Structural determinants for activity of the antidepressant vortioxetine at human and rodent 5-HT3 receptors

Vortioxetine (VTX) is a recently approved antidepressant that targets a variety of serotonin receptors. Here, we investigate the drug's molecular mechanism of operation at the serotonin 5-HT3 receptor (5-HT3R), which features two properties: VTX acts differently on rodent and human 5-HT3R, and VTX appears to suppress any subsequent response to agonists. Using a combination of cryo-EM, electrophysiology, voltage-clamp fluorometry and molecular dynamics, we show that VTX stabilizes a resting inhibited state of the mouse 5-HT3R and an agonist-bound-like state of human 5-HT3R, in line with the functional profile of the drug. We report four human 5-HT3R structures and show that the human receptor transmembrane domain is intrinsically fragile. We also explain the lack of recovery after VTX administration via a membrane partition mechanism.

Clinical benefits of vortioxetine 20 mg/day in patients with major depressive disorder

BACKGROUND

Vortioxetine has demonstrated dose-dependent efficacy in patients with major depressive disorder (MDD), with the greatest effect observed with vortioxetine 20 mg/day. This analysis further explored the clinical relevance of the more rapid and greater improvement in depressive symptoms observed with vortioxetine 20 mg/day vs 10 mg/day.

METHODS

Analysis of pooled data from six short-term (8-week), randomized, placebo-controlled, fixed-dose studies of vortioxetine 20 mg/day in patients with MDD (N = 2620). Symptomatic response (≥50% decrease in Montgomery-Åsberg Depression Rating Scale [MADRS] total score), sustained symptomatic response, and remission (MADRS total score ≤10) were assessed by vortioxetine dosage (20 or 10 mg/day).

RESULTS

After 8 weeks, 51.4% of patients receiving vortioxetine 20 mg/day had achieved symptomatic response vs 46.0% of those receiving vortioxetine 10 mg/day (P < .05). Significantly more patients achieved symptomatic response vs placebo from week 2 onwards for vortioxetine 20 mg/day and from week 6 onwards for vortioxetine 10 mg/day (both P ≤ .05). Sustained response was achieved from week 4 for 26.0% of patients receiving vortioxetine 20 mg/day vs 19.1% of those receiving vortioxetine 10 mg/day (P < .01), increasing to 36.0% and 29.8%, respectively, over the 8-week treatment period (P < .05). At week 8, 32.0% of patients receiving vortioxetine 20 mg/day were in remission vs 28.2% of those receiving vortioxetine 10 mg/day (P = .09). Rates of adverse events and treatment withdrawal were not increased during the week following vortioxetine dose up-titration to 20 mg/day.

CONCLUSION

Vortioxetine 20 mg/day provides more rapid and more sustained symptomatic response than vortioxetine 10 mg/day in patients with MDD, without compromising tolerability.

Fluoxetine increased adult neurogenesis is mediated by 5-HT3 receptor

Adult neurogenesis is an aspect of structural plasticity that remains active during adulthood in some brain regions. One of them is the subgranular zone (SGZ) of the dentate gyrus of the hippocampus. Adult neurogenesis is reduced by different factors and in disorders of the CNS, including major depression. Antidepressant treatments, such as chronic fluoxetine administration, recover the normal level of adult neurogenesis. Fluoxetine treatment increases the free concentration of the neurotransmitter serotonin and this monoamine is implicated in the regulation of the neurogenic process; however, the target of the action of this neurotransmitter has not been fully elucidated. In this study, we have tried to determine the relevance of the serotonin receptor 3 (5-HT3) in the hippocampal neurogenesis of adult rats. We have used fluorescent immunohistochemistry to study the expression of the 5-HT3 receptor in different neurogenesis stages in the SGZ, identifying its expression in stem cells, amplifying neural progenitors and immature neurons. Moreover, we have studied the impact of a 5-HT3 antagonist (ondansetron) in the fluoxetine-induced adult neurogenesis. We observed that fluoxetine alone increases the number of both proliferating cells (ki67 positive) and immature neurons (DCX positive) in the SGZ. By contrast, co-treatment with ondansetron blocked the increase in proliferation and neurogenesis. This study demonstrates that the activation of 5-HT3 receptors is necessary for the increase of adult neurogenesis induced by fluoxetine.

Binding and Activation of Serotonergic G-Protein Coupled Receptors by the Multimodal Antidepressant Vortioxetine

G-protein coupled receptors (GPCRs) are important pharmacological targets. Despite substantial progress, important questions still remain concerning the details of activation: how can a ligand act as an agonist in one receptor but as an antagonist in a homologous receptor, and how can agonists activate a receptor despite lacking polar functional groups able to interact with helix 5 as is the case for the related adrenergic receptors? Studying vortioxetine (VXT), an important multimodal antidepressant drug, may elucidate both questions. Herein, we present a thorough in silico analysis of VXT binding to 5-HT_1A, 5-HT_1B, and 5-HT₇ receptors and compare it with available experimental data. We are able to rationalize the differential mode of action of VXT at different receptors, but also, in the case of the 5-HT_1A receptor, we observe the initial steps of activation that inform about an activation mechanism that does not involve polar interaction with helix 5. The results extend our current understanding of agonist and antagonist action at aminergic GPCRs.

Vortioxetine in major depressive disorder: from mechanisms of action to clinical studies. An updated review

INTRODUCTION

Vortioxetine is a multimodal-acting antidepressant that provides improvements on cognitive function aside from antidepressants and anxiolytic effects. Vortioxetine has been found to be one of the most effective and best tolerated options for major depressive disorder (MDD) in head-to-head trials.

AREAS COVERED

The present review intends to gather the most relevant and pragmatic data of vortioxetine in MDD, specially focusing on new studies that emerged between 2015 and 2020.

EXPERT OPINION

Vortioxetine is the first antidepressant that has shown improvements both in depression and cognitive symptoms, due to the unique multimodal mechanism of action that combine the 5-HT reuptake inhibition with modulations of other key pre- and post-synaptic 5-HT receptors (agonism of 5-HT_1A receptor, partial agonism of 5-HT_1B receptor, and antagonism of 5-HT₃, 5-HT_1D and 5-HT₇ receptors). This new mechanism of action can explain the dose-dependent effect and can be responsible for its effects on cognitive functioning and improved tolerability profile. Potential analgesic and anti-inflammatory properties observed in preclinical studies as well as interesting efficacy and tolerability results of clinical studies with specific target groups render it a promising therapeutic option for patients with MDD and concomitant conditions (as menopause symptoms, pain, inflammation, apathy, sleep and/or metabolic abnormalities).

Probing the molecular basis for signal transduction through the Zinc-Activated Channel (ZAC)

The molecular basis for the signal transduction through the classical Cys-loop receptors (CLRs) has been delineated in great detail. The Zinc-Activated Channel (ZAC) constitutes a so far poorly elucidated fifth branch of the CLR superfamily, and in this study we explore the molecular mechanisms underlying ZAC signaling in Xenopus oocytes by two-electrode voltage clamp electrophysiology. In studies of chimeric receptors fusing either the extracellular domain (ECD) or the transmembrane/intracellular domain (TMD-ICD) of ZAC with the complementary domains of 5-HT₃A serotonin or α₁ glycine receptors, serotonin and Zn²⁺/H⁺ evoked robust concentration-dependent currents in 5-HT₃A/ZAC- and ZAC/α₁-Gly-expressing oocytes, respectively, suggesting that Zn²⁺ and protons activate ZAC predominantly through its ECD. The molecular basis for Zn²⁺-mediated ZAC signaling was probed further by introduction of mutations of His, Cys, Glu and Asp residues in this domain, but as none of the mutants tested displayed substantially impaired Zn²⁺ functionality compared to wild-type ZAC, the location of the putative Zn²⁺ binding site(s) in the ECD was not identified. Finally, the functional importance of Leu²⁴⁶ (Leu9') in the transmembrane M2 α-helix of ZAC was investigated by Ala, Val, Ile and Thr substitutions. In concordance with findings for this highly conserved residue in classical CLRs, the ZAC^L9'X mutants exhibited left-shifted agonist concentration-response relationships, markedly higher degrees of spontaneous activity and slower desensitization kinetics compared to wild-type ZAC. In conclusion, while ZAC is an atypical CLR in terms of its (identified) agonists and channel characteristics, its signal transduction seems to undergo similar conformational transitions as those in the classical CLR.

Central 5-HT receptors and their function; present and future

Since our review of central 5-HT receptors and their function twenty years ago, no new 5-HT receptor has been discovered and there is little evidence that this situation will change in the near future. Nevertheless, over this time significant progress has been made in our understanding of the properties of these receptors and in the clinical translation of this information, and some of these developments are highlighted herein. Such highlights include extensive mapping of 5-HT receptors in both animal and human brain, culminating in readily-accessible brain atlases of 5-HT receptor distribution, as well as emerging data on how 5-HT receptors are distributed within complex neural circuits. Also, a range of important pharmacological and genetic tools have been developed that allow selective 5-HT receptor manipulation, in cells through to whole organism models. Moreover, unexpected complexity in 5-HT receptor function has been identified including agonist-dependent signalling that goes beyond the pharmacology of canonical 5-HT receptor signalling pathways set down in the 1980s and 1990s. This new knowledge of 5-HT signalling has been extended by the discovery of combined signalling of 5-HT and co-released neurotransmitters, especially glutamate. Another important advance has been the progression of a large number of 5-HT ligands through to experimental medicine studies and clinical trials, and some such agents have already become prescribed therapeutic drugs. Much more needs to be discovered and understood by 5-HT neuropharmacologists, not least how the diverse signalling effects of so many 5-HT receptor types interact with complex neural circuits to generate neurophysiological changes which ultimately lead to altered cognitions and behaviour. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

CSTI-300 (SMP-100); a Novel 5-HT3 Receptor Partial Agonist with Potential to Treat Patients with Irritable Bowel Syndrome or Carcinoid Syndrome

The 5-hydroxytryptamine (5-HT) (serotonin) 5-HT₃ receptor represents a clinical target for antagonists to deliver symptomatic relief to patients with diarrhea-predominant irritable bowel syndrome (IBS-d) or carcinoid syndrome. Unfortunately, this pharmacological strategy can present side effects (e.g., severe constipation). The present study investigates the potential of a novel 5-HT₃ receptor partial agonist, CSTI-300, to treat patients with IBS-d and other conditions associated with discomfort from colonic distension, with a predicted reduced side-effect profile. The in vitro and in vivo preclinical pharmacology of the drug CSTI-300 was investigated to explore the potential to treat patients with IBS-d. CSTI-300 displayed selective high affinity for the human and rat 5-HT₃ receptor (K_i approximately 2.0 nM) and acted as a partial agonist (approximately 30%-50% intrinsic efficacy) in vitro. In an in vivo model of IBS-d, the rat colon distension model, CSTI-300 displayed dose-dependent efficacy. In addition, oral administration of CSTI-300 to dogs that achieved plasma levels of the drug exceeding the K_i value for the 5-HT₃ receptor failed to either evoke emesis or alter the state of feces. Pharmacokinetics for CSTI-300 in rat and dog identified high levels of oral availability with t _1/2 range of 1.6-4.4 hours. The preclinical pharmacology of the lead candidate drug, CSTI-300, supports the potential of this novel drug to offer symptomatic relief to patients with irritable bowel syndrome and carcinoid syndrome with a rationale for a reduced "on-target" side-effect profile relative to 5-HT₃ receptor antagonists, such as alosetron. SIGNIFICANCE STATEMENT: There is a lack of effective current treatment for diarrhea-predominant irritable bowel syndrome and carcinoid syndrome, and in both conditions, overactivity of the 5-hydroxytryptamine (5-HT) 5-HT₃ receptor is thought to be implicated in the pathophysiology. Because 5-HT₃ receptor blockade with antagonists results in significant side effects, we present evidence that treatment with a suitable 5-HT₃ receptor partial agonist will alleviate some symptoms associated with these conditions yet, without fully inhibiting the receptor, predict a less pronounced side-effect profile associated with this therapeutic strategy.

Conformational Changes in the 5-HT3A Receptor Extracellular Domain Measured by Voltage-Clamp Fluorometry

The 5-hydroxytryptamine (5-HT) type 3 receptor is a member of the cysteine (Cys)-loop receptor super family of ligand-gated ion channels in the nervous system and is a clinical target in a range of diseases. The 5-HT₃ receptor mediates fast serotonergic neurotransmission by undergoing a series of conformational changes initiated by ligand binding that lead to the rapid opening of an intrinsic cation-selective channel. However, despite the availability of high-resolution structures of a mouse 5-HT₃ receptor, many important aspects of the mechanistic basis of 5-HT₃ receptor function and modulation by drugs remain poorly understood. In particular, there is little direct evidence for the specific conformational changes predicted to occur during ligand-gated channel activation and desensitization. In the present study, we used voltage-clamp fluorometry (VCF) to measure conformational changes in regions surrounding the orthosteric binding site of the human 5-HT_3A (h5-HT_3A) receptor during binding of 5-HT and different classes of 5-HT₃ receptor ligands. VCF utilizes parallel measurements of receptor currents with photon emission from fluorescent reporter groups covalently attached to specific positions in the receptor structure. Reporter groups that are highly sensitive to the local molecular environment can, in real time, report conformational changes as changes in fluorescence that can be correlated with changes in receptor currents reporting the functional states of the channel. Within the loop C, D, and E regions that surround the orthosteric binding site in the h5-HT_3A receptor, we identify positions that are amenable to tagging with an environmentally sensitive reporter group that reports robust fluorescence changes upon 5-HT binding and receptor activation. We use these reporter positions to characterize the effect of ligand binding on the local structure of the orthosteric binding site by agonists, competitive antagonists, and allosterically acting channel activators. We observed that loop C appears to show distinct fluorescence changes for ligands of the same class, while loop D reports similar fluorescence changes for all ligands binding at the orthosteric site. In contrast, the loop E reporter position shows distinct changes for agonists, antagonists, and allosteric compounds, suggesting the conformational changes in this region are specific to ligand function. Interpretation of these results within the framework of current models of 5-HT₃ and Cys-loop mechanisms are used to expand the understanding of how ligand binding in Cys-loop receptors relates to channel gating. SIGNIFICANCE STATEMENT: The 5-HT₃ receptor is an important ligand-gated ion channel and drug target in the central and peripheral nervous system. Determining how ligand binding induced conformational changes in the receptor is central for understanding the structural mechanisms underlying 5-HT₃ receptor function. Here, we employ voltage-gated fluorometry to characterize conformational changes in the extracellular domain of the human 5-HT₃ receptor to identify intrareceptor motions during binding of a range of 5-HT₃ receptor agonists and antagonists.