Joan Lecina

Comparison of New Tau PET-Tracer Candidates With [18F]T808 and [18F]T807

Early clinical results of two tau tracers, [18F]T808 and [18F]T807, have recently been reported. In the present study, the biodistribution, radiometabolite quantification, and competition-binding studies were performed in order to acquire comparative preclinical data as well as to establish the value of T808 and T807 as benchmark compounds for assessment of binding affinities of eight new/other tau tracers. Biodistribution studies in mice showed high brain uptake and fast washout. In vivo radiometabolite analysis using high-performance liquid chromatography showed the presence of polar radiometabolites in plasma and brain. No specific binding of [18F]T808 was found in transgenic mice expressing mutant human P301L tau. In semiquantitative autoradiography studies on human Alzheimer disease slices, we observed more than 50% tau selective blocking of [18F]T808 in the presence of 1 µmol/L of the novel ligands. This study provides a straightforward comparison of the binding affinity and selectivity for tau of the reported radiolabeled tracers BF-158, BF-170, THK5105, lansoprazole, astemizole, and novel tau positron emission tomography ligands against T807 and T808. Therefore, these data are helpful to identify structural requirements for selective interaction with tau and to compare the performance of new highly selective and specific radiolabeled tau tracers.

New Chelators for Low Temperature Al 18 F‑Labeling of Biomolecules

The Al(18)F labeling method is a relatively new approach that allows radiofluorination of biomolecules such as peptides and proteins in a one-step procedure and in aqueous solution. However, the chelation of the {Al(18)F}(2+) core with the macrocyclic chelators NOTA or NODA requires heating to 100-120 °C. Therefore, we have developed new polydentate ligands for the complexation of {Al(18)F}(2+) with good radiochemical yields at a temperature of 40 °C. The stability of the new Al(18)F-complexes was tested in phosphate buffered saline (PBS) at pH 7.4 and in rat serum. The stability of the Al(18)F-L3 complex was found to be comparable to that of the previously reported Al(18)F-NODA complex up to 60 min in rat serum. Moreover, the biodistribution of Al(18)F-L3 in healthy mice showed the absence of in vivo defluorination since no significant bone uptake was observed, whereas the major fraction of activity at 60 min p.i. was observed in liver and intestines, indicating hepatobiliary clearance of the radiolabeled ligand. The acyclic chelator H3L3 proved to be a good lead candidate for labeling of heat-sensitive biomolecules with fluorine-18. In order to obtain a better understanding of the different factors influencing the formation and stability of the complex, we carried out more in-depth experiments with ligand H3L3. As a proof of concept, we successfully conjugated the new AlF-chelator with the urea-based PSMA inhibitor Glu-NH-CO-NH-Lys to form Glu-NH-CO-NH-Lys(Ahx)L3, and a biodistribution study in healthy mice was performed with the Al(18)F-labeled construct. This new class of AlF-chelators may have a great impact on PET radiochemical space as it will stimulate the rapid development of new fluorine-18 labeled peptides and other heat-sensitive biomolecules.

Al18F-Labeling Of Heat-Sensitive Biomolecules for Positron Emission Tomography Imaging

Positron emission tomography (PET) using radiolabeled biomolecules is a translational molecular imaging technology that is increasingly used in support of drug development. Current methods for radiolabeling biomolecules with fluorine-18 are laborious and require multistep procedures with moderate labeling yields. The Al18F-labeling strategy involves chelation in aqueous medium of aluminum mono[18F]fluoride ({Al18F}2+) by a suitable chelator conjugated to a biomolecule. However, the need for elevated temperatures (100-120 °C) required for the chelation reaction limits its widespread use. Therefore, we designed a new restrained complexing agent (RESCA) for application of the AlF strategy at room temperature. Methods. The new chelator RESCA was conjugated to three relevant biologicals and the constructs were labeled with {Al18F}2+ to evaluate the generic applicability of the one-step Al18F-RESCA-method. Results. We successfully labeled human serum albumin with excellent radiochemical yields in less than 30 minutes and confirmed in vivo stability of the Al18F-labeled protein in rats. In addition, we efficiently labeled nanobodies targeting the Kupffer cell marker CRIg, and performed µPET studies in healthy and CRIg deficient mice to demonstrate that the proposed radiolabeling method does not affect the functional integrity of the protein. Finally, an affibody targeting HER2 (PEP04314) was labeled site-specifically, and the distribution profile of (±)-[18F]AlF(RESCA)-PEP04314 in a rhesus monkey was compared with that of [18F]AlF(NOTA)-PEP04314 using whole-body PET/CT. Conclusion. This generic radiolabeling method has the potential to be a kit-based fluorine-18 labeling strategy, and could have a large impact on PET radiochemical space, potentially enabling the development of many new fluorine-18 labeled protein-based radiotracers.

Recent Progress in Metal Catalyzed Direct Carboxylation of Aryl Halides and Pseudo Halides Employing CO2: Opportunities for 11C radiochemistry

Carbon dioxide (CO2) as a synthon in organic transformations is very useful, especially when combined with transition metal catalysts. CO2 insertion on carbon(Aryl)‐metal is are an elegant route to construct carbon–carbon bonds. A limited number of reports have been published to date on the direct conversion of aryl halides (or pseudo halides) to aryl carboxylic acids using transition metal catalysts. These reactions place great demand on the choice of catalyst, starting material, and reducing agent. However, these approaches could be of interest for the synthesis of a wide range of biologically active small molecules. Such transformations have already been applied for the synthesis of radiolabeled compounds for imaging with positron emission tomography (PET) using cyclotron produced [11C]CO2 and may be applicable to the production of a diverse range of PET tracers.

Direct fluorine-18 labeling of heat-sensitive biomolecules for positron emission tomography imaging using the Al 18 F-RESCA method

Positron emission tomography (PET) is a quickly expanding, non-invasive molecular imaging technology, and there is high demand for new specific imaging probes. Herein, we present a generic protocol for direct radiolabeling of heat-sensitive biomolecules with the positron-emitting radioisotope fluorine-18 (18F) using the aluminum fluoride restrained complexing agent (Al18F-RESCA) method. The Al18F-RESCA method combines the chemical advantages of a chelator-based radiolabeling method with the unique physical properties of the radionuclide of choice, fluorine-18. Proteins of interest can be conjugated to RESCA via amine coupling using (±)-H3RESCA-TFP, followed by purification using size-exclusion chromatography (SEC). Next, RESCA-derivatized biomolecules can be labeled in one step, at room temperature (~20 °C) in an aqueous medium with aluminum fluoride (Al18F). Al18F-labeled proteins can be obtained with moderate (12–17 GBq/µmol) to good (80–85 GBq/µmol) apparent molar activity, depending on the starting activity of 18F–. In addition, satisfactory radiochemical yields (35–55%, non–decay corrected) and high radiochemical purity (>98%, using gel filtration or solid-phase purification) are obtained. The mild radiolabeling procedure takes 0.5 h to complete and can be used for direct labeling of vector molecules such as peptides, protein scaffolds, and engineered antibody fragments.This protocol describes a method for direct fluorine-18 labeling of heat-sensitive proteins for PET imaging. After conjugation to RESCA chelators, proteins of interest can be radiolabeled with Al18F at room temperature in an aqueous medium.