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Drug-discovery projects frequently employ structure-based information through protein modeling and ligand docking, and there is a plethora of reports relating successful use of them in virtual screening. Hit/lead optimization, which represents the next step and the longest for the medicinal chemist, is very rarely considered. This is not surprising because lead optimization is a much more complex task. Here, a homology model of the histamine H(3) receptor was built and tested for its ability to discriminate ligands above a defined threshold of affinity. In addition, drug safety is also evaluated during lead optimization, and "antitargets" are studied. So, we have used the same benchmarking procedure with the HERG channel and CYP2D6 enzyme, for which a minimal affinity is strongly desired. For targets and antitargets, we report here an accuracy as high as at least 70%, for ligands being classified above or below the chosen threshold. Such a good result is beyond what could have been predicted, especially, since our test conditions were particularly stringent. First, we measured the accuracy by means of AUC of ROC plots, i. e. considering both false positive and false negatives. Second, we used as datasets extensive chemical libraries (nearly a thousand ligands for H(3)). All molecules considered were true H(3) receptor ligands with moderate to high affinity (from microM to nM range). Third, the database is issued from concrete SAR (Bioprojet H(3) BF2.649 library) and is not simply constituted by few active ligands buried in a chemical catalogue.

A series of 4-(ω-taryialkyloxy)alkyl)-1H-imidazoles, and related sulphur-containing compounds have been prepared and evaluated for their histamine H 3 -autoreceptor antagonist in vuro potency in an assay on synaptosomes of rat cerebral cortex. in addition, the in vivo potency has been determined from the changes in N τ -methylhistamine levels in brain after p.o. administration to mice. Compounds with different alkyl chains and various aryl moities nave been synthesized and tested to explore structure-activity relationships. Within this series of novei antagonists, (1H-imidazol-4-yl)methyl and 2-(1H-imidazol-4-yl)ethyl ether derivatives showed low to moderate H 3 -receptor antagonist potency, whereas the corresponding allyl and propyl derivatives were compounds with high antagonist in vivo potency. Corresponding thioether or sulphoxide derivatives also showed antagonist activity. Additionally, some ether derivatives possessed high in vivo potency as well. The most active ether derivatives under in vivo conditions were 4- 3-(3-(4-fluorophenyl)propyloxy)propyl)-1H-imidazole (11b) and the corresponding chloro compound 11c (FUB 181) with ED 50 values of 0.76 and 0.80 mg/kg, respectively. On the other hand, all compounds tested showed weak activity at histamine H 1 or H 2 receptors. Furthermore, the most promising ether FUB 181 exhibited low activity at adrenergic μ 1 , β 1/2 , serotonergic 5-HT 2A , 5-HT; and muscarinic M 3 receptors. Time-course investigations of FUB 181 in mice showed a rapid mode of action with the highest value 3 h after p.o. application. Thus, FUB 181 appears to block histamine H 3 receptors potently and selectively.

In search of new histamine H 3 -receptor ligands sixteen ether derivatives of 3-(1H-imidazol-4-yl)propanol with benzylic partial structure or related moieties were prepared and investigated as H 3 -receptor antagonists. The new compounds belong to a general construction pattern developed by other histamine H 3 -receptor antagonists. Structural modifications were introduced in an attempt to optimize in vitro as well as in vivo activity. Structure-activity relationships of the new histamine H 3 -receptor antagonists are discussed. All ether derivatives showed in vitro activities in the nanomolar concentration range, but only compounds with bulky lipophilic residues were also active under in vivo conditions. The most active compound within this series was 3-(1H-imidazol-4-yl)propyl 1-naphthylmethyl ether (4n) presenting an ED 50 or 3.2 ± 1.9 mg/kg regarding enhancement of endogenous histamine in brain after p.o. administration to mice. Furthermore, comparison of the H 3 -receptor activities measured on synaptosomes of rat cerebral cortex and on guinea pig ileum gave a good correlation indicating homogeneity of central and peripheral H 3 -receptor test models. The most interesting compounds were also evaluated in functional in vitro assays with regard to their activities at histamine H 1 -, H 2 -, and muscarinic M 3 -receptors. The tested compounds showed very weak activities at these receptor subtypes demonstrating their H 3 -receptor selectivity.

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.

In an effort to design new hybrid compounds with dual properties, i.e. binding affinity at histamine H(3) receptors and inhibitory potency at the catabolic enzyme histamine N(tau)-methyltransferase (HMT), a novel series of 1-substituted piperidine derivatives was synthesized. This alicyclic heterocycle is structurally linked via aminoalkyl spacers of variable lengths to additional aromatic carbo- or hetero-cycles. These new hybrid drugs were pharmacologically evaluated regarding their binding affinities at recombinant human H(3) receptors, stably expressed in CHO cells, and in a functional assay for their inhibitory potencies at rat kidney HMT. All compounds investigated proved to be H(3) receptor ligands with binding affinities in the micro- to nanomolar concentration range despite significant differences in the type of the aromatic moiety introduced. The most potent compound in this series was the quinoline derivative 20 (K(i) = 5.6 nM). Likewise, all new ligands studied showed impressive HMT inhibitory activities. Here, compounds 5, 10, 14 and 18-20 exhibited submicromolar potencies (IC(50) = 0.061-0.56 microM). The aminomethylated quinoline 19 showed almost the same, well balanced nanomolar activities on both targets. In this study, new hybrid compounds with a dual mode biological action were developed. These pharmacological agents are valuable leads for further development and candidates for treatment of histamine-dependent disorders.

Para-substituted aromatic ethers with benzophenone or related structural elements and a 3-(1H-imidazol-4-yl)propyloxy moiety were prepared by Mitsunobu-type ether synthesis or SNAr reaction. Most of the title compounds possess high antagonist potency in histamine H3-receptor assays in vitro as well as in vivo in mouse CNS following oral administration. After defining 4-(3-(1H-imidazol-4-yl)propyloxy)phenyl phenyl methanone as a new lead, structure-activity relationships were investigated for this new class of compounds. Substitution of the meta'-position of the benzophenone moiety with halogen atoms (e.g., iodine, fluorine) led to compounds with high antagonist potency in vitro as well as in vivo (Ki = 9.3 and 4.3 nM, ED50 = 0.7 and 0.47 mg/kg p.o., 18 and 12, respectively). A receptor profile of several functional in vitro assays for several biogenic amine receptors for the meta'-iodinated derivative demonstrated high selectivity toward the histamine H3 receptor.

Cyclopropyl 4-(3-(1H-imidazol-4-yl)propyloxy)phenyl methanone (ciproxifan) is a novel reference antagonist for the histamine H3 receptor. Despite the former Mitsunobu reaction the actual key reaction for preparation based on SNAr for acylated fluoroaromatics with an additional cyclization in a one-pot procedure needs no chromatographic purification steps and results in good yields.

In search of structure-activity relationships among histamine H3-receptor antagonists the imidazole ring of known H3-receptor antagonists was replaced by different heteroaromatic ring systems. Thus, azines and diazines with ether (6-13) and carbamate (15-24) moieties as functional groups were synthesized. The obtained compounds did not show significant H3-receptor antagonist activity in vitro (rat brain cortex) or in vivo (mice brain). The new compounds were also screened for H1-receptor antagonist activity on the isolated guinea-pig ileum and for H2-receptor antagonist activity on the isolated spontaneously beating guinea-pig right atrium. The substances showed only weak antagonistic activity at both histamine receptors, H1 and H2.