H3 receptor antagonists(Gedeon Richter Pharma)

The histamine H3receptor (H3R) is an important modulator of numerous central control mechanisms. Novel lead optimizations for H3R antagonists/inverse agonists involved studies of structure–activity relationships, cross‐affinities, and pharmacokinetic properties of promising ligands. Blockade of inhibitory histamine H3autoreceptors reinforces histaminergic transmission, while antagonism of H3 heteroreceptors accelerates the corticolimbic liberation of acetylcholine, norepinephrine, glutamate, dopamine, serotonin and gamma‐aminobutyric acid (GABA). The H3R positioned at numerous neurotransmission crossroads indicates therapeutic applications of small‐molecule H3R modulators in a number of psychiatric and neurodegenerative diseases with various clinical candidates available. Dual target drugs displaying H3R antagonism/inverse agonism with inhibition of acetylcholine esterase (AChE), histamine N‐methyltransferase (HMT), or serotonin transporter (SERT) are novel class of procognitive agents. Main chemical diversities, pharmacophores, and pharmacological profiles of procognitive agents acting as H3R antagonists/inverse agonists and dual H3R antagonists/inverse agonists with inhibiting activity on AChE, HMT, or SERT are highlighted here.

The putative partial H1-receptor agonism of some H3-receptor antagonists belonging to the proxifan series was characterized in a functional in-vitro assay using guinea-pig ileum.

Whole segments of guinea-pig ileum were mounted in Tyrode's solution under isotonic conditions in the presence of atropine (10(-7) M) and were cumulatively treated with histamine as an internal reference. After washout, the putative H1-receptor agonists were added cumulatively to determine agonist potency (pEC50) and intrinsic activity (Emax) relative to histamine. Maximal or supramaximal concentrations of partial agonists, or sufficient concentrations of H1-receptor antagonists were incubated for 3-15 min prior to construction of a second concentration-effect curve to histamine in order to calculate partial agonist or antagonist affinity for the H1 receptor (pKP or pA2 value, respectively).

Several analogues of FUB 372 displayed low H1-receptor affinities (pA2 or pKP 4.2-5.5) except for a methyl benzoate derivative (pA2 = 6.81, Schild plot slope unity). FUB 372, four ortho-substituted derivatives (R = F, CH3, OCH3, CF3), and ciproxifan were weak contractile agents (Emax 9-38%, pEC50 4.73-5.68, histamine: 6.70) susceptible to antagonism by the H1-antihistaminergic drug mepyramine (2.10(-9)-10(-7) M). Agonist potency and H1-receptor affinity of these compounds did not correlate with the data of a set of H1-histaminergic 2-phenylhistamines bearing the same substituents.

A specific subset of proxifans related to FUB 372 and ciproxifan represent a unique type of H1-receptor agonists lacking a basic side chain.