Robert E. West

Biochemical characterization of desloratadine, a potent antagonist of the human histamine H1 receptor

We have characterized desloratadine (5H-benzo[5,6]cyclohepta[1,2-b]pyridine, 8-chloro-6,11-dihydro-11-(4-piperidinylidene), CAS 100643-71-8) as a potent antagonist of the human histamine H(1) receptor. [3H]Desloratadine bound to membranes expressing the recombinant human histamine H(1) receptor in Chinese hamster ovary cells (CHO-H(1)) in a specific and saturable manner with a K(d) of 1.1+/-0.2 nM, a B(max) of 7.9+/-2.0 pmol/mg protein, and an association rate constant of 0.011 nM(-1) x min(-1). The K(d) calculated from the kinetic measurements was 1.5 nM. Dissociation of [3H]desloratadine from the human histamine H(1) receptor was slow, with only 37% of the binding reversed at 6 h in the presence of 5 microM unlabeled desloratadine. Seventeen histamine H(1)-receptor antagonists were evaluated in competition-binding studies. Desloratadine had a K(i) of 0.9+/-0.1 nM in these competition studies. In CHO-H(1) cells, histamine stimulation resulted in a concentration-dependent increase in [Ca(2+)](i) with an EC(50) of 170+/-30 nM. After a 90-min preincubation with desloratadine, the histamine-stimulated increase in [Ca(2+)](i) was shifted to the right, with a depression of the maximal response at higher concentrations of antagonist. The apparent K(b) value was 0.2+/-0.14 nM with a slope of 1.6+/-0.1. The slow dissociation from the receptor and noncompetitive antagonism suggests that desloratadine may be a pseudoirreversible antagonist of the human histamine H(1) receptor. The mechanism of desloratadine antagonism of the human histamine H(1) receptor may help to explain the high potency and 24-h duration of action observed in clinical studies.

Identification of a novel, orally bioavailable histamine H3 receptor antagonist based on the 4-benzyl-(1H-imidazol-4-yl) template

A novel series of histamine H(3) receptor antagonists, based on the 4-benzyl-(1H-imidazole-4-yl) template, incorporating urea and carbamate linkers has been prepared. Compound 3j is a selective H(3) antagonist and demonstrates excellent oral plasma levels in the rat and monkey.

Biexponential Kinetics of (R)‐α‐[3H]Methylhistamine Binding to the Rat Brain H3 Histamine Receptor

Abstract: The H3 histamine receptor is a high‐affinity receptor reported to mediate inhibition of CNS histidine decarboxylase activity and depolarization‐induced histamine release. We have used (R)‐α‐[3H]methylhistamine, a specific, high‐affinity agonist, to characterize ligand binding to this receptor. Saturation binding studies with rat brain membranes disclosed a single class of sites (KD= 0.68 nM; Bmax= 78 fmol/mg of protein). Competition binding assays also yielded an apparently single class of sites with a rank order of potency for ligands characteristic of an H3 histamine receptor: Nα‐methylhistamine, (R)‐α‐methylhistamine > histamine, thioperamide > impromidine > burimamide > dimaprit. In contrast, kinetic studies disclosed two classes of sites, one with fast, the other with slow on‐and‐off rates. Density of (R)‐α‐[3H]methylhistamine binding followed the order: caudate, midbrain (thalamus and hippocampus), cortex > hypothalamus > brainstem > cerebellum. These data are consistent with an H3 histamine receptor, distinct from H1 and H2 receptors, that occurs in two conformations with respect to agonist association and dissociation or with multiple H3 receptor subtypes that are at present pharmacologically undif‐ferentiated.

Desloratadine Inhibits Constitutive and Histamine-Stimulated Nuclear Factor-κB Activity Consistent with Inverse Agonism at the Histamine H1 Receptor

Background: The human histamine H1 receptor is constitutively active and exhibits basal activation of nuclear factor-ĸB (NF-ĸB), an important modulator of allergic inflammation. Certain H1 antihistamines have recently been shown to inhibit basal NF-ĸB activity by stabilizing the H1 receptor in an inactive state, a phenomenon called ‘inverse agonism’. Methods: We evaluated the effect of the new H1 antihistamine, desloratadine, on basal and histamine-stimulated NF-ĸB activity and compared it with the activities of other H1 antihistamines. Results: Transiently transfected COS-7 cells co-expressing NF-ĸB-luciferase and the H1 receptor exhibited constitutive NF-ĸB activity. H1 antihistamines reduced basal NF-ĸB activity (rank order of potency: desloratadine > pyrilamine > cetirizine > loratadine > fexofenadine). Histamine stimulated basal NF-ĸB activity 8-fold, which was blocked by H1 antihistamines (rank order of potency: desloratadine > cetirizine > pyrilamine > loratadine > fexofenadine). Neither histamine nor antihistamines had any effect on NF-ĸB activity in the absence of the H1 receptor. Conclusions: Desloratadine, acting through the histamine H1 receptor, inhibited basal NF-ĸB activity and can thus be classified as an inverse agonist. Inhibition of basal and histamine-stimulated NF-ĸB activity may help to explain previously reported inhibitory effects of desloratadine on allergic inflammatory mediators.

Identification of a dual histamine H1/H3 receptor ligand based on the H1 antagonist chlorpheniramine.

Combining the first generation H(1) antihistamine chlorpheniramine (1) with H(3) ligands of the alkylamine type has led to the identification of compound 9d, a dual ligand of both the H(1) and H(3) receptors.

Characterization of a digitonin-solubilized bovine brain H3 histamine receptor coupled to a guanine nucleotide-binding protein.

Abstract: The H3 receptor is a high‐affinity histamine receptor that inhibits release of several neurotransmitters, including histamine. We have characterized H3 receptor binding in bovine brain and developed conditions for its solubilization. Particulate [3H]histamine binding showed an apparently single class of sites (KD= 4.6 nM; Bmax= 78 fmol/mg of protein). Of the detergents tested, digitonin at a detergent/protein ratio of 1:1 (wt/wt) yielded the greatest amount of solubilized receptors, typically 15–30% of particulate binding. Neither equilibrium binding of [3H]histamine to receptors (KD= 6.1 nM; Bmax= 92 fmol/mg of protein) nor the inhibitor profile was substantially altered by digitonin solubilization. However, solubilization did increase the rate of [3H]histamine association with and dissociation from the receptor. Size‐exclusion chromatography indicated an apparent molecular weight of 220,000 for the solubilized receptor, and peak binding from this column retained its guanine nucleotide sensitivity. These last two observations are consistent with the solubilized receptor occurring in complex with a guanine nucleotide‐binding protein.

Substituted morpholine-2S-acetic acid derivatives: Sch 50911 and related compounds as novel GABAB antagonists

Abstract The synthesis and GABA B antagonist activity of a series of substituted morpholine-2-acetic acid derivatives is described. Resolution of the lead compound from the series produces one active and one inactive enantiomer. X-ray analysis of a halogenated derivative ( 25 ) of the active enantiomer Sch 50911 ( 23 ) shows that it possesses the 2 S configuration.

The synthesis of substituted fluorenes as novel non-imidazole histamine H3 inhibitors

A novel non-imidazole fluorene oxime 1a has been identified as a histamine H(3) inhibitor, and its structure-activity relationship has been evaluated.

Benzimidazole-substituted (3-phenoxypropyl)amines as histamine H3 receptor ligands.

A series of non-imidazole histamine H(3) receptor antagonists based on the (3-phenoxypropyl)amine motif, which is a common pharmacophore for H(3) antagonists, has been identified. A preliminary SAR study around the amine moiety has identified 8a as a potent H(3) antagonist possessing a good pharmacokinetic profile in the rat.

Trans-4-methyl-3-imidazoyl pyrrolidine as a potent, highly selective histamine H3 receptor agonist in vivo.

Extensive structural modification of immepyr (+)-2 led to the discovery of trans-4-methyl-3-imidazoyl pyrrolidine (+/-)-3a as a potent and highly selective H3 agonist. The pyrroline (+/-)-3a was resolved, and its (+) enantiomer, Sch 50971 [(+)-3a], showed a greater separation of H3 and H1 activities in vivo (H3/H1 ratio >> 330) than (R)-alpha-methylhistamine (+)-1 (H3/H1 ratio = 17), the standard H3 agonist.