attempts to establish an in situ canine model to evaluate direct PDT effect on peripheral nerves. Methods: PDT was performed by irradiating the cutaneous branches of the saphenous nerve at 763 nm with light doses of 50-200 J/cm 2 after i.v. infusion of the photosensitizer Tookad (0-2 mg/kg). Evoked compound-action potentials (CAP) were recorded directly from the surface of the saphenous nerve. The latencies to onset and conduction velocities were determined during PDT and 1-week post-PDT. Results: Nerve and surrounding tissue damage corresponded well with drug/light doses. With Tookad doses of 2 mg/kg, treatment with 50 J/cm 2 induced little change in saphenous nerve conduction properties. However, treatment with 100 J/cm 2 resulted in localized nerve injury and decreases in nerve conduction velocities, and treatment with 200 J/cm 2 severely damaged the nerve. Conclusions: This canine model adequately demonstrates effects of Tookad PDT on peripheral nerves. Direct irradiation of 100-200 J/cm 2 can alter nerve conduction and induce nerve damage. Therefore, possible side effects of interstitial PDT on the pelvic plexus need to be investigated in future studies.