Development of Positron Emission Tomography Radiotracers for the GABA Transporter 1

Abstract

In vivo positron emission tomography (PET) imaging of the γ-aminobutyric acid (GABA) receptor complex has been accomplished using radiolabeled benzodiazepine derivatives, but development of specific presynaptic radioligands targeting the neuronal membrane GABA transporter type 1 (GAT-1) has been less successful. The availability of new structure-activity studies of GAT-1 inhibitors and the introduction of a GAT-1 inhibitor (tiagabine, Gabatril) into clinical use prompted us to reinvestigate the syntheses of PET ligands for this transporter. Initial synthesis and rodent PET studies of N-[C]methylnipecotic acid confirmed the low brain uptake of that small and polar molecule. The common design approach to improve blood-brain barrier permeability of GAT-1 inhibitors is the attachment of a large lipophilic substituent. We selected an unsymmetrical bis-aromatic residue attached to the ring nitrogen by a vinyl ether spacer from a series recently reported by Wanner and coworkers. Nucleophilic aromatic substitution of an aryl chloride precursor with [F]fluoride was used to prepare the desired candidate radiotracer ( R, E/ Z)-1-(2-((4-fluoro-2-(4-[F]fluorobenzoyl)styryl)oxy)ethyl)piperidine-3-carboxylic acid (( R, E/ Z)-[F]10). PET studies in rats showed no brain uptake, which was not altered by pretreatment of animals with the P-glycoprotein inhibitor cyclosporine A, indicating efflux by Pgp was not responsible. Subsequent PET imaging studies of ( R, E/ Z)-[F]10 in rhesus monkey brain showed very low brain uptake. Finally, to test if the free carboxylic acid group was the likely cause of poor brain uptake, PET studies were done using the ethyl ester derivative of ( R, E/ Z)-[F]10. Rapid and significant monkey brain uptake of the ester was observed, followed by a slow washout over 90 min. The blood-brain barrier permeability of the ester supports a hypothesis that the free acid function limits brain uptake of nipecotic acid-based GAT-1 radioligands, and future radiotracer efforts should investigate the use of carboxylic acid bioisosteres.

[18F]ALX5406: A Brain-Penetrating Prodrug for GlyT1-Specific PET Imaging

“…However, it should not interfere with passive transfer across the BBB per se and could even partly compensate for the unfavorably high molecular weight. , However, owing to the presence of a free carboxylic acid function adjacent to the basic amino group in the sarcosine motif, the predominant form of ALX5407 at physiological pH values is a zwitterion. Previous findings indicate that such compounds are characterized by poor BBB penetration unless they are subject to active transport into the brain. − The corresponding methyl ester ALX5406 lacks a free carboxylic acid function and reached a CNS MPO PET score of 3.5 (Table ), indicating that it may be more effective in crossing the BBB. With this in mind, we next followed a prodrug strategy and examined if the methyl ester could be used as a BBB-penetrating prodrug for delivery of [ 18 F]­ALX5407 into the brain.…”

Classics in Neuroimaging: Development of Positron Emission Tomography Tracers for Imaging the GABAergic Pathway

“…While tiagabine crosses the BBB at therapeutic concentrations, it appears that the nipecotic acid moiety hinders the BBB permeability of tiagabine and its derivatives when administered at the microdose required for PET imaging. Evidence of this is presented by Sowa and colleagues, who demonstrate that esterification of the carboxylic acid group on a tiagabine analogue enables sufficient penetration into the brain of the rhesus macaque for imaging, though affinity for GAT-1 is presumably compromised in the absence of the free acid moiety. This finding supports the idea that direct radiolabeling of therapeutic drugs may not be the optimal strategy for approaching the development of GABAergic tracers, particularly when those molecules share structural similarity with GABA.…”

Visualizing GABA transporters in vivo: an overview of reported radioligands and future directions

“…4 ) [ 40 , 41 ]. In later attempts, 11 C-methylated nipecotic acid [ 11 C]7 was developed as potential GAT1 inhibitor, but also proved unsuccessful [ 42 ].

Fig.

…”

“…Lastly, Sowa et al . developed the radioligand [ 18 F]40 [ 42 ], which was inspired by a series of GAT1 inhibitors developed by Quandt et al . [ 74 ].…”

“…cortex/striatum ratio of 1.44) was obtained, which is similar to [ 3 H]tiagabine [ 76 ]. However, no further studies were done to optimize these radioligands due to, among other reasons, the found toxicity of Cl-966 [ 42 , 77 , 78 ]. Similar results were obtained for the structurally related 125 I-labelled CIPCA [ 125 I]33 .…”