Development of PET radiopharmaceuticals and their clinical applications at the Positron Medical Center

Development of an α-synuclein positron emission tomography tracer for imaging synucleinopathies

Synucleinopathies are characterized by the accumulation of α-synuclein (α-Syn) aggregates in the brain. Positron emission tomography (PET) imaging of synucleinopathies requires radiopharmaceuticals that selectively bind α-Syn deposits. We report the identification of a brain permeable and rapid washout PET tracer [18F]-F0502B, which shows high binding affinity for α-Syn, but not for Aβ or Tau fibrils, and preferential binding to α-Syn aggregates in the brain sections. Employing several cycles of counter screenings with in vitro fibrils, intraneuronal aggregates, and neurodegenerative disease brain sections from several mice models and human subjects, [18F]-F0502B images α-Syn deposits in the brains of mouse and non-human primate PD models. We further determined the atomic structure of the α-Syn fibril-F0502B complex by cryo-EM and revealed parallel diagonal stacking of F0502B on the fibril surface through an intense noncovalent bonding network via inter-ligand interactions. Therefore, [18F]-F0502B is a promising lead compound for imaging aggregated α-Syn in synucleinopathies.

Automated Radiosynthesis and Molecular Docking Studies of Coumarin-Triazole Hybrid with fluorine-18: A feasibility study

BACKGROUND

Fluorine-18 is one of the promising radiotracers that can report target specific information related to its physiology to understand the disease status through PET modality. In current study, the radiochemical synthesis, purification and molecular docking studies of fluorine-18 (18F) radiolabeled coumarin-triazole hybrid have been performed.

OBJECTIVE

To develop target specific fluorine-18 radiotracer for the diagnosis in oncology.

METHODS

GE Tracer-lab FX2N module with few modifications in the line connections was used for the radiosynthesis and purification of target molecule [18F]SG-2, 4-((2,6-dimethylmorpholino) methyl)-7-((1-(4-(fluoro-18F) benzyl)-1H-1,2,3-triazol-4-yl) oxy)-2H-chromen-2-one, through the nucleophilic radiofluorination mechanism. The radiochemical purity was measured by HPLC, and TLC analytical methods. The kryptofix levels were also evaluated by using TLC method. The residual solvents like DMF, ethanol was measured using GC. The Schrödinger drug discovery suite 2018 was used to study the protein and ligand interactions.

RESULTS

The quality control parameters revealed the purity, chemical identity, and limits of residual solvents. The radiochemical purity was 95.5 ± 2.3 %, and dimethyl formamide solvent limit was 89 ± 3 ppm. The molecular docking results had suggested that the cold target molecule has made strong electronic interactions and showed the possible pharmacokinetic (ADME) properties with galectin-1 protein. Overall, these results showed that [18F]SG-2 radiolabeling with 18F radionuclide was feasible, and support of molecular docking studies suggest possible interactions with Galectin-1.

CONCLUSION

we reported feasibility study for labeling coumarin-triazole hybrid with fluorine-18 through aromatic nucleophilic fluorination reaction (SNAr).

In Vivo Positron Emission Tomography Imaging of Adenosine A2A Receptors

As an invasive nuclear medical imaging technology, positron emission tomography (PET) possess the possibility to imaging the distribution as well as the density of selective receptors via specific PET tracers. Inspired by PET, the development of radio-chemistry has greatly promoted the progress of innovative imaging PET tracers for adenosine receptors, in particular adenosine A2A receptors (A2ARs). PET imaging of A2A receptors play import roles in the research of adenosine related disorders. Several radio-tracers for A2A receptors imaging have been evaluated in human studies. This paper reviews the recent research progress of PET tracers for A2A receptors imaging, and their applications in the diagnosis and treatment of related disease, such as cardiovascular diseases, autoimmune diseases, neurodegenerative and psychiatric disease. The future development of A2A PET tracers were also discussed.

Radiosynthesis and in Vivo Evaluation of Two PET Radioligands for Imaging α-Synuclein

Two α-synuclein ligands, 3-methoxy-7-nitro-10H-phenothiazine (2a, K_i = 32.1 ± 1.3 nM) and 3-(2-fluoroethoxy)-7-nitro-10H-phenothiazine (2b, K_i = 49.0 ± 4.9 nM), were radiolabeled as potential PET imaging agents by respectively introducing ¹¹C and ¹⁸F. The syntheses of [¹¹C]2a and [¹⁸F]2b were accomplished in a good yield with high specific activity. Ex vivo biodistribution studies in rats revealed that both [¹¹C]2a and [¹⁸F]2b crossed the blood-brain barrier (BBB) and demonstrated good brain uptake 5 min post-injection. MicroPET imaging of [¹¹C]2a in a non-human primate (NHP) confirmed that the tracer was able to cross the BBB with rapid washout kinetics from brain regions of a healthy macaque. The initial studies suggested that further structural optimization of [¹¹C]2a and [¹⁸F]2b is necessary in order to identify a highly specific positron emission tomography (PET) radioligand for in vivo imaging of α-synuclein aggregation in the central nervous system (CNS).

Adenosine A(2A) receptors measured with [C]TMSX PET in the striata of Parkinson's disease patients

Adenosine A_2A receptors (A2ARs) are thought to interact negatively with the dopamine D₂ receptor (D2R), so selective A2AR antagonists have attracted attention as novel treatments for Parkinson's disease (PD). However, no information about the receptor in living patients with PD is available. The purpose of this study was to investigate the relationship between A2ARs and the dopaminergic system in the striata of drug-naïve PD patients and PD patients with dyskinesia, and alteration of these receptors after antiparkinsonian therapy. We measured binding ability of striatal A2ARs using positron emission tomography (PET) with [7-methyl-¹¹C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine ([¹¹C]TMSX) in nine drug-naïve patients with PD, seven PD patients with mild dyskinesia and six elderly control subjects using PET. The patients and eight normal control subjects were also examined for binding ability of dopamine transporters and D2Rs. Seven of the drug-naïve patients underwent a second series of PET scans following therapy. We found that the distribution volume ratio of A2ARs in the putamen were larger in the dyskinesic patients than in the control subjects (p<0.05, Tukey-Kramer post hoc test). In the drug-naïve patients, the binding ability of the A2ARs in the putamen, but not in the head of caudate nucleus, was significantly lower on the more affected side than on the less affected side (p<0.05, paired t-test). In addition, the A2ARs were significantly increased after antiparkinsonian therapy in the bilateral putamen of the drug-naïve patients (p<0.05, paired t-test) but not in the bilateral head of caudate nucleus. Our study demonstrated that the A2ARs in the putamen were increased in the PD patients with dyskinesia, and also suggest that the A2ARs in the putamen compensate for the asymmetrical decrease of dopamine in drug-naïve PD patients and that antiparkinsonian therapy increases the A2ARs in the putamen. The A2ARs may play an important role in regulation of parkinsonism in PD.