Using X-rays in photodynamic therapy: an overview.

Photodynamic therapy is a therapeutic option to treat cancer and other diseases. PDT is used every day in dermatology, and recent developments in the treatment of glioblastoma, mesothelioma or prostate have demonstrated the efficacy of this modality. In order to improve the efficacy of PDT, different strategies are under development, such as the use of targeted PS or nanoparticles to improve selectivity and the design of light devices to better monitor the light dose. Due to the low penetration of light into tissue, another way to improve the efficacy of PDT to treat deep tumors is the use of upconversion NPs or bi-photon absorption compounds. These compounds can be excited in the red part of the spectrum. A relatively new approach, which we will call PDTX, is the use of X-rays instead of UV-visible light for deeper penetration into tissue. The principle of this technique will be described, and the state-of-art literature concerning this modality will be discussed. First, we will focus on various photosensitizers that have been used in combination with X-ray irradiation. To improve the efficacy of this modality, nanoparticles have been designed that allow the conversion of high-energy ionizing radiation into UV-visible light; these are potential candidates for the PDTX approach. They will be discussed at the end of this review.

Inorganic Nanoparticles for Photodynamic Therapy.

Photodynamic therapy (PDT) is a well-established technique employed to treat aged macular degeneration and certain types of cancer, or to kill microbes by using a photoactivatable molecule (a photosensitizer, PS) combined with light of an appropriate wavelength and oxygen. Many PSs are used against cancer but none of them are highly specific. Moreover, most are hydrophobic, so are poorly soluble in aqueous media. To improve both the transportation of the compounds and the selectivity of the treatment, nanoparticles (NPs) have been designed. Thanks to their small size, these can accumulate in a tumor because of the well-known enhanced permeability effect. By changing the composition of the nanoparticles it is also possible to achieve other goals, such as (1) targeting receptors that are over-expressed on tumoral cells or neovessels, (2) making them able to absorb two photons (upconversion or biphoton), and (3) improving singlet oxygen generation by the surface plasmon resonance effect (gold nanoparticles). In this chapter we describe recent developments with inorganic NPs in the PDT domain. Pertinent examples selected from the literature are used to illustrate advances in the field. We do not consider either polymeric nanoparticles or quantum dots, as these are developed in other chapters.