PET Tracers for Mapping Adenosine Receptors as Probes for Diagnosis of CNS Disorders

Adenosine is an endogenous modulator of several physiological functions in the central nervous system (CNS). The effect is mediated by a receptor family that consists of at least four subtypes: A1, A2A, A2B and A3 receptors. The adenosine receptors play a role in neurological and psychiatric disorders such as Alzheimers disease, Parkinsons disease, epilepsy and schizophrenia. Knowledge on adenosine receptor densities and status are important for understanding the mechanisms underlying the pathogenesis of diseases and for developing new therapeutics. Positron emission tomography (PET) offers a non-invasive tool to investigate these features in vivo, provided that suitable radiopharmaceuticals are available. As a consequence of the development of xanthine-type adenosine receptor antagonists with high affinity and high selectivity, several PET ligands labeled with carbon-11 (half-life of 20.4 min) and fluorine-18 (half-life of 109.8 min) have been proposed for mapping the adenosine A1 and A2A receptors (A1R and A2AR, respectively) and the adenosine uptake site in the CNS since 1995. Later non-xanthine-type antagonists for A2AR were radiolabeled. So far two tracers for A1R, [18F]CPFPX and [11C]MPDX, and a tracer for A2AR, [11C]TMSX (also called [11C]KF18446), have been applied to humans. For the other subtypes and the adenosine uptake site no suitable radioligands have been developed yet. This paper gives an overview of the current status on PET tracers for mapping adenosine receptors and the development of new compounds that may lead to new PET tracers

Application of Click Chemistry for PET

Sharpless et al. presented, in 2001, a review in which they introduced the concept of "click chemistry". In this review a "new way" of making chemicals, with a particular emphasis on drugs, is presented. Current drugs are often based on natural products that were first extracted from plants or other organisms and then with enormous effort were synthetically reproduced by chemists. Sharpless et al. propose to shift the focus away from the structure, which chemists focus on when they synthesize natural products, towards the function of molecules. Rather than making natural products with known biological activity and using these as templates for small modifications, it is proposed to make large libraries of compounds using (mainly) modular chemistry. After all, when looking for new and better drugs, it is the function that matters rather than the structure. This approach mimics nature in that it involves making a great variety of different compounds starting from a relatively small number of building blocks via a set amount of reactions. These sets of reactions have been termed "click reactions" in which simple molecules with specific functionalities can be "clicked" to each other to form a large variety of compounds with relative ease that can subsequently be tested as potential drug candidates. For these "click reactions" Sharpless also looks to nature for inspiration. Ideally, the reaction conditions should be simple, involving no or benign solvents and the reaction itself should be insensitive to oxygen and water. It was found that copper not only accelerates the reaction but also controls the regioselectivity since in the presence of copper, only the 1,4-isomer is formed. The reaction proceeds in water, with or without co-solvent at room temperature and is relatively fast. The reaction is 100% atom efficient which means that there are no side products so the work up is usually simple. It can take place in a wide pH range which makes is suitable for biological compounds that require a specific pH. Furthermore the azide and alkyne functionalities are bioorthogonal, so theoretically, other functional groups present in a biological environment will not touch them. Finally the triazole product is biologically stable

PET imaging of beta-adrenoceptors in human brain: A realistic goal or a mirage?

Beta-adrenoceptors are predominantly located in the cerebral cortex, nucleus accumbens and striatum. At lower densities, they are also present in amygdala, hippocampus and cerebellum. Beta-2 sites regulate glial proliferation during ontogenic development, after trauma and in neurodegenerative diseases. The densities of beta-1 adrenoceptors are changed by stress, in several mood disorders (depression, excessive hostility, schizophrenia) and during treatment of patients with antidepressants. A technique for beta-adrenoceptor imaging in the human brain is not yet available. Although 24 (ant)agonists have been labeled with either 11C or 18F and some of these are successful myocardial imaging agents, only two (S-1’-18F-fluorocarazolol and S-1’-18F-fluoroethylcarazolol) could actually visualize ß-adrenoceptors within the central nervous system. Unfortunately, these radiopharmaceuticals showed a positive Ames test. They may be mutagenic and cannot be employed for human studies. Screening of more than 150 beta-blockers described in the literature yields only two compounds (exaprolol and L643,717) which can still be radiolabeled and evaluated for ß-adenoceptor imaging. However, other imaging techniques could be examined. Cerebral ß-adrenoceptors might be labeled after temporary opening of the blood-brain barrier (BBB) and simultaneous administration of a hydrophilic ligand such as S-11C-CGP12388. Another approach to target ß-adrenoceptor ligands to the CNS is esterification of a myocardial imaging agent (such as 11C-CGP12177), resulting in a lipophilic prodrug which can cross the BBB and is split by tissue esterases. BBB opening is not feasible in healthy subjects, but the prodrug approach may be successful and deserves to be explored

New sensitive method for HEPES quantification in (68)Ga-radiopharmaceuticals

Contains fulltext : 220814.pdf (publisher's version ) (Open Access)BACKGROUND: The introduction of a GMP-certified (68)Ga-generator spurred the application of (68)Ga-radiopharmaceuticals. Several radiosynthesis of (68)Ga-radiopharmaceuticals are more efficient and robust when performed with 2-[4-(2-hydroxyethyl)piperazin-1-yl] ethanesulfonic acid (HEPES) buffer, which is considered as an impurity in the quality control (QC) procedure. Thus, prior to clinical use, QC must be conducted to ensure that HEPES does not exceed the maximum dose of 200 mug/V Injected as described in European Pharmacopoeia (Ph Eur) for edotreotide. However, when applying the thin-layer chromatography (TLC) method described in the Ph Eur to quantify the HEPES amount present in the (68)Ga-octreotide or in the remaining (68)Ga-radiopharmaceuticals that were tested, no amount was detectable after 4 min of iodine incubation. Here we tested our modified TLC method and validate a new high-performance liquid chromatography (HPLC) method to quantify HEPES in (68)Ga-radiopharmaceuticals and compare it to the TLC-method described in Ph Eur. In addition, samples collected from various institutes were tested to evaluate whether the synthesis of different (68)Ga-radiopharmaceuticals or the use of different synthesis methods could affect the amounts of HEPES. RESULTS: HEPES could not be detected by the TLC method described in the Ph Eur within 4 min incubation in an iodine-saturated chamber. As for our modified TLC method, only after 2 h, spots were only visible > 1 mg/mL. The HPLC method had a limit-of-quantification (LOQ) of 3 mug/mL and a limit-of-detection (LOD) of 1 mug/mL. From the three (68)Ga-radiopharmaceuticals tested, only in the [(68)Ga]Ga-NODAGA-Exendin samples exceeding amounts of HEPES were found and its concentration in the [(68)Ga]Ga-NODAGA-Exendin was significantly higher, when compared to [(68)Ga]Ga-DOTATOC and [(68)Ga]Ga-PSMA-11. CONCLUSION: The TLC method described in Ph Eur and our modified TLC method may not be sufficiently sensitive and thus unsuitable to use for QC release. The new HPLC method was sensitive, quantitative, reproducible and suitable for QC release. With this method, we were able to determine that some (68)Ga-radiopharmaceuticals may exceed the HEPES limit of 200 mug/ V Injected. This new analytical system would allow correcting for the maximum injected dose in order not to exceed this amount