Current Topics in Medicinal Chemistry - Volume 2, Issue 6, 2002

5-hydroxytryptamine (5-HT serotonin) is a neurotransmitter essential for a large number of physiological processes including the regulation of vascular and non-vascular smooth muscle contraction, modulation of platelet aggregation, and the regulation of appetite, mood, anxiety, wakefulness and perception. To mediate this astonishing array of functions, no fewer than 15 separate receptors have evolved, of which all but two (5-HT3A and 5-HT3B) are G-protein coupled receptors. This review will summarize our current understanding of the structure and function of the G-protein coupled 5-HT receptors. In particular, a systematic review of the available mutagenesis studies of 5-HT receptors will be presented. This information will be synthesized to provide a working model of agonist and antagonist actions at a prototypic 5-HT receptor-the 5-HT2A receptor. Finally, examples will be given to demonstrate that a detailed knowledge of the predicted structure of one receptor can be useful for structure-based drug design.

Several lines of evidence indicate that G-protein coupled receptors (GPCR) may exist in a state that allows a tonic level of stimulation in vivo (constitutive activity). Several native forms of GPCR, when expressed in recombinant cell lines, display significant signal transduction stimulation in the absence of activating ligand. Many GPCR, including three serotonin receptors, display robust constitutive activation upon the mutation of a single amino acid, indicating mutations producing inappropriate constitutive activation may be etiological factors in diseases. If constitutive activity of GPCR is as common a phenomenon as some researchers suspect, this would suggest significant alterations in the classical model of ligand-receptor interactions. One of the most significant implications of constitutive activity for pharmacologists and medicinal chemists, is the possibility of developing drugs that lower the level of constitutive activity. Such compounds have been termed “inverse agonists”. These drugs, in theory, would have different physiological effects, and therefore possibly different therapeutic potential, than classical competitive receptor antagonists (“neutral antagonists”). Theoretical issues concerning constitutive activity in the GPCR family and some of the evidence supporting the existence of constitutive activity in the GPCR family is reviewed. Studies are presented demonstrating the procedures for producing and characterizing constitutive activated forms of serotonin receptors, including the demonstration of inverse agonist activity of drugs on these receptors.

The structures of ketanserin (1) and spiperone (2) were examined in detail to determine the role of various substituent groups on 5-HT2A receptor affinity and selectivity. It was found that the presence of the quinazoline ring of ketanserin detracts from selectivity and that various ring-opened analogs displayed ketanserin-like affinity and up to 30-fold enhanced selectivity. The triazaspirodecanone portion of spiperone is a major determinant of its 5-HT2A affinity and selectivity. The conformational rigidity imposed by the ring, as well as the nature of the N1-substituent, are important factors in controlling binding at 5-HT2A, 5-HT2C, 5-HT1A, and dopamine D2 receptors. Replacement of the N1-phenyl ring of spiperone with a methyl group (KML-010 48) resulted in a compound that binds at 5-HT2A receptors with slightly lower affinity than spiperone, but that lacked affinity (Ki >10,000 nM) for 5-HT2C and 5-HT1A receptors and binds with 400-fold reduced affinity at D2 receptors.

The human 5-HT1B and 5-HT1D receptors are especially similar in sequence despite being encoded by two distinct genes. Although, human 5-HT1B and 5-HT1D receptors have been pharmacologically differentiated using nonselective 5-HT1B / D receptor antagonists such as ketanserin (1), ritanserin (2) and methiothepin (3), the precise function of these receptors remains undefined, and progress toward this has been hampered by the lack of selective ligands. The interest of the major pharmaceutical companies in 5-HT1B / 1D antagonists increased by the discovery of potent and selective tools, combined with the fact that the blockade of terminal 5-HT1B receptors by selective antagonists has been proposed as a new approach for more efficient and / or fast-acting antidepressant drugs, since the acute blockade of these 5-HT autoreceptors will, in theory, immediately mimic their desensitization. Furthermore, it has been also suggested that supersensitive 5- HT1B / 1D receptors may be involved in the pathophysiology of obsessive compulsive disorders (OCD). In the 5-HT1B / 1D agonist field, since the discovery of sumatriptan (26) (a 5-HT1B / 1D receptor agonist) as an effective treatment for migraine headache, intensive research in this area has led to several second-generation compounds, a few of which have either entered the market place or are in late clinical trials. Beside the antimigraine activity of the 5-HT1B / 1D agonists in clinical evaluation or already on the market, other potential therapeutic evaluations (such as gastric motor effect, bipolar disorder, autism, anti-aggressive effects) with these drugs are being investigated.This article highlights and reviews the research advances published in the 5-HT1B / 1D antagonist and agonist literature. The article is supplemented with selected references on the design, synthesis and development of novel 5-HT1B / 1D agents, and on studies to understand their mechanism and pathophysiology. Emphasis is given to recent advances in the potential therapeutic applications of 5-HT1B / 1D serotonergic agents. By no means has any attempt been made to exhaustively review the literature but rather, primary references along with citations to recent literature reviews have been included in each section.

The present review describes our approach to the development of a structurally unique class of 5- HT2A ligands. On the basis of an abbreviated graphics model of a 5-HT2A serotonin receptor, it was hypothesized that introduction of an additional aromatic ring might enhance the affinity of phenylethylamine (an agent that lacks significant affinity for the 5-HT2A receptors). Continued work with such structures, and the continual refinement of graphics receptor models, ultimately led to the identification of AMDA (27, 5-HT2A Ki = 20 nM). AMDA is a 5-HT2A antagonist that, unlike certain other tricyclic 5-HT2A antagonists, binds with very low affinity at dopamine D2 receptors, the serotonin transporter, and the norepinephrine transporter. Comparative structure-affinity studies indicate that AMDA binds in a manner distinct from the tricyclic antagonists Graphics models were employed to identify possible modes of binding. This investigation illustrates the impact of a combination of classical medicinal chemistry, receptor modeling, and molecular biology on novel drug design.

The serotonin 5-HT3 receptor subtype is unique among the receptors for this neurotransmitter because it has been demonstrated to be a ligand-gated ion channel capable of mediating rapid intercellular communication. This review covers the authors'work performed during more than a decade in the development of 5-HT3 receptor ligands belonging to the classes of arylpiperazines, tropanes, and quinuclidine derivatives. The discussion is focused mainly on what the authors have learned about the interaction of these structurally different ligands with their receptor and shows the way their ideas evolved along with the progress of the project. Furthermore, a summary of the most significant structure-affinity relationships, derived from the original work, is reported to support the discussion.

Among serotonin receptors (5-HTRs), the 5-HT4 subtype is of considerable interest because it is involved in (patho)physiological processes both in peripheral and central nervous systems. In addition to the clinical use of 5-HT4R agonists in the treatment of gastrointestinal motility disorders, the potential use of antagonists in the treatment of irritable bowel syndrome, arrhythmias and micturition disturbances are currently under investigation. This article will review the development of the most important classes of 5- HT4R antagonists with an emphasis on benzimidazole derivatives, their structure-affinity relationships, ligand-receptor interactions and pharmacological applications.

The possible role of 5-HT6 receptor antagonists in the treatment of learning and memory disorders has stimulated significant recent work in this area. The first selective antagonists of this receptor were identified by Roche (Ro 04-6790 and Ro 63-0563) and SmithKline Beecham (SB-271046), although they only had poor to modest brain penetration, respectively. Recently, several structurally different series of selective antagonists have been reported. Glennon's group and Merck Sharp & Dohme have discovered N,N-dimethyl-1- benzenesulfonyl-5-methoxytryptamine as a reasonably selective, high affinity antagonist, while Allelix have gone on to find that a 6-bicyclopiperazinyl-1-naphthylsulfonylindole had improved affinity and selectivity. Roche have reported subsequently on more lipophilic analogs of Ro 04-6790 that appear to penetrate the brain better. Reversing the sulfonamide linkage of SB-271046 led to a new series of compounds, producing SB-357134, which also had increased CNS penetration. A series of selective partial agonists containing a 4-piperazinylquinoline system has also been described. Recent studies in the Morris water maze with both Ro 04-6790 and SB-271046 have concluded that 5-HT6 receptor antagonists improved retention performance, although these results are open to interpretation. Other behavioural studies have also implicated a role for 5-HT6 in cognition enhancement and this has been supported by in vivo microdialysis studies that showed SB-271046 produced an increase in extracellular glutamate levels in the frontal cortex. However, we have been unable to replicate these effects with either SB-271046 or Ro 04-6790, and clearly further work is required before we can be certain of the functional role of this receptor.