Cherenkov Radiation induced photodynamic therapy - repurposing older photosensitizers, and radionuclides.

CONTEXT: Old-generation photosensitizers are minimally used in current photodynamic therapy (PDT) because they absorb in the UV/blue/green region of the spectrum where biological tissues are generally highly absorbing. The UV/blue light of Cherenkov Radiation (CR) from nuclear disintegration of beta-emitter radionuclides shows promise as an internal light source to activate these photosensitizers within tissue. Outline of the study: 1) radionuclide choice and Cherenkov Radiation, 2) Photosensitizer choice, synthesis and radiolabeling, 3) CR-induced fluorescence, 4) Verification of ROS formation, 5) CR-induced PDT with either free eosine and free CR emitter, or with radiolabelled eosin. RESULTS: Cherenkov Radiation Energy Transfer (CRET) from therapeutic radionuclides (90Y) and PET imaging radionuclides (18F, 68Ga) to eosin was shown by spectrofluorimetry and in vitro, and was shown to result in a PDT process. The feasibility of CR-induced PDT (CR-PDT) was demonstrated in vitro on B16F10 murine melanoma cells mixing free eosin (λabs = 524 nm, ΦΔ 0.67) with free CR-emitter [18F]-FDG under their respective intrinsic toxicity levels (0.5 mM/8 MBq) and by trapping singlet oxygen with diphenylisobenzofuran (DPBF). An eosin-DOTAGA-chelate conjugate 1 was synthesized and radiometallated with CR-emitter [68Ga] allowed to reach 25 % cell toxicity at 0.125 mM/2 MBq, i.e. below the toxicity threshold of each component measured on controls. Incubation time was carefully examined, especially for CR emitters, in light of its toxicity, and its CR-emitting yield expected to be 3 times as much for 68Ga than 18F (considering their ß particle energy) per radionuclide decay, while its half-life is about twice as small. PERSPECTIVE: This study showed that in complete darkness, as it is at depth in tissues, PDT could proceed relying on CR emission from radionuclides only. Interestingly, this study also repurposed PET imaging radionuclides, such as 68Ga, to trigger a therapeutic event (PDT), albeit in a modest extent. Moreover, although it remains modest, such a PDT approach may be used to achieve additional tumoricidal effect to RIT treatment, where radionuclides, such as 90Y, are strong CR emitters, i.e. very potent light source for photosensitizer activation.

Cherenkov luminescence imaging is a fast and relevant preclinical tool to assess tumour hypoxia in vivo.

PURPOSE: Molecular imaging techniques visualise biomarkers for both drug development and personalised medicine. In this field, Cherenkov luminescence imaging (CLI) seems to be very attractive by allowing imaging with clinical PET radiotracers with high-throughput capabilities. In this context, we developed a fast CLI method to detect tumour hypoxia with 18F-fluoromisonidazole (FMISO) for drug development purposes. METHODS: Colon cancer model was induced in mice by subcutaneous injection of 1 × 106 CT-26 cells. FMISO was injected, and simultaneous PET-blood oxygen level dependent (BOLD)-MRI followed by CLI were performed along with immunohistochemistry staining with pimonidazole. RESULTS: There was a significant correlation between FMISO PET and CLI tumour uptakes, consistent with the BOLD-MRI mapping. Tumour-to-background ratio was significantly higher for CLI compared with PET and MRI. Immunohistochemistry confirmed tumour hypoxia. The imaging workflow with CLI was about eight times faster than the PET-MRI procedure. CONCLUSION: CLI is a fast and relevant tool to assess tumour hypoxia. This approach could be particularly interesting for hypoxia-targeting drug development.