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Yan X, Siméon FG, Liow JS, Morse CL, Montero Santamaria JA, Jenkins M, Manly LS, Van Buskirk M, Zoghbi SS, Pike VW, Innis RB, Zanotti-Fregonara P. In vivo evaluation of a novel 18F-labeled PET radioligand for translocator protein 18 kDa (TSPO) in monkey brain. Eur J Nucl Med Mol Imaging 2023; 50:2962-2970. [PMID: 37249618 PMCID: PMC10382351 DOI: 10.1007/s00259-023-06270-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/14/2022] [Accepted: 05/18/2023] [Indexed: 05/31/2023]
Abstract
PURPOSE [18F]SF51 was previously found to have high binding affinity and selectivity for 18 kDa translocator protein (TSPO) in mouse brain. This study sought to assess the ability of [18F]SF51 to quantify TSPO in rhesus monkey brain. METHODS Positron emission tomography (PET) imaging was performed in monkey brain (n = 3) at baseline and after pre-blockade with the TSPO ligands PK11195 and PBR28. TSPO binding was calculated as total distribution volume corrected for free parent fraction in plasma (VT/fP) using a two-tissue compartment model. Receptor occupancy and nondisplaceable uptake were determined via Lassen plot. Binding potential (BPND) was calculated as the ratio of specific binding to nondisplaceable uptake. Time stability of VT was used as an indirect probe to detect radiometabolite accumulation in the brain. In vivo and ex vivo experiments were performed in mice to determine the distribution of the radioligand. RESULTS After [18F]SF51 injection, the concentration of brain radioactivity peaked at 2.0 standardized uptake value (SUV) at ~ 10 min and declined to 30% of the peak at 180 min. VT/fP at baseline was generally high (203 ± 15 mL· cm-3) and decreased by ~ 90% after blockade with PK11195. BPND of the whole brain was 7.6 ± 4.3. VT values reached levels similar to terminal 180-min values by 100 min and remained relatively stable thereafter with excellent identifiability (standard errors < 5%), suggesting that no significant radiometabolites accumulated in the brain. Ex vivo experiments in mouse brain showed that 96% of radioactivity was parent. No significant uptake was observed in the skull, suggesting a lack of defluorination in vivo. CONCLUSION The results demonstrate that [18F]SF51 is an excellent radioligand that can quantify TSPO with a good ratio of specific to nondisplaceable uptake and has minimal radiometabolite accumulation in brain. Collectively, the results suggest that [18F]SF51 warrants further evaluation in humans.
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Affiliation(s)
- Xuefeng Yan
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA.
| | - Fabrice G Siméon
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
| | - Jeih-San Liow
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
| | - Cheryl L Morse
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
| | - Jose A Montero Santamaria
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
| | - Madeline Jenkins
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
| | - Lester S Manly
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
| | - Maia Van Buskirk
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
| | - Sami S Zoghbi
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
| | - Victor W Pike
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
| | - Robert B Innis
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
| | - Paolo Zanotti-Fregonara
- Molecular Imaging Branch, National Institute of Mental Health, NIH, 10 Center Drive, Bethesda, MD, 20892, USA
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Shetty HU, Morse CL, Pike VW. Tandem Mass Spectrometry as an Independent Method for Corroborating Fluorine-18 Radioactivity Measurements in Positron Emission Tomography. ACS MEASUREMENT SCIENCE AU 2022; 2:370-376. [PMID: 35996540 PMCID: PMC9389646 DOI: 10.1021/acsmeasuresciau.2c00018] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Positron emission tomography (PET) uses many tracers labeled with fluorine-18 (t 1/2 = 109.8 min; β+ 97%) for quantitative imaging of biochemical and physiological processes in animal and human subjects. In PET methodology, the radioactivity in a dose of an 18F-labeled tracer to be administered to a living subject is measured with a calibrated ionization chamber. This type of detector measures the radioactivity of a sample relative to those of certified amounts of longer-lived surrogate isotopes that are recommended for detector calibration. No alternative means for corroborating widely varying fluorine-18 radioactivity measurements from calibrated ionization chambers has been available. Here, we describe an independent nonradiometric method for this purpose. In this method, highly sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) is used to quantify the relative masses of the radioactive isotopologue ([18F]1) and the accompanying nonradioactive counterpart (carrier 1) in an 18F-labeled tracer preparation to give the mole ratio of [18F]1. High-performance liquid chromatography (HPLC) with a mass-calibrated absorbance detection is used alongside to provide a separate measurement of the aggregate mass of all isotopologues. The radioactivity of the radiotracer is then derived in becquerels (Bq) from these two measurements, plus Avogadro's number and the decay constant of fluorine-18. For the chosen example [18F]LSN3316612, the radioactivity values determined nonradiometrically and with a selected ionization chamber were in fair agreement. In addition, LC-MS/MS alone was found to provide an accurate measure of the half-life of fluorine-18.
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