Challenges for Developing PET Tracers: Isotopes, Chemistry, and Regulatory Aspects

Automation of simplified two-step radiolabeling via ditosylate synthon for 18F-labeled radiotracers using AllinOne module

Direct fluorination of a tosylate or mesylate precursor has been a wide-spread and reliable way for radio-fluorination. This approach can be difficult to achieve when the precursor cannot be easily obtained or is chemically unstable. A possible alternative method is to radiolabel ethylene ditosylate or 1,3-propanediol di-p-tosylate to form a radiofluorinated synthon. Here we present the automation of a simplified and reliable approach for the two-step fluorination using [18F]FP-TMP, an analog of antibacterial agent trimethoprim. We demonstrate the feasibility of purifying the fluorinated synthon via filtration, and one final HPLC purification on a commercially available Trasis AllinOne module. The overall reaction time for the two-step reaction is around 90 min andthe decay-corrected yield for more than fifty preparations of [18F]FP-TMP is 22 ± 5 % with high radiochemical purity (≥ 90 %) and specific activities (147 ± 107 GBq/μmol). All batches passed pre-established quality control specifications, demonstrating the utility of using this method in tracer syntheses that meet good manufacturing practice (GMP) requirement. This method can be adopted to the syntheses of other radiotracers, such as [18F]FE-TMP, (+)-[18F]F-PHNO and [18F]FFMZ.

ImmunoPET Directed to the Brain: A New Tool for Preclinical and Clinical Neuroscience

Immuno-positron emission tomography (immunoPET) is a non-invasive in vivo imaging method based on tracking and quantifying radiolabeled monoclonal antibodies (mAbs) and other related molecules, such as antibody fragments, nanobodies, or affibodies. However, the success of immunoPET in neuroimaging is limited because intact antibodies cannot penetrate the blood-brain barrier (BBB). In neuro-oncology, immunoPET has been successfully applied to brain tumors because of the compromised BBB. Different strategies, such as changes in antibody properties, use of physiological mechanisms in the BBB, or induced changes to BBB permeability, have been developed to deliver antibodies to the brain. These approaches have recently started to be applied in preclinical central nervous system PET studies. Therefore, immunoPET could be a new approach for developing more specific PET probes directed to different brain targets.

Last-step 18F-fluorination of supported 2-(aryl-di-tert-butylsilyl)-N-methyl-imidazole conjugates for applications in positron emission tomography

Aiming for potential applications in positron emission tomography, fully automated productions of ¹⁸F-labelled bioconjugates were achieved using heterogenous precursors obtained by anchoring imidazole-di-tert-butyl-arylsilanes to a polystyrene resin. The reactions were performed using either "batch" or "flow" procedures, avoiding both the time-consuming azeotropic drying and HPLC purifications usually required.

Bioconjugated arylpalladium complexes on solid supports for a convenient last-step synthesis of 11C-labelled tracers for positron emission tomography

Bioconjugated arylpalladium complexes anchored onto polystyrene beads provided [¹¹C]CO-labelled compounds with excellent radiochemical purities after a simple filtration.

Cobalt-Catalyzed C-F Bond Borylation of Aryl Fluorides

A mild and practical cobalt-catalyzed defluoroborylation of fluoroarenes is presented for the first time. The method permits straightforward functionalization of fluoroarenes, with high selectivity for borylation of C-F over C-H bonds, and a tolerance for aerobic conditions. Furthermore, two-step ¹⁸F-fluorination was achieved for expanding the scope of ¹⁸F-positron emission tomography probes.

Automation of the Radiosynthesis of Six Different 18F-labeled radiotracers on the AllinOne

Background

Fast implementation of positron emission tomography (PET) into clinical and preclinical studies highly demands automated synthesis for the preparation of PET radiopharmaceuticals in a safe and reproducible manner. The aim of this study was to develop automated synthesis methods for these six ¹⁸F-labeled radiopharmaceuticals produced on a routine basis at the University of Pennsylvania using the AllinOne synthesis module.

Results

The development of automated syntheses with varying complexity was accomplished including HPLC purification, SPE procedures and final formulation with sterile filtration. The six radiopharmaceuticals were obtained in high yield and high specific activity with full automation on the AllinOne synthesis module under current good manufacturing practice (cGMP) guidelines.

Conclusion

The study demonstrates the versatility of this synthesis module for the preparation of a wide variety of ¹⁸F-labeled radiopharmaceuticals for PET imaging studies.

Late stage benzylic C-H fluorination with [¹⁸F]fluoride for PET imaging

We describe the first late-stage (18)F labeling chemistry for aliphatic C-H bonds with no-carrier-added [(18)F]fluoride. The method uses Mn(salen)OTs as an F-transfer catalyst and enables the facile labeling of a variety of bioactive molecules and building blocks with radiochemical yields (RCY) ranging from 20% to 72% within 10 min without the need for preactivation of the labeling precursor. Notably, the catalyst itself can directly elute [(18)F]fluoride from an ion exchange cartridge with over 90% efficiency. Using this feature, the conventional and laborious dry-down step prior to reaction is circumvented, greatly simplifying the mechanics of this protocol and shortening the time for automated synthesis. Eight drug molecules, including COX, ACE, MAO, and PDE inhibitors, have been successfully [(18)F]-labeled in this way.

Fluoride-18 radiolabeling of peptides bearing an aminooxy functional group to a prosthetic ligand via an oxime bond

We have developed a novel F-18 prosthetic ligand named fluoro-PEG-benzaldehyde (FPBA) 1. [(18)F]-FPBA 1 is formed in situ from its radiolabeled precursor [(18)F]6. Compound 6 is efficiently synthesized in four steps starting from commercially available 6-bromo-3-pyridine carbaldehyde 2. [(18)F]-FPBA was evaluated as a prosthetic ligand to radiolabel three cyclic peptides bearing an aminooxy functional group at the N-terminus position. Acetal [(18)F]6 is purified by either solid-phase extraction (SPE) or reverse-phase HPLC with the overall radiochemical yields (RCY) and radiochemical purity (RCP) in very close agreement. The SPE purification process has the advantage of shorter reaction times (71-87 min for entire reaction sequence), while the use of the reverse-phase HPLC purification process allows the use of up to fifty times less of the expensive synthetic peptides (~ 50 nmol) in the oxime coupling reaction. We have demonstrated an efficient methodology in the production of [(18)F]-FPBA 1 and demonstrated its use as a prosthetic ligand for the labeling of peptides possessing an aminooxy functional group.

New F-18 prosthetic group via oxime coupling

A novel fluorine-18 prosthetic ligand, 5-(1,3-dioxolan-2-yl)-2-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)pyridine [(18)F]2, has been synthesized. The prosthetic ligand is formed in high radiochemical yield (rcy = 71 ± 2%, n = 3) with excellent radiochemical purity (rcp = 99 ± 1%, n = 3) in a short reaction time (10 min). [(18)F]2 is a small, neutral, organic complex, easily synthesized in four steps from a readily available starting material. It can be anchored onto a target molecule containing an aminooxy functional group under acidic conditions by way of an oxime bond. We report herein two examples [(18)F]23 and [(18)F]24, potential imaging agents for β-amyloid plaques, which were labeled with this prosthetic group. This approach could be used for labeling proteins and peptides containing an aminooxy group. Biodistribution in male ICR mice for both oxime labeled complexes [(18)F]23 and [(18)F]24 were compared to that of the known β-amyloid plaque indicator, [(18)F]-AV-45, florbetapir 1. Oximes [(18)F]23 and [(18)F]24 are larger in size and therefore should reduce the blood-brain barrier (BBB) penetration. The brain uptake for oxime [(18)F]23 appeared to be reduced, but still retained some capability to cross the BBB. Oxime [(18)F]24 showed promising results after 2 min post injection (0.48% dose/gram); however, the uptake increased after 30 min post injection (0.92% dose/gram) suggesting an in vivo decomposition/metabolism of compound [(18)F]24. We have demonstrated a general protocol for the fluoride-18 labeling with a new prosthetic ligand [(18)F]2 that is tolerant toward several functional groups and is formed via chemoselective oxime coupling.