N1-Aryl(heteroaryl)alkyl-N2-[3-(1H-imidazol-4-yl)propyl]guanidines are potent histamine H2-receptor (H2R) agonists, but their applicability is compromised by the lack of oral bioavailability and CNS penetration. To improve pharmacokinetics, we introduced carbonyl instead of methylene adjacent to the guanidine moiety, decreasing the basicity of the novel H2R agonists by 4-5 orders of magnitude. Some acylguanidines with one phenyl ring were even more potent than their diaryl analogues. As demonstrated by HPLC-MS, the acylguanidines (bioisosteres of the alkylguanidines) were absorbed from the gut of mice and detected in brain. In GTPase assays using recombinant receptors, acylguanidines were more potent at the guinea pig than at the human H2R. At the hH1R and hH3R, the compounds were weak to moderate antagonists or partial agonists. Moreover, potent partial hH4R agonists were identified. Receptor subtype selectivity depends on the imidazolylpropylguanidine moiety (privileged structure), opening an avenue to distinct pharmacological tools including potent H4R agonists.

Species isoforms of histamine H 2 -, H 3 -, and H 4 -receptors differ in their pharmacological properties. The study aim was to dissect differences between the human H 1 R (hH 1 R) and guinea pig H 1 R (ghH 1 R). We coexpressed hH 1 R and gpH 1 R with regulators of G-protein signaling in Sf9 insect cells and analyzed the GTPase activity of G q -proteins. Small H 1 R agonists showed similar effects at hH 1 R and gpH 1 R, whereas bulkier 2-phenylhistamines and histaprodifens were up to ϳ10-fold more potent at gpH 1 R than at hH 1 R. Most 2-phenylhistamines and histaprodifens were more efficacious at gpH 1 R than at hH 1 R. Several first-generation H 1 R antagonists were ϳ2-fold, and arpromidine-type H 1 R antagonists up to ϳ10-fold more potent at gpH 1 R than at hH 1 R. [ The Phe-1533 Leu-153/Ile-4333 Val-433 double mutant expressed excellently but only partially changed the pharmacological properties of hH 1 R. Small H 1 R agonists and 2-phenylhistamines interacted differentially with human and guinea pig H 2 R in terms of potency and efficacy, respectively. Our data show the following: 1) there are differences in agonist-and antagonistpharmacology of hH 1 R and gpH 1 R encompassing diverse classes of bulky ligands. These differences may be explained by higher conformational flexibility of gpH 1 R relative to hH 1 R; 2) Phe-153 and Ile-433 are critical for proper folding and expression of hH 1 R; and 3) H 2 R species isoforms distinguish between H 1 R agonists.Histamine serves as a neurotransmitter and autacoid and acts through specific H x Rs designated as H 1 R, H 2 R, H 3 R, and H 4 R, respectively (Hill et al., 1997;Hough, 2001). The H 1 R couples to G q -proteins. Numerous H 1 R agonists and antagonists are known. H 1 R agonists are divided into three classes ( Fig. 1): 1) small agonists (2-4) derived from histamine (1), 2) histamine derivatives with bulkier aromatic substituents at position 2 of the imidazole ring (5-18), and 3) histaprodifens, e.g., compounds 19 to 23 Zingel et al., 1995;Elz et al., 2000). H 1 R agonists are important experimental tools to analyze H 1 R function in cellular and organ systems (Zingel et al., 1995;Hill et al., 1997). H 1 R antagonists are commonly divided into sedating (first-generation, 24-32) and nonsedating (second-generation, 41-45) antagonists (Fig. 2). Today, especially the second-generation H 1 R antagonists are of great importance for the treatment of allergic diseases (Hill et al., 1997). Guanidines 33, 34, and 36 to 39 derived from arpromidine (35) are dual H 2 R agonists/ H 1 R antagonists (Buschauer, 1989).The availability of H x R cDNAs allowed for the comparison of the pharmacological properties of H x R species isoforms in recombinant systems under identical experimental conditions. Such expression studies uncovered species differences This work was supported by the National Institutes of Health COBRE Award 1 P20 RR15563 and matching support from the State of Kansas and the University of Kansas (R.S.), a grant from the Army Research Office (DAAD 19-00-...

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.

Epigenetic modifiers of the histone deacetylase (HDAC) family contribute to autoimmunity, cancer, HIV infection, inflammation, and neurodegeneration. Hence, histone deacetylase inhibitors (HDACi), which alter protein acetylation, gene expression patterns, and cell fate decisions, represent promising new drugs for the therapy of these diseases. Whereas pan-HDACi inhibit all 11 Zn-dependent histone deacetylases (HDACs) and cause a broad spectrum of side effects, specific inhibitors of histone deacetylase 6 (HDAC6i) are supposed to have less side effects. We present the synthesis and biological evaluation of Marbostats, novel HDAC6i that contain the hydroxamic acid moiety linked to tetrahydro-β-carboline derivatives. Our lead compound Marbostat-100 is a more potent and more selective HDAC6i than previously established well-characterized compounds in vitro as well as in cells. Moreover, Marbostat-100 is well tolerated by mice and effective against collagen type II induced arthritis. Thus, Marbostat-100 represents a most selective known HDAC6i and the possibility for clinical evaluation of a HDAC isoform-specific drug.

The synthesis and biological evaluation of a new class of histamine H2 antagonists with N-cyano-N'-[omega-[3-(1-piperidinylmethyl)phenoxy] alkyl]guanidine partial structure are described as part of an extensive research program to find model compounds for the development of new radioligands with high H2 affinity and specific activity. High receptor affinity is achieved by an additional (substituted) aromatic ring, which is connected with the third guanidine N by a carbon chain spacer and an amine, carboxamide, ester, or sulfonamide link ("polar group"). In functional studies for H2 antagonistic activity and other pharmacological actions [e.g. H1 antihistaminic, antimuscarinic, antiadrenergic (alpha 1, beta 1), 5-HT2 blocking activity] in the isolated guinea pig atrium and ileum and rat aorta and tail artery, the compounds proved to be highly potent and selective histamine H2 receptor antagonists. The H2 antagonistic activity is mainly depending on the length of both the N'-alkyl chain (chain A) and the N"-spacer (chain B). Compounds with a C3 chain A and a C2 chain B are most potent in the preferred group of substances, i.e., the carboxamide series. A wide variety of substituents at the aromatic ring is tolerated, among them iodine, amino, and azido groups. These compounds are up to 32 times more potent than cimetidine in the isolated guinea pig right atrium. The replacement of the carboxamide by an ester group (44c) is well tolerated, while replacement of the cyanoguanidine by an urea group results in nearly 100-fold decrease in activity (46c,e). The iodinated benzamides are among the most potent H2 antagonists known so far. The [125I]-labeled form of 31f ([125I]iodoaminopotentidine, [125I]-N-[2-(4-amino-3-iodobenzamido) ethyl]-N'-cyano-N"-[3-[3-(1-piperidinylmethyl) phenoxy]propyl]guanidine) and its photolabile analogue 31h ([125I]iodoazidopotentidine, [125I]-N-[2-(4-azido-3- iodobenzamido)ethyl]-N'-cyano-N"-[3-[3-(1-piperidinyl-methyl)pheno xy] propyl]guanidine) proved to be useful probes for reversible and irreversible labeling of the histamine H2 receptor. Radioligand binding studies in guinea pig cerebral membranes revealed considerably higher H2 receptor affinity for 31f (pKi = 9.15), 31h (pKi = 8.58), and some analogues than functional experiments (guinea pig atrium), presumably reflecting an easier access to the H2 receptors in membranes.

A series of new piperidinomethylphenoxypropylamine-type histamine H2 receptor (H2 R) antagonists with different substituted "urea equivalents" was synthesized and characterized in functional in vitro assays. Based on these data as selection criteria, radiosynthesis of N-[6-(3,4-dioxo-2-{3-[3-(piperidin-1-ylmethyl)phenoxy]propylamino}cyclobut-1-enylamino)hexyl]-(2,3-(3) H2 )propionic amide ([(3) H]UR-DE257) was performed. The radioligand (specific activity: 63 Ci mmol(-1) ) had high affinity for human, rat, and guinea pig H2 R (hH2 R, Sf9 cells: Kd , saturation binding: 31 nM, kinetic studies: 20 nM). UR-DE257 revealed high H2 R selectivity on membranes of Sf9 cells, expressing the respective hHx R subtype (Ki values: hH1 R: >10000 nM, hH2 R: 28 nM, hH3 R: 3800 nM, hH4 R: >10000 nM). In spite of insurmountable antagonism, probably due to rebinding of [(3) H]UR-DE257 to the H2 R (extended residence time), the title compound proved to be a valuable pharmacological tool for the determination of H2 R affinities in competition binding assays.

The bioisosteric replacement of the guanidino group in arpromidine-like histamine H(2) receptor (H(2)R) agonists by an acylguanidine moiety is a useful approach to obtain potent H(2)R agonists with improved oral bioavailability and blood-brain barrier penetration. Unfortunately, the selectivity of such N(G)-acylated imidazolylpropylguanidines for the H(2)R is poor, in particular versus histamine H(3) (H(3)R) and H(4) receptors (H(4)R). This drawback appears to depend on the "privileged" imidazolylpropylguanidine structure. The 2-amino-4-methylthiazol-5-yl moiety is a bioisostere of the imidazole ring in the moderately potent H(2)R-selective histamine analogue amthamine. This approach was successfully applied to acylguanidine-type H(2)R agonists. The aminothiazoles are nearly equipotent to the corresponding imidazoles as H(2)R agonists. Compared with histamine, the potency is increased up to 40-fold on the guinea pig right atrium, and up to 125- and 280-fold in GTPase assays with human and guinea pig H(2)R-G(salphaS) fusion proteins expressed in Sf9 insect cells, respectively. Docking studies on H(2)R models support the hypothesis that 2-aminothiazolyl and imidazolyl derivatives interact with H(2)Rs as bioisosteres. In contrast to the imidazoles, the aminothiazoles are devoid of agonistic or relevant antagonistic effects on H(1), H(3), and H(4) receptors. Moreover, unlike amthamine, the 4-methyl group does not significantly contribute to the H(2)R agonism of N(G)-acylated 2-amino-4-methylthiazol-5-ylpropylguanidines.

On the basis of the long-known prototypic pharmacophore 3-(1H-imidazol-4-yl)propylguanidine (SK&F 91486, 2), monomeric, homodimeric, and heterodimeric bisalkylguanidine-type histamine H2 receptor (H2R) agonists with various alkyl spacers were synthesized. Aiming at increased H2R selectivity of the ligands, the imidazol-4-yl moiety was replaced by imidazol-1-yl, 2-aminothiazol-5-yl or 2-amino-4-methylthiazol-5-yl according to a bioisosteric approach. All compounds turned out to be partial or full agonists at the h/gp/rH2R. The most potent analogue, the thiazole-type heterodimeric ligand 63 (UR-Po461), was a partial agonist (Emax = 88%) and 250 times more potent than histamine (pEC50: 8.56 vs 6.16, gpH2R, atrium). The homodimeric structures 56 (UR-Po395) and 58 (UR-Po448) exhibited the highest hH2R affinities (pKi: 7.47, 7.33) in binding studies. Dimeric amino(methyl)thiazole derivatives, such as 58, generated an increased hH2R selectivity compared to the monomeric analogues, e.g., 139 (UR-Po444). Although monomeric ligands showed up lower affinities and potencies at the H2R, compounds with a short alkylic side chain like 129 (UR-Po194) proved to be highly affine hH4R ligands.