|hypocretin (orexin) receptor 1|
|Locus||Chr. 1 p33|
|hypocretin (orexin) receptor 2|
|Locus||Chr. 6 p11-q11|
|Orexin receptor type 2|
The orexin receptor (also referred to as the hypocretin receptor) is a G-protein-coupled receptor that binds the neuropeptide orexin. There are two variants, OX1 and OX2, each encoded by a different gene (HCRTR1, HCRTR2).
Both orexin receptors exhibit a similar pharmacology - the 2 orexin peptides, orexin-A and orexin-B, bind to both receptors and, in each case, agonist binding results in an increase in intracellular calcium levels. However, orexin-B shows a 10-fold selectivity for orexin receptor type 2, whilst orexin-A is equipotent at both receptors.
Several orexin receptor antagonists are in development for potential use in sleep disorders. Only the crystal structure of OX2 is known.
Several drugs acting on the orexin system are under development, either orexin agonists for the treatment of conditions such as narcolepsy, or orexin antagonists for insomnia. No non-peptide agonists are yet available, although synthetic Orexin-A polypeptide has been made available as a nasal spray and tested on monkeys. Several non-peptide antagonists are in development however; SB-649,868 by GlaxoSmithKline for sleep disorders is a non-selective orexin receptor antagonist. Another dual orexin antagonist, almorexant (ACT-078573) by Actelion, was abandoned because of side effects. A third entry is Merck's suvorexant (Belsomra), which has recently been approved for use. A new antagonist compound, ACT-462206, was recently studied in humans.
Most ligands acting on the orexin system so far are polypeptides modified from the endogenous agonists Orexin-A and Orexin-B, however there are some subtype-selective non-peptide antagonists available for research purposes.
- SB-334,867 – selective OX1 antagonist
- SB-408,124 – selective OX1 antagonist
- TCS-OX2-29 – selective OX2 antagonist
- EMPA (N-Ethyl-2-[(6-methoxy-pyridin-3-yl)-(toluene-2-sulfonyl)-amino]-N-pyridin-3-ylmethyl-acetamide) – selective OX2 antagonist
- RTIOX-276 – selective OX1 antagonist
- Spinazzi R, Andreis PG, Rossi GP, Nussdorfer GG (2006). "Orexins in the regulation of the hypothalamic-pituitary-adrenal axis". Pharmacol. Rev. 58 (1): 46–57. doi:10.1124/pr.58.1.4. PMID 16507882.
- Smart D, Jerman JC, Brough SJ, Rushton SL, Murdock PR, Jewitt F, Elshourbagy NA, Ellis CE, Middlemiss DN, Brown F (September 1999). "Characterization of recombinant human orexin receptor pharmacology in a Chinese hamster ovary cell-line using FLIPR". Br. J. Pharmacol. 128 (1): 1–3. doi:10.1038/sj.bjp.0702780. PMC 1571615. PMID 10498827.
- Yin J, Mobarec JC, Kolb P, Rosenbaum DM (December 2014). "Crystal Structure of the Human Ox2 Orexin Receptor Bound to the Insomnia Drug Suvorexant". Nature. doi:10.1038/nature14035.
- Heifetz A, Morris GB, Biggin PC, Barker O, Fryatt T, Bentley J, Hallett D, Manikowski DP, Pal S, Reifegerste R, Slack M, Law R (2012). "Study of Human Orexin-1 and -2 G-Protein-Coupled Receptors with Novel and Published Antagonists by Modeling, Molecular Dynamics Simulations, and Site-Directed Mutagenesis". Biochemistry. 51 (15): 3178–3197. doi:10.1021/bi300136h.
- Baxter CA, Cleator ED, Karel MJ, Edwards JS, Reamer RA, Sheen FJ, Stewart GW, Strotman NA, Wallace DJ (2011). "The First Large-Scale Synthesis of MK-4305: A Dual Orexin Receptor Antagonist for the Treatment of Sleep Disorder". Organic Process Research & Development. 15 (2): 367–375. doi:10.1021/op1002853.
- "Entry-into-humans study with ACT-462206, a novel dual orexin receptor antagonist, comparing its pharmacodynamics with almorexant". J Clin Pharmacol. 54: 979–86. Sep 2014. doi:10.1002/jcph.297. PMID 24691844.
- "Orexin Receptors". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology.
- Orexin Receptors at the US National Library of Medicine Medical Subject Headings (MeSH)