Synthesis and structural and pharmacological properties of cyclopropane-based conformationally restricted analogs of 4-methylhistamine as histamine H3/H4 receptor ligands.

On the basis of the previous results on a histamine H(4) receptor agonist 4-methylhistamine and a cyclopropane-based conformationally restricted analog CEIC (3) with potent H(3)/H(4) receptor antagonistic effect, 4-methylhistamine analogs 4 and 5 of CEIC were designed and synthesized. Compound 4 showed strong affinity (K(i)=38.7 nM) for the H(3) receptor, which was more potent than a well-known H(3) antagonist thioperamide. Stable tautomer and conformation of 3 and 4, which can affect the pharmacological activity, were analyzed by ab initio calculations.

Enzyme-catalyzed prodrug approaches for the histamine H3-receptor agonist (R)-alpha-methylhistamine.

Five novel prodrug types of the potent and selective histamine H3-receptor agonist (R)-alpha-methylhistamine (1) were prepared and pharmacologically tested in vitro as well as in vivo. In particular, an amide of fatty acid, mono- and dicarbamates, an (acyloxy)alkylcarbamate, and a diphthalidyl derivative were synthesized, all of which require initial prodrug activation through an enzyme-catalyzed reaction in contrast to formerly developed azomethine prodrugs which are cleaved by chemical hydrolysis only. Further drug liberation may ensue spontaneously in a cascade to give 1. Since they have diverse stabilities the prodrugs were investigated for drug liberation in vitro under neutral, acidic, and basic conditions at different temperatures as well as with liver homogenates. In vivo investigation of prodrugs after oral administration to mice proved that the fatty amide 2, the Nalpha-methylcarbamate 4a, and the Nalpha-(1-(acetyloxy)ethylcarbamate) 5 showed moderate to high plasma levels of 1. Compound 5 displayed even more than 2.5 times the AUC for 1 than that of the reference azomethine prodrug BP2.94 in the periphery and also displayed a detectable drug level in the central nervous system. It was shown that prodrug approaches based on an initial enzyme-catalyzed liberation step are successfully applicable to different pro-moieties for improved bioavailability and prolonged half-live. These approaches may also be used for other aminergic compounds of this class to optimize pharmacokinetic behavior.

Histaprodifens: synthesis, pharmacological in vitro evaluation, and molecular modeling of a new class of highly active and selective histamine H(1)-receptor agonists.

A new class of histamine analogues characterized by a 3, 3-diphenylpropyl substituent at the 2-position of the imidazole nucleus has been prepared outgoing from 4,4-diphenylbutyronitrile (4b) via cyclization of the corresponding methyl imidate 5b with 2-oxo-4-phthalimido-1-butyl acetate or 2-oxo-1,4-butandiol in liquid ammonia, followed by standard reactions. The title compounds displayed partial agonism on contractile H(1) receptors of the guinea-pig ileum and endothelium-denuded aorta, respectively, except 10 (histaprodifen; 2-[2-(3, 3-diphenylpropyl)-1H-imidazol-4-yl]ethanamine) which was a full agonist in the ileum assay. While 10 was equipotent with histamine (1), methylhistaprodifen (13) and dimethylhistaprodifen (14) exceeded the functional potency of 1 by a factor of 3-5 (13) and 2-3 (14). Compounds 10 and 13-17 relaxed precontracted rat aortic rings (intact endothelium) with relative potencies of 3.3- up to 28-fold (compared with 1), displaying partial agonism as well. Agonist effects were sensitive to blockade by the selective H(1)-receptor antagonist mepyramine (pA(2) approximately 9 (guinea-pig) and pA(2) approximately 8 (rat aorta)). The affinity of 10 and 13-17 for guinea-pig H(1) receptors increased 20- to 100-fold compared with 1. Two lower homologues of 10 were weak partial H(1)-receptor agonists while two higher homologues of 10 were silent antagonists endowed with micromolar affinity for rat and guinea-pig H(1) receptors. In functional selectivity experiments, 10, 13, and 14 did not stimulate H(2), H(3), and several other neurotransmitter receptors. They displayed only low to moderate affinity for these sites (pA(2) < 6). For a better understanding of structure-activity relationships, the interaction of 1 and 10, 13 and 14 within the transmembrane (TM) domains of the human histamine H(1) receptor were studied using molecular dynamics simulations. Remarkable differences were found between the binding modes of 10, 13, and 14 and that of 1. The imidazole ring of 10, 13, and 14 was placed 'upside down' compared with 1, making the interaction of the N(pi)-atom with Tyr431 possible. This new orientation was mainly caused by the space filling substitution at the 2-position of the imidazole ring and influenced the location of the protonated N(alpha)-atom which was positioned more between TM III and TM VI. This orientation can explain both the increased relative potency and the maximum effect of 10, 13, and 14 compared with 1. Compound 13 (methylhistaprodifen; N(alpha)-methyl-2-[2-(3, 3-diphenylpropyl)-1H-imidazol-4-yl]ethanamine) is the most potent histamine H(1)-receptor agonist reported so far in the literature and may become a valuable tool for the study of physiological and pathophysiological H(1)-receptor-mediated effects.

Synthesis of tritium-labelled N tau-methylhistamine for the improvement of extraction efficiency of N tau-methylhistamine from biological fluids.

In order to trace the loss of N tau-methylhistamine, a principal metabolite of histamine, during extraction and purification from human plasma and urine samples, N tau-[3H]methylhistamine was prepared in two steps from N alpha t-butoxycarbonylhistamine (II). In the first step, compound II was deprotonated with NaH in an aprotic solvent and treated with [3H]methyl iodide. The products, N alpha t-butoxycarbonyl-N tau-[3H]methylhistamine (III) and N alpha t-butoxycarbonyl-N pi-[3H]methylhistamine (IV), were then hydrolysed with iodotrimethylsilane under mild and short reaction conditions. Facile purification with Sep-Pak silica cartridges gave the combined two isomers of N tau-[3H]methylhistamine and N pi-[3H]methylhistamine in 10.7% radiochemical yield with a radiochemical purity of > 94% and a ratio of approximately 2:1. Improvements in the extraction of methylhistamine using chromatography on Sep-Pak silica cartridges led to an overall recovery of 82.5 +/- 0.3% (n = 3) based upon total [3H]methylhistamine from normal human plasma.

Structure-activity relationships of novel azomethine prodrugs of the histamine H3-receptor agonist (R)-alpha-methylhistamine: from alkylaryl to substituted diaryl derivatives.

This study was performed on the basis of recently developed prodrugs of the histamine H3-receptor agonist (R)-alpha-methylhistamine (1) to determine structure-activity relationships of azomethine prodrugs of 1, in which the primary amine functionality is bioreversibly linked to aromatic ketones. Therefore, the pro-moiety was systematically altered from alkylaryl over benzylaryl to diaryl substitution. Those compounds that emerged to be stable enough during preparation were tested for their in vitro hydrolysis rates. Apparently, bulky alkyl residues were capable of preventing previously observed intramolecular cyclization, but the obtained azomethines 12a-c were far too unstable to serve as prodrugs. However, the benzylaryl imines 12d, e were stable compounds, but 12d decomposed too rapidly under in vitro conditions. Distinctly greater stability was provided by diaryl pro-moieties, even if strongly electron-withdrawing functionalities were introduced. Selected compounds were also tested in vivo following p.o. application to mice. Particularly the trifluoromethyl substituted imine 12i proved to be highly effective as stability and rate of conversion were well-balanced, so that brain penetration of 1 was strikingly facilitated. Thus 12i, a highly potent azomethine prodrug, may serve as an important pharmacological tool and, possibly, a therapeutic agent.

New potent azomethine prodrugs of the histamine H3-receptor agonist (R)-alpha-methylhistamine containing a heteroarylphenyl partial structure.

The therapeutic value of histamine H3-receptor ligands is under current investigation. On the basis of recently described diary limine prodrugs of the histamine H3-receptor agonist (R)-alpha-methylhistamine (1) as a series of new azomethine prodrugs containing five- and six-membered heterocycles were synthesized and tested for their in vitro hydrolysis rates and in vivo activity after oral application. It was found that electron-deficient six-membered heterocycles drastically destabilized the imine double bond so that these prodrugs decomposed unsuitably fast. On the contrary, prodrugs containing five-membered heterocycles appeared to be highly effective for the CNS delivery of 1, and a remarkable correlation between chemical structure and pharmacokinetic profile was observed. Particularly (R)-4-fluoro-2-[[N-[1-(1H-imidazol-4-yl)-2-propyl]imino] (1H-pyrrol-2-yl)methyl]phenol (8c), the 2-furanyl analogue 8d, and its 3-furanyl isomer 8e proved to be equipotent to the most potent of recently described halogenated diaryl imine prodrugs of 1. However, in contrast to any other azomethine prodrug, 8c exhibited an incomparably long lasting delivery of 1 in the CNS and can thus be regarded as a 'retard' prodrug. Assuming that a therapeutic indication of histamine H3-receptor agonists will soon be established, these highly potent heteroarylphenyl azomethine prodrugs, which already serve as valuable pharmacological tools, may also become potential drugs in clinical use.

Synthesis, X-ray crystallography, and pharmacokinetics of novel azomethine prodrugs of (R)-alpha-methylhistamine: highly potent and selective histamine H3 receptor agonists.

Since various neuroregulatory functions of the histamine H3 receptor have been proved during the last few years, the H3 receptor is of current interest. Azomethine derivatives of the highly potent histamine H3 receptor agonist (R)-alpha-methylhistamine (1) were prepared as lipophilic prodrugs to improve the bioavailability of the hydrophilic drug, particularly its entry into the brain. Additionally, azomethine derivatization provides protection against histamine methyltransferase, the major metabolizing enzyme in man, and thus efficiently enhances the bioavailability of 1. The molecular conformations of (R)-2(-)[[N(-)[1-(1H-imidazol-4-yl)-2-propyl]- imino]phenylmethyl]phenol (9a) and (R)-4-fluoro-2(-)[[N(-)[1-(1H-imidazol-4-yl)-2-propyl[imino]- (4-chlorophenyl)methyl]phenol (9p) were determined by X-ray structure analysis. An intramolecular hydrogen bond which is essential for the stability of these azomethines was thereby confirmed. Moreover, the pharmacokinetic parameters of the prodrugs were investigated in vitro as well as in vivo. The halogenated azomethines have an effect following peroral administration in mice, and some of them seem to be highly potent for the central nervous system (CNS) delivery of 1. At present the most potent prodrug of 1 is (R)-4-chloro-2(-)[[N(-)[1-(1H-imidazol-4-yl)-2-propyl]imino](4- chlorophenyl)methyl]phenol (9q), reaching by far the highest CNS level of 1 (Cmax = 71 ng/g). Prodrugs of this type are not only valuable pharmacological tools but may also become H3 histaminergic drugs for therapeutic use.

Cactus alkaloids. XLVII. N alpha-dimethylhistamine, a hypotensive component of Echinocereus triglochidiatus.

Echinocereus triglochidiatus Engelm. var. neomexicanus (Standley) Standley ex W. T. Marshall is believed to cause psychotropic effects when consumed by the Mexican Tarahumara Indians. A phytochemical study was initiated with E. triglochidiatus Engelm. var. paucispinus Engelm. ex W. T. Marshall, which is more abundant in Texas. In the fractionated extracts three compounds, detected by thin-layer chromatography, were positive to Ehrlich's reagent, indicating the possible presence of indole alkaloids; however, a non-Erhlich positive alkaloid was crystallized as the dihydrochloride and subsequently identified (spectrometrically and via synthesis as N alpha, N alpha-dimethylhistamine dihydrochloride. The same compound was then detected chromatographically in the neomexicanus variety. This compound has peripheral hypotensive effects similar to histamine, and this action may help to explain the alleged psychotrophic effects of the cactus.