Functional analysis of a novel positive allosteric modulator of AMPA receptors derived from a structure-based drug design strategy

The [3+2] annulation of in situ formed trifluorodiazoethane with 4-nitroisoxazoles: access to trifluoromethylpyrazolo[3,4-d]isoxazoles

A general and practical K2CO3-promoted [3+2] annulation between trifluoroacetaldehyde N-triftosylhydrazone (TFHZ-Tfs) and 4-nitroisoxazoles has been developed to access structurally diverse trifluoromethylpyrazolo[3,4-d]isoxazoles. Mechanistically, CF3CHN2 is formed in situ by TFHZ-Tfs decomposition and involves a formal [3+2] dearomative cycloaddition with 4-nitroisoxazoles, followed by aromatization via the elimination of the nitro group. Furthermore, this protocol is metal-free, scalable, mild, and operationally safe, with a broad substrate scope and high functional group compatibility.

AMPA receptor neurotransmission and therapeutic applications: A comprehensive review of their multifaceted modulation

The neuropharmacological community has shown a strong interest in AMPA receptors as critical components of excitatory synaptic transmission during the last fifteen years. AMPA receptors, members of the ionotropic glutamate receptor family, allow rapid excitatory neurotransmission in the brain. AMPA receptors, which are permeable to sodium and potassium ions, manage the bulk of the brain's rapid synaptic communications. This study thoroughly examines the recent developments in AMPA receptor regulation, focusing on a shift from single chemical illustrations to a more extensive investigation of underlying processes. The complex interplay of these modulators in modifying the function and structure of AMPA receptors is the main focus, providing insight into their influence on the speed of excitatory neurotransmission. This research emphasizes the potential of AMPA receptor modulation as a therapy for various neurological disorders such as epilepsy and Alzheimer's disease. Analyzing these regulators' sophisticated molecular details enhances our comprehension of neuropharmacology, representing a significant advancement in using AMPA receptors for treating intricate neurological conditions.

5-Nitroisoxazoles in SNAr Reactions: A Novel Chemo- and Regioselective Approach to Isoxazole-Based Bivalent Ligands of AMPA Receptors

An efficient regioselective approach to novel functionalized bis(isoxazoles) with a variety of aromatic and aliphatic linkers was elaborated, based on the heterocyclization reaction of electrophilic alkenes under the treatment with tetranitromethane-triethylamine complex affording 3-EWG-5-nitroisoxazoles. The subsequent SNAr reactions of 5-nitroisoxazoles with various O,O-, N,N- and S,S-bis(nucleophiles) provide a wide range of bis(isoxazole) derivatives in good isolated yields. Employing an elaborated method, a series of novel bis(3-EWG-isoxazoles) as the promising allosteric modulators of AMPA receptors were designed and synthesized. The effect of the compounds on the kainate-induced currents was studied in the patch clamp experiments, revealing modulator properties for several of them. The best positive modulator potency was found for dimethyl 5,5'-(ethane-1,2-diylbis(sulfanediyl))bis(isoxazole-3-carboxylate), which potentiated the kainate-induced currents in a wide concentration range (10-12-10-6 M) with maximum potentiation of 77% at 10-10 M. The results were rationalized using molecular docking and molecular dynamics simulations of modulator complexes with the dimeric ligand-binding domain of the GluA2 AMPA receptor. The predicted physicochemical, ADMET, and PAINS properties confirmed that the AMPA receptor modulators based on the bis(isoxazole) scaffold may serve as potential lead compounds for the development of neuroprotective drugs.

Diversity of AMPA Receptor Ligands: Chemotypes, Binding Modes, Mechanisms of Action, and Therapeutic Effects

L-Glutamic acid is the main excitatory neurotransmitter in the central nervous system (CNS). Its associated receptors localized on neuronal and non-neuronal cells mediate rapid excitatory synaptic transmission in the CNS and regulate a wide range of processes in the brain, spinal cord, retina, and peripheral nervous system. In particular, the glutamate receptors selective to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) also play an important role in numerous neurological disorders and attract close attention as targets for the creation of new classes of drugs for the treatment or substantial correction of a number of serious neurodegenerative and neuropsychiatric diseases. For this reason, the search for various types of AMPA receptor ligands and studies of their properties are attracting considerable attention both in academic institutions and in pharmaceutical companies around the world. This review focuses mainly on the advances in this area published since 2017. Particular attention is paid to the structural diversity of new chemotypes of agonists, competitive AMPA receptor antagonists, positive and negative allosteric modulators, transmembrane AMPA regulatory protein (TARP) dependent allosteric modulators, ion channel blockers as well as their binding sites. This review also presents the studies of the mechanisms of action of AMPA receptor ligands that mediate their therapeutic effects.

Novel Nanomolar Allosteric Modulators of AMPA Receptor of Bis(pyrimidine) Series: Synthesis, Biotesting and SAR Analysis

Positive allosteric modulators (PAMs) of AMPA receptors represent attractive candidates for the development of drugs for the treatment of cognitive and neurodegenerative disorders. Dimeric molecules have been reported to have an especially potent modulating effect, due to the U-shaped form of the AMPA receptor's allosteric binding site. In the present work, novel bis(pyrimidines) were studied as AMPA receptor modulators. A convenient and flexible preparative approach to bis(pyrimidines) containing a hydroquinone linker was elaborated, and a series of derivatives with varied substituents was obtained. The compounds were examined in the patch clamp experiments for their influence on the kainate-induced currents, and 10 of them were found to have potentiating properties. The best potency was found for 2-methyl-4-(4-((2-methyl-5,6,7,8-tetrahydroquinazolin-4-yl)oxy)phenoxy)-6,7,8,9-tetrahydro-5H-cyclohepta[d]pyrimidine, which potentiated the kainate-induced currents by up to 77% in all tested concentrations (10^-12-10^-6 M). The results were rationalized via the modeling of modulator complexes with the dimeric ligand binding domain of the GluA2 AMPA receptor, using molecular docking and molecular dynamics simulation. The prediction of ADMET, physicochemical, and PAINS properties of the studied bis(pyrimidines) confirmed that PAMs of this type may act as the potential lead compounds for the development of neuroprotective drugs.

A Facile Approach to Bis(isoxazoles), Promising Ligands of the AMPA Receptor

A convenient synthetic approach to novel functionalized bis(isoxazoles), the promising bivalent ligands of the AMPA receptor, was elaborated. It was based on the heterocyclization reactions of readily available electrophilic alkenes with the tetranitromethane-triethylamine complex. The structural diversity of the synthesized compounds was demonstrated. In the electrophysiological experiments using the patch clamp technique on Purkinje neurons, the compound 1,4-phenylenedi(methylene)bis(5-aminoisoxazole-3-carboxylate) was shown to be highly potent positive modulator of the AMPA receptor, potentiating kainate-induced currents up to 70% at 10⁻¹¹ M.

Structural basis for positive allosteric modulation of AMPA and kainate receptors

This paper summarizes the present knowledge on how positive allosteric modulators (PAMs) interact with the ligand-binding domain (LBD) of AMPA and kainate receptors, based on structure determinations. AMPA and kainate receptors belong to the family of ionotropic glutamate receptors that are responsible for mediating the majority of fast excitatory neurotransmission. These receptors have been related to brain disorders, e.g. Alzheimer's disease and attention deficit hyperactivity disorder. PAMs are small molecules that potentiate AMPA and kainate receptor currents by interfering with receptor desensitization. Therefore, PAMs are considered to be of interest for the development of pharmacological tools. Whereas PAMs for AMPA receptors have been known for several years, only recently have PAMs for kainate receptors been reported. Today, >80 structures are available for AMPA receptors with PAMs. These PAMs bind at the interface between two LBD subunits in the vicinity of residue 775, which is important for functional differences between flip and flop isoforms of AMPA receptors. PAMs can be divided into five classes based on their binding mode. The most potent PAM reported to date belongs to class 3, which comprises dimerized PAMs. Three structures of the kainate receptor GluK1 were determined with PAMs belonging to class 2. One PAM enhances kainate receptor currents 5- to 59-fold but shows 100-fold lower potency compared to AMPA receptors. Selective PAMs for kainate receptors will be of great use as pharmacological tools for functional investigations in vivo and might potentially prove useful as drugs in controlling the activity of neuronal networks.

Novel Positive Allosteric Modulators of AMPA Receptors Based on 3,7-Diazabicyclo[3.3.1]nonane Scaffold

A series of new positive allosteric modulators (PAMs) of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors based on 3,7-diazabicyclo[3.3.1]nonane scaffold have been designed, synthesized, and analyzed. In electrophysiological patch clamp studies, several compounds have demonstrated a sub-nanomolar potency. Compound 4 in in vivo tests showed anti-amnestic properties in the scopolamine-induced model of amnesia in the step-through passive avoidance or maximal electroshock experiments in rats at 0.01 mg/kg showing a significant "dose-response" advantage over memantine. Based on the analysis of the flexible docking results of PAMs, the cyclothiazide-like mechanism of binding mode was suggested as the major site for the interaction with AMPA receptors.

Disruption of the GluA2/GAPDH complex using TAT-GluA2NT1-3-2 peptide protects against AMPAR-mediated excitotoxicity after epilepsy

Excitotoxicity and neuronal death following epilepsy involve α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). It forms a protein complex with glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and co-internalizes upon activation of AMPA receptors after epilepsy. Disruption of the GluA2/GAPDH complex with an interfering peptide, TAT-GluA2NT1-3-2, protects cells against AMPAR-mediated excitotoxicity, which have been identified in in-vitro and in-vivo models of brain ischemia. We postulated that disruption of the GluA2/GAPDH interaction with the TAT-GluA2NT1-3-2 peptide would also protect against AMPAR-induced neuronal injury in an in-vivo model of status epilepticus (SE). In the present study, we divided pilocarpine-induced SE Wistar rats into three main groups: the TAT-GluA2NT1-3-2 peptide group, the TAT-GluA2NT-scram peptide group, and the normal saline group, and injected different doses of peptides stereotaxically into the hippocampus of SE rats to investigate whether the GluA2/GAPDH interaction could be disrupted by our TAT-GluA2NT1-3-2 peptide and determine its most appropriate dose. Then, the dose was administered stereotaxically at different time points after SE to determine the best administration time of neuronal protection. We found that the TAT-GluA2NT1-3-2 peptide can disrupt the GluA2/GAPDH interaction and protects against epilepsy-induced neuronal damage. The GluA2/GAPDH interaction may be a novel therapeutic target for epilepsy.

The Impact and Mechanism of a Novel Allosteric AMPA Receptor Modulator LCX001 on Protection Against Respiratory Depression in Rodents

Analgesics and sedative hypnotics in clinical use often give rise to significant side effects, particularly respiratory depression. For emergency use, specific antagonists are currently administered to counteract respiratory depression. However, antagonists are often short-lasting and eliminate drug generated analgesia. To resolve this issue, novel positive AMPA modulators, LCX001, was tested to alleviate respiratory depression triggered by different drugs. The acetic acid writhing and hot-plate test were conducted to evaluate analgesic effect of LCX001. Binding assay, whole-cell recording, live cell imaging, and Ca²⁺ imaging were used to clarify mechanism and impact of LCX001 on respiratory protection. Results showed that LCX001 effectively rescued and prevented opioid (fentanyl and TH-030418), propofol, and pentobarbital-induced respiratory depression by strengthening respiratory frequency and minute ventilation. The acetic acid writhing test and hot-plate test revealed potent anti-nociceptive efficacy of LCX001, in contrast to other typical ampakines that did not affect analgesia. Furthermore, LCX001 potentiated [³H]AMPA and L-glutamate binding affinity to AMPA receptors, and facilitated glutamate-evoked inward currents in HEK293 cells stably expressing GluA2(R). LCX001 had a typical positive modulatory impact on AMPAR-mediated function. Importantly, application of LCX001 generated a significant increase in GluA2(R) surface expression, and restrained opioid-induced abnormal intracellular Ca²⁺ load, which might participate in breathing modulation. Our study improves therapeutic interventions for the treatment of drug induced respiratory depression, and increases understanding of potential mechanism of AMPA receptor modulators.

Pharmacological characterisation of S 47445, a novel positive allosteric modulator of AMPA receptors

S 47445 is a novel positive allosteric modulator of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors (AMPA-PAM). S 47445 enhanced glutamate’s action at AMPA receptors on human and rat receptors and was inactive at NMDA and kainate receptors. Potentiation did not differ among the different AMPA receptors subtypes (GluA1/2/4 flip and flop variants) (EC₅₀ between 2.5–5.4 μM), except a higher EC₅₀ value for GluA4 flop (0.7 μM) and a greater amount of potentiation on GluA1 flop. A low concentration of S 47445 (0.1 μM) decreased receptor response decay time of GluA1flop/GluA2flip AMPA receptors and increased the sensitivity to glutamate. Furthermore, S 47445 (0.1 and 0.3 μM) in presence of repetitive glutamate pulses induced a progressive potentiation of the glutamate-evoked currents from the second pulse of glutamate confirming a rapid-enhancing effect of S 47445 at low concentrations. The potentiating effect of S 47445 (1 μM) was concentration-dependently reversed by the selective AMPA receptor antagonist GYKI52466 demonstrating the selective modulatory effect of S 47445 on AMPA receptors. Using an AMPA-kainate chimera approach, it was confirmed that S 47445 binds to the common binding pocket of AMPA-PAMs. S 47445 did not demonstrate neurotoxic effect against glutamate-mediated excitotoxicity in vitro, in contrast significantly protected rat cortical neurons at 10 μM. S 47445 was shown to improve both episodic and spatial working memory in adult rodents at 0.3 mg/kg, as measured in the natural forgetting condition of object recognition and T-maze tasks. Finally, no deleterious effect on spontaneous locomotion and general behavior was observed up to 1000 mg/kg of S 47445 given acutely in rodents, neither occurrence of convulsion or tremors. Collectively, these results indicate that S 47445 is a potent and selective AMPA-PAM presenting procognitive and potential neuroprotective properties. This drug is currently evaluated in clinical phase 2 studies in Alzheimer’s disease and in Major Depressive Disorder.

Beyond Membrane Protein Structure: Drug Discovery, Dynamics and Difficulties

Most of the previous content of this book has focused on obtaining the structures of membrane proteins. In this chapter we explore how those structures can be further used in two key ways. The first is their use in structure based drug design (SBDD) and the second is how they can be used to extend our understanding of their functional activity via the use of molecular dynamics. Both aspects now heavily rely on computations. This area is vast, and alas, too large to consider in depth in a single book chapter. Thus where appropriate we have referred the reader to recent reviews for deeper assessment of the field. We discuss progress via the use of examples from two main drug target areas; G-protein coupled receptors (GPCRs) and ion channels. We end with a discussion of some of the main challenges in the area.

The discovery and characterization of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor potentiator N-{(3S,4S)-4-[4-(5-cyano-2-thienyl)phenoxy]tetrahydrofuran-3-yl}propane-2-sulfonamide (PF-04958242)

A unique tetrahydrofuran ether class of highly potent α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor potentiators has been identified using rational and structure-based drug design. An acyclic lead compound, containing an ether-linked isopropylsulfonamide and biphenyl group, was pharmacologically augmented by converting it to a conformationally constrained tetrahydrofuran to improve key interactions with the human GluA2 ligand-binding domain. Subsequent replacement of the distal phenyl motif with 2-cyanothiophene to enhance its potency, selectivity, and metabolic stability afforded N-{(3S,4S)-4-[4-(5-cyano-2-thienyl)phenoxy]tetrahydrofuran-3-yl}propane-2-sulfonamide (PF-04958242, 3), whose preclinical characterization suggests an adequate therapeutic index, aided by low projected human oral pharmacokinetic variability, for clinical studies exploring its ability to attenuate cognitive deficits in patients with schizophrenia.

Rational Design of a Novel AMPA Receptor Modulator through a Hybridization Approach

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are a family of glutamate ion channels of considerable interest in excitatory neurotransmission and associated disease processes. Here, we demonstrate how exploitation of the available X-ray crystal structure of the receptor ligand binding domain enabled the development of a new class of AMPA receptor positive allosteric modulators (7) through hybridization of known ligands (5 and 6), leading to a novel chemotype with promising pharmacological properties.

The multifaceted subunit interfaces of ionotropic glutamate receptors

The past fifteen years has seen a revolution in our understanding of ionotropic glutamate receptor (iGluR) structure, starting with the first view of the ligand binding domain (LBD) published in 1998, and in many ways culminating in the publication of the full-length structure of GluA2 in 2009. These reports have revealed not only the central role played by subunit interfaces in iGluR function, but also myriad binding sites within interfaces for endogenous and exogenous factors. Changes in the conformation of inter-subunit interfaces are central to transmission of ligand gating into pore opening (itself a rearrangement of interfaces), and subsequent closure through desensitization. With the exception of the agonist binding site, which is located entirely within individual subunits, almost all modulatory factors affecting iGluRs appear to bind to sites in subunit interfaces. This review seeks to summarize what we currently understand about the diverse roles interfaces play in iGluR function, and to highlight questions for future research.

Functional insight into development of positive allosteric modulators of AMPA receptors

Positive allosteric modulators of α-amino-3-hydroxy-5-methyl-isoxazole-propionic acid (AMPA) ionotropic glutamate receptors facilitate synaptic plasticity and contribute essentially to learning and memory, properties which make AMPA receptors targets for drug discovery and development. One region at which several different classes of positive allosteric modulators bind lies at the dimer interface between the ligand-binding core of the second, membrane-proximal, extracellular domain of AMPA receptors. This solvent-accessible binding pocket has been the target of drug discovery efforts, leading to the recent delineation of five "subsites" which differentially allow access to modulator moieties, and for which distinct modulator affinities and apparent efficacies are attributed. Here we use the voltage-clamp technique in conjunction with rapid drug application to study the effects of mutants lining subsites "A" and "B" of the allosteric modulator pocket to assess affinity and efficacy of allosteric modulation by cyclothiazide, CX614, CMPDA and CMPDB. A novel analysis of the decay of current produced by the onset of desensitization has allowed us to estimate both affinity and efficacy from single concentrations of modulator. Such an approach may be useful for effective high throughput screening of new target compounds.

Hitting the target: fragment screening with acoustic in situ co-crystallization of proteins plus fragment libraries on pin-mounted data-collection micromeshes

Acoustic droplet ejection (ADE) is a powerful technology that supports crystallographic applications such as growing, improving and manipulating protein crystals. A fragment-screening strategy is described that uses ADE to co-crystallize proteins with fragment libraries directly on MiTeGen MicroMeshes. Co-crystallization trials can be prepared rapidly and economically. The high speed of specimen preparation and the low consumption of fragment and protein allow the use of individual rather than pooled fragments. The Echo 550 liquid-handling instrument (Labcyte Inc., Sunnyvale, California, USA) generates droplets with accurate trajectories, which allows multiple co-crystallization experiments to be discretely positioned on a single data-collection micromesh. This accuracy also allows all components to be transferred through small apertures. Consequently, the crystallization tray is in equilibrium with the reservoir before, during and after the transfer of protein, precipitant and fragment to the micromesh on which crystallization will occur. This strict control of the specimen environment means that the crystallography experiments remain identical as the working volumes are decreased from the few microlitres level to the few nanolitres level. Using this system, lysozyme, thermolysin, trypsin and stachydrine demethylase crystals were co-crystallized with a small 33-compound mini-library to search for fragment hits. This technology pushes towards a much faster, more automated and more flexible strategy for structure-based drug discovery using as little as 2.5 nl of each major component.

A charge-inverting mutation in the "linker" region of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors alters agonist binding and gating kinetics independently of allosteric modulators

AMPA receptors are gated through binding of glutamate to a solvent-accessible ligand-binding domain. Upon glutamate binding, these receptors undergo a series of conformational rearrangements regulating channel function. Allosteric modulators can bind within a pocket adjacent to the ligand-binding domain to stabilize specific conformations and prevent desensitization. Yelshansky et al. (Yelshansky, M. V., Sobolevsky, A. I., Jatzke, C., and Wollmuth, L. P. (2004) J. Neurosci. 24, 4728-4736) described a model of an electrostatic interaction between the ligand-binding domain and linker region to the pore that regulated channel desensitization. To test this hypothesis, we have conducted a series of experiments focusing on the R628E mutation. Using ultrafast perfusion with voltage clamp, we applied glutamate to outside-out patches pulled from transiently transfected HEK 293 cells expressing wild type or R628E mutant GluA2. In response to a brief pulse of glutamate (1 ms), mutant receptors deactivated with significantly slower kinetics than wild type receptors. In addition, R628E receptors showed significantly more steady-state current in response to a prolonged (500-ms) glutamate application. These changes in receptor kinetics occur through a pathway that is independent of that of allosteric modulators, which show an additive effect on R628E receptors. In addition, ligand binding assays revealed the R628E mutation to have increased affinity for agonist. Finally, we reconciled experimental data with computer simulations that explicitly model mutant and modulator interactions. Our data suggest that R628E stabilizes the receptor closed cleft conformation by reducing agonist dissociation and the transition to the desensitized state. These results suggest that the AMPA receptor external vestibule is a viable target for new positive allosteric modulators.

Targeting α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors in epilepsy

INTRODUCTION

Despite epilepsies being between the oldest and most studied neurological diseases, new treatment remains an unmet need of scientific research due to the high percentage of refractory patients. Several studies have identified new suitable anti-seizure targets. Glutamate activation of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) have long ago been identified as suitable targets for the development of anti seizure drugs.

AREAS COVERED

Here, we describe: i) AMPARs' structure and their involvement and role during seizures and in epilepsy and ii) the efficacy of AMPAR antagonists in preclinical models of seizures and epilepsy.

EXPERT OPINION

The physiological and pathological role of AMPAR in the CNS and the development of AMPAR antagonists have recently gained attention considering their recent involvement in status epilepticus and the marketing of perampanel. The need for new anti-seizure drugs represents a major challenge in both preclinical and clinical epilepsy. The introduction into the market of perampanel for the treatment of epilepsy will shed new light on the real potential of AMPAR antagonism in clinical settings outside the limited world of clinical trials. While research will go on in this area, fundamental will be the post-marketing evaluation of perampanel efficacy and tolerability and a better definition of the role of this receptor in the epileptic brain.

Positive allosteric modulation of AMPA receptors from efficacy to toxicity: the interspecies exposure-response continuum of the novel potentiator PF-4778574

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) positive allosteric modulation (i.e., "potentiation") has been proposed to overcome cognitive impairments in schizophrenia, but AMPAR overstimulation can be excitotoxic. Thus, it is critical to define carefully a potentiator's mechanism-based therapeutic index (TI) and to determine confidently its translatability from rodents to higher-order species. Accordingly, the novel AMPAR potentiator N-{(3R,4S)-3-[4-(5-cyano-2-thienyl)phenyl]tetrahydro-2H-pyran-4-yl}propane-2-sulfonamide (PF-4778574) was characterized in a series of in vitro assays and single-dose animal studies evaluating AMPAR-mediated activities related to cognition and safety to afford an unbound brain compound concentration (Cb,u)-normalized interspecies exposure-response relationship. Because it is unknown which AMPAR subtype(s) may be selectively potentiated for an optimal TI, PF-4778574 binding affinity and functional potency were determined in rodent tissues expected to express a native mixture of AMPAR subunits and their associated proteins to afford composite pharmacological values. Functional activity was also quantified in recombinant cell lines stably expressing human GluA2 flip or flop homotetramers. Procognitive effects of PF-4778574 were evaluated in both rat electrophysiological and nonhuman primate (nhp) behavioral models of pharmacologically induced N-methyl-d-aspartate receptor hypofunction. Safety studies assessed cerebellum-based AMPAR activation (mouse) and motor coordination disruptions (mouse, dog, and nhp), as well as convulsion (mouse, rat, and dog). The resulting empirically derived exposure-response continuum for PF-4778574 defines a single-dose-based TI of 8- to 16-fold for self-limiting tremor, a readily monitorable clinical adverse event. Importantly, the Cb,u mediating each physiological effect were highly consistent across species, with efficacy and convulsion occurring at just fractions of the in vitro-derived pharmacological values.

Use of allosteric targets in the discovery of safer drugs

The need for drugs with fewer side effects cannot be overemphasized. Today, most drugs modify the actions of enzymes, receptors, transporters and other molecules by directly binding to their active (orthosteric) sites. However, orthosteric site configuration is similar in several proteins performing related functions and this leads to a lower specificity of a drug for the desired protein. Consequently, such drugs may have adverse side effects. A new basis of drug discovery is emerging based on the binding of the drug molecules to sites away (allosteric) from the orthosteric sites. It is possible to find allosteric sites which are unique and hence more specific as targets for drug discovery. Of many available examples, two are highlighted here. The first is caloxins - a new class of highly specific inhibitors of plasma membrane Ca²⁺ pumps. The second concerns the modulation of receptors for the neurotransmitter acetylcholine, which binds to 12 types of receptors. Exploitation of allosteric sites has led to the discovery of drugs which can selectively modulate the activation of only 1 (M1 muscarinic) out of the 12 different types of acetylcholine receptors. These drugs are being tested for schizophrenia treatment. It is anticipated that the drug discovery exploiting allosteric sites will lead to more effective therapeutic agents with fewer side effects.