Alpha-Synuclein PET Tracer Development-An Overview about Current Efforts.

Neurodegenerative diseases such as Parkinson's disease (PD) are manifested by inclusion bodies of alpha-synuclein (α-syn) also called α-synucleinopathies. Detection of these inclusions is thus far only possible by histological examination of postmortem brain tissue. The possibility of non-invasively detecting α-syn will therefore provide valuable insights into the disease progression of α-synucleinopathies. In particular, α-syn imaging can quantify changes in monomeric, oligomeric, and fibrillic α-syn over time and improve early diagnosis of various α-synucleinopathies or monitor treatment progress. Positron emission tomography (PET) is a non-invasive in vivo imaging technique that can quantify target expression and drug occupancies when a suitable tracer exists. As such, novel α-syn PET tracers are highly sought after. The development of an α-syn PET tracer faces several challenges. For example, the low abundance of α-syn within the brain necessitates the development of a high-affinity ligand. Moreover, α-syn depositions are, in contrast to amyloid proteins, predominantly localized intracellularly, limiting their accessibility. Furthermore, another challenge is the ligand selectivity over structurally similar amyloids such as amyloid-beta or tau, which are often co-localized with α-syn pathology. The lack of a defined crystal structure of α-syn has also hindered rational drug and tracer design efforts. Our objective for this review is to provide a comprehensive overview of current efforts in the development of selective α-syn PET tracers.

EPR assessment of protein sites for incorporation of Gd(III) MRI contrast labels.

We have engineered apolipoprotein A-I (apoA-I), a major protein constituent of high-density lipoprotein (HDL), to contain DOTA-chelated Gd(III) as an MRI contrast agent for the purpose of imaging reconstituted HDL (rHDL) biodistribution, metabolism and regulation in vivo. This protein contrast agent was obtained by attaching the thiol-reactive Gd[MTS-ADO3A] label at Cys residues replaced at four distinct positions (52, 55, 76 and 80) in apoA-I. MRI of infused mice previously showed that the Gd-labeled apoA-I migrates to both the liver and the kidney, the organs responsible for HDL catabolism; however, the contrast properties of apoA-I are superior when the ADO3A moiety is located at position 55, compared with the protein labeled at positions 52, 76 or 80. It is shown here that continuous wave X-band (9 GHz) electron paramagnetic resonance (EPR) spectroscopy is capable of detecting differences in the Gd(III) signal when comparing the labeled protein in the lipid-free with the rHDL state. Furthermore, the values of NMR relaxivity obtained for labeled variants in both the lipid-free and rHDL states correlate to the product of the X-band Gd(III) spectral width and the collision frequency between a nitroxide spin label and a polar relaxation agent. Consistent with its superior relaxivity measured by NMR, the rHDL-associated apoA-I containing the Gd[MTS-ADO3A] probe attached to position 55 displays favorable dynamic and water accessibility properties as determined by X-band EPR. While room temperature EPR requires >1 m m Gd(III)-labeled and only >10 µ m nitroxide-labeled protein to resolve the spectrum, the volume requirement is exceptionally low (~5 µl). Thus, X-band EPR provides a practical assessment for the suitability of imaging candidates containing the site-directed ADO3A contrast probe.

Evaluation of an unshielded luminescence flow-through radio-HPLC detector for LC quality control and preparation of PET radiopharmaceuticals.

Radio-HPLC is an essential method to assess the purity of PET radiopharmaceuticals. The usual NaI scintillator radiodetector requires heavy, costly and cumbersome lead shielding. The luminescence LB 500 fLumo detector has been developed to tackle these drawbacks and achieve high sensitivity. The fLumo uses a photon counting detector combined with a flow-through cell modified with a solid melt-on scintillator only sensitive to the positron. This study demonstrates the usefulness of the fLumo for analysis and purification of PET radiopharmaceuticals.

Synthesis of two new alkyne-bearing linkers used for the preparation of siRNA for labeling by click chemistry with fluorine-18.

Oligonucleotides (ONs) and more particularly siRNAs are promising drugs but their pharmacokinetics and biodistribution are widely unknown. Positron Emission Tomography (PET) using fluorine-18 is a suitable technique to quantify these biological processes. Click chemistry (Huisgen cycloaddition) is the current method for labeling siRNA. In order to study the influence of a linker bearing by [(18)F] labeled ONs, on the in vivo pharmacokinetic and metabolism, we have developed two modified ONs by two new linkers. Here we report the synthesis of two alkyne-bearing linkers, the incorporation onto a ONs and the conjugation by click chemistry with a [(18)F] prosthetic group.

Gadolinium DOTA chelates featuring alkyne groups directly grafted on the tetraaza macrocyclic ring: synthesis, relaxation properties, "click" reaction, and high-relaxivity micelles.

This paper reports on the synthesis and relaxivity properties of tetraacetic DOTA-type chelating agents featuring one or two alkyne groups directly grafted on the tetraaza macrocyclic ring and available for "click" reactions with azide-bearing substrates. The racemic DOTAma ligand bearing one alkyne group was obtained by a bisaminal template route. The same approach was used to prepare ligand DOTAda substituted by two alkyne groups located on two adjacent carbon atoms. The S,S enantiomer of DOTAda was also prepared by a "crab-like" condensation. This ligand is the first example of a DOTA derivative featuring two reactive functions adjacent to each other on the macrocyclic ring. A triacetic monoalkyne ligand (DO3ma) was also synthesized for comparison purposes. NMR studies indicate that the Yb(III) chelates of DOTAma and DOTAda adopt two conformations in solutions in which the tetraaza ring is rigidified. The hydration state of the Eu(III) chelates was determined by luminescence spectroscopy, and the water exchange time of the Gd(III) complexes was measured by (17)O NMR. Ring substitution accelerates the water exchange. These data were used to interpret nuclear magnetic relaxation dispersion curves of the Gd(III) chelates. Two long aliphatic chains have been added to DOTAda by a "click" procedure to form the (C18)(2)DOTAda ligand. The corresponding Gd(III) complex forms micelles of unusually high relaxivity presumably because of the close proximity of the aliphatic chains on the macrocyclic ring that ensures a rigid double anchoring into the micelles.

Fully automated preparation and conjugation of N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB) with RGD peptide using a GE FASTlab™ synthesizer.

PURPOSE: The aim of this work was to automate the radiosynthesis of [(18)F]SFB, a widely used reagent for the labeling of biomolecules with (18)F on a new generation commercial synthesis module (FASTLab™, GE Healthcare). PROCEDURES: Two synthesis approaches were implemented on this module: the classical "two-pot radiosynthesis" and the more recently described "one-pot" method. RESULTS: The "two-pot" approach affords [(18)F]SFB with a 42% decay-corrected yield in 57 min (n = 24) with a chemical purity sufficient to avoid an intermediate HPLC purification. The recently established "one-pot" method, afforded a product with a lower chemical purity, in the conditions used in this report. The lower d.c. yield obtained (32% (n = 15)) was related to the low (18)F labeling yields obtained in MeCN compared with DMSO. The subsequent conjugation step with a RGD (PRGD2) peptide was also successfully automated. CONCLUSIONS: The formulated [(18)F]FPRGD2 was obtained without any operator manipulation with a d.c. yield of 13% ± 3% (n = 13) in 130 min, a radiochemical purity >98% and a specific activity of 140 ± 40 TBq/mmol.

Imaging apolipoprotein AI in vivo.

Coronary disease risk increases inversely with high-density lipoprotein (HDL) level. The measurement of the biodistribution and clearance of HDL in vivo, however, has posed a technical challenge. This study presents an approach to the development of a lipoprotein MRI agent by linking gadolinium methanethiosulfonate (Gd[MTS-ADO3A]) to a selective cysteine mutation in position 55 of apo AI, the major protein of HDL. The contrast agent targets both liver and kidney, the sites of HDL catabolism, whereas the standard MRI contrast agent, gadolinium-diethylenetriaminepentaacetic acid-bismethylamide (GdDTPA-BMA, gadodiamide), enhances only the kidney image. Using a modified apolipoprotein AI to create an HDL contrast agent provides a new approach to investigate HDL biodistribution, metabolism and regulation in vivo.

General method for labeling siRNA by click chemistry with fluorine-18 for the purpose of PET imaging.

The alkyne-azide Cu(I)-catalyzed Huisgen cycloaddition, a click-type reaction, was used to label a double-stranded oligonucleotide (siRNA) with fluorine-18. An alkyne solid support CPG for the preparation of monostranded oligonucleotides functionalized with alkyne has been developed. Two complementary azide labeling agents (1-(azidomethyl)-4-[(18)F]fluorobenzene) and 1-azido-4-(3-[(18)F]fluoropropoxy)benzene have been produced with 41% and 35% radiochemical yields (decay-corrected), respectively. After annealing with the complementary strand, the siRNA was directly labeled by click chemistry with [(18)F]fluoroazide to produce the [(18)F]-radiolabeled siRNA with excellent radiochemical yield and purity.

New strategy for the preparation of clickable peptides and labeling with 1-(azidomethyl)-4-[(18)F]-fluorobenzene for PET.

The alkyne-azide Cu(I)-catalyzed Huisgen cycloaddition, a click type reaction was used to label a peptide with fluorine-18. A novel solid phase synthesis approach for the preparation of clickable peptides has been developed and has also permitted the straightforward preparation of reference compounds. A complementary azide labeling agent (1-(azidomethyl)-4-[(18)F]-fluorobenzene) has been produced in a four step procedure in 75 min with a 34% radiochemical yield (decay corrected). Conjugation of [(18)F]fluoroazide with a model alkyne-neuropeptide produced the desired (18)F-radiolabeled peptide in less than 15 min with a yield of 90% and excellent radiochemical purity.

A gadolinium triacetic monoamide DOTA derivative with a methanethiosulfonate anchor group. Relaxivity properties and conjugation with albumin and thiolated particles.

The gadolinium(III) complex with a new DOTA-based ligand bearing a methanethiosulfonate group (MTS) was synthesized and its relaxivity properties were investigated. MTS-ADO3A is a triacid DOTA derivative with an amide arm substituted by an ethylmethanethiosulfonate function. This ligand was obtained in two steps: tri-tert-butyl 2,2',2''-(1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetate was reacted with S-(2-aminoethyl)methanesulfonothioate and the tert-butyl groups were removed with trifluoroacetic acid. The Gd(III) MTS-ADO3A complex readily formed disulfide bonds with albumin (BSA) in its native and reduced forms and with thiolated silica particles. Four- to five-fold relaxivity increases at 20 MHz were measured on the isolated adducts. The EuMTS-ADO3A chelate was found to be monohydrated by fluorescence and the relaxivity parameters of the Gd(III) complex were obtained by (17)O NMR and by measuring the nuclear magnetic relaxation dispersion between 0.01 and 80 MHz. The water exchange time tau(m) is increased upon forming disulfide bonds with macromolecules and particles and the relaxivity gains of all the complexes are limited by the tau(m) factor. Forming covalent or hydrophobic/electrostatic bonds with BSA seems to bring about similar relaxivity changes but the covalent BSA adducts can be isolated and their properties can be directly studied. The addition of dithiothreitol or glutathione leads to the removal of the metal chelates from the macromolecules, as indicated by the relaxation times reverting to their values before binding. It is thus expected that the chelate will stay in the body long enough for imaging but will still be excreted through the kidneys.