Photodynamic therapy is now recognized as a legitimate therapy for both palliative and potentially curative treatment of solid tumors. This represents a major achievement for investigators who have committed their careers to PDT. Scientific research characteristically runs well ahead of accepted views and technologies. This Symposium-in-Print perhaps provides a snapshot of the potential for this technology during the next two decades.

Summary Hexadecafluorinated zinc phthalocyanine (ZnPcF,6), an analogue of zinc phthalocyanine (ZnPc) in which all hydrogen atoms have been substituted by fluorine, was prepared as a single isomeric product via the condensation of tetrafluorophthalonitrile with zinc acetate. Fluorination renders the ZnPc soluble in most common solvents. The photodynamic properties and pharmacokinetics of the ZnPcF,6 were evaluated in EMT-6 tumour-bearing Balb/c mice using 65Zn-radiolabelled analogues. Both dyes, administered i.v. at fLmol kg-' as Cremophor emulsions, revealed good tumour uptake [approximately 8-9 per cent of the injected dose per g tissue (%ID g-')] at 24 h post injection (p.i.), with the fluorinated dye reaching higher concentrations (approximately 11%ID g -') at 48 h p.i. and subsequently higher tumour -blood ratios due to rapid blood clearance. ZnPcF,6 at a dose of 5 pmol kg-' (4.3 mg kg-') induced complete tumour regression after phototherapy (24 h p.i., 650 -700 nm band, 360 J cm-2, 200 mW cm -'). At a dose of 2 fimol kg-' and phototherapy at 24 h p.i., the tumour volume doubling time increased to 11 days vs 6 days for the control tumours. A similar tumour growth delay was observed when phototherapy was conducted at 48 h or 72 h after dye injection implying that tumour response correlates with tumour dye concentrations rather than serum concentrations. As a result of its low solubility, the administered dose of ZnPc was limited to 1 fimol kg-' and at this drug level significant tumour response was only observed when the dye was solubilised as the pyridinium salt. Isolation of the neoplastic cells after in vivo dye administration and in vitro exposure to red light followed by a colony formation assay showed that the ZnPcF,6 exhibited a 1-2 order of magnitude higher potential for direct cell killing as compared with Photofrin and about a five times lower efficiency than ZnPc. However, all three photosensitisers induced complete occlusion of tumour vasculature immediately after PDT, suggesting that tumour regression mainly resulted from vascular stasis. The ZnPcF,6 offers several advantages over ZnPc for clinical applications, including improved solubility in most solvents, resulting in facilitated drug formation, favourable pharmacokinetics as well as the potential use in fluorine magnetic resonance (F-MR) imaging.

Since engaging T cell receptors in the absence of costimulation results in suboptimal activation of T cells and ultimately anergy, it appears that the immunomodulatory effects of low-dose PDT associated with extended engraftment may depend upon decreased LC expression of major histocompatibility complex and costimulatory molecules.

Benzoporphyrin derivative monoacid ring A (BPD), a hydrophobic chlorin-like porphyrin derivative, which fluoresces strongly at 690 nm, may have potential for both oncologic and nononcologic applications in photodynamic therapy (PDT). To study the influence of cellular characteristics on the uptake of BPD, the murine tumor cell line (P815), and in vitro and in vivo concanavalin A (Con A) -stimulated and unstimulated murine splenic lymphocytes were incubated with 2 micrograms/mL BPD at 37 degrees C for 0-60 min. At various times, cells were lysed and the amount of BPD taken up by cells was quantified by fluorescence measurements. The subsets of cells taking up BPD were analyzed using a panel of monoclonal antibodies and the Coulter XL fluorescence-activated cell sorter. Furthermore, Con A-stimulated and unstimulated spleen cells were incubated with 0-50 ng/mliter of BPD for 1 h prior to exposure to red light (7.2 J/cm2). Cell survival 24 h post-PDT was measured by the MTT assay. We found that the rapidly dividing tumor cell line and mitogen-stimulated murine T cells (mainly CD4+/IL-2R+) took up significantly more BPD (5-10-fold) than do unstimulated splenic lymphocytes. Increased BPD uptake correlated with greater photoinactivation when these cells were exposed to light at a wavelength of 690 nm. These findings suggest that activated cells of the immune system may be a target for photoinactivation by BPD.

In vitro experiments with benzoporphyrin derivative monoacid ring A (BPD) confirmed earlier studies that it was taken up rapidly (within 30 min) to maximum concentrations by all cells tested. It was also confirmed that rapidly dividing tumor cell lines and mitogen-activated murine T lymphocytes took up significantly more (5-10-fold) BPD than did normal splenic lymphocytes. Further experiments were undertaken to determine whether BPD could be activated by whole-body irradiation with red light in the blood of animals, shortly after intravenous (i.v.) administration, in the absence of skin photosensitivity. It was found that shaved and depilated mice injected i.v. 60 min earlier with BPD at between 0.5 and 1.0 mg/kg could tolerate 160 J/cm2 of broad-band red light (560-900 nm) delivered, at a relatively low rate, over a 90 min time interval without developing skin photosensitivity or general phototoxicity. During the treatment time, plasma levels of BPD were between 0.7 and 1.0 micrograms/mL. The light treatment resulted in between 70 and 80% photoinactivation of circulating BPD. When L1210 tumor cells were preincubated with BPD and injected i.v. into mice immediately before total-body light treatment (160 J/cm2 of 590-900 nm light delivered over 90 min), significant reductions in circulating clonogenic tumor cells were observed in blood samples taken immediately following treatment. This indicated that sufficient light was being delivered to BPD in the blood flowing in the peripheral vasculature to effect cytotoxicity to cells containing the photosensitizer without causing either vascular or skin photosensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)