Introduction: In this communication we report on a novel non-invasive methodology in utilizing "soft" energy diagnostic X-rays to indirectly activate a photo-agent utilized in photodynamic therapy (PDT): Photofrin II (Photo II) through X-ray induced luminescence from Gadolinium Oxysulfide (20 micron dimension) particles doped with Terbium: Gd $_{2}$ O $_{2}$ S:Tb. Photodynamic agents such as Photo II utilized in PDT possess a remarkable property to become preferentially retained within the tumor's micro-environment. Upon the photo-agent's activation through (visible light) photon absorption, the agents exert their cellular cytotoxicity through type I and type II pathways through extensive generation of reactive oxygen species (ROS); namely, singlet oxygen $^{1}$ O $_{2}$ , superoxide anion O $_{2}^{-}$ , and hydrogen peroxide H $_{2}$ O $_{2}$ , within the intra-tumoral environment. Unfortunately, due to shallow visible light penetration depth (∼ 2 mm to 5 mm) in tissues, the current PDT strategy has largely been restricted to the treatment of surface tumors, such as the melanomas. Additional invasive strategies through optical fibers are currently utilized in getting the visible light into the intended deep seated targets within the body for PDT.

Methods: X-ray induced visible luminescence from Gd $_{2}$ O $_{2}$ S:Tb particles were spectroscopically characterized, and the potential in-vitro cellular cytotoxicity of Gd $_{2}$ O $_{2}$ S:Tb particles on human glioblastoma cells (due to 48 Hrs Gd $_{2}$ O $_{2}$ S:Tb particle exposure) was screened through the MTS cellular metabolic assay. In-vitro human glioblastoma cellular exposures in presence of Photo II with Gd $_{2}$ O $_{2}$ S:Tb particles were performed in the dark in sterile 96 well tissue culture plates, and the corresponding changes in the metabolic activities of the glioblastoma due to 15 minutes of (diagnostic energy) X-ray exposure was determined 48 Hrs after treatment through the MTS assay.

Results: Severe suppression (> 90% relative to controls) in the cellular metabolic activity of human glioblastoma was measured due to the treatment of clinically relevant concentrations of 20 μg/ml Photo II, with Gd $_{2}$ O $_{2}$ S:Tb particles, and (120 kVp) diagnostic X-rays. Taken together, the in-vitro findings herein provide the basis for future studies in determining the safety and efficacy of this non-invasive X-ray induced luminescence strategy in activating photo-agent in deep seated tumors.

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Novel applications of diagnostic X-rays in activating a clinical photodynamic drug: Photofrin II through X-ray induced visible luminescence from "rare-earth" formulated particles