Photodynamic Therapy for Treatment of Malignant Dysphagia:

Photodynamic therapy induced esophageal stricture—an animal model: From mouse to pig

INTRODUCTION

A major limitation of photodynamic therapy (PDT) for Barrett's esophagus is the development of esophageal stricture. We developed an animal model of PDT-induced esophageal stricture to elucidate the mechanism of stricture development. Our studies began in a mouse but, due to its limitations, we advanced to a porcine model.

MATERIALS AND METHODS

In the mouse model, 62 mice were injected with Photofrin (2-10 mg/kg) 48 h prior to photoactivation. Light energy (20-400 Joules/cm (J)) was delivered with a laser probe as a single dose, or fractionated doses (20-150 J). Animals were sacrificed when showing signs of distress or 6 to 18 weeks post-illumination. Esophagus was removed, with gross and microscopic examination performed on frozen specimens. To develop a pig model, six pigs were injected with Photofrin (2 mg/kg) 48 h prior to photoactivation. Light energy (400 J) was delivered via an endoscope using a laser probe as a single dose or repeated at 48 h. Animals were sacrificed if they could not eat soft food or lost more than 10% of their original weight according to the University of Pittsburgh Institutional Animal Care and Use Committee.

RESULTS

Exposure of mice to doses of 400 J x 1, 125 J x 3, or 150 J x 3 fractions resulted in severe lung damage and death in 90% of the mice without any evidence of esophageal stricture. Lower energy levels caused minor lung damage and no change in the endothelial layer or a stricture. In pigs, exposure of 400 J as one or two fractions resulted in weight loss of 10% within 3 weeks. Endoscopy, upper GI, contrast swallow, and pathological and histological examination showed evidence of esophageal stricture at the exposed area.

CONCLUSIONS

In the mouse model, pulmonary toxicity is the limiting factor following esophageal PDT exposure. In the pig model we induced esophageal stricture following PDT. This is the first animal model created to study esophageal strictures resulting from PDT.

Photodynamic therapy and the alimentary tract

Photodynamic therapy offers the possibility of relatively selective tumour necrosis and normal tissue healing. It has many potential applications but as yet no clear role. Articles, editorials and case reports published primarily in English and listed in Medline/ISI up to April 2000 or identified by a manual search have been reviewed in an attempt to provide a comprehensive overview of the use of photodynamic therapy in the alimentary tract. It is concluded that photodynamic therapy can be an effective treatment for superficial pre-malignant mucosal lesions and early cancers, especially in diffuse disease. Suitable patients include those wishing to avoid surgery, high risk subjects or those in whom other forms of treatment have failed. Superiority over other methods of ablation has not so far been demonstrated. Cheaper and more effective photosensitizers and improved techniques of light delivery are likely to increase the application of photodynamic therapy.