Successful application of anticancer
therapy, and especially
photodynamic therapy (
PDT) mediated by type II (PDTII) processes, depends on the
oxygen content within the
tumor before, during and
after treatment. The high consumption of
oxygen during type II
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 situ 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 (pO2) during
PDT. Highly pigmented M2R mouse
melanoma tumors implanted in CD1 nude mice were treated with
bacteriochlorophyll-
serine (Bchl-Ser; a new photodynamic
reagent) and were subjected to fractionated illumination (700 < lambda < 900 nm) at a fluence rate of 12 mW cm-2. This illumination led to total
oxygen depletion with an average consumption rate of 7.2 microM(O2) s-1. Spontaneous reoxygenation (at an average rate of 2.5 microM(O2)/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., Radiat. Res. 142, 221, 1995). The observed patterns of oxygen consumption and recovery during prolonged periods of light/dark cycles were interpreted in terms of vasculature damage and sensitizer clearance. The presented data support the previously suggested advantages of fractionated illumination for type II photodynamic processes.