Characterization of novel selective H1-antihistamines for clinical evaluation in the treatment of insomnia.

Analogues of the known H(1)-antihistamine R-dimethindene were profiled as potential agents for the treatment of insomnia. Several highly selective compounds were efficacious in rodent sleep models. On the basis of overall profile, indene 1d and benzothiophene 2a had pharmacokinetic properties suitable for evaluation in night time dosing. Compound 2a did not show an in vivo cardiovascular effect from weak hERG channel inhibition.

Analytical method for simultaneously measuring ex vivo drug receptor occupancy and dissociation rate: application to (R)-dimethindene occupancy of central histamine H1 receptors.

We introduce a novel experimental method to determine both the extent of ex vivo receptor occupancy of administered compound and its dissociation rate constant (k4). [Here, we reference k4 as the rate of offset of unlabeled ligand in convention with Motulsky and Mahan (1)]. We derived a kinetic rate equation based on the dissociation rate constant for an unlabeled compound competing for the same site as a labeled compound and describe a model to simulate fractional occupancy. To validate our model, we performed in vitro kinetics and ex vivo occupancy experiments in rat cortex with varying concentrations of (R)-dimethindene, a sedating antihistamine. Brain tissue was removed at various times post oral administration, and histamine H1 receptor ligand [3H]-doxepin binding to homogenates from drug-treated or vehicle-treated rats was measured at multiple time points at room temperature. Fractional occupancy and k4 for (R)-dimethindene binding to H1 receptors were calculated by using our proposed model. Rats dosed with 30 and 60 mg/kg (R)-dimethindene showed 42% and 67% occupancy of central H1 receptors, respectively. These results were comparable to occupancy data determined by equilibrium radioligand binding. In addition, drug k4 rate determined by using our ex vivo method was equivalent to k4 determined by in vitro competition kinetics (dissociation half-life t(1/2) approximately 30 min). The outlined method can be used to assess, by simulation and experiment, occupancy for compounds based on dissociation rate constants and contributes to current efforts in drug optimization to profile antagonist efficacy in terms of its kinetic drug-target binding parameters. Data described by the method may be analyzed with commercially available software. Suggested fitting procedures are given in the appendix.

Studies of the metabolism of the antihistaminic drug dimethindene by high-performance liquid chromatography and capillary electrophoresis including enantioselective aspects.

A reversed-phase high-performance liquid chromatography method for the determination of dimethindene and its main metabolites N-demethyldimethindene, 6-hydroxydimethindene and 6-hydroxy-N-demethyldimethindene in human urine was developed. The assay was also applied to the quantification of dimethindene-N-oxide in rat urine. Conjugates of the hydroxylated metabolites were determined after enzymatic deconjugation. Moreover the direct determination of dimethindene and its metabolites without prior extraction from urine was performed by capillary electrophoresis. The direct simultaneous determination of the enantiomers of dimethindene and N-demethyldimethindene was achieved on a Chiralcel OD column. Urinary data after oral administration of dimethindene are presented. The assays were used to study dimethindene and it metabolites in urine upon oral administration of the drug to rats and human volunteers.

Metabolism of dimetindene in rats.

The preparative separation of dimetindene (CAS 5636-83-9) enantiomers was achieved by fractionated crystallization of the diastereomeric tartrate salts. HPLC on alpha-AGP (alpha 1-acid glycoprotein) was used for confirmation of the enantiomeric purity. After administration of the enantiomers to rats the AUC and Cmax of S(+) dimetindene and (-)N-demethyldimetindene were slightly increased. In agreement with the serum concentration data significantly more (-)N-demethyldimethindene was excreted into urine after administration of the individual enantiomers. S(+) dimetindene was metabolised to a lesser extent in vivo (see above) as well as in vitro in rat liver homogenate. After prior enzyme induction conversion of the enantioselectivity with respect to unmetabolised dimetindene occurred. 6-Methoxydimetindene, 6-hydroxydimetindene and 6-hydroxy-N-demethyldimetindene were synthesized. 6-Methoxydimetindene is likely to be a metabolite since the retention times of synthetic compound and the substance extracted from serum and urine are identical. The formation of 6-hydroxy-N-demethyldimetindene was proved for the first time by comparison of the mass spectra of metabolite isolated from in vitro incubations and synthetic compound after derivatisation with acetic anhydride.

Sedation and histamine H1-receptor antagonism: studies in man with the enantiomers of chlorpheniramine and dimethindene.

1. The effects of 10 mg (+)- and (-)-chlorpheniramine and 5 mg (+)- and (-)-dimethindene on daytime sleep latencies, digit symbol substitution and subjective assessments of mood and well-being were studied in 6 healthy young adult humans. Each subject also took 5 mg triprolidine hydrochloride as an active control and two placebos. 2. Daytime sleep latencies were reduced with triprolidine, (+)-chlorpheniramine and (-)-dimethindene, and subjects also reported that they felt more sleepy after (+)-chlorpheniramine and (-)-dimethindene. Performance on digit symbol substitution was impaired with (+)-chlorpheniramine. 3. Changes in measures with (-)-chlorpheniramine and (+)-dimethindene were not different from changes with placebo. 4. In the present study, changes in measures of drowsiness and performance were limited to the enantiomers with high affinity for the histamine H1-receptor. These findings strongly suggest that sedation can arise from H1-receptor antagonism alone, and provide further support for the belief that the histaminergic system is concerned with the regulation of alertness in man.

Enantioselective determination of dimethindene in urine after oral administration of racemic dimethindene.

A sensitive high-performance liquid chromatographic determination of dimethindene and its metabolite N-demethyldimethindene in urine has been developed. The quantitative analysis was followed by determination of the enantiomeric ratio of dimethindene on an alpha 1-AGP column (EnantioPac). The urinary data for nine volunteers after oral administration of racemic dimethindene are presented. The isolation, identification and synthesis of the metabolite N-demethyldimethindene are reported.

Identification of the molecular structure of the phenolic primary metabolite of dimetindene in animals and man.

The metabolic pathway of dimetindene (dimetindene maleate, Fenistil), an antiallergic drug commercially available for more than 20 years, has remained unknown. By means of HPLC, GC-MS and MS-MS, dimetindene was found to be hydroxylated on the indene moiety of the molecule both in vitro with hepatic microsomes of several species and in vivo in man. Cochromatographic analysis (HPLC and GC-MS) of the primary metabolites produced in vitro and in vivo demonstrated their similarities but revealed differences from the chemically synthesized 5-hydroxy-dimetindene. Using large volumes of in vitro incubations of rat microsomes with dimetindene and cofactors, 1.15 mg of pure primary metabolite were first produced and then extensively purified. The study of the NMR proton in one and two dimensions (COSY) clearly demonstrated that this metabolite is hydroxylated on the C6 of the indene moiety as compared to the C5 for the synthetic product.