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H3-receptor antagonists: synthesis and structure-activity relationships of para- and meta-substituted 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazoles

Mor, M.
Bordi, F.
Silva, C.
Rivara, S.
Crivori, P.
Plazzi, P. V.
Ballabeni, V.
Caretta, A.
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Published in Helvetica chimica acta. 1997, vol. 40, no. 16, p. 2571-2578
Abstract We report the synthesis, octanol/water partition coefficient (log P), dissociation constants (pKa), H3-receptor affinity (pKi in rat brain membranes, [3H]-N alpha-methylhistamine), and H3-antagonist potency (pA2 in guinea ileum, (R)-alpha-methylhistamine) of novel H3-receptor antagonists obtained by introducing a para or meta substituent on the phenyl ring of the lead compound 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazole (3a). The substituents were chosen to obtain broad and uncorrelated variation in their lipophilic, electronic, and steric properties. The log P values of the neutral species cover almost 3 orders of magnitude (from 1.40 to 4.11). The pKa,2 values (protonation of the 2-thioimidazole fragment) vary from 3.13 to 4.34, indicating that this fragment, which incorporates the so-called polar group common to many H3-receptor antagonists, is neutral at physiological pH. The compounds had pKi values in a range too narrow (from 7.28 to 8.03) to derive QSAR equations. In one case (3g), a biphasic displacement curve was observed (pKi,1 = 8.53; pKi,2 = 6.90). The pA2 values ranged 2 orders of magnitude (from 6.83 to 8.87) and yielded a QSAR model (PLS) indicating that antagonist potency depends parabolically on lipophilicity and is decreased by bulky para substituents. The compounds of this series, therefore, maintain a fair-to-good affinity for rat brain H3-receptor and a fair-to-good H3-antagonist potency on guinea pig ileum, although varying markedly in their lipophilicity. The series thus appears as a good candidate for pharmacokinetic optimization leading to brain-penetrating H3-receptor antagonists.
Keywords AnimalsBinding, CompetitiveCerebral Cortex/metabolismDimaprit/pharmacologyElectric StimulationEvoked PotentialsGuinea PigsHistamine Antagonists/*chemical synthesis/pharmacologyIleum/metabolismImidazoles/*chemical synthesis/pharmacologyMethylhistamines/metabolismRatsRats, WistarReceptors, Histamine H3/*metabolismStructure-Activity Relationship
PMID: 9258364
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MOR, M. et al. H3-receptor antagonists: synthesis and structure-activity relationships of para- and meta-substituted 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazoles. In: Helvetica chimica acta, 1997, vol. 40, n° 16, p. 2571-2578. doi: 10.1021/jm970070p

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Deposited on : 2010-08-06

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