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Lead optimization guided by histamine H3 receptor (H3R) affinity and calculated physico-chemical properties enabled simultaneous improvement in potency and PK properties leading to the identification of a potent, selective, devoid of hERG issues, orally bioavailable, and CNS penetrable H3R antagonist/inverse agonist 3h. The compound was active in forced-swimming tests suggesting its potential therapeutic utility as an anti-depressive agent. This Letter further includes its cardiovascular and neuropsychological/behavioral safety assessments.

Previous studies have shown that compound 1 displayed high affinity towards histamine H3 receptor (H3R), (human (h-H3R), K(i)=8.6 nM, rhesus monkey (rh-H3R), K(i)=1.2 nM, and rat (r-H3R), K(i)=16.5 nM), but exhibited high affinity for hERG channel. Herein, we report the discovery of a novel, potent, and highly selective H3R antagonist/inverse agonist 5a(SS) (SAR110068) with acceptable hERG channel selectivity and desirable pharmacological and pharmacokinetic properties through lead optimization sequence. The significant awakening effects of 5a(SS) on sleep-wake cycles studied by using EEG recording in rats during their light phase support its potential therapeutic utility in human sleep-wake disorders.

Lead compounds 5-fluoro-2-methyl-N-[2-methyl-4-(2-methyl-[1,3′]bipyrrolidinyl-1′-yl)-phenyl]-benzamide (1), tetrahydro-pyran-4-carboxylic acid [((2S,3′S)-2-methyl-[1,3′]bipyrrolidinyl-1′-yl)-phenyl]-amide (2), and 3,5-dimethyl-isoxazole-4-carboxylic acid [((2S,3′S)-2-methyl-[1,3′]bipyrrolidinyl-1′-yl)-phenyl]-amide (3) discovered in our laboratory, displayed high histamine H3 receptor (H3R) affinity, good selectivity and weak human Ether-a-go-go-Related Gene (hERG) channel affinity with desirable overall physico-chemical and pharmacokinetic (PK) profiles. Herein, we describe the design and synthesis of a novel series of H3R antagonists utilizing a scaffold hopping strategy. Further structure–activity relationship (SAR) studies of the series culminated in the identification of ((2S,3′S)-2-methyl-[1,3′]bipyrrolidinyl-1′-yl)-naphthalene-2-carboxylic acid (tetrahydro-pyran-4-yl)-amide (4c) and -[4-((2S,3′S)-2-methyl-[1,3′]bipyrrolidinyl-1′-yl)-phenyl]-N-(tetrahydro-pyran-4-yl)-acetamide (4d), which exhibited good H3R affinity in vitro, good selectivity, and desirable PK properties. Compounds 4c and 4d were also assessed in cardiac safety experiments. In particular, the effects of the compounds on action potentials recorded from ventricular myocytes isolated from guinea pigs were used to screen compounds that not only displayed a low affinity towards hERG channel, but also had lower interference with other cardiac ion channels. Compound 4c did not alter the major parameters in this model system at ⩽10 μM, and no significant induction of any major haemodynamic effect when intravenously administered at 3 mg/kg dose to anaesthetized mongrel dogs. Compound 4c is a new promising lead as orally potent and selective H3R antagonist belonging to a distinct structural class.

This Letter describes the asymmetric synthesis of the four stereoisomers (8a-8d) of a potent and highly selective histamine H3 receptor (H3R) antagonist, 5-fluoro-2-methyl-N-[2-methyl-4-(2-methyl[1,3']bipyrrolidinyl-1'-yl) phenyl]benzamide (1). The physico-chemical properties, in vitro H3R affinities and ADME of 8a-8d were determined. Stereoisomer 8c (2S,3'S) displayed superior in vitro H3R affinity over other three stereoisomers and was selected for further profiling in in vivo PK and drug safety. Compound 8c exhibited excellent PK properties with high exposure, desired brain to plasma ratio and reasonable brain half life. However, all stereoisomers showed similar unwanted hERG affinities.

A novel series of histamine H3 receptor (H3R) antagonists was derived from an arylurea lead series (1) via bioisosteric replacement of the urea functionality by an amide linkage. The arylamide series was optimized through SAR studies by a broad variation of substituents in the left-hand side benzoyl residue (analogs 2a-2ag) or replacement of the benzoyl moiety by heteroarylcarbonyl residues (analogs 5a-5n). Compounds 2p and 2q were identified within the series as potent and selective H3R antagonists/inverse agonists with acceptable overall profile. Compound 2q was orally active in food intake inhibition in diet-induced obese (DIO) mice. Compound 2q represents a novel H3R antagonist template with improved in vitro potency and oral efficacy and has its merits as a new lead for further optimization.