Mode of action of photodynamic therapy with sulfonated aluminum phthalocyanine in induced squamous cell carcinomas in animal models.

In order to investigate the mechanism of action of photodynamic therapy (PDT) with sulfonated aluminum phthalocyanine (AlSPc) in squamous cell carcinoma, animal experiments were performed in induced carcinomas of the mucosa of the hamster's cheek pouch and skin of the laboratory mouse. Histological examinations revealed signs of massive interstitial bleeding, indicating a vascular response to PDT with AlSPc. It was also possible to induce similar change adjacent to newly formed vessels at the margin of an inflammatory reaction in the cheek pouch of five hamsters in the absence of tumor cells. Implanted human squamous cell carcinoma cells in athymic nude mice showed that carcinoma cells removed immediately following PDT remained viable, while tumors left in situ became necrotic. These results suggest that the primary effect of PDT with AlSPc in vivo is not the malignant cell itself, but the vascular stroma of the tumor or in the immediate vicinity of the latter.