(S)- and (R)-8-(di-n-propylamino)-6,7,8,9-tetrahydro-3H-benz[e]indole-1- carbaldehyde: a new class of orally active 5-HT1A-receptor agonists

Development and challenges in the discovery of 5-HT1A and 5-HT7 receptor ligands

Serotonin (5-hydroxytryptamine) is a small molecule that acts both in the central and peripheral nervous system as a neurotransmitter and a hormone, respectively. Serotonin is synthesized via a multi-stage pathway beginning with l-tryptophan, which is converted by an enzyme called tryptophan hydroxylase into L-5-Hydroxytryptophan. It is well-known for its significance in the control of mood, anxiety, depression, and insomnia as well as in normal human functions such as sleep, sexual activity, and appetite. Thus, for medical chemists and pharmaceutical firms, serotonin is one of the most desirable targets. Among the seven different classes of serotonin receptors, the 5-HT_1A was one of the first discovered serotonin receptors, and the 5-HT₇ was the last addition to the serotonin receptor family. Both the classes were thoroughly examined. 5-HT_1A neurotransmission-related dysfunctions are linked to many psychological conditions such as anxiety, depression, and movement disorders. 5-HT₇ is a member of the cell surface receptor GPCR superfamily and is regulated by the serotonin neurotransmitter. It has been the focus of intensive research efforts since its discovery, which was prompted by its presence in functionally important regions of the brain. The thalamus and hypothalamus have the highest 5-HT₇ receptor densities. They are also found in the hippocampus and cortex at higher densities. Thermoregulation, circadian rhythm, learning and memory, and sleep are all associated with the 5-HT₇ receptor. It is also suspected that this receptor may be involved in the control of mood, indicating that it may be a beneficial target for depression treatment. Several differently structured molecules such as aminotetralins, ergolines, arylpiperazines, indolylalkylamines, aporphines, and aryloxyalkyl-amines are known to bind to 5-HT_1A and 5-HT₇ receptor sites. In brain serotonin receptors 5-HT_1A and 5-HT₇ are strongly co-expressed in regions involved in depression. However, their functional interaction has not been identified. An overview of the 5-HT_1A and 5-HT₇ receptor ligands belonging to different chemical groups is mentioned in this review.

The structural evolution of dopamine D3 receptor ligands: structure-activity relationships and selected neuropharmacological aspects

"Evolution consists largely of molecular tinkering."-Following the famous concept of the molecular geneticist and medicine Nobel laureate François Jacob, in this review we describe the structural evolution of dopamine D3 receptor ligands from the natural agonist dopamine (DA) to highly potent and subtype selective new agents by bioisosteric tinkering with well-established and privileged or novel and fancy chemical functionalities and scaffolds. Some of the more than 200 ligands presented herein have already achieved therapeutic or scientific value up to now, some will most likely achieve it in the future. Hence, great importance is not only attached to the relationship between structure and activity of the ligands, but also to their utility as pharmacological tools in animal models or as therapeutics in patients with neurological diseases or other disorders.

The 5-HT1A receptor and its ligands: structure and function

An overview is presented on progress made in research on 5-HT1A receptors and their ligands since their discovery in 1983. Molecular biology has offered new tools, for example cloned 5-HT1A receptors, their mutants and chimeras to study structure and function. Many compounds, belonging to different chemical classes, display high affinity and selectivity for 5-HT1A receptors. The majority of these compounds are agonists or partial agonists, full antagonists are still scarce. Agonists and partial agonists are active in various animal models of anxiety and depression. Partial receptor agonists have been proven to be effective in general anxiety disorder and depression in man. Potential therapeutic applications for 5-HT1A receptor antagonists are evaluated, for example, in cognition disorders.

Reduction of wrap restraint stress-induced defecation by MKC-242, a novel benzodioxan derivative, via 5-HT1A-receptor agonist action in rats

Effects of MKC-242 (5-[3-[((2S)-1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3-b enzodioxole HC1), a novel 5-HT1A-receptor agonist, and reference compounds on wrap restraint stress-induced defecation were evaluated in rats. Wrapping restraint stress increased defecation in rats. The increase was attenuated by putative 5-HT1A-receptor agonists, MKC-242 and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The suppressive effect of MKC-242 on wrap stress-induced defecation was antagonized by prior administration of a 5-HT1A-receptor antagonist, WAY100135. MKC-242 did not affect spontaneous defecation and 5-HT-induced defecation. Diazepam and amitriptyline also significantly reduced the stress-induced defecation. However, amitriptyline showed a potent anti-cholinergic effect in the oxotremorine-induced tremor test and reduced spontaneous defecation. In contrast to MKC-242 and 8-OH-DPAT, buspirone and tandospirone tended to suppress the increase at high doses. A major metabolite of buspirone and tandospirone, 1-(2-pyrimidinyl)piperazine, antagonized the suppressive effect of MKC-242. These findings suggest that stimulation of 5-HT1A receptors reduces stress-induced defecation but not spontaneous and 5-HT-induced defecation and that MKC-242 may be useful for the treatment of irritable bowel syndrome.

Pharmacokinetics of the 5-hydroxytryptamine1A agonist 8-hydroxy-2-(N,N-di-n-propylamino)tetralin (8-OHDPAT) in the rat after intravenous and oral administration

1. Plasma levels of 3H and unchanged drug were measured in the non-anaesthetized male rat after intravenous (i.v.) or oral administration of (+/-)-(R,S)-[propyl-3H]-8-OHD-PAT, at three dose levels per route of administration. The excretion of conjugated metabolites in bile was also studied following i.v. administration. 2. For unchanged 8-OHDPAT following i.v. administration, terminal t1/2 was 1.56 +/- 0.01 h (mean +/- SD, n > or = 4), kelim 0.45 +/- 0.01 h-1, volume of distribution 0.14 +/- 0.02 litres and clearance 1.10 +/- 0.17 mlmin-1. After oral administration, terminal t1/2, kelim, apparent volume of distribution and clearance were essentially the same when bioavailability was taken into account. Neither dose size nor route of administration had any significant effect on either terminal t1/2 or kelim. Comparison of AUCs following i.v. and oral administration yielded a mean for absolute oral bioavailability of 2.60 +/- 0.24%. 3. Comparison of AUCB for total plasma 3H showed that the extent of absorption was 80.1%, indicating that the low oral bioavailability of 8-OHDPAT is due to first-pass metabolism, rather than poor absorption from the GI tract. 4. Following i.v. administration, irrespective of dose, some 10% of the 3H dose was excreted in the bile in 6 h, 8.5% as 8-OHDPAT-glucuronide and 1.5% as the glucuronide of the N-despropylated metabolite, 8-OHDPAT. The majority of the biliary excretion occurred within 3 h of dosing.

Design of drugs through a consideration of drug metabolism and pharmacokinetics

Drug metabolism input to the discovery process has been to date largely on an empirical case by case basis. Considerable advances have been made, such that basic rules can be applied to the behaviour of a compound in man based on physico-chemistry and structure. This is particularly true in the area of the cytochrome P-450 enzymes, the principal enzymes involved in the primary clearance of drugs. The major human forms, CYP2D6, CYP2C9 and CYP3A4 all have distinct substrate preferences which are being catalogued and rationalised. Such understandings will not only impact on existing drugs. Since the enzymes and systems will remain the same, these understandings can be applied to the design of molecules for the targets of the future, whilst the structure activity relationships of those targets are being researched and revealed.