Identification of a pyridinium metabolite in human urine following a single oral dose of 1-[2-[bis[4-(trifluoromethyl)phenyl]methoxy]ethyl]- 1,2,5,6-tetrahydro-3-pyridinecarboxylic acid monohydrochloride, a gamma-aminobutyric acid uptake inhibitor

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

By clearing GABA from the synaptic cleft, GABA transporters (GATs) play an essential role in inhibitory neurotransmission. Consequently, in vivo visualization of GATs can be a valuable diagnostic tool and biomarker for various psychiatric and neurological disorders. Not surprisingly, in recent years several research attempts to develop a radioligand have been conducted, but so far none have led to suitable radioligands that allow imaging of GATs. Here, we provide an overview of the radioligands that were developed with a focus on GAT1, since this is the most abundant transporter and most of the research concerns this GAT subtype. Initially, we focus on the field of GAT1 inhibitors, after which we discuss the development of GAT1 radioligands based on these inhibitors. We hypothesize that the radioligands developed so far have been unsuccessful due to the zwitterionic nature of their nipecotic acid moiety. To overcome this problem, the use of non-classical GAT inhibitors as basis for GAT1 radioligands or the use of carboxylic acid bioisosteres may be considered. As the latter structural modification has already been used in the field of GAT1 inhibitors, this option seems particularly viable and could lead to the development of more successful GAT1 radioligands in the future.

In silico ADMET analysis of the A-, B- and D-modified androstane derivatives with potential anticancer effects

The major challenge in the fight against cancer is to design new drugs that will be more selective for cancer cells, with fewer side effects. Synthetic steroids such as cyproterone, fulvestrant, exemestane and abiraterone are approved powerful drugs for the treatment of hormone-dependent diseases such as breast and prostate cancers. Therefore, androstane derivatives in 17-substituted, 17a-homo lactone and 16,17-seco series, with potent anticancer activity, were selected for pharmacokinetic and druglike predictions from the absorption, distribution, metabolism and excretion (ADME) models. In silico determination of physico-chemical and ADMET properties was performed using SwissADME and ProTox-II web tools. The possibility of gastrointestinal absorption and brain penetration was analyzed using the BOILED-Egg model, while the in silico evaluation of the similarities between selected steroid derivatives and FDA-approved drugs was carried out using the SwissSimilarity tool. Of all tested, two compounds that showed good in silico ADMET results, in addition to promising cytotoxicity and molecular docking results, could potentially be evaluated in in vivo tests.

Development of Positron Emission Tomography Radiotracers for the GABA Transporter 1

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-[¹¹C]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 [¹⁸F]fluoride was used to prepare the desired candidate radiotracer ( R, E/ Z)-1-(2-((4-fluoro-2-(4-[¹⁸F]fluorobenzoyl)styryl)oxy)ethyl)piperidine-3-carboxylic acid (( R, E/ Z)-[¹⁸F]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)-[¹⁸F]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)-[¹⁸F]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.

Glutamate and GABA synthesis, release, transport and metabolism as targets for seizure control

The synthesis, release, reuptake, and metabolism of the excitatory and inhibitory neurotransmitters glutamate and GABA, respectively, are tightly controlled. Given the role that these two neurotransmitters play in normal and abnormal neurotransmission, it is important to consider the processes whereby they are regulated. This brief review is focused entirely on the metabolic aspects of glutamate and GABA synthesis and neurotransmission. It describes in limited detail the synthesis, release, reuptake, metabolism, cellular compartmentation and pharmacology of the glutamatergic and GABAergic synapse. This review also provides a summary and brief description of the pathologic and phenotypic features of the various genetic animal models that have been developed in an effort to provide a greater understanding of the role that each of the aforementioned metabolic processes plays in controlling excitatory and inhibitory neurotransmission and how their use will hopefully facilitate the development of safer and more efficacious therapies for the treatment of epilepsy and other neurological disorders.

First Demonstration of a Functional Role for Central Nervous System Betaine/γ-Aminobutyric Acid Transporter (mGAT2) Based on Synergistic Anticonvulsant Action among Inhibitors of mGAT1 and mGAT2

In a recent study, EF1502 [N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo [d]isoxazol-3-ol], which is an N-substituted analog of the GAT1-selective GABA uptake inhibitor exo-THPO (4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol), was found to inhibit GABA transport mediated by both GAT1 and GAT2 in human embryonic kidney (HEK) cells expressing the mouse GABA transporters GAT1 to 4 (mGAT1-4). In the present study, EF1502 was found to possess a broad-spectrum anticonvulsant profile in animal models of generalized and partial epilepsy. When EF1502 was tested in combination with the clinically effective GAT1-selective inhibitor tiagabine [(R)-N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]nipecotic acid] or LU-32-176B [N-[4,4-bis(4-fluorophenyl)-butyl]-3-hydroxy-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol], another GAT1-selective N-substituted analog of exo-THPO, a synergistic rather than additive anticonvulsant interaction was observed in the Frings audiogenic seizure-susceptible mouse and the pentylenetetrazol seizure threshold test. In contrast, combination of the two mGAT1-selective inhibitors, tiagabine and LU-32-176B, resulted in only an additive anticonvulsant effect. Importantly, the combination of EF1502 and tiagabine did not result in a greater than additive effect in the rotarod behavioral impairment test. In subsequent in vitro studies conducted in HEK-293 cells expressing the cloned mouse GAT transporters mGAT1 and mGAT2, EF1502 was found to noncompetitively inhibit both mGAT1 and the betaine/GABA transporter mGAT2 (K(i) of 4 and 5 muM, respectively). Furthermore, in a GABA release study conducted in neocortical neurons, EF1502 did not act as a substrate for the GABA carrier. Collectively, these findings support a functional role for mGAT2 in the control of neuronal excitability and suggest a possible utility for mGAT2-selective inhibitors in the treatment of epilepsy.

Potential metabolic bioactivation pathways involving cyclic tertiary amines and azaarenes

A major theme explored in this review is the MAO-and cytochrome P450-catalyzed alpha-carbon oxidations of selected cyclic tertiary amines to give iminium metabolites that undergo further chemical modifications to form known or potentially toxic products. The most dramatic illustration of this type of bioactivation process is the conversion of the parkinsonian-inducing neurotoxin MPTP (23) by brain MAO-B to the iminium (dihydropyridinium) metabolite 24 which is oxidized further to the pyridinium species MPP+ (25). The selective destruction of nigrostriatal neurons by MPP+ is dependent on a unique sequence of events (transport into the nerve terminals by the dopamine transporter, localization in the inner mitochondrial membrane by electromotive forces, and inhibition of complex I of the mitochondrial electron transport chain) that, fortunately, are unlikely to be encountered with many substances. A second example of a well-documented metabolic bioactivation sequence involves the highly toxic pyrrolizidine alkaloids (102). These compounds undergo cytochrome P450-catalyzed alpha-carbon oxidation which converts the 3-pyrrolinyl moiety present in the parent alkaloids into a pyrrolyl-containing metabolite (105). The presence of labile functional groups results in the spontaneous conversion of 105 to reactive electrophilic products (106 and 108) that undergo Michael addition reactions with nucleophiles on biomacromolecules leading to a variety of toxic outcomes. Less clearly defined are the potential contributions to neurodegenerative processes that may be mediated by low-level, long term exposure to less potent toxins. Examples of potential proneurotoxins are the endogenously formed tetrahydroisoquinolines (such as 40-50) and tetrahydro-beta-carbolines (such as 54) that may be biotransformed to neurotoxic isoquinolinium (such as 51) and beta-carbolinium (such as 52) species in the brain. A similar argument can be made for 4-piperidinols (compounds that are at the same oxidation state as the tetrahydropyridines) which may be metabolized via iminium intermediates to amino enols that spontaneously convert to dihydropyridinium species and hence to pyridinium metabolites (67-->68-->69-->70-->71, Scheme 10). This type of reaction sequence has been well documented with the parkinsonian-inducing neuroleptic agent haloperidol (72) which is metabolized in humans, baboons, and rodents to the pyridinium species HPP+ (75), a potent inhibitor of mitochondrial respiration. Finally, an appreciation of the alpha-carbon oxidations of fully reduced azacycles such as (S)-nicotine (61) and phencyclidine (82) to chemically reactive metabolites that form covalent adducts with proteins, including the enzymes that are responsible for their formation, may prove of toxicological importance when attempting to account for the effects of chronic abuse of these potent drugs.1

Biomedical and biochemical applications of liquid chromatography-mass spectrometry

This review centres on the application of various LC-MS and LC-MS-MS techniques to the study and solution of practical problems in biomedical research. For this purpose it covers a selection of publications in this area included in the MEDLINE database for the period 1991-mid-1994. As shown herein, LC-MS is increasingly gaining in importance in the biomedical field, especially after the revolution brought about by the introduction of the new liquid-phase atmospheric pressure ionization (API) techniques, such as electrospray (ES) and ionspray. The information in this database shows that thermospray (TS), which clearly dominated LC-MS coupling in the 1980s, is on a downward trend relative to the more modern API techniques which will certainly dominate this application field in the present decade. Studies on drug metabolism, therapeutic drug monitoring and pharmacology have been traditionally carried out by GC-MS. However, LC-MS has lately been replacing classical GC-MS techniques in many of these applications. For instance, LC-ES-MS has greatly facilitated the application of both qualitative and quantitative LC-MS methods to highly polar molecules. This is possible without the need to resort to elaborate sample handling and derivatization procedures for relatively high-molecular-mass compounds such as drug conjugates, biosynthetic and natural peptides and therapeutic proteins obtained by recombinant DNA technology, all of them formerly totally inaccessible to the standard GC-MS or LC-MS methods. With regard to studies on metabolism and biochemical phenomena of endogenous compounds, LC-ES-MS is also becoming very strong in the analysis of structural biopolymers such as peptides, proteins, glycoproteins and glycolipids, and also lower molecular mass compounds such as fatty acids, vitamins, steroids and nucleic acids. For example, structural verification of post-translational modifications in proteins can be efficiently obtained in the time frame of an LC run and suitable MS methods for the location of glycopeptide-containing fractions in proteolytic digests of glycoproteins have been developed. Interesting examples are also shown of the use of LC-MS in clinical studies and the determination of biological markers of disease.