Radiolabeling and in vitro and in vivo evaluation of [18F]-FE-DABP688 as a PET radioligand for the metabotropic glutamate receptor subtype 5

Synthesis, Biodistribution, and Radiation Dosimetry of a Novel mGluR5 Radioligand: 18F-AZD9272

The metabotropic glutamate receptor subtype mGluR5 has been proposed as a potential drug target for CNS disorders such as anxiety, depression, Parkinson’s disease, and epilepsy. The AstraZeneca compound AZD9272 has previously been labeled with carbon-11 and used as a PET radioligand for mGluR5 receptor binding. The molecular structure of AZD9272 allows one to label the molecule with fluorine-18 without altering the structure. The aim of this study was to develop a fluorine-18 analogue of AZD9272 and to examine its binding distribution in the nonhuman primate brain in vivo as well as to obtain whole body radiation dosimetry. ¹⁸F-AZD9272 was successfully synthesized from a nitro precursor. The radioligand was stable, with a radiochemical purity of >99% at 2 h after formulation in a sterile phosphate buffered solution (pH = 7.4). After injection of ¹⁸F-AZD9272 in two cynomolgus monkeys, the maximum whole brain radioactivity concentration was 4.9–6.7% of the injected dose (n = 2) and PET images showed a pattern of regional radioactivity consistent with that previously obtained for ¹¹C-AZD9272. The percentage of parent radioligand in plasma was 59 and 64% (n = 2) at 120 min after injection of ¹⁸F-AZD9272, consistent with high metabolic stability. Two whole body PET scans were performed in nonhuman primates for a total of 231 min after injection of ¹⁸F-AZD9272. Highest uptakes were seen in liver and small intestine, followed by brain and kidney. The estimated effective dose was around 0.017 mSv/MBq. ¹⁸F-AZD9272 shows suitable properties as a PET radioligand for in vivo imaging of binding in the primate brain. ¹⁸F-labeled AZD9272 offers advantages over ¹¹C-AZD9272 in terms of higher image resolution, combined with a longer half-life. Moreover, based on the distribution and the estimated radiation burden, imaging of ¹⁸F-AZD9272 could be used as an improved tool for quantitative assessment and characterization of AZD9272 binding sites in the human brain by using PET.

Radiation dosimetry of [18F]-PSS232-a PET radioligand for imaging mGlu5 receptors in humans

Purpose

(E)-3-(pyridin-2-ylethynyl)cyclohex-2-enone O-(3-(2-[¹⁸F]-fluoroethoxy)propyl) oxime ([¹⁸F]-PSS232) is a new PET tracer for imaging of metabotropic glutamate receptor subtype 5 (mGlu5), and has shown promising results in rodents and humans. The aim of this study was to estimate the radiation dosimetry and biodistribution in humans, to assess dose-limiting organs, and to demonstrate safety and tolerability of [¹⁸F]-PSS232 in healthy volunteers.

Methods

PET/CT scans of six healthy male volunteers (mean age 23.5 ± 1.7; 21–26 years) were obtained after intravenous administration of 243 ± 3 MBq of [¹⁸F]-PSS232. Serial whole-body (vertex to mid-thigh) PET scans were assessed at ten time points, up to 90 min after tracer injection. Calculation of tracer kinetics and cumulated organ activities were performed using PMOD 3.7 software. Dosimetry estimates were calculated using the OLINDA/EXM software.

Results

Injection of [¹⁸F]-PSS232 was safe and well tolerated. Organs with highest absorbed doses were the gallbladder wall (0.2295 mGy/MBq), liver (0.0547 mGy/MBq), and the small intestine (0.0643 mGy/MBq). Mean effective dose was 3.72 ± 0.12 mSv/volunteer (range 3.61–3.96 mSv; 0.0153 mSv/MBq).

Conclusion

[¹⁸F]-PSS232, a novel [¹⁸F]-labeled mGlu5 tracer, showed favorable dosimetry values. Additionally, the tracer was safe and well tolerated.

Preclinical evaluation and test-retest studies of [(18)F]PSS232, a novel radioligand for targeting metabotropic glutamate receptor 5 (mGlu5)

PURPOSE

A novel, (18)F-labelled metabotropic glutamate receptor subtype 5 (mGlu5) derivative of [(11)C]ABP688 ([(11)C]1), [(18)F]PSS232 ([(18)F] ]5), was evaluated in vitro and in vivo for its potential as a PET agent and was used in test-retest reliability studies

METHODS

The radiosynthesis of [(18)F]5 was accomplished via a one-step reaction using a mesylate precursor. In vitro stability was determined in PBS and plasma, and with liver microsomal enzymes. Metabolite studies were performed using rat brain extracts, blood and urine. In vitro autoradiography was performed on horizontal slices of rat brain using 1 and 8, antagonists for mGlu5 and mGlu1, respectively. Small-animal PET, biodistribution, and test-retest studies were performed in Wistar rats. In vivo, dose-dependent displacement studies were performed using 6 and blocking studies with 7.

RESULTS

[(18)F]5 was obtained in decay-corrected maximal radiochemical yield of 37 % with a specific activity of 80 - 400 GBq/μmol. Treatment with rat and human microsomal enzymes in vitro for 60 min resulted in 20 % and 4 % of hydrophilic radiometabolites, respectively. No hydrophilic decomposition products or radiometabolites were found in PBS or plasma. In vitro autoradiography on rat brain slices showed a heterogeneous distribution consistent with the known distribution of mGlu5 with high binding to hippocampal and cortical regions, and negligible radioactivity in the cerebellum. Similar distribution of radioactivity was found in PET images. Under displacement conditions with 6, reduced [(18)F]5 binding was found in all brain regions except the cerebellum. 7 reduced binding in the striatum by 84 % on average. Test-retest studies were reproducible with a variability ranging from 6.8 % to 8.2 %. An extended single-dose toxicity study in Wistar rats showed no compound-related adverse effects.

CONCLUSION

The new mGlu5 radiotracer, [(18)F]5, showed specific and selective in vitro and in vivo properties and is a promising radioligand for PET imaging of mGlu5 in humans.

Synthesis and evaluation of [11C]MMPIP as a potential radioligand for imaging of metabotropic glutamate 7 receptor in the brain

BACKGROUND

Metabotropic glutamate 7 (mGlu7) receptor is a crucial target protein for the development of pharmaceuticals against central nervous system disorders. In the present study, we synthesized [11C]MMPIP, a putative radioligand for mGlu7 (binding constant KB = 30 nM), and evaluated its potential for imaging of mGlu7 via in vitro and in vivo techniques.

METHODS

[11C]MMPIP was synthesized by the reaction of phenol precursor 3 with [11C]CH3I. In vitro autoradiography using [11C]MMPIP was performed on rat brain sections. To determine in vitro specific binding of [11C]MMPIP with mGlu7, a blocking study was conducted by co-incubation with excess AMN082, a selective antagonist for mGlu7, or unlabeled MMPIP. Positron emission tomography (PET) studies and ex vivo metabolite analysis were carried out on rat brains.

RESULTS

[11C]MMPIP was obtained with two specific activity (SA) levels of average 58 (conventional) and 3,800 (high SA) GBq/μmol, respectively. High radioactive signals derived from conventional [11C]MMPIP in the in vitro autoradiography were seen in the thalamus, medulla oblongata, and striatum, corresponding with comprehensive brain distributions of mGlu7. Co-incubation with ANM082 or unlabeled MMPIP reduced the radioactive signals in the brain sections, respectively. In the PET studies with [11C]MMPIP, no specific uptake relative to mGlu7 was found in the examined brain regions.

CONCLUSION

Despite in vitro specific binding of [11C]MMPIP with mGlu7, visualization of mGlu7 in the living brain using PET was not successful. Development of new ligand candidates with higher affinity for mGlu7 is necessary.

Probing the metabotropic glutamate receptor 5 (mGlu₅) positive allosteric modulator (PAM) binding pocket: discovery of point mutations that engender a "molecular switch" in PAM pharmacology

Positive allosteric modulation of metabotropic glutamate receptor subtype 5 (mGlu₅) is a promising novel approach for the treatment of schizophrenia and cognitive disorders. Allosteric binding sites are topographically distinct from the endogenous ligand (orthosteric) binding site, allowing for co-occupation of a single receptor with the endogenous ligand and an allosteric modulator. Negative allosteric modulators (NAMs) inhibit and positive allosteric modulators (PAMs) enhance the affinity and/or efficacy of the orthosteric agonist. The molecular determinants that govern mGlu₅ modulator affinity versus cooperativity are not well understood. Focusing on the modulators based on the acetylene scaffold, we sought to determine the molecular interactions that contribute to PAM versus NAM pharmacology. Generation of a comparative model of the transmembrane-spanning region of mGlu₅ served as a tool to predict and interpret the impact of mutations in this region. Application of an operational model of allosterism allowed for determination of PAM and NAM affinity estimates at receptor constructs that possessed no detectable radioligand binding as well as delineation of effects on affinity versus cooperativity. Novel mutations within the transmembrane domain (TM) regions were identified that had differential effects on acetylene PAMs versus 2-methyl-6-(phenylethynyl)-pyridine, a prototypical NAM. Three conserved amino acids (Y658, T780, and S808) and two nonconserved residues (P654 and A809) were identified as key determinants of PAM activity. Interestingly, we identified two point mutations in TMs 6 and 7 that, when mutated, engender a mode switch in the pharmacology of certain PAMs.

PET and SPECT tracers for glutamate receptors

Radioligands for PET imaging of glutamate receptors will have the potential for studying neurological and neuropsychiatric disorders and their diagnosis and therapeutic intervention. Glutamate is the major excitatory neurotransmitter in the brain and is implicated in the pathophysiology of many neurodegenerative and neuropsychiatric disorders. Glutamate and its receptors are potential targets in the treatment of these disorders. Glutamate signaling is mediated through ionotropic and metabotropic receptors. The abundant concentration of these receptors can facilitate their in vivo quantification using positron emission tomography (PET). Glutamate receptors are a potentially important set of targets for monitoring disease progression, for evaluating the effect of therapy and for new treatment development based on the quantification of receptor occupancy. Here, we review the PET and single-photon emission computed tomography (SPECT) radioligands that have been developed for imaging glutamate receptors in living brain.

Synthesis and evaluation of novel α-fluorinated (E)-3-((6-methylpyridin-2-yl)ethynyl)cyclohex-2-enone-O-methyl oxime (ABP688) derivatives as metabotropic glutamate receptor subtype 5 PET radiotracers

In the search for an optimal fluorine-18-labeled positron emission tomography (PET) radiotracer for imaging metabotropic glutamate receptor subtype 5 (mGluR5), we have prepared a series of five α-fluorinated derivatives based on the ABP688 structural manifold by application of a two-step enolization/NFSI α-fluorination method. Their binding affinities were evaluated in vitro, and the most promising candidate (Z)-16 exhibited a K(i) of 5.7 nM and a clogP value of 2.3. The synthesis of the precursor tosylate (E)-22 revealed a preference for the (E)-configurational isomer (K(i) = 31.2 nM), and successful radiosynthesis afforded (E)-[(18)F]-16 which was used as a model PET tracer to establish plasma and PBS stability. (E)-[(18)F]-16 (K(d) = 70 nM) exhibited excellent specificity for mGluR5 in autoradiographic studies on horizontal rat brain slices in vitro.

In vivo positron emission tomography imaging with [¹¹C]ABP688: binding variability and specificity for the metabotropic glutamate receptor subtype 5 in baboons

PURPOSE

Metabotropic glutamate receptor subtype 5 (mGluR5) dysfunction has been implicated in several disorders. [(11)C]ABP688, a positron emission tomography (PET) ligand targeting mGluR5, could be a valuable tool in the development of novel therapeutics for these disorders by establishing in vivo drug occupancy. Due to safety concerns in humans, these studies may be performed in nonhuman primates. Therefore, in vivo characterization of [(11)C]ABP688 in nonhuman primates is essential.

METHODS

Test-retest studies were performed in baboons (Papio anubis) to compare modeling approaches and determine the optimal reference region. The mGluR5-specific antagonist 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP) was then used in test-block studies, in which ligand binding was measured before and after MTEP administration. Test/block data were analyzed both by calculating changes in binding and using a graphical approach, which allowed estimation of both MTEP occupancy and nonspecific binding.

RESULTS

Test-retest results, which have not been previously reported for [(11)C]ABP688, indicated that [(11)C]ABP688 variability is low using an unconstrained two-tissue compartment model. The most appropriate, though not ideal, reference region was found to be the gray matter of the cerebellum. Using these optimal modeling techniques on the test/block data, about 90% occupancy was estimated by the graphical approach.

CONCLUSION

These studies are the first to demonstrate the specificity of [(11)C]ABP688 for mGluR5 with in vivo PET in nonhuman primates. The results indicate that, in baboons, occupancy of mGluR5 is detectable by in vivo PET, a useful finding for proceeding to human studies, or performing further baboon studies, quantifying the in vivo occupancy of novel therapeutics targeting mGluR5.