Novel radioiodinated {gamma}-hydroxybutyric acid analogues for radiolabeling and Photolinking of high-affinity {gamma}-hydroxybutyric acid binding sites

Further Investigation of Synaptic Vesicle Protein 2A (SV2A) Ligands Designed for Positron Emission Tomography and Single-Photon Emission Computed Tomography Imaging: Synthesis and Structure-Activity Relationship of Substituted Pyridinylmethyl-4-(3,5-difluorophenyl)pyrrolidin-2-ones

A series of synaptic vesicle protein 2A (SV2A) ligands were synthesized to explore the structure-activity relationship and to help further investigate a hydrogen bonding pharmacophore hypothesis. Racemic SynVesT-1 was used as a lead compound to explore the replacement of the 3-methyl group on the pyridinyl moiety with halogens and hydrocarbons. Pyridinyl isomers of racemic SynVesT-1 were also investigated. Highly potent analogs were discovered including a 3-iodo pyridinyl ligand amenable to investigation as a PET or SPECT imaging agent.

GHB analogs confer neuroprotection through specific interaction with the CaMKIIα hub domain

GHB is a natural brain metabolite of GABA, previously reported to be neuroprotective. However, the high-affinity binding site for GHB has remained elusive for almost 40 y. We here unveil CaMKIIα, a highly important neuronal kinase, as the long-sought-after GHB high-affinity target. Via a specific interaction within the central hub domain of CaMKIIα, GHB analogs act to stabilize the hub oligomer complex. This interaction potentially explains pronounced neuroprotective effects of GHB analogs in cultured neurons exposed to a chemical insult and in mice exposed to ischemia. The postischemic treatment effects of GHB analogs underline these compounds as selective and high-affinity potential drug candidates and CaMKIIα as a relevant pharmacological target for stroke therapy.

Ca²⁺/calmodulin-dependent protein kinase II alpha subunit (CaMKIIα) is a key neuronal signaling protein and an emerging drug target. The central hub domain regulates the activity of CaMKIIα by organizing the holoenzyme complex into functional oligomers, yet pharmacological modulation of the hub domain has never been demonstrated. Here, using a combination of photoaffinity labeling and chemical proteomics, we show that compounds related to the natural substance γ-hydroxybutyrate (GHB) bind selectively to CaMKIIα. By means of a 2.2-Å x-ray crystal structure of ligand-bound CaMKIIα hub, we reveal the molecular details of the binding site deep within the hub. Furthermore, we show that binding of GHB and related analogs to this site promotes concentration-dependent increases in hub thermal stability believed to alter holoenzyme functionality. Selectively under states of pathological CaMKIIα activation, hub ligands provide a significant and sustained neuroprotection, which is both time and dose dependent. This is demonstrated in neurons exposed to excitotoxicity and in a mouse model of cerebral ischemia with the selective GHB analog, HOCPCA (3-hydroxycyclopent-1-enecarboxylic acid). Together, our results indicate a hitherto unknown mechanism for neuroprotection by a highly specific and unforeseen interaction between the CaMKIIα hub domain and small molecule brain-penetrant GHB analogs. This establishes GHB analogs as powerful tools for investigating CaMKII neuropharmacology in general and as potential therapeutic compounds for cerebral ischemia in particular.

The γ-hydroxybutyric acid (GHB) analogue NCS-382 is a substrate for both monocarboxylate transporters subtypes 1 and 4

The small-molecule ligand (E)-2-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[7]annulen-6-ylidene)acetic acid (NCS-382) is an analogue of γ-hydroxybutyric acid (GHB) and is widely used for probing the brain-specific GHB high-affinity binding sites. To reach these, brain uptake is imperative, and it is therefore important to understand the molecular mechanisms of NCS-382 transport in order to direct in vivo studies. In this study, we hypothesized that NCS-382 is a substrate for the monocarboxylate transporter subtype 1 (MCT1) which is known to mediate blood-brain barrier (BBB) permeation of GHB. For this purpose, we investigated NCS-382 uptake by MCT subtypes endogenously expressed in tsA201 and MDA-MB-231 cell lines in assays of radioligand-based competition and fluorescence-based intracellular pH measurements. To further verify the results, we measured NCS-382 uptake by means of mass spectrometry in Xenopus laevis oocytes heterologously expressing MCT subtypes. As expected, we found that NCS-382 is a substrate for MCT1 with half-maximal effective concentrations in the low millimolar range. Surprisingly, NCS-382 also showed substrate activity at MCT4 as well as uptake in water-injected oocytes, suggesting a component of passive diffusion. In conclusion, transport of NCS-382 across membranes differs from GHB as it also involves MCT4 and/or passive diffusion. This should be taken into consideration when designing pharmacological studies with this compound and its closely related analogues. The combination of MCT assays used here exemplifies a setup that may be suitable for a reliable characterization of MCT ligands in general.

Lack of evidence for synaptic high-affinity γ-hydroxybutyric acid (GHB) transport in rat brain synaptosomes and 11 Na+ -dependent SLC neurotransmitter transporters

γ-Hydroxybutyric acid (GHB) is an endogenous compound proposed to act as a neurotransmitter. Na⁺ -dependent, high-affinity GHB transport has long been considered important evidence supporting this hypothesis. However, the molecular identity of such a high-affinity transporter remains unknown. In this study, we sought to identify and characterize GHB synaptic transport through a series of studies using both native and recombinant systems with the ultimate aim of providing evidence to clarify the proposed role of GHB as a neurotransmitter in the mammalian brain. Native [³ H]GHB transport was studied in isolated rat brain synaptosomes and compared to synaptic membranes. As a targeted approach, GHB was also screened against a panel of Na⁺ -dependent SLC6 neurotransmitter transporters recombinantly expressed in Xenopus laevis oocytes or tsA201 cells. Finally, the low-affinity GHB transporters, MCT1/2 and SMCT1, were probed as GHB transporters in L-[¹⁴ C]lactate uptake assays in synaptosomes. We found no evidence of high-affinity [³ H]GHB transport in purified rat brain cortical or striatal synaptosomes or at any of the 11 SLC6 transporters tested. Instead, our results indicate the binding of [³ H]GHB to an unidentified membrane component, distinct from any of the known GHB targets. In accordance with others, we found that GHB and the analog 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) can, in millimolar concentrations, inhibit L-[¹⁴ C]lactate uptake at MCT1 and/or MCT2 and that this also can occur in synaptosomes. In conclusion, through a variety of in vitro pharmacological studies, we were unsuccessful in identifying a specific synaptic high-affinity transporter for GHB. Our findings emphasize the need to reevaluate GHB's role as a potential neurotransmitter. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Radiosynthesis and Evaluation of [11C]3-Hydroxycyclopent-1-enecarboxylic Acid as Potential PET Ligand for the High-Affinity γ-Hydroxybutyric Acid Binding Sites

γ-Hydroxybutyric acid (GHB) is an endogenous neuroactive substance and proposed neurotransmitter with affinity for both low- and high-affinity binding sites. A radioligand with high and specific affinity toward the high-affinity GHB binding site would be a unique tool toward a more complete understanding of this population of binding sites. With its high specific affinity and monocarboxylate transporter (MCT1) mediated transport across the blood-brain barrier in pharmacological doses, 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) seems like a suitable PET radiotracer candidate. Here, we report the ¹¹C-labeling and subsequent evaluation of [¹¹C]HOCPCA in a domestic pig, as a PET-radioligand for visualization of the high-affinity GHB binding sites in the live pig brain. To investigate the regional binding of HOCPCA in pig brain prior to in vivo PET studies, in vitro quantitative autoradiography on sections of pig brain was performed using [³H]HOCPCA. In vivo evaluation of [¹¹C]HOCPCA showed no brain uptake, possibly due to a limited uptake of HOCPCA by the MCT1 transporter at tracer doses of [¹¹C]HOCPCA.

Autoradiographic imaging and quantification of the high-affinity GHB binding sites in rodent brain using 3H-HOCPCA

GHB (γ-hydroxybutyric acid) is a compound endogenous to mammalian brain with high structural resemblance to GABA. GHB possesses nanomolar-micromolar affinity for a unique population of binding sites, but the exact nature of these remains elusive. In this study we utilized the highly selective GHB analogue, 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) as a tritiated version (³H-HOCPCA) to radioactively label the specific GHB high-affinity binding site and gain further insight into the density, distribution and developmental profile of this protein. We show that, in low nanomolar concentrations, ³H-HOCPCA displays excellent signal-to-noise ratios using rodent brain autoradiography, which makes it a valuable ligand for anatomical quantification of native GHB binding site levels. Our data confirmed that ³H-HOCPCA labels only the high-affinity specific GHB binding site, found in high density in cortical and hippocampal regions. The experiments revealed markedly stronger binding at pH 6.0 (K_d 73.8 nM) compared to pH 7.4 (K_d 2312 nM), as previously reported for other GHB radioligands but similar B_max values. Using ³H-HOCPCA we analyzed the GHB binding protein profile during mouse brain development. Due to the high sensitivity of this radioligand, we were able to detect low levels of specific binding already at E15 in mouse brain, which increased progressively until adulthood. Collectively, we show that ³H-HOCPCA is a highly sensitive radioligand, offering advantages over the commonly used radioligand ³H-NCS-382, and thus a very suitable in vitro tool for qualitative and quantitative autoradiography of the GHB high-affinity site.

In Vitro and In Vivo Evidence for Active Brain Uptake of the GHB Analog HOCPCA by the Monocarboxylate Transporter Subtype 1

γ-Hydroxybutyric acid (GHB) is a recreational drug, a clinically prescribed drug in narcolepsy and alcohol dependence, and an endogenous substance that binds to both high- and low-affinity sites in the brain. For studying the molecular mechanisms and the biologic role of the GHB high-affinity binding sites, ligands with high and specific affinity are essential. The conformationally restricted GHB analog HOCPCA (3-hydroxycyclopent-1-enecarboxylic acid) is one such compound. The objective of this study was to investigate the transport of HOCPCA across the blood-brain barrier in vitro and in vivo and to investigate the hypothesis that HOCPCA, like GHB, is a substrate for the monocarboxylate transporters (MCTs). For in vitro uptake studies, MCT1, -2, and -4 were recombinantly expressed in Xenopus laevis oocytes, and the previously reported radioligand [(3)H]HOCPCA was used as substrate. HOCPCA inhibited the uptake of the endogenous MCT substrate l-[(14)C]lactate, and [(3)H]HOCPCA was shown to act as substrate for MCT1 and 2 (Km values in the low- to mid-millimolar range). Introducing single-point amino acid mutations into positions essential for MCT function supported that HOCPCA binds to the endogenous substrate pocket of MCTs. MCT1-mediated brain entry of HOCPCA (10 mg/kg s.c.) was further confirmed in vivo in mice by coadministration of increasing doses of the MCT inhibitor AR-C141990 [(R)-5-(3-hydroxypyrrolidine-1-carbonyl)-1-isobutyl-3-methyl-6-(quinolin-4-ylmethyl)thieno[2,3-d]pyrimidine-2,4(1H,3H)-dione], which inhibited brain penetration of HOCPCA in a dose-dependent manner (ID50 = 4.6 mg/kg). Overall, our study provides evidence that MCT1 is an important brain entry site for HOCPCA and qualifies for future in vivo studies with HOCPCA.

Positive allosteric modulation of the GHB high-affinity binding site by the GABAA receptor modulator monastrol and the flavonoid catechin

γ-Hydroxybutyric acid (GHB) is a metabolite of γ-aminobutyric acid (GABA) and a proposed neurotransmitter in the mammalian brain. We recently identified α4βδ GABAA receptors as possible high-affinity GHB targets. GABAA receptors are highly sensitive to allosteric modulation. Thus to investigate whether GHB high-affinity binding sites are also sensitive to allosteric modulation, we screened both known GABAA receptor ligands and a library of natural compounds in the rat cortical membrane GHB specific high-affinity [3H]NCS-382 binding assay. Two hits were identified: Monastrol, a positive allosteric modulator of GABA function at δ-containing GABAA receptors, and the naturally occurring flavonoid catechin. These compounds increased [3H]NCS-382 binding to 185-272% in high micromolar concentrations. Monastrol and (+)-catechin significantly reduced [3H]NCS-382 dissociation rates and induced conformational changes in the binding site, demonstrating a positive allosteric modulation of radioligand binding. Surprisingly, binding of [3H]GHB and the GHB high-affinity site-specific radioligands [125I]BnOPh-GHB and [3H]HOCPCA was either decreased or only weakly increased, indicating that the observed modulation was critically probe-dependent. Both monastrol and (+)-catechin were agonists at recombinant α4β3δ receptors expressed in Xenopus laevis oocytes. When monastrol and GHB were co-applied no changes were seen compared to the individual responses. In summary, we have identified the compounds monastrol and catechin as the first allosteric modulators of GHB high-affinity binding sites. Despite their relatively weak affinity, these compounds may aid in further characterization of the GHB high-affinity sites that are likely to represent certain GABAA receptors.

Modulation of ionotropic GABA receptors by 6-methoxyflavanone and 6-methoxyflavone

We evaluated the effects of 6-methoxyflavanone and 6-methoxyflavone on wild-type α1/α2β2γ2L GABAA and ρ1 GABAC receptors and on mutant ρ1I307S, ρ1W328 M, ρ1I307S/W328 M GABAC receptors expressed in Xenopus oocytes using two-electrode voltage clamp and radioligand binding. 6-Methoxyflavanone and 6-methoxyflavone act as a flumazenil-insensitive positive allosteric modulator of GABA responses at human recombinant α1β2γ2L and α2β2γ2L GABAA receptors. However, unlike 6-methoxyflavone, 6-methoxyflavanone was relatively inactive at α1β2 GABAA receptors. 6-Methoxyflavanone inhibited [(3)H]-flunitrazepam binding to rat brain membranes. Both flavonoids were found to be inactive as modulators at ρ1, ρ1I307S and ρ1W328 M GABA receptors but acted as positive allosteric modulators of GABA at the benzodiazepine sensitive ρ1I307S/W328 M GABA receptors. This double mutant retains ρ1 properties of being insensitive to bicuculline and antagonised by TPMPA and THIP. Additionally, 6-methoxyflavanone was also a partial agonist at ρ1W328 M GABA receptors. The relative inactivity of 6-methoxyflavanone at α1β2 GABAA receptors and it's partial agonist action at ρ1W328 M GABA receptors suggest that it exhibits a unique profile not matched by other flavonoids.

Design, synthesis, and pharmacological characterization of N- and O-substituted 5,6,7,8-tetrahydro-4H-isoxazolo[4,5-d]azepin-3-ol analogues: novel 5-HT(2A)/5-HT(2C) receptor agonists with pro-cognitive properties

The isoxazol-3-one tautomer of the bicyclic isoxazole, 5,6,7,8-tetrahydro-4H-isoxazolo[4,5-d]azepin-3-ol (THAZ), has previously been shown to be a weak GABA(A) and glycine receptor antagonist. In the present study, the potential in this scaffold has been explored through the synthesis and pharmacological characterization of a series of N- and O-substituted THAZ analogues. The analogues N-Bn-THAZ (3d) and O-Bn-THAZ (4d) were found to be potent agonists of the human 5-HT(2A) and 5-HT(2C) receptors. Judging from an elaborate pharmacological profiling at numerous other CNS targets, the 3d analogue appears to be selective for the two receptors. Administration of 3d substantially improved the cognitive performance of mice in a place recognition Y-maze model, an effect fully reversible by coadministration of the selective 5-HT(2C) antagonist SB242084. In conclusion, as novel bioavailable cognitive enhancers that most likely mediate their effects through 5-HT(2A) and/or 5-HT(2C) receptors, the isoxazoles 3d and 4d constitute interesting leads for further medicinal chemistry development.

α4βδ GABA(A) receptors are high-affinity targets for γ-hydroxybutyric acid (GHB)

γ-Hydroxybutyric acid (GHB) binding to brain-specific high-affinity sites is well-established and proposed to explain both physiological and pharmacological actions. However, the mechanistic links between these lines of data are unknown. To identify molecular targets for specific GHB high-affinity binding, we undertook photolinking studies combined with proteomic analyses and identified several GABA(A) receptor subunits as possible candidates. A subsequent functional screening of various recombinant GABA(A) receptors in Xenopus laevis oocytes using the two-electrode voltage clamp technique showed GHB to be a partial agonist at αβδ- but not αβγ-receptors, proving that the δ-subunit is essential for potency and efficacy. GHB showed preference for α4 over α(1,2,6)-subunits and preferably activated α4β1δ (EC(50) = 140 nM) over α4β(2/3)δ (EC(50) = 8.41/1.03 mM). Introduction of a mutation, α4F71L, in α4β1(δ)-receptors completely abolished GHB but not GABA function, indicating nonidentical binding sites. Radioligand binding studies using the specific GHB radioligand [(3)H](E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene)acetic acid showed a 39% reduction (P = 0.0056) in the number of binding sites in α4 KO brain tissue compared with WT controls, corroborating the direct involvement of the α4-subunit in high-affinity GHB binding. Our data link specific GHB forebrain binding sites with α4-containing GABA(A) receptors and postulate a role for extrasynaptic α4δ-containing GABA(A) receptors in GHB pharmacology and physiology. This finding will aid in elucidating the molecular mechanisms behind the proposed function of GHB as a neurotransmitter and its unique therapeutic effects in narcolepsy and alcoholism.