Muneer Ahamed

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.

Synthesis, Biodistribution and In vitro Evaluation of Brain Permeable High Affinity Type 2 Cannabinoid Receptor Agonists [11C]MA2 and [18F]MA3

The type 2 cannabinoid receptor (CB2) is a member of the endocannabinoid system and is known for its important role in (neuro)inflammation. A PET-imaging agent that allows in vivo visualization of CB2 expression may thus allow quantification of neuroinflammation. In this paper, we report the synthesis, radiosynthesis, biodistribution and in vitro evaluation of a carbon-11 ([11C]MA2) and a fluorine-18 ([18F]MA3) labeled analog of a highly potent N-arylamide oxadiazole CB2 agonist (EC50 = 0.015 nM). MA2 and MA3 behaved as potent CB2 agonist (EC50: 3 nM and 0.1 nM, respectively) and their in vitro binding affinity for hCB2 was found to be 87 nM and 0.8 nM, respectively. Also MA3 (substituted with a fluoro ethyl group) was found to have higher binding affinity and EC50 values when compared to the originally reported trifluoromethyl analog 12. [11C]MA2 and [18F]MA3 were successfully synthesized with good radiochemical yield, high radiochemical purity and high specific activity. In mice, both tracers were efficiently cleared from blood and all major organs by the hepatobiliary pathway and importantly these compounds showed high brain uptake. In conclusion, [11C]MA2 and [18F]MA3 are shown to be high potent CB2 agonists with good brain uptake, these favorable characteristics makes them potential PET probes for in vivo imaging of brain CB2 receptors. However, in view of its higher affinity and selectivity, further detailed evaluation of MA3 as a PET tracer for CB2 is warranted.

Evaluation of PET radioligands for in vivo visualization of phosphodiesterase 5 (PDE5)

INTRODUCTION The cyclic guanosine monophosphate (cGMP) specific phosphodiesterase type 5 (PDE5) is considered to play an important role in various etiologies such as pulmonary arterial hypertension (PAH) and chronic heart failure. This PDE5 modulation represents an important prognostic and/or therapeutic target; however, there is currently no method to non-invasively evaluate the PDE5 expression levels in vivo. METHODS Radiolabeled tracers were prepared by N-alkylation of the corresponding precursors with [(11)C]methyl trifluoromethanesulfonate ([(11)C]CH3OTf) or 2-[(18)F]fluoroethyl trifluoromethanesulfonate ([(18)F]FEtOTf). Biodistribution of radiolabeled tracers was studied in NMRI mice and their specific binding to PDE5 was investigated by comparing their lung retention as the enzyme is abundantly expressed in this organ. RESULTS The overall radiochemical yields ranged between 24% and 60% for labeled radiotracers with radiochemical purity of>99%. The highest retention in the lungs at 30min post injection was observed for vardenafil derivatives [(11)C]-7 and [(18)F]-11 and the retention of the ethoxyethyl pyrazolopyrimidine derivative [(11)C]-37 was moderate. The other investigated compounds [(11)C]-8, [(11)C]-14, [(11)C]-21 and [(11)C]-33 showed lower retention in lungs in agreement with their lower in-vitro affinity for PDE5. CONCLUSION Among the different radiolabeled PDE5 inhibitors evaluated in this study, the vardenafil derivatives [(11)C]-7 and [(18)F]-11 are found to be promising tracers for in vivo visualization of PDE5.

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.

Synthetic strategies for radioligands for in vivo imaging of brain cannabinoid type-1 receptors.

Despite its abundant expression in the brain, the cannabinoid type-1 (CB1) receptor was discovered only in 1988. During the last decade, efforts from several research groups have made it possible to visualize the CB1 receptor in vivo to unravel its role in the brain physiology and pathology. This review discusses strategies for (radio)synthesis of radioligands for in vivo imaging of CB1 receptors.

The Outcome of the Oxidations of Unusual Enediamide Motifs Is Governed by the Stabilities of the Intermediate Iminium Ions

We compare the results from the oxidation of two unusual “enediamide” motifs (3,4-dihydropyrazin-2(1H)-ones), where a double bond is flanked by two amides. In one case the oxidation led to a ring-opened product arising from the cleavage of the double bond, and in the other a rare cis-dioxygenated compound was obtained. Both products have been characterized by X-ray crystallography. The outcomes of the key reactions are rationalized based on calculated free energies of intermediates.

Synthesis, Enzyme Assays and Molecular Docking Studies of Fluorina ted Bioisosteres of Santacruzamate A as Potential HDAC Tracers

Background: Histone deacetylases (HDACs) emerged as important epigenetic regulators of gene expression. Method: In order to identify potential positron emission tomography (PET) tracers for imaging HDACs, we evaluated in vitro and in cellulo activities of some compounds that were reported as potent HDAC2-selective inhibitors. We observed marked differences between reported activity values and the values obtained in our assays for some of the compounds. To understand the structural basis of the activity of some of these inhibitors, we also performed molecular docking studies to understand their interaction patterns and binding modes with HDAC2. Results and Conclusion: We observed the low affinity compounds 4, 6 and 7 did not showed equal number of key ?-? interactions and hydrogen bonding when compared to high affinity compounds, and could be the possible reason for poor inhibition as reflected in in vitro assays. These preliminary experimental and computational results will help to interpret the HDAC affinity values of these key compounds with caution.

Carbon-11 and Fluorine-18 Radiolabeled Pyridopyrazinone Derivatives for Positron Emission Tomography (PET) Imaging of Phosphodiesterase-5 (PDE5)

The cyclic guanosine monophosphate (cGMP) specific phosphodiesterase type 5 (PDE5) plays an important role in various pathologies including pulmonary arterial hypertension and cardiomyopathy. PDE5 represents an important therapeutic and/or prognostic target, but noninvasive assessment of PDE5 expression is lacking. The purpose of this study was to develop and evaluate pyridopyrazinone derivatives labeled with carbon-11 or fluorine-18 as PDE5-specific PET tracers. In biodistribution studies, highest PDE5-specific retention was observed for [11C]-12 and [18F]-17 in the lungs of wild-type mice and in the myocardium of transgenic mice with cardiomyocyte-specific PDE5 overexpression at 30 min postinjection. In vivo dynamic microPET images in rats revealed that both tracers crossed the blood-brain barrier but brain retention was not PDE5-specific. Both [11C]-12 and [18F]-17 showed specific binding to PDE5 in myocardium of transgenic mice; however [18F]-17 showed significantly higher PDE5-specific inhibitable binding than [11C]-12.