The overall goal of our research is to develop effective new photosensitizers for tumor-selective photodynamic therapy. Phenoxazine dyes, including several Nile blue analogues, are known to localize selectively in animal tumors. Structural modifications yielded several series of analogues with substantially higher 1O2 yields and different photochemical and physicochemical properties. This study examined the photosensitization potency, cellular uptake, and retention of these derivatives in human bladder carcinoma cells (MGH-U1) in culture. Nile blue derivatives containing halogens and/or sulfur substitutes were selected to exhibit different 1O2 yields, pKa values, and hydrophobicities.
The effectiveness of these derivatives in mediating photokilling of tumor cells in vitro corresponded well with the 1O2 yields of these compounds, indicating that structural modifications which resulted in increased 1O2 yields enhanced potency in mediating photocytotoxicity in vitro. Using derivatives (sat-NBS and sat-NBS-6I) with the highest 1O2 quantum yield (0.35 and 0.821), over 90% cell kill was achieved at a sensitizer concentration of 5 × 10-8 m, about 3 orders of magnitude more effective than hematoporphyrin derivative, the only sensitizer currently available clinically. This result suggests that some of the oxazine derivatives could potentially be effective photosensitizers. The correspondence between 1O2 yield and photosensitizing potency, together with results showing enhanced photocytotoxicity in the presence of D2O and reduced photocytotoxicity under hypoxic conditions, strongly suggests that the generation of 1O2 is a major mechanism mediating the photocytotoxic effect.
The uptake of Nile blue derivatives by cells in culture exhibited a pattern of rapid initial uptake followed by a gradual increase in cellular dye contents. The uptake does not correlate directly with the individual pKa values or hydrophobicities of the derivatives, indicating that the structural modifications that increased 1O2 yields did not significantly alter the uptake and retention of Nile blue derivatives. The highly concentrative uptake by and slow efflux from dye-loaded cells were consistent with an active mechanism for the cellular accumulation of these dyes. On the other hand, the retention of the compounds was directly proportional to dye concentration in the medium over a 1000-fold range of concentrations, and the uptake could proceed at temperatures below 2°C; these observations excluded endocytosis or a carriermediated mechanism for the uptake. The uptake was also unaffected by the presence of serum in the medium. Based on these results, we hypothesize that Nile blue derivatives transport across the cell membrane possibly as deprotonated forms and, upon entering the cell, either partition into lipophilic areas of the cell membranes and/or become sequestered in certain intracellular organelles.
This work was supported by grants from the National Cancer Institute (CA 32259), the Beinecke Foundation, and the Thomas Anthony Pappas Charitable Foundation and by the Rowland Institute of Science.