Photodynamic Therapy (PDT) using Photofrin has been recently approved by the FDA for the treatment of esophageal cancer and Barrett's esophagus. A major limitation of PDT for Barrett's esophagus is the development of esophageal stricture in up to 53% of patients. Mechanisms of PDT stricture formation have not been elucidated. The major difficulty is the lack of an animal model for PDT-induced stricture. We have used a pig model in which the esophagus is very similar to that of the human esophagus. Two (Scrofa) domestic pigs were injected with Photofrin at dosage of 2 mg/kg 48 hours prior to photoactivation with 630 nm light. Following anesthesia, a laser probe (2.5 cm in length) was passed through the oral cavity to approximately the mid-point of the esophagus via an endoscope. Light energy (400 Joules (J)/cm) was delivered as a single dose in one pig or repeated at 72 hours in the second pig. In this pig model, upper endoscopy, Barium swallow and pathological studies confirmed stricture formation following esophageal PDT exposure of 400 J as one or two fractions. We believe that this is the first animal model created to study esophageal strictures resulting from PDT.
A major limitation of PDT for Barrett's esophagus is the development of esophageal strictures. This report summarizes the effects of PDT delivered to mouse esophagus. Sixty-two C3H/Nsd mice were injected with Photofrin (2-10mg/Kg) intraperitoneally. Forty-eight hours later a 1 cm laser probe was passed orally to the mid-esophagus. Light energy (630nm) ranged from 0 to 400 Joules/cm (J). Animals were sacrificed if death was imminent, otherwise at 6 weeks and 3 months. Gross and microscopic exams were performed on paraffin embedded esophagus and lung specimens. Exposure to 400J as a single fraction, 125 X 3 or 150 X 3 fractions resulted in a lethal pulmonary injury in 90% of mice within 48 hours. There was no esophageal mucosal damage at this early time point. Lower doses caused minor pulmonary injury allowing long-term survival but no change in the esophageal endothelium and no stricture. In the mouse, this histopathologic study demonstrates that pulmonary toxicity is the limiting factor following esophageal PDT. At lower PDT doses, minimal pulmonary damage occurred but no effect was observed on the esophagus. We believe the 5 mm depth of PDT injury leads to lethal pulmonary damage preventing subsequent study of the effects on the esophagus.