Comparison of [11C]TZ1964B and [18F]MNI659 for PET imaging brain PDE10A in nonhuman primates

Synthesis and in vivo biological characterization of six carbon-11 sigma-1 receptor radiotracers in rodent and nonhuman primate

Six enantiomers of three racemic sigma-1 receptor (σ1R) ligands were resolved, and absolute configuration was determined. Their high σ1R potency and selectivity were determined through in vitro binding assays, further validated by molecular docking analysis. Central Nervous System Multiparameter Optimization algorithm (CNS MPO) predicts efficient brain penetration for these enantiomers. Six C-11 radiotracers were radiosynthesized successfully, ex vivo biodistribution in rats showed that (-)-[11C]7 had high brain uptake of ∼4.8-fold for 5 min versus 60 min. Mouse brain PET imaging studies showed (-)-[11C]7 and (-)-[11C]16 have in vivo binding specificity for σ1R. Macaque PET scans showed high brain uptake for all six radiotracers, with (-)-[11C]7 peaked at ∼45 min (SUV 2.5), possessing the best washout kinetics and highest cerebellum-to-white matter ratio (∼3.1), in agreement with in vitro or ex vivo measures of σ1R expression. Radiometabolite analysis showed that no newly formed radiometabolite was observed post-injection of (-)-[11C]7. Our data suggest that further evaluation is warranted to determine that (-)-[11C]7 is a suitable PET radiotracer for imaging σ1R in the brain of animal and human.

Radiation dosimetry of [11C]TZ1964B as determined by whole-body PET imaging of nonhuman primates

BACKGROUND

Phosphodiesterase 10A (PDE10A) is a postsynaptic, membrane bound cyclic nucleotide phosphodiesterase that is highly enriched in the striatal medium spiny neurons and regulates dopaminergic neurotransmission. The objective of this study is to determine the absorbed radiation dosimetry of a novel radiotracer for PDE10A: 3-(Methoxy-11C)-2-((4-(1-methyl-4-(pyridine-4yl)-1H-pyrazol-3-yl)phenoxy)methyl)quinolone ([11C]TZ1964B) based on whole body PET imaging in nonhuman primates, a critical step before translating this radiotracer to imaging studies in humans. [11C]TZ1964B may contribute to the clinical investigation of multiple neuropsychiatric conditions including Parkinson disease, Huntington disease and schizophrenia. For absorbed radiation measures, two males and one female cynomolgus monkeys (Macaca fascicularis) had intravenous injections of 302.3-384.4 MBq of [11C]TZ1964B followed by sequential whole body PET imaging in a MicroPET-Focus220 scanner. Volumes of interest (VOIs) that either encompassed the entire organ or sampled regions of highest activity within larger organs were defined. Time-activity curves were derived from the PET data for each VOI, and analytical integration of its multi-exponential fit yielded the organ time-integrated activity. We generated human radiation dose estimates based on the scaled organ residence using OLINDA/EXM2.2.

RESULTS

Highest retention was observed in the liver with total time-integrated activity of ~ 0.23 h. Absorbed organ dosimetry was highest in the liver (53.3 μGy/MBq), making it the critical organ. Gallbladder (35.9 μGy/MBq) and spleen (35.4 μGy/MBq) were the next highest organs for absorbed radiation dose. Effective doses were estimated to be 5.02 and 5.84 μSv/MBq for males and females, respectively.

CONCLUSIONS

This nonhuman primate dosimetry study suggests intravenous doses up to 938 MBq of [11C]TZ1964B can be safely administered to human subjects for PET measurements of PDE10A activity. The tracer kinetic data is consistent with a hepatobiliary clearance pathway for the radiotracer.

Genetically modified non-human primate models for research on neurodegenerative diseases

Neurodegenerative diseases (NDs) are a group of debilitating neurological disorders that primarily affect elderly populations and include Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS). Currently, there are no therapies available that can delay, stop, or reverse the pathological progression of NDs in clinical settings. As the population ages, NDs are imposing a huge burden on public health systems and affected families. Animal models are important tools for preclinical investigations to understand disease pathogenesis and test potential treatments. While numerous rodent models of NDs have been developed to enhance our understanding of disease mechanisms, the limited success of translating findings from animal models to clinical practice suggests that there is still a need to bridge this translation gap. Old World non-human primates (NHPs), such as rhesus, cynomolgus, and vervet monkeys, are phylogenetically, physiologically, biochemically, and behaviorally most relevant to humans. This is particularly evident in the similarity of the structure and function of their central nervous systems, rendering such species uniquely valuable for neuroscience research. Recently, the development of several genetically modified NHP models of NDs has successfully recapitulated key pathologies and revealed novel mechanisms. This review focuses on the efficacy of NHPs in modeling NDs and the novel pathological insights gained, as well as the challenges associated with the generation of such models and the complexities involved in their subsequent analysis.

Preliminary PET Imaging of Microtubule-Based PET Radioligand [11C]MPC-6827 in a Nonhuman Primate Model of Alzheimer's Disease

The microtubule (MT) instability observed in Alzheimer's disease (AD) is commonly attributed to hyperphosphorylation of the MT-associated protein, tau. In vivo PET imaging offers an opportunity to gain critical information about MT changes with the onset and development of AD and related dementia. We developed the first brain-penetrant MT PET ligand, [11C]MPC-6827, and evaluated its in vivo imaging utility in vervet monkeys. Consistent with our previous in vitro cell uptake and in vivo rodent imaging experiments, [11C]MPC-6827 uptake increased with MT destabilization. Radioactive uptake was inversely related to (cerebrospinal fluid) CSF Aβ42 levels and directly related to age in a nonhuman primate (NHP) model of AD. Additionally, in vitro autoradiography studies also corroborated PET imaging results. Here, we report the preliminary results of PET imaging with [11C]MPC-6827 in four female vervet monkeys with high or low CSF Aβ42 levels, which have been shown to correlate with the Aβ plaque burden, similar to humans.

Easily automated radiosynthesis of [18F]P10A-1910 and its clinical translation to quantify phosphodiesterase 10A in human brain

Our previous work showed that [18F]P10A-1910 was a potential radioligand for use in imaging phosphodiesterase 10A (PDE10A). Specifically, it had high brain penetration and specific binding that was demonstrated in both rodents and non-human primates. Here, we present the first automatic cGMP-level production of [18F]P10A-1910 and translational PET/MRI study in living human brains. Successful one-step radiolabeling of [18F]P10A-1910 on a GE TRACERlab FX2N synthesis module was realized via two different methods. First, formulated [18F]P10A-1910 was derived from heating spirocyclic iodonium ylide in a tetra-n-butyl ammonium methanesulfonate solution. At the end of synthesis, it was obtained in non-decay corrected radiochemical yields (n.d.c. RCYs) of 12.4 ± 1.3%, with molar activities (MAs) of 90.3 ± 12.6 μmol (n = 7) (Method I). The boronic pinacol ester combined with copper and oxygen also delivered the radioligand with 16.8 ± 1.0% n. d.c. RCYs and 77.3 ± 20.7 GBq/μmol (n = 7) MAs after formulation (Method II). The radiochemical purity, radionuclidic purity, solvent residue, sterility, endotoxin content and other parameters were all validated for human use. Consistent with the distribution of PDE10A in the brain, escalating uptake of [18F]P10A-1910 was observed in the order of cerebellum (reference region), substantial nigra, caudate and putamen. The non-displaceable binding potential (BPND) was estimated by simplified reference-tissue model (SRTM); linear regressions demonstrated that BPND was well correlated with the most widely used semiquantitative parameter SUV. The strongest correlation was observed with SUV(50–60 min) (R2 = 0.966, p < 0.01). Collectively, these results indicated that a static scan protocol could be easily performed for PET imaging of PDE10A. Most importantly, that [18F]P10A-1910 is a promising radioligand to clinically quantify PDE10A.

Discovery of a highly specific 18F-labeled PET ligand for phosphodiesterase 10A enabled by novel spirocyclic iodonium ylide radiofluorination

As a member of cyclic nucleotide phosphodiesterase (PDE) enzyme family, PDE10A is in charge of the degradation of cyclic adenosine (cAMP) and guanosine monophosphates (cGMP). While PDE10A is primarily expressed in the medium spiny neurons of the striatum, it has been implicated in a variety of neurological disorders. Indeed, inhibition of PDE10A has proven to be of potential use for the treatment of central nervous system (CNS) pathologies caused by dysfunction of the basal ganglia–of which the striatum constitutes the largest component. A PDE10A-targeted positron emission tomography (PET) radioligand would enable a better assessment of the pathophysiologic role of PDE10A, as well as confirm the relationship between target occupancy and administrated dose of a given drug candidate, thus accelerating the development of effective PDE10A inhibitors. In this study, we designed and synthesized a novel ¹⁸F-aryl PDE10A PET radioligand, codenamed [¹⁸F]P10A-1910 ([¹⁸F]9), in high radiochemical yield and molar activity via spirocyclic iodonium ylide-mediated radiofluorination. [¹⁸F]9 possessed good in vitro binding affinity (IC₅₀ = 2.1 nmol/L) and selectivity towards PDE10A. Further, [¹⁸F]9 exhibited reasonable lipophilicity (logD = 3.50) and brain permeability (P_app > 10 × 10⁻⁶ cm/s in MDCK-MDR1 cells). PET imaging studies of [¹⁸F]9 revealed high striatal uptake and excellent in vivo specificity with reversible tracer kinetics. Preclinical studies in rodents revealed an improved plasma and brain stability of [¹⁸F]9 when compared to the current reference standard for PDE10A-targeted PET, [¹⁸F]MNI659. Further, dose–response experiments with a series of escalating doses of PDE10A inhibitor 1 in rhesus monkey brains confirmed the utility of [¹⁸F]9 for evaluating target occupancy in vivo in higher species. In conclusion, our results indicated that [¹⁸F]9 is a promising PDE10A PET radioligand for clinical translation.

Challenges on Cyclic Nucleotide Phosphodiesterases Imaging with Positron Emission Tomography: Novel Radioligands and (Pre-)Clinical Insights since 2016

Cyclic nucleotide phosphodiesterases (PDEs) represent one of the key targets in the research field of intracellular signaling related to the second messenger molecules cyclic adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP). Hence, non-invasive imaging of this enzyme class by positron emission tomography (PET) using appropriate isoform-selective PDE radioligands is gaining importance. This methodology enables the in vivo diagnosis and staging of numerous diseases associated with altered PDE density or activity in the periphery and the central nervous system as well as the translational evaluation of novel PDE inhibitors as therapeutics. In this follow-up review, we summarize the efforts in the development of novel PDE radioligands and highlight (pre-)clinical insights from PET studies using already known PDE radioligands since 2016.

Development of 2-(2-(3-(4-([18F]Fluoromethoxy- d2)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione for Positron-Emission-Tomography Imaging of Phosphodiesterase 10A in the Brain

Phosphodiesterase 10A (PDE10A) is a newly identified therapeutic target for central-nervous-system disorders. 2-(2-(3-(4-([¹⁸F]Fluoroethoxy)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([¹⁸F]MNI-659, [¹⁸F]5) is a useful positron-emission-tomography (PET) ligand for imaging of PDE10A in the human brain. However, the radiolabeled metabolite of [¹⁸F]5 can accumulate in the brain. In this study, using [¹⁸F]5 as a lead compound, we designed four new ¹⁸F-labeled ligands ([¹⁸F]6-9) to find one more suitable than [¹⁸F]5. Of these, 2-(2-(3-(4-([¹⁸F]fluoromethoxy- d₂)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([¹⁸F]9) exhibited high in vitro binding affinity ( K_i = 2.9 nM) to PDE10A and suitable lipophilicity (log D = 2.2). In PET studies, the binding potential (BP_ND) of [¹⁸F]9 (5.8) to PDE10A in the striatum of rat brains was significantly higher than that of [¹⁸F]5 (4.6). Furthermore, metabolite analysis showed much lower levels of contamination with radiolabeled metabolites in the brains of rats given [¹⁸F]9 than in those given [¹⁸F]5. In conclusion, [¹⁸F]9 is a useful PET ligand for PDE10A imaging in brain.

In Vivo Characterization of Two 18F-Labeled PDE10A PET Radioligands in Nonhuman Primate Brains

Positron emission tomography (PET) with phosphodiesterase 10A (PDE10A) specific radioligands provides a noninvasive and quantitative imaging tool to access the expression of this enzyme in vivo under normal and diseased conditions. We recently reported two potent 18F-labeled PDE10A radioligands (18F-TZ19106B and 18F-TZ8110); initial evaluation in rats and nonhuman primates indicated stable metabolic profiles and excellent target-to-nontarget ratio (striatum/cerebellum) for both tracers. Herein, we focused on in vivo characterization of 18F-TZ19106B and 18F-TZ8110 to identify a suitable radioligand for imaging PDE10A in vivo. We directly compared microPET studies of these two radiotracers in adult male Macaca fascicularis nonhuman primates (NHPs). 18F-TZ19106B had higher striatal uptake and tracer retention in NHP brains than 18F-TZ8110, quantified by either standardized uptake values (SUVs) or nondisplaceable binding potential (BPND) estimated using reference-based modeling analysis. Blocking and displacement studies using the PDE10A inhibitor MP-10 indicated the binding of 18F-TZ19106B to PDE10A was specific and reversible. We also demonstrated sensitivity of 18F-TZ19106B binding to varying number of specific binding sites using escalating doses of MP-10 blockade (0.3, 0.5, 1.0, 1.5, and 2.0 mg/kg). Pretreatment with a dopamine D2-like receptor antagonist enhanced the striatal uptake of 18F-TZ19106B. Our results indicate that 18F-TZ19106B is a promising radioligand candidate for imaging PDE10A in vivo and it may be used to determine target engagement of PDE10A inhibitors and serve as a tool to evaluate the effect of novel antipsychotic therapies.

Comparison between [18F]fluorination and [18F]fluoroethylation reactions for the synthesis of the PDE10A PET radiotracer [18F]MNI-659

INTRODUCTION

2-(2-(3-(4-(2-[¹⁸F]Fluoroethoxy)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([¹⁸F]MNI-659, [¹⁸F]1) is a useful PET radiotracer for imaging phosphodiesterase 10A (PDE10A) in human brain. [¹⁸F]1 has been previously prepared by direct [¹⁸F]fluorination of a tosylate precursor 2 with [¹⁸F]F^-. The aim of this study was to determine the conditions for the [¹⁸F]fluorination reaction to obtain [¹⁸F]1 of high quality and with sufficient radioactivity for clinical use in our institute. Moreover, we synthesized [¹⁸F]1 by [¹⁸F]fluoroethylation of a phenol precursor 3 with [¹⁸F]fluoroethyl bromide ([¹⁸F]FEtBr), and the outcomes of [¹⁸F]fluorination and [¹⁸F]fluoroethylation were compared.

METHODS

We performed the automated synthesis of [¹⁸F]1 by [¹⁸F]fluorination and [¹⁸F]fluoroethylation using a multi-purpose synthesizer. We determined the amounts of tosylate precursor 2 and potassium carbonate as well as the reaction temperature for direct [¹⁸F]fluorination.

RESULTS

The efficiency of the [¹⁸F]fluorination reaction was strongly affected by the amount of 2 and potassium carbonate. Under the determined reaction conditions, [¹⁸F]1 with 0.82±0.2GBq was obtained in 13.6%±3.3% radiochemical yield (n=8, decay-corrected to EOB and based on [¹⁸F]F^-) at EOS, starting from 11.5±0.4GBq of cyclotron-produced [¹⁸F]F^-. On the other hand, the [¹⁸F]fluoroethylation of 3 with [¹⁸F]FEtBr produced [¹⁸F]1 with 1.0±0.2GBq and in 22.5±2.5 % radiochemical yields (n=7, decay-corrected to EOB and based on [¹⁸F]F^-) at EOS, starting from 7.4GBq of cyclotron-produced [¹⁸F]F^-. Clearly, [¹⁸F]fluoroethylation resulted in a higher radiochemical yield of [¹⁸F]1 than [¹⁸F]fluorination.

CONCLUSION

[¹⁸F]1 of high quality and with sufficient radioactivity was successfully radiosynthesized by two methods. [¹⁸F]1 synthesized by direct [¹⁸F]fluorination has been approved and will be provided for clinical use in our institute.

Comparison of [11C]TZ1964B and [18F]MNI659 for PET imaging brain PDE10A in nonhuman primates

Phosphodiesterase 10A (PDE10A) inhibitors show therapeutic effects for diseases with striatal pathology. PET radiotracers have been developed to quantify in vivo PDE10A levels and target engagement for therapeutic interventions. The aim of this study was to compare two potent and selective PDE10A radiotracers, [¹¹C]TZ1964B and [¹⁸F]MNI659 in the nonhuman primate (NHP) brain. Double scans in the same cynomolgus monkey on the same day were performed after injection of [¹¹C]TZ1964B and [¹⁸F]MNI659. Specific uptake was determined in two ways: nondisplaceable binding potential (BP_ND) was calculated using cerebellum as the reference region and the PDE‐10A enriched striatum as the target region of interest (ROI); the area under the time–activity curve (AUC) for the striatum to cerebellum ratio was also calculated. High‐performance liquid chromatography (HPLC) analysis of solvent‐extracted NHP plasma identified the percentage of intact tracer versus radiolabeled metabolites samples post injection of each radiotracer. Both radiotracers showed high specific accumulation in NHP striatum. [¹¹C]TZ1964B has higher striatal retention and lower specific striatal uptake than [¹⁸F]MNI659. The BP_ND estimates of [¹¹C]TZ1964B were 3.72 by Logan Reference model (LoganREF) and 4.39 by simplified reference tissue model (SRTM); the BP_ND estimates for [¹⁸F]MNI659 were 5.08 (LoganREF) and 5.33 (SRTM). AUC ratios were 5.87 for [¹¹C]TZ1964B and 7.60 for [¹⁸F]MNI659. Based on BP_ND values in NHP striatum, coefficients of variation were ~10% for [¹¹C]TZ1964B and ~30% for [¹⁸F]MNI659. Moreover, the metabolism study showed the percentage of parent compounds were ~70% for [¹¹C]TZ1964B and ~50% for [¹⁸F]MNI659 60 min post injection. These data indicate that either [¹¹C]TZ1964B or [¹⁸F]MNI659 could serve as suitable PDE10A PET radiotracers with distinguishing features for particular clinical application.