Ultrastructural Studies on the Mechanism of the Photodynamic Therapy of Tumors

Balb/c mice bearing a transplanted MS‐2 fibrosarcoma were injected with 2.5 mg kg 1 of either tetra(4‐sulfonatophenyl/porphine (TPPS) in phosphate‐buffered saline or 0.5 mg kg−1 of Zn2+‐phthalocyanine (Zn‐Pc) incorporated into unilamellar liposomes of dipalmitoyl‐phosphatidylcholine. Chromatographic studies showed that TPPS is mainly transported in the serum by globulins and albumin, while Zn‐Pc is specifically bound by lipoproteins. Exposure of the injected mice to red light (300 J cm−2) caused extensive tumor necrosis. The ultrastructural analysis of tumor specimens taken from mice at 15 h after PDT showed that TPPS photoinduces a preferential necrosis of the neoplastic cells, while Zn‐Pc causes severe photodamage to both the vascular system and the neoplastic cells. The different modes of tumor photosensitization by TPPS and Zn‐Pc are discussed on the basis of the transport mechanism of the two dyes.