A new photodynamic therapy (PDT) system promises to address some issues that exist with current PDT agents. For example, most agents repel water, a property incompatible with the aqueous environment of cells. Furthermore, they do not effectively target cancer cells and are not optimized for two-photon PDT, a technique that enables treatment of deeper tissues. The new system has been used recently to treat living cancer cells.As a result of two-photon irradiation, cervical cancer cells treated with a novel PDT system are dead. The images on the left show cells treated with nanoparticles before irradiation and the ones on the right, after irradiation. The top images show merged transmission (blue) and fluorescence (red) mode pictures, and the bottom monochrome images were taken in transmission mode. Reprinted with permission of the Journal of the American Chemical Society.PDT selectively kills cancer cells via targeted photosensitizing drugs that are activated by a laser. Paras N. Prasad and colleagues from State University of New York at Buffalo and from Roswell Park Cancer Institute, also in Buffalo, have developed a PDT system that uses organically modified silica (Ormosil) nanoparticles to encapsulate the photosensitizer and a two-photon-absorbing fluorophore. The photosensitizer and fluorophore interact via Förster resonance energy transfer (FRET), causing the photosensitizer to produce singlet oxygen that kills cancer cells. The researchers developed the fluorophore, the photosensitizer and the nanoparticles.The Ormosil nanoparticles provide several advantages. They are hydrophilic and can be modified easily to target cancer cells. Therefore, encapsulating the photosensitizer in the nanoparticles obviates the complex chemistry required to make photosensitizers more water-soluble or to direct them to targets. The nanoparticles are thought to accumulate in tumors more rapidly than others because tumors have been observed to take up molecules of that size more quickly. The nanoparticles are inert and not known to be immunogenic, and in vitro cell viability assays have demonstrated that they are not cytotoxic.Prasad said that they used a fluorophore and a photosensitizer separately because, unlike most photosensitizers, the fluorophore that they developed has enhanced two-photon absorptivity and produces intense fluorescence when aggregated. They wanted a system compatible with two-photon PDT because it increases the depth penetration of the laser down to centimeters. They used a photosensitizer currently in Phase I and II clinical trials at Roswell Park Cancer Institute. Prasad said that they believed using this photosensitizer would be more meaningful because it is undergoing clinical trials. In addition, it has an absorption spectrum overlapping the fluorescence of the fluorophore, a necessity for FRET. The fluorophore and the photosensitizer have the chemical formulas 9,10-bis[4'-(4'-aminostyryl)styryl]anthracene and 2-devinyl-2-(1-hexyloxyethyl)pyropheophorbide, respectively. The researchers excited the two-photon-absorbing fluorophore at 850 nm using a Ti:sapphire laser pumped by a diode laser, both from Spectra-Physics of Mountain View, Calif. They then employed Bio-Rad and Nikon microscopes to image cells from a HeLa cervical cancer line.After performing PDT, they observed cell shrinkage and breaks in the cell membrane with release of cytosol, changes in cancer cell morphology consistent with cell death (see figure). Prasad said that they have demonstrated proof of concept and are developing new photosensitizers and improved nanoparticle formulations for in vivo study.Journal of the American Chemical Society, March 7, 2007, pp. 2669-2675.