[Experimental studies of photodynamic therapy of squamous cell cancers with hematoporphyrin derivative]

The target tissue of photodynamic therapy (PDT) of tumours by haematoporphyrin derivative (HPD) in vivo is controversial. A direct attack on the malignant cell itself, an indirect effect via the vascular system supplying the tumour, or a combination of both factors, have been suggested. We investigated this question in squamous cell carcinoma in experimental animals. Histology showed massive interstitial bleeding in carcinomas induced in the mucosa of the cheek pouch of hamsters or in the skin of mice, indicating a vascular response to PDT. However, it was also possible to induce similar changes in the neighbourhood of newly formed vessels at the margin of an inflammatory reaction induced in the cheek pouch of the hamster in the absence of tumour cells. We implanted cells from human squamous cell carcinomas into athymic nude mice and found that carcinoma cells removed immediately after PDT remained viable, whereas tumour cells left in situ died. Thus, the point of action of PDT and HPD in vivo is not the tumour cell itself, but the vascular stroma of the tumour or its immediate vicinity.