Structure-activity relationship studies and sleep-promoting activity of novel 1-chloro-5,6,7,8-tetrahydroimidazo[1,5-a]pyrazine derivatives as dual orexin receptor antagonists. Part 2.

Replacement of the dimethoxyphenyl moiety in the core skeleton of almorexant by appropriately substituted imidazoles afforded novel 1-chloro-5,6,7,8-tetrahydroimidazo[1,5-a]pyrazine derivatives as potent dual orexin receptor antagonists. We describe in this Letter our efforts to further optimize the potency and brain penetration of these derivatives by fine-tuning of the pivotal phenethyl motif, and we comment on the sleep-promoting activity of selected compounds in a rat electroencephalographic (EEG) model.

Synthesis, structure-activity relationship studies, and identification of novel 5,6,7,8-tetrahydroimidazo[1,5-a]pyrazine derivatives as dual orexin receptor antagonists. Part 1.

A novel series of non-peptidic OX1R/OX2R orexin receptor antagonists was prepared by heterocyclic replacement of the dimethoxyphenyl moiety contained in the tetrahydroisoquinoline core skeleton of almorexant. Introduction of substituted imidazole moieties delivered potent dual orexin receptor antagonists with nanomolar potency for hOX1R and hOX2R suitable for further fine-tuning. The preparation of these novel orexin receptor antagonists and the outcome of preliminary structure-activity relationship studies are described in this communication.

Novel pyrazolo-tetrahydropyridines as potent orexin receptor antagonists.

A novel series of dual orexin receptor antagonists was prepared by heteroaromatic five-membered ring system replacement of the dimethoxyphenyl moiety contained in the tetrahydroisoquinoline core skeleton of almorexant. Thus, replacement of the dimethoxyphenyl by a substituted pyrazole and additional optimization of the substitution pattern of the phenethyl motif allowed the identification of potent antagonists with low nanomolar affinity for hOX(1)R and hOX(2)R. The synthesis and structure-activity relationship of these novel antagonists will be discussed in this communication. These investigations furnished several suitable candidates for further evaluation in in vivo studies in rats.

N-Glycine-sulfonamides as potent dual orexin 1/orexin 2 receptor antagonists.

A series of dual OX(1)R/OX(2)R orexin antagonists was prepared based on a N-glycine-sulfonamide core. SAR studies of a screening hit led to compounds with low nanomolar affinity for both receptors and good oral bioavailability. One of these compounds, 47, has demonstrated in vivo activity in rats following oral administration.

Promotion of sleep by targeting the orexin system in rats, dogs and humans.

Orexins are hypothalamic peptides that play an important role in maintaining wakefulness in mammals. Permanent deficit in orexinergic function is a pathophysiological hallmark of rodent, canine and human narcolepsy. Here we report that in rats, dogs and humans, somnolence is induced by pharmacological blockade of both orexin OX(1) and OX(2) receptors. When administered orally during the active period of the circadian cycle, a dual antagonist increased, in rats, electrophysiological indices of both non-REM and, particularly, REM sleep, in contrast to GABA(A) receptor modulators; in dogs, it caused somnolence and increased surrogate markers of REM sleep; and in humans, it caused subjective and objective electrophysiological signs of sleep. No signs of cataplexy were observed, in contrast to the rodent, dog or human narcolepsy syndromes. These results open new perspectives for investigating the role of endogenous orexins in sleep-wake regulation.