Further optimization of the M5 NAM MLPCN probe ML375: tactics and challenges

Characterization of Selective M5 Acetylcholine Muscarinic Receptor Modulators on Dopamine Signaling in the Striatum

The type-5 muscarinic acetylcholine receptor (mAChR, M5) is almost exclusively expressed in dopamine (DA) neurons of the ventral tegmental area and substantia nigra pars compacta; therefore, they are ideally located to modulate DA signaling and underlying behaviors. However, the role of M5 in shaping DA release is still poorly characterized. In this study, we first quantitatively mapped the expression of M5 in different neurons of the mouse midbrain, then used voltammetry in mouse striatum to evaluate the effect of M5-selective modulators on DA release. The M5 negative allosteric modulator ML375 significantly decreased electrically evoked DA release and blocked the effect of Oxotremorine-M (Oxo-M; nonselective mAChR agonist) on DA release in the presence of an acetylcholine nicotinic receptor blocker. Conversely, the M5 positive allosteric modulator VU 0365114 significantly increased electrically evoked DA release and the Oxo-M effect on DA release. We then assessed M5's impact on mesolimbic circuit function in vivo. Although psychostimulant-induced locomotor activity models in knockout mice have previously been used to characterize the role of M5 in DA transmission, the results of these studies conflict, leading us to select a different in vivo model, namely a cocaine self-administration paradigm. In contrast to a previous study that also used this model, in the current study, administration of ML375 did not decrease cocaine self-administration in rats (using fixed and progressive ratio). These conflicting results illustrate the complexity of M5 modulation and the need to further characterize its involvement in the regulation of dopamine signaling, central to multiple neuropsychiatric diseases. SIGNIFICANCE STATEMENT: This work describes the type-5 muscarinic receptor (M5) pattern of expression within the midbrain as well as its physiological modulation by selective compounds at the axon terminal level in the striatum, where M5 directly shapes dopamine transmission. It offers the first direct readout of mesolimbic dopamine release modulation by M5, highlighting its role in regulating neurocircuits implicated in the pathophysiology of neuropsychiatric disorders such as substance use disorders, major depressive disorder, and schizophrenia.

Ventral Tegmental Area M5 Muscarinic Receptors Mediate Effort-Choice Responding and Nucleus Accumbens Dopamine in a Sex-Specific Manner

Optimization of effort-related choices is impaired in depressive disorders. Acetylcholine (ACh) and dopamine (DA) are linked to depressive disorders, and modulation of ACh tone in the ventral tegmental area (VTA) affects mood-related behavioral responses in rats. However, it is unknown if VTA ACh mediates effort-choice behaviors. Using a task of effort-choice, rats can choose to lever press on a fixed-ratio 5 (FR5) schedule for a more-preferred food or consume freely available, less-preferred food. VTA administration of physostigmine (1 μg and 2 μg/side), a cholinesterase inhibitor, reduced FR5 responding for the more-preferred food while leaving consumption of the less-preferred food intact. VTA infusion of the M5 muscarinic receptor negative allosteric modulator VU6000181 (3 μM, 10 μM, 30 μM/side) did not affect lever pressing or chow consumption. However, VU6000181 (30 μM/side) coadministration with physostigmine (2 μg/side) attenuated physostigmine-induced decrease in lever pressing in female and male rats and significantly elevated lever pressing above vehicle baseline levels in male rats. In in vivo voltammetry experiments, VTA infusion of combined physostigmine and VU6000181 did not significantly alter evoked phasic DA release in the nucleus accumbens core (NAc) in female rats. In male rats, combined VTA infusion of physostigmine and VU6000181 increased phasic evoked DA release in the NAc compared with vehicle, physostigmine, or VU6000181 infusion alone. These data indicate a critical role and potential sex differences of VTA M5 receptors in mediating VTA cholinergic effects on effort choice behavior and regulation of DA release. SIGNIFICANCE STATEMENT: Effort-choice impairments are observed in depressive disorders, which are often treatment resistant to currently available thymoleptics. The role of ventral tegmental area (VTA) acetylcholine muscarinic M5 receptors, in a preclinical model of effort-choice behavior, is examined. Using the selective negative allosteric modulator of the M5 receptor VU6000181, we show the role of VTA M5 receptors on effort-choice and regulation of dopamine release in the nucleus accumbens core. This study supports M5 receptors as therapeutic targets for depression.

Unified Approach to Diverse Fused Fragments via Catalytic Dehydrative Cyclization

A range of highly functionalized polycyclic fragments have been synthesized, employing a catalytic dehydrative cyclization. A range of nucleophiles are shown to be successful, with the reaction producing numerous high value motifs.

Identification of a Novel Allosteric Site at the M5 Muscarinic Acetylcholine Receptor

The M₅ muscarinic acetylcholine receptor (mAChR) has emerged as an exciting therapeutic target for the treatment of addiction and behavioral disorders. This has been in part due to promising preclinical studies with the M₅ mAChR selective negative allosteric modulator (NAM), ML375. The binding site of ML375 remains unknown, however, making it difficult to develop improved M₅ mAChR selective modulators. To determine the possible location of the ML375 binding site, we used radioligand binding and functional assays to show that ML375 does not interact with the well-characterized "common" mAChR allosteric site located in the receptor's extracellular vestibule, nor a previously proposed second allosteric site recognized by the modulator, amiodarone. Molecular docking was used to predict potential allosteric sites within the transmembrane (TM) domain of the M₅ mAChR. These predicted sites were assessed using M₅-M₂ mAChR receptor chimeras and further targeted with site-directed mutagenesis, which enabled the identification of a putative binding site for ML375 at the interface of TMs 2-4. Collectively, these results identify a third allosteric site at the M₅ mAChR and highlight the ability of allosteric modulators to selectively target highly conserved proteins.

Examining the role of muscarinic M5 receptors in VTA cholinergic modulation of depressive-like and anxiety-related behaviors in rats

Acetylcholine is implicated in mood disorders including depression and anxiety. Increased cholinergic tone in humans and rodents produces pro-depressive and anxiogenic-like effects. Cholinergic receptors in the ventral tegmental area (VTA) are known to mediate these responses in male rats, as measured by the sucrose preference test (SPT), elevated plus maze (EPM), and the forced swim test (FST). However, these effects have not been examined in females, and the VTA muscarinic receptor subtype(s) mediating the pro-depressive and anxiogenic-like behavioral effects of increased cholinergic tone are unknown. We first examined the behavioral effects of increased VTA cholinergic tone in male and female rats, and then determined whether VTA muscarinic M5 receptors were mediating these effects. VTA infusion of the acetylcholinesterase inhibitor physostigmine (0.5 μg, 1 μg and 2 μg/side) in males and females produced anhedonic-like, anxiogenic, pro-depressive-like responses on the SPT, EPM, and FST. In females, VTA administration of the muscarinic M5 selective negative allosteric modulator VU6000181 (0.68 ng, 2.3 ng, 6.8 ng/side for a 3 μM, 10 μM, 30 μM/side infusion) did not alter SPT, EPM nor FST behavior. However, in males intra-VTA infusion of VU6000181 alone reduced time spent immobile on the FST. Furthermore, co-infusion of VU6000181 with physostigmine, in male and female rats, attenuated the pro-depressive and anxiogenic-like behavioral responses induced by VTA physostigmine alone, in the SPT, EPM, and FST. Together, these data reveal a critical role of VTA M5 receptors in mediating the anhedonic, anxiogenic, and depressive-like behavioral effects of increased cholinergic tone in the VTA.

An Agonist Dependent Allosteric Antagonist of Prostaglandin EP2 Receptors

All reported prostaglandin EP2 receptor antagonists have a purely orthosteric, competitive mode of action. Herein, we report the characterization of compound 1 (pubchem CID 664888) as the first EP2 antagonist that features a reversible, agonist dependent allosteric mode of action. Compound 1 displayed an unsurmountable inhibition of cAMP accumulation stimulated by different EP2 agonists in C6 glioma cells overexpressing human EP2 (C6G-hEP2). The degree of reduction of agonist potency and efficacy depended on the agonist employed. Negative allosteric modulation was not observed in C6G cells overexpressing human EP4, IP, or DP1 receptors. Moreover, in the murine microglial cell line that stably expresses human EP2 receptors (BV2-hEP2), compound 1 reduced the EP2 agonist-induced elevation of interleukin 6 (IL-6), IL-1β, and hEP2 mRNA levels and increased that of tumor necrosis factor (TNF)-α. Compound 1 was docked into a homology model of hEP2. The predicted binding site on the cytoplasmic receptor surface was similar to that of allosteric inhibitors of the β2-adrenergic, CC chemokine receptor 9 (CCR9), and CC chemokine receptor 2 (CCR2) receptors, which supports the notion of a conserved G-protein-coupled receptor (GPCR) binding pocket for allosteric inhibitors. As the first agonist dependent negative allosteric modulator of EP2 receptor, the structure of this compound may provide a basis for developing improved allosteric modulators of EP2 receptors.

Allosteric Modulation of Class A GPCRs: Targets, Agents, and Emerging Concepts

G-protein-coupled receptors (GPCRs) have been tractable drug targets for decades with over one-third of currently marketed drugs targeting GPCRs. Of these, the class A GPCR superfamily is highly represented, and continued drug discovery for this family of receptors may provide novel therapeutics for a vast range of diseases. GPCR allosteric modulation is an innovative targeting approach that broadens the available small molecule toolbox and is proving to be a viable drug discovery strategy, as evidenced by recent FDA approvals and clinical trials. Numerous class A GPCR allosteric modulators have been discovered recently, and emerging trends such as the availability of GPCR crystal structures, diverse functional assays, and structure-based computational approaches are improving optimization and development. This Perspective provides an update on allosterically targeted class A GPCRs and their disease indications and the medicinal chemistry approaches toward novel allosteric modulators and highlights emerging trends and opportunities in the field.

1-Aminobenzotriazole: A Mechanism-Based Cytochrome P450 Inhibitor and Probe of Cytochrome P450 Biology

1-Aminobenzotriazole (1-ABT) is a pan-specific, mechanism-based inactivator of the xenobiotic metabolizing forms of cytochrome P450 in animals, plants, insects, and microorganisms. It has been widely used to investigate the biological roles of cytochrome P450 enzymes, their participation in the metabolism of both endobiotics and xenobiotics, and their contributions to the metabolism-dependent toxicity of drugs and chemicals. This review is a comprehensive evaluation of the chemistry, discovery, and use of 1-aminobenzotriazole in these contexts from its introduction in 1981 to the present.

Allosteric modulators targeting CNS muscarinic receptors

Muscarinic acetylcholine receptors are G protein-coupled receptors (GPCRs) which are broadly expressed in the central nervous system (CNS) and other tissues in the periphery. They emerge as important drug targets for a number of diseases including Alzheimer's disease, Parkinson's disease, and schizophrenia. Muscarinic receptors are divided into five subtypes (M₁-M₅) of which M₁-M₄ have been crystalized. All subtypes possess at least one allosteric binding site which is located in the extracellular region of the receptor on top of the ACh (i.e. orthosteric) binding site. The former can be specifically targeted by chemical compounds (mostly small molecules) and binding of such allosteric modulators affects the affinity and/or efficacy of orthosteric ligands. This allows highly specific modulation of GPCR function and, from a drug discovery point of view, may be advantageous in terms of subtype selectivity and biased signaling. There is a plethora of allosteric modulators for all five muscarinic receptor subtypes. This review presents the basic principles of allosteric modulation of GPCRs on both the molecular and structural level focusing on allosteric modulators of the muscarinic receptor family. Further we discuss dualsteric (i.e. bitopic orthosteric/allosteric) ligands emphasizing their potential in modulating muscarinic receptor dynamics and signaling. The common mechanisms of muscarinic receptor allosteric modulation have been proven to be generalizable and are at play at many, if not all GPCRs. Given this paradigmatic role of muscarinic receptors we suggest that also new developments in muscarinic allosteric modulation may also be extended to other members of the GPCR superfamily. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Discovery and optimization of 3-(4-aryl/heteroarylsulfonyl)piperazin-1-yl)-6-(piperidin-1-yl)pyridazines as novel, CNS penetrant pan-muscarinic antagonists

This letter describes the synthesis and structure activity relationship (SAR) studies of structurally novel M4 antagonists, based on a 3-(4-aryl/heteroarylsulfonyl)piperazin-1-yl)-6-(piperidin-1-yl)pyridazine core, identified from a high-throughput screening campaign. A multi-dimensional optimization effort enhanced potency at human M4 (hM4 IC50s<200nM), with only moderate species differences noted, and with enantioselective inhibition. Moreover, CNS penetration proved attractive for this series (rat brain:plasma Kp=2.1, Kp,uu=1.1). Despite the absence of the prototypical mAChR antagonist basic or quaternary amine moiety, this series displayed pan-muscarinic antagonist activity across M1-5 (with 9- to 16-fold functional selectivity at best). This series further expands the chemical diversity of mAChR antagonists.

Synthesis and evaluation of 4,6-disubstituted pyrimidines as CNS penetrant pan-muscarinic antagonists with a novel chemotype

This letter describes the synthesis and structure activity relationship (SAR) studies of structurally novel M₄ antagonists, based on a 4,6-disubstituted core, identified from a high-throughput screening campaign. A multi-dimensional optimization effort enhanced potency at both human and rat M₄ (IC₅₀s<300nM), with no substantial species differences noted. Moreover, CNS penetration proved attractive for this series (brain:plasma K_p,uu=0.87), while other DMPK attributes were addressed in the course of the optimization effort, providing low in vivo clearance in rat (CL_p=5.37mL/min/kg). Surprisingly, this series displayed pan-muscarinic antagonist activity across M_1-5, despite the absence of the prototypical basic or quaternary amine moiety, thus offering a new chemotype from which to develop a next generation of pan-muscarinic antagonist agents.

Continued optimization of the M5 NAM ML375: Discovery of VU6008667, an M5 NAM with high CNS penetration and a desired short half-life in rat for addiction studies

This letter describes the continued optimization of M₅ NAM ML375 (VU0483253). While a valuable in vivo tool compound, ML375has an excessively long elimination half-life in rat (t_1/2=80h), which can be problematic in certain rodent addiction paradigms (e.g., reinstatement). Thus, we required an M₅ NAM of comparable potency to ML375, but with a rat t_1/2 of less than 4h. Steep SAR plagued this chemotype, and here we detail aniline replacements that offered some improvements over ML375, but failed to advance. Ultimately, incorporation of a single methyl group to the 9b-phenyl ring acted as a metabolic shunt, providing (S)-11 (VU6008667), an equipotent M₅ NAM, with high CNS penetration, excellent selectivity versus M_1-4 and the desired short half-life (t_1/2=2.3h) in rat.

Ligand-based virtual screen for the discovery of novel M5 inhibitor chemotypes

This Letter describes a ligand-based virtual screening campaign utilizing SAR data around the M5 NAMs, ML375 and VU6000181. Both QSAR and shape scores were employed to virtually screen a 98,000-member compound library. Neither approach alone proved productive, but a consensus score of the two models identified a novel scaffold which proved to be a modestly selective, but weak inhibitor (VU0549108) of the M5 mAChR (M5 IC50=6.2μM, M1-4 IC50s>10μM) based on an unusual 8-((1,3,5-trimethyl-1H-pyrazol-4-yl)sulfonyl)-1-oxa-4-thia-8-azaspiro[4,5]decane scaffold. [(3)H]-NMS binding studies showed that VU0549108 interacts with the orthosteric site (Ki of 2.7μM), but it is not clear if this is negative cooperativity or orthosteric binding. Interestingly, analogs synthesized around VU0549108 proved weak, and SAR was very steep. However, this campaign validated the approach and warranted further expansion to identify additional novel chemotypes.

Practical Strategies and Concepts in GPCR Allosteric Modulator Discovery: Recent Advances with Metabotropic Glutamate Receptors

Allosteric modulation of GPCRs has initiated a new era of basic and translational discovery, filled with therapeutic promise yet fraught with caveats. Allosteric ligands stabilize unique conformations of the GPCR that afford fundamentally new receptors, capable of novel pharmacology, unprecedented subtype selectivity, and unique signal bias. This review provides a comprehensive overview of the basics of GPCR allosteric pharmacology, medicinal chemistry, drug metabolism, and validated approaches to address each of the major challenges and caveats. Then, the review narrows focus to highlight recent advances in the discovery of allosteric ligands for metabotropic glutamate receptor subtypes 1-5 and 7 (mGlu1-5,7) highlighting key concepts ("molecular switches", signal bias, heterodimers) and practical solutions to enable the development of tool compounds and clinical candidates. The review closes with a section on late-breaking new advances with allosteric ligands for other GPCRs and emerging data for endogenous allosteric modulators.