Diether derivatives of homo- or substituted piperidines as non-imidazole histamine H3 receptor ligands

Chemical Probes for Histamine Receptor Subtypes

Ligands with different properties and different selectivity are highly needed for in vitro and in vivo studies on the (patho)physiological influence of the chemical mediator histamine and its receptor subtypes. A selection of well-described ligands for the different receptor subtypes and different studies is shown with a particular focus on affinity and selectivity. In addition, compounds with radioactive or fluorescence elements will be presented with their beneficial use for other species or different investigations.

Histamine H3 receptor and cholinesterases as synergistic targets for cognitive decline: Strategies to the rational design of multitarget ligands

The role of histamine and acetylcholine in cognitive functions suggests that compounds able to increase both histaminergic and cholinergic neurotransmissions in the brain should be considered as promising therapeutic options. For this purpose, dual inhibitors of histamine H₃ receptors (H₃ R) and cholinesterases (ChEs) have been designed and assessed. In this context, this paper reviews the strategies used to obtain dual H₃ R/ChEs ligands using multitarget design approaches. Hybrid compounds designed by linking tacrine or flavonoid motifs to H₃ R antagonists were obtained with high affinity for both targets, and compounds designed by merging the H₃ R antagonist pharmacophore with known anticholinesterase molecules were also reported. These reports strongly suggest that key modifications in the lipophilic region (including a second basic group) seem to be a strategy to reach novel compounds, allied with longer linker groups to a basic region. Some compounds have already demonstrated efficacy in memory models, although the pharmacokinetic and toxicity profile should be considered when designing further compounds. In conclusion, the key features to be considered when designing novel H₃ R/ChEs inhibitors with improved pharmacological profile were herein summarized.

Biphenyloxy-alkyl-piperidine and azepane derivatives as histamine H3 receptor ligands

Novel biphenyloxy-alkyl derivatives of piperidine and azepane were synthesized and evaluated for their binding properties at the human histamine H₃ receptor. Two series of compounds were obtained with a meta- and a para-biphenyl moiety. The alkyl chain spacer contained five and six carbon atoms. The highest affinity among all compounds was shown by 1-(6-(3-phenylphenoxy)hexyl)azepane (13) with a K_i value of 18nM. Two para-biphenyl derivatives, 1-(5-(4-phenylphenoxy)pentyl)piperidine (14; K_i=25nM) and 1-(5-(4-phenylphenoxy)pentyl)azepane (16; K_i=34nM), classified as antagonists in a cAMP accumulation assay (IC₅₀=4 and 9nM, respectively), were studied in detail. Compounds 14 and 16 blocked RAMH-induced dipsogenia in rats (ED₅₀ of 2.72mg/kg and 1.75mg/kg respectively), and showed high selectivity (hH₄R vs hH₃R>600-fold) and low toxicity (hERG inhibition: IC₅₀>1.70µM; hepatotoxicity IC₅₀>12.5µM; non-mutagenic up to 10µM). Furthermore, the metabolic stability was evaluated in vitro on human liver microsomes (HLMs) and/or rat liver microsomes (RLMs). Metabolites produced were analyzed and tentatively identified by UPLC-MS techniques. The results demonstrated easy hydroxylation of the biphenyl ring.

Cholinesterase inhibitory activity of chlorophenoxy derivatives-Histamine H3 receptor ligands

In recent years, multitarget-directed ligands have become an interesting strategy in a search for a new treatment of Alzheimer's disease. Combination of both: a histamine H3 receptor antagonist/inverse agonist and a cholinesterases inhibitor in one molecule could provide a new therapeutic opportunity. Here, we present biological evaluation of histamine H3 receptor ligands-chlorophenoxyalkylamine derivatives against cholinesterases: acetyl- and butyrylcholinesterase. The target compounds showed cholinesterase inhibitory activity in a low micromolar range. The most potent in this group was 1-(7-(4-chlorophenoxy)heptyl)homopiperidine (18) inhibiting the both enzymes (EeAChE IC50=1.93μM and EqBuChE IC50=1.64μM). Molecular modeling studies were performed to explain the binding mode of 18 with histamine H3 receptor as well as with cholinesterases.

Chlorophenoxy aminoalkyl derivatives as histamine H(3)R ligands and antiseizure agents

A series of twenty new chlorophenoxyalkylamine derivatives (9-28) was synthesized and evaluated on their binding properties at the human histamine H3 receptor (hH3R). The spacer alkyl chain contained five to seven carbon atoms. The highest affinities have shown the 4-chloro substituted derivatives 10 and 25 (Ki=133 and 128 nM, respectively) classified as antagonists in cAMP accumulation assay (EC50=72 and 75 nM, respectively). Synthesized compounds were also evaluated for anticonvulsant activity in Antiepileptic Screening Program (ASP) at National Institute of Neurological Disorders and Stroke (USA). Two compounds (4-chloro substituted derivatives: 20 and 26) were the most promising and showed in the MES seizure model in rats (after ip administration) ED50 values of 14 mg/kg and 13.18 mg/kg, respectively. Protective indexes (PI=TD50/ED50) were 3.2 for 20 and 3.8 for 26. Moreover, molecular modeling and docking studies were undertaken to explain affinity at hH3R of target compounds, and the experimentally and in silico estimation of properties like lipophilicity and metabolism was performed. Antiproliferative effects have been also investigated in vitro for selected compounds (10 and 25). These compounds neither possessed significant antiproliferative and antitumor activity, nor modulated CYP3A4 activity up to concentration of 10 μM.