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Revisiting the Radiosynthesis of [¹⁸F]FPEB and Preliminary PET Imaging in a Mouse Model of Alzheimer's Disease.

ABSTRACT: [¹⁸F]FPEB is a positron emission tomography (PET) radiopharmaceutical used for imaging the abundance and distribution of mGluR5 in the central nervous system (CNS). Efficient radiolabeling of the aromatic ring of [¹⁸F]FPEB has been an ongoing challenge. Herein, five metal-free precursors for the radiofluorination of [¹⁸F]FPEB were compared, namely, a chloro-, nitro-, sulfonium salt, and two spirocyclic iodonium ylide (SCIDY) precursors bearing a cyclopentyl (SPI5) and a new adamantyl (SPIAd) auxiliary. The chloro- and nitro-precursors resulted in a low radiochemical yield (<10% RCY), whereas both SCIDY precursors and the sulfonium salt precursor produced [¹⁸F]FPEB in the highest RCYs of 25% and 36%, respectively. Preliminary PET/CT imaging studies with [¹⁸F]FPEB were conducted in a transgenic model of Alzheimer's Disease (AD) using B6C3-Tg(APPswe,PSEN1dE9)85Dbo/J (APP/PS1) mice, and data were compared with age-matched wild-type (WT) B6C3F1/J control mice. In APP/PS1 mice, whole brain distribution at 5 min post-injection showed a slightly higher uptake (SUV = 4.8 ± 0.4) than in age-matched controls (SUV = 4.0 ± 0.2). Further studies to explore mGluR5 as an early biomarker for AD are underway.

SUBMITTER: Varlow C

PROVIDER: S-EPMC7070414 | biostudies-literature | 2020 Feb

REPOSITORIES: biostudies-literature

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Revisiting the Radiosynthesis of [18F]FPEB and Preliminary PET Imaging in a Mouse Model of Alzheimer's Disease.

Varlow Cassis C Murrell Emily E Holland Jason P JP Kassenbrock Alina A Shannon Whitney W Liang Steven H SH Vasdev Neil N Stephenson Nickeisha A NA

Molecules (Basel, Switzerland) 20200222 4

[18F]FPEB is a positron emission tomography (PET) radiopharmaceutical used for imaging the abundance and distribution of mGluR5 in the central nervous system (CNS). Efficient radiolabeling of the aromatic ring of [18F]FPEB has been an ongoing challenge. Herein, five metal-free precursors for the radiofluorination of [18F]FPEB were compared, namely, a chloro-, nitro-, sulfonium salt, and two spirocyclic iodonium ylide (SCIDY) precursors bearing a cyclopentyl (SPI5 ...[more]

PMID: 32098347

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Project description:INTRODUCTION:Our previous work demonstrated that the 99mTc renal tracer, 99mTc(CO)3(FEDA) (99mTc-1), has a rapid clearance comparable in rats to that of 131I-OIH, the radioactive gold standard for the measurement of effective renal plasma flow. The uncharged fluoroethyl pendant group of 99mTc-1 provides a route to the synthesis of a structurally analogous rhenium-tricarbonyl 18F renal imaging agent, Re(CO)3([18F]FEDA) (18F-1). Our goal was to develop an efficient one-step method for the preparation of 18F-1 and to compare its pharmacokinetic properties with those of 131I-OIH in rats. METHODS:18F-1 was prepared by the nucleophilic 18F-fluorination of its tosyl precursor. The labeled compound was isolated by HPLC and subsequently evaluated in Sprague-Dawley rats using 131I-OIH as an internal control and by dynamic PET/CT imaging. Plasma protein binding (PPB) and erythrocyte uptake (RCB) were determined and the urine was analyzed for metabolites. RESULTS:18F-1 was efficiently prepared as a single species with high radiochemical purity (>99%) and it displayed high radiochemical stability in vitro and in vivo. PPB was 87% and RCB was 21%. Biodistribution studies confirmed rapid renal extraction and high specificity for renal excretion, comparable to that of 131I-OIH, with minimal hepatic/gastrointestinal elimination. The activity in the urine, as a percentage of 131I-OIH, was 92% and 95% at 10 and 60 min, respectively. All other organs (heart, spleen, lungs) showed a negligible tracer uptake (<0.4% ID). Dynamic microPET/CT imaging demonstrated rapid transit of 18F-1 through the kidneys and into the bladder; there was no demonstrable activity in bone verifying the absence of free [18F]fluoride. CONCLUSIONS:18F-1 exhibited a high specificity for the kidney, rapid renal excretion comparable to that of 131I-OIH and high in vivo radiochemical stability. Not only is 18F-1 a promising PET renal tracer, but it provides a route to the development of a pair of analogous 18F/99mTc renal imaging agents with almost identical structures and comparable pharmacokinetic properties. These promising in vivo results warrant subsequent evaluation in humans.

Project description:BACKGROUND:Peptides labeled with positron-emitting isotopes are emerging as a versatile class of compounds for the development of highly specific, targeted imaging agents for diagnostic imaging via positron-emission tomography (PET) and for precision medicine via theranostic applications. Despite the success of peptides labeled with gallium-68 (for imaging) or lutetium-177 (for therapy) in the clinical management of patients with neuroendocrine tumors or prostate cancer, there are significant advantages of using fluorine-18 for imaging. Recent developments have greatly simplified such labeling: in particular, labeling of organotrifluoroborates via isotopic exchange can readily be performed in a single-step under aqueous conditions and without the need for HPLC purification. Though an automated synthesis has not yet been explored, microfluidic approaches have emerged for 18F-labeling with high speed, minimal reagents, and high molar activity compared to conventional approaches. As a proof-of-concept, we performed microfluidic labeling of an octreotate analog ([18F]AMBF3-TATE), a promising 18F-labeled analog that could compete with [68Ga]Ga-DOTATATE with the advantage of providing a greater number of patient doses per batch produced. METHODS:Both [18F]AMBF3-TATE and [68Ga]Ga-DOTATATE were labeled, the former by microscale methods adapted from manual labeling, and were imaged in mice bearing human SSTR2-overexpressing, rat SSTR2 wildtype, and SSTR2-negative xenografts. Furthermore, a dosimetry analysis was performed for [18F]AMBF3-TATE. RESULTS:The micro-synthesis exhibited highly-repeatable performance with radiochemical conversion of 50?±?6% (n?=?15), overall decay-corrected radiochemical yield of 16?±?1% (n?=?5) in ~40?min, radiochemical purity >99%, and high molar activity. Preclinical imaging with [18F]AMBF3-TATE in SSTR2 tumor models correlated well with [68Ga]Ga-DOTATATE. The favorable biodistribution, with the highest tracer accumulation in the bladder followed distantly by gastrointestinal tissues, resulted in 1.26?×?10-2?mSv/MBq maximal estimated effective dose in human, a value lower than that reported for current clinical 18F- and 68Ga-labeled compounds. CONCLUSIONS:The combination of novel chemical approaches to 18F-labeling and microdroplet radiochemistry have the potential to serve as a platform for greatly simplified development and production of 18F-labeled peptide tracers. Favorable preclinical imaging and dosimetry of [18F]AMBF3-TATE, combined with a convenient synthesis, validate this assertion and suggest strong potential for clinical translation.

Dataset Information

Revisiting the Radiosynthesis of [¹⁸F]FPEB and Preliminary PET Imaging in a Mouse Model of Alzheimer's Disease.

Publications

Revisiting the Radiosynthesis of [<sup>18</sup>F]FPEB and Preliminary PET Imaging in a Mouse Model of Alzheimer's Disease.

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