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There are currently a large number of photosensitizers under investigation for their possible applications to PDT. Many of them appear to share the property of PhotofrinR of selective accumulation in malignant or other abnormal tissue. In developing the area of photodynamic therapy there are a number of investigative approaches that might be taken to add rigour to the application of photosensitizers for treatment of disease. One is to try to understand the mechanisms whereby selective accumulation takes place, and the other is to try to develop technology to improve selective uptake by targeted tissue. The following report covers, at preliminary level, our attempts to address both these issues using a new group of photosensitizers, the benzoporphyrin derivative (BPD) analogues described previously (1-3). The BPD analogues all have an identical reduced tetrapyrrol ring, and differ by the position of a cyclohexadiene ring (fused at either ring A or ring B of the porphyrin) and the presence of either two acid groups or one acid and one ester group at rings C and D of the porphyrin. The monoacid derivatives (ring A or B designated as BPD-MA or BPD-MB) are considerably more active in in vitro phototoxic killing of cell lines and in PDT using a murine tumour model, than are the diacids (designated BPD-DA or BPD-DB (3) Because of the close similarity in structures between these compounds and the clear differences in photodynamic effect, we considered them to be useful molecular models in which to determine whether there were basic differences between them in serum distribution, and if these differences would affect the efficacy of PDT.
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