Kinetic analysis of the metabotropic glutamate subtype 5 tracer [(18)F]FPEB in bolus and bolus-plus-constant-infusion studies in humans

A systematic review of molecular imaging (PET and SPECT) in autism spectrum disorder: current state and future research opportunities

Non-invasive positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are techniques used to quantify molecular interactions, biological processes and protein concentration and distribution. In the central nervous system, these molecular imaging techniques can provide critical insights into neurotransmitter receptors and their occupancy by neurotransmitters or drugs. In recent years, there has been an increase in the number of studies that have investigated neurotransmitters in autism spectrum disorder (ASD), while earlier studies mostly focused on cerebral blood flow and glucose metabolism. The underlying and contributing mechanisms of ASD are largely undetermined and ASD diagnosis relies on the behavioral phenotype. Discovery of biochemical endophenotypes would represent a milestone in autism research that could potentially lead to ASD subtype stratification and the development of novel therapeutic drugs. This review characterizes the prior use of molecular imaging by PET and SPECT in ASD, addresses methodological challenges and highlights areas of future opportunity for contributions from molecular imaging to understand ASD pathophysiology.

Iodonium ylide-mediated radiofluorination of 18F-FPEB and validation for human use

Translation of new methodologies for labeling nonactivated aromatic molecules with (18)F remains a challenge. Here, we report a one-step, regioselective, metal-free (18)F-labeling method that uses a hypervalent iodonium(III) ylide precursor, to prepare the radiopharmaceutical (18)F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ((18)F-FPEB).

METHODS

Automated radiosynthesis of (18)F-FPEB was achieved by reaction of the ylide precursor (4 mg) with (18)F-Et4NF in dimethylformamide at 80°C for 5 min and formulated for injection within 1 h.

RESULTS

(18)F-FPEB was synthesized in 20% ± 5% (n = 3) uncorrected radiochemical yields relative to (18)F-fluoride, with specific activities of 666 ± 51.8 GBq (18 ± 1.4 Ci)/μmol at the end of synthesis and was validated for human use.

CONCLUSION

Radiofluorination of iodonium (III) ylides proved to be an efficient radiosynthetic strategy for synthesis of (18)F-labeled radiopharmaceuticals.

In vivo ketamine-induced changes in [¹¹C]ABP688 binding to metabotropic glutamate receptor subtype 5

BACKGROUND

At subanesthetic doses, ketamine, an N-methyl-D-aspartate glutamate receptor antagonist, increases glutamate release. We imaged the acute effect of ketamine on brain metabotropic glutamatergic receptor subtype 5 with a high-affinity positron emission tomography (PET) ligand [(11)C]ABP688 (E)-3-[2-(6-methyl-2-pyridinyl)ethynyl]-2-cyclohexen-1-one-O-(methyl-11C)oxime, a negative allosteric modulator of the metabotropic glutamatergic receptor subtype 5.

METHODS

Two [(11)C]ABP688 PET scans were performed in 10 healthy nonsmoking human volunteers (34 ± 13 years old); the two PET scans were performed on the same day-before (scan 1) and during intravenous ketamine administration (.23 mg/kg over 1 min, then .58 mg/kg over 1 hour; scan 2). The PET data were acquired for 90 min immediately after [(11)C]ABP688 bolus injection. Input functions were obtained through arterial blood sampling with metabolite analysis.

RESULTS

A significant reduction in [(11)C]ABP688 volume of distribution was observed in scan 2 relative to scan 1 of 21.3% ± 21.4%, on average, in the anterior cingulate, medial prefrontal cortex, orbital prefrontal cortex, ventral striatum, parietal lobe, dorsal putamen, dorsal caudate, amygdala, and hippocampus. There was a significant increase in measurements of dissociative state after ketamine initiation (p < .05), which resolved after completion of the scan.

CONCLUSIONS

This study provides first evidence that ketamine administration decreases [(11)C]ABP688 binding in vivo in human subjects. The results suggest that [(11)C]ABP688 binding is sensitive to ketamine-induced effects, although the high individual variation in ketamine response requires further examination.

Preparation of the metabotropic glutamate receptor 5 (mGluR5) PET tracer [(18)F]FPEB for human use: An automated radiosynthesis and a novel one-pot synthesis of its radiolabeling precursor

The radiotracer 3-[(18)F]fluoro-5-(2-pyridinylethynyl)benzonitrile, or [(18)F]FPEB, is a promising PET imaging agent for the metabotropic glutamate subtype 5 receptor (mGluR5). In an effort to develop a routine production method of this radiotracer for use in clinical research we adapted its radiosynthesis to an automated chemistry module. In the meanwhile, we also developed a simplified "one-pot" method for the preparation of the nitrobenzonitrile radiolabeling precursor for [(18)F]FPEB and its reference standard to replace the existing multi-step synthetic approach.

Radiation Dosimetry of (18)F-FPEB in Humans

(18)F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ((18)F-FPEB) is a potent and specific radioligand for the metabotropic glutamate receptor subtype 5 (mGluR5). Before undertaking clinical research studies with (18)F-FPEB, we performed studies of human radiation dosimetry.

METHODS

Serial whole-body scans were obtained in 9 healthy human subjects (5 men, 4 women) for 190-440 min after the intravenous administration of (18)F-FPEB. Radiation doses were estimated using the OLINDA/EXM software.

RESULTS

Peak organ doses were to the urinary bladder wall, 0.258 mGy/MBq (0.955 rad/mCi), and gallbladder wall, 0.193 mGy/MBq (0.716 rad/mCi). The effective dose was 0.025 mSv/MBq (0.0922 rem/mCi). The doses to the red marrow and spleen were 0.00797 mGy/MBq (0.0295 rad/mCi) and 0.00709 mGy/MBq (0.0262 rad/mCi), respectively. Reducing the urinary voiding interval to 60 or 90 min lowered the urinary bladder wall dose to 0.0885 mGy/MBq (0.327 rad/mCi) or 0.128 mGy/MBq (0.473 rad/mCi), respectively, and the effective dose to 0.0149 mSv/MBq (0.0551 rem/mCi) or 0.0171 mSv/MBq (0.0634 rem/mCi), respectively.

CONCLUSION

Urinary voiding should be performed during (18)F-FPEB studies to minimize radiation exposure to research subjects.

Imaging glutamate in schizophrenia: review of findings and implications for drug discovery

Currently, all treatments for schizophrenia (SCZ) function primarily by blocking D(2)-type dopamine receptors. Given the limitations of these medications, substantial efforts have been made to identify alternative neurochemical targets for treatment development in SCZ. One such target is brain glutamate. The objective of this article is to review and synthesize the proton magnetic resonance spectroscopy ((1)H MRS) and positron emission tomography (PET)/single-photon emission computed tomography (SPECT) investigations that have examined glutamatergic indices in SCZ, including those of modulatory compounds such as glutathione (GSH) and glycine, as well as data from ketamine challenge studies. The reviewed (1)H MRS and PET/SPECT studies support the theory of hypofunction of the N-methyl-D-aspartate receptor (NMDAR) in SCZ, as well as the convergence between the dopamine and glutamate models of SCZ. We also review several advances in MRS and PET technologies that have opened the door for new opportunities to investigate the glutamate system in SCZ and discuss some ways in which these imaging tools can be used to facilitate a greater understanding of the glutamate system in SCZ and the successful and efficient development of new glutamate-based treatments for SCZ.

Microfluidic continuous-flow radiosynthesis of [18F]FPEB suitable for human PET imaging

The synthesis of fluorine-18 labeled 3-fluoro-5-[(pyridin-3-yl)ethynyl] benzonitrile ([¹⁸F]FPEB) for imaging metabotropic glutamate receptor subtype type 5 (mGluR5) was achieved with a commercial continuous-flow microfluidics device. This work represents the first positron emission tomography (PET) radiopharmaceutical that is suitable for human use with this technology. We also describe a validated synthesis of [¹⁸F]FPEB with a commercial reactor-based system.