New Molecular Targets for Antidepressant Drugs
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pharmaceuticals Review New Molecular Targets for Antidepressant Drugs Johannes Kornhuber 1,* and Erich Gulbins 2,3 1 Department of Psychiatry and Psychotherapy, University Hospital, Friedrich-Alexander-University of Erlangen-Nuremberg, 91054 Erlangen, Germany 2 Department of Molecular Biology, University of Duisburg-Essen, 45117 Essen, Germany; [email protected] 3 Department of Surgery, University of Cincinnati, Cincinnati, OH 45267, USA * Correspondence: [email protected] Abstract: Major depressive disorder (MDD) is a common and severe mental disorder that is usually recurrent and has a high risk of suicide. This disorder manifests not only with psychological symptoms but also multiple changes throughout the body, including increased risks of obesity, diabetes, and cardiovascular disease. Peripheral markers of oxidative stress and inflammation are elevated. MDD is therefore best described as a multisystem whole-body disease. Pharmacological treatment with antidepressants usually requires several weeks before the desired effects manifest. Previous theories of depression, such as the monoamine or neurogenesis hypotheses, do not explain these characteristics well. In recent years, new mechanisms of action have been discovered for long-standing antidepressants that also shed new light on depression, including the sphingolipid system and the receptor for brain-derived neurotrophic factor (BDNF). Keywords: acid sphingomyelinase; ceramide; sphingomyelin; FIASMA; antidepressant drug; lyso- some; neurogenesis; hippocampus; autophagy; brain-derived neurotrophic factor (BDNF) Citation: Kornhuber, J.; Gulbins, E. New Molecular Targets for 1. Introduction Antidepressant Drugs. Pharmaceuticals 2021, 14, 894. https:// For many clinically successful drugs, new and relevant mechanisms of action have doi.org/10.3390/ph14090894 been discovered over time. For example, we have demonstrated relevant antagonistic effects of memantine, amantadine and budipine, which until then had been considered Academic Editor: Marco Scarselli dopaminomimetic substances, on N-methyl-D-aspartate (NMDA) receptors [1–3]. A new mechanism of action has only recently been demonstrated for the very well established Received: 16 August 2021 and effective drug metformin in the treatment of type 2 diabetes [4]. This is also true for an- Accepted: 25 August 2021 tidepressants, which have been used successfully to treat major depressive disorder (MDD) Published: 2 September 2021 since 1957 [5]. These compounds were initially used clinically without the knowledge of their molecular targets. Over time, their mechanisms of action were initially discovered to Publisher’s Note: MDPI stays neutral be the inhibition of monoamine oxidases and the inhibition of monoaminergic transporters. with regard to jurisdictional claims in Later, other molecular mechanisms were discovered, such as the inhibition of postsynaptic published maps and institutional affil- serotonin receptors, the inhibition of presynaptic alpha 2 receptors, and the activation of iations. melatonergic receptors. In recent years, a number of new antidepressants with novel mechanisms of action have been developed and clinically evaluated. The inhibition of NMDA receptors by esketamine shows a rapid antidepressant effect [6]. However, NMDA receptor inhibition Copyright: © 2021 by the authors. as an antidepressant mode of action is in doubt, since a ketamine metabolite has low Licensee MDPI, Basel, Switzerland. affinity for the NMDA receptor but has antidepressant activity [7]. A single i.v. adminis- This article is an open access article tration of brexanolone, a neuroactive steroid (SAGE-547) with gamma-aminobutyric acid distributed under the terms and (GABA)A receptor-positive allosteric modulatory activity, shows a rapid antidepressant conditions of the Creative Commons effect that lasts more than a week in postpartum depression [8,9]. Zuranolone, an orally Attribution (CC BY) license (https:// available neuroactive steroid with a comparable mechanism of action (SAGE-217), also creativecommons.org/licenses/by/ shows antidepressant effects on MDD [10] and postpartum depression [11]. In addition, 4.0/). Pharmaceuticals 2021, 14, 894. https://doi.org/10.3390/ph14090894 https://www.mdpi.com/journal/pharmaceuticals Pharmaceuticals 2021, 14, x FOR PEER REVIEW 2 of 16 Pharmaceuticals 2021, 14, 894 2 of 15 number of other compounds, each with different molecular targets and potential antide- therepressant are aeffects, number that of otherare currently compounds, being each evaluated with different [12]. molecular targets and potential antidepressantIn parallel effects, with the that development are currently of being new evaluated compounds [12]. and the exploitation of new molecularIn parallel targets, with important the development cellular and of newsystemic compounds mechanisms and theof action exploitation of antidepres- of new molecularsants have targets, been discovered, important such cellular as neurog and systemicenesis [13], mechanisms neuroplasticity of action and ofinflammation. antidepres- santsHowever, have it been is not discovered, yet clear through such as which neurogenesis molecular [13 targets], neuroplasticity neurogenesis, and neuroplasticity inflammation. However,and inflammation it is not yet are clear affected. through which molecular targets neurogenesis, neuroplasticity and inflammationThe focus of this are affected.review is relatively narrow. We discuss only novel molecular targets otherThe than focus the ofpreviously this review described is relatively targets narrow. for the We groups discuss of onlylicensed novel antidepressants molecular targets (i.e., otherGroup than N06A the in previously the ATC system), described and targets we only for consider the groups developments of licensed during antidepressants the last 10 (i.e.,years. Group The mechanisms N06A in the should ATC system), apply not and only we to only individual consider substances developments but also during to many the lastantidepressants, 10 years. The even mechanisms structura shouldlly different apply ones. not We only discuss to individual in detail substances the effect of but antide- also topressants many antidepressants, on the sphingolipid even system structurally and the different tyrosine ones. kinase We receptor discuss in 2 detail(TRKB) the receptor effect offor antidepressants BDNF. on the sphingolipid system and the tyrosine kinase receptor 2 (TRKB) receptor for BDNF. 2. MDD as a Whole-Body Multisystem Disease 2. MDD as a Whole-Body Multisystem Disease MDD is a common mental disorder with a lifetime prevalence of approximately 10%. PatientsMDD suffer is a common a high overall mental burden disorder of with disease a lifetime due to prevalence severe impairment of approximately during 10%.each Patientsepisode sufferof illness, a high long overall individual burden episodes of disease of illness, due to an severe often impairmentearly age of onset, during and each an episodeoften recurrent of illness, course long individual[14]. MDD episodesis a risk fa ofctor illness, for anthe oftendevelopment early age of of cardiovascular onset, and an oftendisease, recurrent diabetes, course metabolic [14]. MDD syndrome, is a risk osteoporosis, factor for the Alzheimer’s development disease, of cardiovascular and hippocam- dis- ease, diabetes, metabolic syndrome, osteoporosis, Alzheimer’s disease, and hippocampal pal atrophy. The adrenocortical stress axis is activated, and changes in serum lipids occur; atrophy. The adrenocortical stress axis is activated, and changes in serum lipids occur; pe- peripheral proinflammatory cytokines, including interleukin-1β (IL-1β), markers of oxi- ripheral proinflammatory cytokines, including interleukin-1β (IL-1β), markers of oxidative dative stress and inflammation, and serum phospholipase A2 activity are increased. Inde- stress and inflammation, and serum phospholipase A2 activity are increased. Independent pendent of suicide, life expectancy is reduced in patients with MDD. Therefore, MDD is of suicide, life expectancy is reduced in patients with MDD. Therefore, MDD is considered considered not only a mental disorder but also a multisystem whole-body disease [15–19] not only a mental disorder but also a multisystem whole-body disease [15–19] (Figure1). (Figure 1). FigureFigure 1.1. MDD is is a a risk risk factor factor for for the the development development of ofmultiple multiple disorders disorders and and diseases, diseases, such such as met- as metabolicabolic syndrome, syndrome, osteoporosis, osteoporosis, an andd cardiovascular cardiovascular disease. disease. MDD MDD can can therefore bebe conceptualizedconceptualized asas aa multisystem,multisystem, whole-body whole-body disease. disease. TheThe diagnosticdiagnostic criteriacriteria forfor majormajor depressiondepression accordingaccording toto thethe DSM-5DSM-5 [[20]20] includeinclude pre-pre- dominantlydominantly psychologicalpsychological symptomssymptoms apart apart from from fatigue fatigue and and weight weight change. change. The The physical physical illnessesillnesses listed listed here, here, which which occur occur frequently frequently as as a follow-upa follow-up to to depression, depression, do do not not belong belong to theto the diagnostic diagnostic criteria. criteria. Pharmaceuticals 2021, 14, 894 3 of 15 3. Current Pharmacological Treatments for MDD The group of antidepressants defined in the ATC system (N06A) includes substances with very different structures and mechanisms of action.