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

Synthesis, preclinical assessment, and first-in-human study of [18F]d4-FET for brain tumor imaging

PURPOSE

Tumor-to-background ratio (TBR) is a critical metric in oncologic PET imaging. This study aims to enhance the TBR of [18F]FET in brain tumor imaging by substituting deuterium ("D") for hydrogen ("H"), thereby improving the diagnostic sensitivity and accuracy.

METHODS

[18F]d4-FET was synthesised by two automated radiochemistry modules. Biodistribution studies and imaging efficacy were evaluated in vivo  and ex vivo in rodent models, while metabolic stability and radiation dosimetry were assessed in non-human primates. Additionally, preliminary imaging evaluations were carried out in five brain tumor patients: three glioma patients underwent imaging with both [18F]d4-FET and [18F]FET, and two patients with brain metastases were imaged using [18F]d4-FET and [18F]FDG.

RESULTS

[18F]d4-FET demonstrated high radiochemical purity and yield. PET/MRI in rodent models demonstrated superior TBR for [18F]d4-FET compared to [18F]FET, and autoradiography showed tumor margins that correlated well with pathological extents. Studies in cynomolgus monkeys indicated comparable in vivo stability and effective dose with [18F]FET. In glioma patients, [18F]d4-FET showed enhanced TBR, while in patients with brain metastases, [18F]d4-FET displayed superior lesion delineation compared to [18F]FDG, especially in smaller metastatic sites.

CONCLUSION

We successfully synthesized the novel PET radiotracer [18F]d4-FET, which retains the advantageous properties of [18F]FET while potentially enhancing TBR for glioma imaging. Preliminary studies indicate excellent stability, efficacy, and sensitivity of [18F]d4-FET, suggesting its potential in clinical evaluations of brain tumors.

TRIAL REGISTRATION

ChiCTR2400081576, registration date: 2024-03-05, https://www.chictr.org.cn/bin/project/edit?pid=206162.

Preclinical characterization of [18F]D2-LW223: an improved metabolically stable PET tracer for imaging the translocator protein 18 kDa (TSPO) in neuroinflammatory rodent models and non-human primates

Positron emission tomography (PET) targeting translocator protein 18 kDa (TSPO) can be used for the noninvasive detection of neuroinflammation. Improved in vivo stability of a TSPO tracer is beneficial for minimizing the potential confounding effects of radiometabolites. Deuteration represents an important strategy for improving the pharmacokinetics and stability of existing drug molecules in the plasma. This study developed a novel tracer via the deuteration of [18F]LW223 and evaluated its in vivo stability and specific binding in neuroinflammatory rodent models and nonhuman primate (NHP) brains. Compared with LW223, D2-LW223 exhibited improved binding affinity to TSPO. Compared with [18F]LW223, [18F]D2-LW223 has superior physicochemical properties and favorable brain kinetics, with enhanced metabolic stability and reduced defluorination. Preclinical investigations in rodent models of LPS-induced neuroinflammation and cerebral ischemia revealed specific [18F]D2-LW223 binding to TSPO in regions affected by neuroinflammation. Two-tissue compartment model analyses provided excellent model fits and allowed the quantitative mapping of TSPO across the NHP brain. These results indicate that [18F]D2-LW223 holds significant promise for the precise quantification of TSPO expression in neuroinflammatory pathologies of the brain.

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.

Automated radiosynthesis of two 18F-labeled tracers containing 3-fluoro-2-hydroxypropyl moiety, [18F]FMISO and [18F]PM-PBB3, via [18F]epifluorohydrin

Background

[¹⁸F]Fluoromisonidazole ([¹⁸F]FMISO) and 1-[¹⁸F]fluoro-3-((2-((1E,3E)-4-(6-(methylamino)pyridine-3-yl)buta-1,3-dien-1-yl)benzo[d]thiazol-6-yl)oxy)propan-2-ol ([¹⁸F]PM-PBB3 or [¹⁸F]APN-1607) are clinically used radiotracers for imaging hypoxia and tau pathology, respectively. Both radiotracers were produced by direct ¹⁸F-fluorination using the corresponding tosylate precursors 1 or 2 and [¹⁸F]F⁻, followed by the removal of protecting groups. In this study, we synthesized [¹⁸F]FMISO and [¹⁸F]PM-PBB3 by ¹⁸F-fluoroalkylation using [¹⁸F]epifluorohydrin ([¹⁸F]5) for clinical applications.

Results

First, [¹⁸F]5 was synthesized by the reaction of 1,2-epoxypropyl tosylate (8) with [¹⁸F]F⁻ and was purified by distillation. Subsequently, [¹⁸F]5 was reacted with 2-nitroimidazole (6) or PBB3 (7) as a precursor for ¹⁸F-labeling, and each reaction mixture was purified by preparative high-performance liquid chromatography and formulated to obtain the [¹⁸F]FMISO or [¹⁸F]PM-PBB3 injection. All synthetic sequences were performed using an automated ¹⁸F-labeling synthesizer. The obtained [¹⁸F]FMISO showed sufficient radioactivity (0.83 ± 0.20 GBq at the end of synthesis (EOS); n = 8) with appropriate radiochemical yield based on [¹⁸F]F⁻ (26 ± 7.5 % at EOS, decay-corrected; n = 8). The obtained [¹⁸F]PM-PBB3 also showed sufficient radioactivity (0.79 ± 0.10 GBq at EOS; n = 11) with appropriate radiochemical yield based on [¹⁸F]F⁻ (16 ± 3.2 % at EOS, decay-corrected; n = 11).

Conclusions

Both [¹⁸F]FMISO and [¹⁸F]PM-PBB3 injections were successfully synthesized with sufficient radioactivity by ¹⁸F-fluoroalkylation using [¹⁸F]5.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-021-00138-9.

Advances in Cyclic Nucleotide Phosphodiesterase-Targeted PET Imaging and Drug Discovery

Cyclic nucleotide phosphodiesterases (PDEs) control the intracellular concentrations of cAMP and cGMP in virtually all mammalian cells. Accordingly, the PDE family regulates a myriad of physiological functions, including cell proliferation, differentiation and apoptosis, gene expression, central nervous system function, and muscle contraction. Along this line, dysfunction of PDEs has been implicated in neurodegenerative disorders, coronary artery diseases, chronic obstructive pulmonary disease, and cancer development. To date, 11 PDE families have been identified; however, their distinct roles in the various pathologies are largely unexplored and subject to contemporary research efforts. Indeed, there is growing interest for the development of isoform-selective PDE inhibitors as potential therapeutic agents. Similarly, the evolving knowledge on the various PDE isoforms has channeled the identification of new PET probes, allowing isoform-selective imaging. This review highlights recent advances in PDE-targeted PET tracer development, thereby focusing on efforts to assess disease-related PDE pathophysiology and to support isoform-selective drug discovery.

Synthesis and Biological Evaluation of [18F]FECNT-d4 as a Novel PET Agent for Dopamine Transporter Imaging

PURPOSE

The dopamine transporter (DAT) is a marker of the occurrence and development of Parkinson's disease (PD) and other diseases with nigrostriatal degeneration. 2β-Carbomethoxy-3β-(4-chlorophenyl)-8-(2-[¹⁸F]-fluoroethyl)nortropane ([¹⁸F]FECNT), an ¹⁸F-labelled tropane derivative, was reported to be a useful positron-emitting probe for DAT. However, the rapid formation of brain-penetrating radioactive metabolites is an impediment to the proper quantitation of DAT in PET studies with [¹⁸F]FECNT. Deuterium-substituted analogues have presented better in vivo stability to reduce metabolites. This study aimed to synthesize a deuterium-substituted DAT radiotracer, [¹⁸F]FECNT-d₄, and to make a preliminary investigation of its properties as a DAT tracer in vivo.

PROCEDURES

The ligand [¹⁸F]FECNT-d₄ was obtained by one-step radiolabelling reaction. The lipophilicity was measured by the shake-flask method. Binding properties of [¹⁸F]FECNT-d₄ were estimated by in vitro binding assay, biodistribution, and microPET imaging in rats. In vivo stability of [¹⁸F]FECNT-d₄ was estimated by radio-HPLC.

RESULTS

[¹⁸F]FECNT-d₄ was synthesized at an average activity yield of 46 ± 17 % (n = 15) and the molar activity was 67 ± 12 GBq/μmol. The deuterated tracer showed suitable lipophilicity and the ability to penetrate the blood-brain barrier (brain uptake of 1.72 % ID at 5 min). [¹⁸F]FECNT-d₄ displayed a high binding affinity for DAT comparable to that of [¹⁸F]FECNT in rat striatum homogenates. Biodistribution results in normal rats showed that [¹⁸F]FECNT-d₄ exhibited a higher ratio of the target to non-target (striatum/cerebellum) at 15 min post administration (5.00 ± 0.44 vs 3.84 ± 0.24 for [¹⁸F]FECNT-d₄ vs [¹⁸F]FECNT). MicroPET imaging studies of [¹⁸F]FECNT-d₄ in normal rats showed that the ligand selectively localized to DAT-rich striatal regions and the accumulation could be blocked with DAT inhibitor. Furthermore, in the unilateral PD model rat, a significant reduction of the signal was found in the lesioned side relative to the unlesioned side. Striatal standardized uptake value of [¹⁸F]FECNT-d₄ remained ~4.02 in the striatum between 5 and 20 min, whereas that of [¹⁸F]FECNT fell rapidly from 4.11 to 2.95. Radio-HPLC analysis of the plasma demonstrated better in vivo stability of [¹⁸F]FECNT-d₄ than [¹⁸F]FECNT.

CONCLUSION

The deuterated compound [¹⁸F]FECNT-d₄ may serve as a promising PET imaging agent to assess DAT-related disorders.

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.

Medicinal chemistry strategies for the development of phosphodiesterase 10A (PDE10A) inhibitors - An update of recent progress

Phosphodiesterase 10A is a member of Phosphodiesterase (PDE)-superfamily of the enzyme which is responsible for hydrolysis of cAMP and cGMP to their inactive forms 5'-AMP and 5'-GMP, respectively. PDE10A is highly expressed in the brain, particularly in the putamen and caudate nucleus. PDE10A plays an important role in the regulation of localization, duration, and amplitude of the cyclic nucleotide signalling within the subcellular domain of these regions, and thereby modulation of PDE10A enzyme can give rise to a new therapeutic approach in the treatment of schizophrenia and other neurodegenerative disorders. Limitation of the conventional therapy of schizophrenia forced the pharmaceutical industry to move their efforts to develop a novel treatment approach with reduced side effects. In the past decade, considerable developments have been made in pursuit of PDE10A centric antipsychotic agents by several pharmaceutical industries due to the distribution of PDE10A in the brain and the ability of PDE10A inhibitors to mimic the effect of D2 antagonists and D1 agonists. However, no selective PDE10A inhibitor is currently available in the market for the treatment of schizophrenia. The present compilation concisely describes the role of PDE10A inhibitors in the therapy of neurodegenerative disorders mainly in psychosis, the structure of PDE10A enzyme, key interaction of different PDE10A inhibitors with human PDE10A enzyme and recent medicinal chemistry developments in designing of safe and effective PDE10A inhibitors for the treatment of schizophrenia. The present compilation also provides useful information and future direction to bring further improvements in the discovery of PDE10A inhibitors.

Deuteration versus ethylation - strategies to improve the metabolic fate of an 18F-labeled celecoxib derivative

The inducible isoenzyme cyclooxygenase-2 (COX-2) is closely associated with chemo-/radioresistance and poor prognosis of solid tumors. Therefore, COX-2 represents an attractive target for functional characterization of tumors by positron emission tomography (PET). In this study, the celecoxib derivative 3-([¹⁸F]fluoromethyl)-1-[4-(methylsulfonyl)phenyl]-5-(p-tolyl)-1H-pyrazole ([¹⁸F]5a) was chosen as a lead compound having a reported high COX-2 inhibitory potency and a potentially low carbonic anhydrase binding tendency. The respective deuterated analog [D₂,¹⁸F]5a and the fluoroethyl-substituted derivative [¹⁸F]5b were selected to study the influence of these modifications with respect to COX inhibition potency in vitro and metabolic stability of the radiolabeled tracers in vivo. COX-2 inhibitory potency was found to be influenced by elongation of the side chain but, as expected, not by deuteration. An automated radiosynthesis comprising ¹⁸F-fluorination and purification under comparable conditions provided the radiotracers [¹⁸F]5a,b and [D₂,¹⁸F]5a in good radiochemical yields (RCY) and high radiochemical purity (RCP). Biodistribution and PET studies comparing all three compounds revealed bone accumulation of ¹⁸F-activity to be lowest for the ethyl derivative [¹⁸F]5b. However, the deuterated analog [D₂,¹⁸F]5a turned out to be the most stable compound of the three derivatives studied here. Time-dependent degradation of [¹⁸F]5a,b and [D₂,¹⁸F]5a after incubation in murine liver microsomes was in accordance with the data on metabolism in vivo. Furthermore, metabolites were identified based on UPLC-MS/MS.

The aim of this study is to investigate the influence of deuteration and elongation on an ¹⁸F-labeled COX-2 inhibitor with focus on metabolic stability to develop suitable COX-2 targeting radiotracers.