We describe here a strategy for photodynamic eradica-tion of solid melanoma tumors that is based on photo-induced vascular destruction. The suggested protocol relies on synchronizing illumination with maximal circulating drug concentration in the tumor vasculature attained within the first minute after administrating the sensitizer. This differs from conventional photodynamic therapy (PDT) of tumors where illumination coincides with a maximal concentration differential of sensitizer in favor of the tumor, relative to the normal surrounding tissue. This time window is often achieved after a delay (3-48 h) following sensitizer administration. We used a novel photosensitizer, bacteriochlorophyll-serine (Bchl-Ser), which is water soluble, highly toxic upon illumination in the near-infrared (max 765-780 nm) and clears from the circulation in less than 24 h. Nude CD1 mice bearing malignant M2R melanotic melanoma xenografts (76-212 mm 3) received a single complete treatment session. Massive vascular damage was already apparent 1 h after treatment. Changes in vascular permeability were observed in vivo using contrast-enhanced magnetic resonance imaging (MRI), with the contrast reagent Gd-DTPA, by shortening spin-spin relaxation time because of hemorrhage formation and by determination of vas-cular macromolecular leakage. Twenty-four hours after treatment a complete arrest of vascular perfusion was observed by Gd-DTPA-enhanced MRI. Histopathology performed at the same time confirmed primary vascular damage with occlusive thrombi, hemorrhage and tumor necrosis. The success rate of cure of over 80% with Bchl-Ser indicates the benefits of the short and effective treatment protocol. Combining the sensitizer administration and illumination steps into one treatment session (30 min) suggests a clear advantage for future PDT of solid tumors. ¶Posted on the website on 20 December 2000.

Successful application of anticancer therapy, and especially photodynamic therapy (PDT) mediated by type I1 (PDTII) processes, depends on the oxygen content within the tumor before, during and after treatment. The high consumption of oxygen during type I1 PDT imposes constraints on therapy strategies. Although rates of oxygen consumption and repletion during PDTII were suggested by theoretical studies, direct measurements have not been reported. Application of a novel oxygen sensor allowed continuous and direct in sifu measurements (up to a depth of 8-9 mm from the tumor surface and for several hours) of temporal variations in the oxygen partial pressure (pOz) during PDT. Highly pigmented M2R mouse melanoma tumors implanted in CD1 nude mice were treated with bacteriochlorophyll+erine (Bchl-Ser; a new photodynamic reagent) and were subjected to fractionated illumination (700 < A < 900 nm) at a fluence rate of 12 mW cm+. This illumination led to total oxygen depletion with an average consumption rate of 7.2 pM(Oz) s-l. Spontaneous reoxygenation (at an average rate of 2.5 pM(O,)/s) was observed during the following dark period. These rates are in good agreement with theoretical considerations (Foster et al., Radiat. Res. 126, 296,1991 and Henning et al., Radiut. Res. 142,221,1995). The observed patterns of oxygen consumption and recovery during prolonged periods of Iightldark cycles were interpreted in terms of vasculature damage and sensitizer clearance. The presented data support the previously suggested advantages of fractionated illumination for type I1 photodynamic processes.

The Cesium salt of BSSB (Cs4B24H22S2), a common boron-neutron-capture-therapy (BNCT) agent, was injected into M2R mouse melanoma xenografts, and detected in vivo by 1H-observed, 10B-edited NMR spectroscopy. The technique of spin-echo difference spectroscopy, in which a proton spin-echo is detected following the alternating presence and absence of a 10B 180 degrees pulse was used. This method provides much higher sensitivity than direct 10B NMR detection, and should thus be suitable for in vivo detection in patients about to undergo BNCT treatment, where the infused agents are 95% isotopically enriched in 10B.

ABSTRACT

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