Assessment of Photodynamic Therapy (PDT) Efficacy of X-ray Activated Nanoscintillators Using ¹⁸F-FLT in PET

Objectives: We have developed Y₂O₃:Eu nanoparticles that exhibit bright, stable phosphorescence upon their x-ray irradiation; with marked linearity of response for changes in x-ray flux and energy, making them suited for dosimetric applications^1,2. Materials with high atomic numbers can efficiently convert incoming x-ray to fast electrons via the photoelectric effect, and in turn produce singlet oxygen which damages DNA in cancer cells. The efficacy of radiotherapy (RT) is often identified by responses such as changes in tumor size, which may not correlate with metabolic and physiological changes that precede a change in size. The correlation between 3′-deoxy-3′-¹⁸F-fluorothymidine (¹⁸F-FLT) uptake and proliferation has been validated for various tumors³. In this study, we used ¹⁸F-FLT PET to evaluate the Europium-doped nanoscintillators as potential radiosensitizers for ovarian cancer photodynamic therapy (PDT).

Methods: Europium-doped yttrium oxide nanoscintillators with silica shell (Y₂O₃:Eu@SiO₂) were synthesized and characterized as previously described^1,2. Nanoscintillators surface was further modified with trimethyl ammonia groups (Y₂O₃:Eu@SiO₂-TA) and resuspended in 10mM HEPES buffer at pH7.4 to increase solubility. Radiation-sensitive CAOV3 ovarian cancer cells⁴ were inoculated into female nude mice. Xenografts were injected intratumorally with 1mg/500 mm³ of Y₂O₃:Eu@SiO₂-TA. X-ray irradiation was performed following a fractionated protocol, of 2 Gy/day for 4 consecutive days. CAOV3 and SKOV3 radiation resistant xenografts without nanoscintillators were used as negative controls. Tumor response to treatment was assessed via ¹⁸F-FLT PET. Baseline tumor cell proliferation was quantified the day prior to initiation of X-ray irradiation and monitored up to 14 days post-treatment. ¹⁸F-FLT was produced by the Cyclotron Facility at the University of Chicago (UChicago) and microPET/CT was performed on the Beta-Cube and X-Cube by Molecubes in the Integrated Small Animal Imaging Research Resource (iSAIRR) at UChicago.

Results: Uniform Y₂O₃:Eu@SiO₂-TA nanoscintillators were reproducibly synthesized and TA surface modification combined with HEPES buffer increased their solubility by 10 folds vs. other buffers. CAOV3 xenografts responded to irradiation. However, while ¹⁸F-FLT mean uptake decreased starting from Day-1 post-treatment, decrease in tumor volumes were measured only starting from Day-7. At Day-14, both CAOV3-Y₂O₃:Eu@SiO₂-TA and CAOV3 groups had an average reduction of 50% in volume, but no significant difference was found between the two groups. In contrast, ¹⁸F-FLT mean uptake decreased about 20% from Day-1. While it remained stable at about 50% of baseline up to Day-14 for the PDT group, CAOV3 tumors that received RT only showed renewed cell proliferation at later time points. Spatial distribution of ¹⁸F-FLT in the tumors revealed an irregular pattern. A layered analysis showed higher uptake in the outer layers with 2-fold reduction in the center of the tumors, likely indicative of necrosis. As anticipated, RT had little effect on SKOV3 tumors, which quadrupled in volume. ¹⁸F-FLT mean uptake remained about the same as baseline following RT, and was significantly higher than in the two CAOV3 groups.

Conclusions: Preliminary results confirmed the therapeutic efficacy of Y₂O₃:Eu@SiO₂-TA nanoscintillators as photosensitizing agents. Intratumoral delivery of Y₂O₃:Eu@SiO₂-TA nanoscintillators resulted in increased response to x-ray radiation therapy and reduction in tumor burden and proliferation as shown by ¹⁸F-FLT PET. Sensitivity to x-rays makes these nanoscintillators suitable for treatment of deep masses, thus overcoming one of the major limitations of PDT. Ongoing efforts aim at increasing their therapeutic effect by modifying the surface to selectively target ovarian cancer biomarkers in animals bearing orthotopic metastatic ovarian cancer, a more clinically relevant model.

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