From an historical perspective ligands for GPCRs adrenaline

From an historical perspective, ligands for GPCRs (adrenaline, serotonin, A 350619 hydrochloride mg or morphine, to name a few) have been identified before their receptor counterparts, at a time when the concept of receptor itself was controversial [15]. Although many cognate receptors for endogenous ligands were rapidly cloned during the eighties and nineties, many investigators came rapidly across unidentified receptors [16] and labeled them as “orphan” for their endogenous ligands [17]. Since their conceptual definition, the number of orphan GPCRs continued to grow significantly until the publication of the human genome sequence [18], the phylogenetic analysis of the human GPCRs repertoire [19] and a detailed list of receptors issued by IUPHAR [20]. GPCRs are usually classified by phylogeny and the most recent analysis proposed to group the receptors in families according to their resemblance to Glutamate, Rhodopsin, Adhesion, Frizzled or Secretin receptors, termed the GRAFS system [19]. A former classification is often found in the literature and assigns the different receptors into classes that roughly overlap the GRAFS categories: class A (Rhodopsin-like), class B (Secretin-like) and class C (Glutamate-like). Not all the families contain orphans, most of them are present in the Adhesion, Rhodopsin (Class A) and Glutamate (Class C) families. A list of orphans is maintained on the receptor database established by a shared effort from the International Union of Basic and Clinical Pharmacology (IUPHAR) and the British Pharmacological Society (BPS) [21]. For example the numerically largest Rhodopsin-class A family totalizes 91 orphan receptors. According to the recommendations of the IUPHAR [22], a receptor is considered “deorphanized” when i) two or more peer-reviewed papers from independent labs report activity of the ligand at the receptor, at potencies compatible with a physiologic function. The assays describing pharmacological activity should be diverse and measure different parameters (affinity with binding assays and second messengers for determination of efficacy and potency). The reproducibility criterion is of prime importance and discrepancies should be carefully analyzed and explained. It can become problematic when two independent labs report a ligand for a receptor but other fail to confirm the result. When such controversies arise, the IUPHAR encourages further investigations before making a definitive statement. ii) The proposed endogenous ligand must be present in the tissues at sufficient levels. Several analytical techniques such as mass spectrometry or radioimmunoassays can be used to measure the concentration of small molecule ligands in tissues. The use of genetically engineered mice lacking the receptor should display a phenotype that is in accordance with the proposed pharmacological link between an endogenous ligand and its cognate receptor. When robustly demonstrated, the identification of an endogenous ligand for an orphan receptor is often the key to understand a novel physiological process. In addition, such discovery opens new avenues in terms of drug discovery. One of the most recent success stories from initial discovery to drug development is the identification of the orexin system. Orexins are 28- and 33-amino acids long peptides located in the hypothalamus and were identified as ligands for the orphans HFGAN72 and D81887 (OX1 and OX2 receptors, respectively) in 1998 [23]. The newly discovered peptides and their receptors were named in reference to the greek word for “appetite” (orexis). However, subsequent investigations demonstrated their importance in the physiology of sleep [24]. In 2014, an orexin receptor antagonist, suvorexant, has been approved by the FDA for the treatment of insomnia (Belsomra®, Merck), a condition with important unmet medical needs [25]. At that time, the only efficient drugs were GABAA positive modulators such as benzodiazepines and related molecules (such as Zolpidem). Therefore, suvorexant, as a dual-orexin receptors antagonist, laid the foundation of a novel class of medicine. Moreover, it hit the market less than 20 years after the orexin receptors have been deorphanized, thus demonstrating the importance of this kind of research in terms of drug discovery.