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Reply to ‘Is Cherenkov luminescence bright enough for photodynamic therapy?’

Nature Nanotechnology volume 13, pages 354–355 (2018)Cite this article

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Kotagiri, Laforest and Achilefu reply — The interaction of beta radionuclides with semiconductors and photocatalysts such as TiO2 remains an underexplored and exciting area of investigation. These interactions could potentially involve many processes, including: (1) radiolysis of bulk water by ionizing radiation, generating electrons and hydroxyl radicals; (2) generation of electron and hole pairs in TiO2 through radiolytic energy conversion1; and (3) generation of electron and hole pairs in TiO2 through Cherenkov luminescence energy transfer and other luminescence-generating phenomena. The various types of energy emission from a decaying radionuclide are complex and diverse. While it would be immensely important and interesting to explore the various phenomena dictating material behaviour on interaction with these sources, we took steps to initially demonstrate the factors that do not play a singular role in the effects we observed.

To minimize the contribution of pathway (1) above, we used an 18F-fludeoxyglucose (18F-FDG) dose that ensured 18F radiolysis did not result in any observable biological effect in the absence of TiO2. Using a HT1080 tumour model, a reduction in tumour burden was not observed up to 30 MBq. In pathways (2) and (3) above, hydrogen peroxide, singlet oxygen, hydroxyl and superoxide radicals are produced on the solid–liquid interface through catalysis2,3. The cumulative effects of these radicals is expected to induce cytotoxic effects in tumours. Superoxide dismutase can convert superoxide radicals to hydrogen peroxide and the increased iron content in tumour cells facilitates the conversion of hydrogen peroxide to hydroxyl radicals. Hydrogen peroxide is sufficiently stable to traverse cells before conversion to the reactive hydroxyl radicals. Though hydroxyl radicals have extremely short lifespan and path lengths, their high reactivity is known to induce localized cytotoxicity4.

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Fig. 1: Clonogenic assay comparing the effect of <250 keV X-ray irradiation on HT1080 cancer cells with and without TiO2 nanoparticles (2.5 μg ml–1).

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  1. Department of Radiology, Washington University School of Medicine, St Louis, MO, USA

    Nalinikanth Kotagiri, Richard Laforest & Samuel Achilefu

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  1. Nalinikanth Kotagiri
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  2. Richard Laforest
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  3. Samuel Achilefu
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Correspondence to Samuel Achilefu.

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Kotagiri, N., Laforest, R. & Achilefu, S. Reply to ‘Is Cherenkov luminescence bright enough for photodynamic therapy?’. Nature Nanotech 13, 354–355 (2018). https://doi.org/10.1038/s41565-018-0143-x

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