5-HT1A-receptor antagonism: N-alkyl derivatives of (R)-(-)-8,11-dimethoxynoraporphine

Gram-Scale Total Synthesis of Carbazomycins A-D through Late-Stage Regioselective Demethylation of Aryl Methyl Ethers

Gram-scale total synthesis of carbazomycins A-D is described. The total synthesis of carbazomycin A was achieved in 44% overall yield over six steps from symmetrical 5-chloro-1,2,3-trimethoxybenzene. The key aryne-mediated carbazole formation and methylation steps provided the multiply substituted carbazole. The regioselective demethylation of the trimethoxycarbazole was performed using boron trichloride. Thereafter, the phenolic hydroxy group was converted into the methyl group to provide carbazomycin A. Subsequent installation of the methoxy group realized the total synthesis of carbazomycin D. Regioselective demethylation was performed using 1-dodecanethiol, effecting the conversions of carbazomycins A and D into carbazomycins B and C, respectively.

C4 phenyl aporphines with selective h5-HT(2B) receptor affinity

A group of aporphine alkaloids related to (±)-nantenine (1) and bearing a C4 phenyl and various C1 or N-substituents, was synthesized and evaluated for affinity to h5-HT receptors. In general, unlike nantenine, the analogs lack affinity for the h5-HT(2A) receptor and other 5-HT receptors but bind selectively to the h5-HT(2B) receptor. With regards to 5-HT(2B) affinity, there appears to be a low tolerance for bulky C1 or N-substituents when the C4 phenyl moiety is present. Compound 5a had the highest 5-HT(2B) affinity of the compounds tested, was found to be an antagonist and is selective vs other CNS receptors.

Nantenine as an acetylcholinesterase inhibitor: SAR, enzyme kinetics and molecular modeling investigations

Nantenine, as well as a number of flexible analogs, were evaluated for acetylcholinesterase (AChE) inhibitory activity in microplate spectrophotometric assays based on Ellman's method. It was found that the rigid aporphine core of nantenine is an important structural requirement for its anticholinesterase activity. Nantenine showed mixed inhibition kinetics in enzyme assays. Molecular docking experiments suggest that nantenine binds preferentially to the catalytic site of AChE but is also capable of interacting with the peripheral anionic site (PAS) of the enzyme, thus accounting for its mixed inhibition profile. The aporphine core of nantenine may thus be a useful template for the design of novel PAS or dual-site AChE inhibitors. Inhibiting the PAS is desirable for prevention of aggregation of the amyloid peptide Aβ, a major causative factor in the progression of Alzheimer's disease (AD).

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.

Serotonin receptors and their ligands: a lack of selective agents

Four major families of serotonin (5-hydroxytryptamine; 5-HT) receptors have been identified: 5-HT1, 5-HT2, 5-HT3 and 5-HT4. At this time, there is a general consensus that the 5-HT1 family can be further subdivided into 5-HT1A, 5-HT1B, 5-HT1C, 5-HT1D, and 5-HT1P subpopulations. In addition, there are several other populations of less well-defined 5-HT receptors. The purpose of this presentation is to discuss 5-HT receptor nomenclature and the agents that are commonly used to investigate each receptor population in as much as it will serve to provide background for the remainder of the symposium. There is presently available an abundance of serotonergic agents; however, these agents are only semiselective, and none can be considered truly selective for a particular population of 5-HT receptors. As useful as these agents have been for the identification and characterization of 5-HT receptors, there remains a need for the development of new, more selective ligands.

Approaches to molecular modeling studies and specific application to serotonin ligands and receptors

Molecular modeling studies are useful in as much as they may allow us to understand the activity and selectivity of currently existing agents, and, furthermore, may aid in the design of completely novel therapeutic agents. There are two basic modeling strategies: the ligand-ligand approach and the ligand-receptor approach. Both approaches possess certain inherent advantages and disadvantages and, in addition, make certain assumptions about the agents and/or receptors being investigated. Keeping with the spirit of this minisymposium, we describe these two approaches, their general usefulness, and their limitations. Using serotonin (5-HT) receptors as a focal point, we review and provide novel examples of molecular modeling studies involving both strategies. Presented for the first time are examples of ligand-receptor models to account for the binding of serotonergic agents at 5-HT2 and 5-HT1C receptors.

Structure-activity relationships at 5-HT1A receptors: binding profiles and intrinsic activity

The 5-HT1A receptor has been one of the most studied 5-HT receptor subtypes in terms of its pharmacologic profile. Comparisons of various studies of structure-activity relationships (SAR) at this receptor shows an emerging profile for this receptor's pharmacophore. The present discussion focuses on the findings generated with relatively small molecules that can be considered as analogs of serotonin itself and that illustrate some of the structural properties that are important for high-affinity recognition by the receptor. Most of the SAR work has been based on the affinities of compounds for the receptor as determined by the radioligand-binding technique, which has a significant limitation in that it cannot define the intrinsic activity of compounds at the receptor. This problem can be addressed by functional assays, and an example of SAR at the 5-HT1A receptor-coupled adenylate cyclase system is provided.