Corresponding to the different insulinotropic signaling the

Corresponding to the different insulinotropic signaling, the competitive radioligand-binding studies demonstrated at least 3 distinct Tenofovir Disoproxil Fumarate on GPR40: orthosteric binding site for endogenous ligands, allosteric binding site for partial agonists and another allosteric binding site for full agonists [8]. The full agonists do not bind to the same site as the partial agonists but exhibit positive heterotropic cooperativity. The different action modes of GPR40 full agonists and partial agonists are related to their distinct binding sites on the receptor, offering the potential to produce synergistic therapeutic benefits [8]. It is thus understandable that the Ca2+ reflux and insulin secretion were improved in the presence of α-linoleic acid, and chronic incubation of insulin-secreting INS832/13 cells with GPR40 agonist TAK-875 did not lead to cell dysfunction or apoptosis in contrast to FFAs which impaired pancreatic beta cell function when present at elevated levels for prolonged periods of time [10], [18], [19]. In 2014, the co-crystal structure of GPR40 in complex with TAK-875 was resolved [7]. Two extra putative ligand-binding pockets in addition to the binding site of TAK-875 were observed [7], supporting the multiple binding sites statement from the radioligand binding interaction studies. As a result, different types of agonists specifically bind to each of these allosteric sites, stabilizing special conformation and inducing different biological outcomes. Notably, the presence of the allosterism on GPR40 offers many therapeutic possibilities although adding an additional level of complexity to its pharmacobiology [20]. Relative to orthosteric agonists, allosteric modulators provide multiple advantages for their use as anti-diabetic therapeutics. First, the allosteric binding pockets are relatively poorly conserved and thus very receptor-specific, enabling the design of highly target-selective ligands to avoid the adverse off-target effects. Second, allosteric modulators can act cooperatively and synergistically with one another and with orthosteric ligands, thus allowing a highly active combination therapy with decreased total doses and fewer side effects. Finally, allosteric ligands can alter the receptor conformation, and thus signaling profile, produced by orthosteric ligands, providing an alternative approach to circumvent the hepatotoxic effect observed with TAK-875. More importantly, the crystallization of GPR40 and the discovery of the multiple putative binding sites initiate the new concept to design bitopic ligands of GPR40 to increase the safety and therapeutic efficacy [21]. Such hybrid molecules possess separate orthosteric and allosteric pharmacophores that concomitantly engage with their respective sites on a single GPCR to mediate novel pharmacology [22], [23]. The two pharmacophores will be properly joined by a linker. Consequently, bitopic ligands theoretically afford improved target affinity and/or selectivity. Moreover, bitopic engagement of GPR40 may stabilize a specific receptor conformation producing favorable biased signaling. A proof-of-principle study has recently been published that shows the rational design of a bitopic adenosine A1 receptor ligand which directs signaling via a therapeutically beneficial pathway while limiting adverse effects by avoiding pathways that mediate adverse outcomes [24]. Considering the safety concerns on the GPR40 agonists for T2DM treatment, the concept of bitopic ligands will provide a promising avenue for future GPR40-based pharmacotherapeutics.
Chemical space of GPR40 agonists From the view point of the chemical structure, the synthetic GPR40 agonists can be regarded as a modular molecule consisting of phenylpropanoic acid core, linker and aromatic fragment (Fig. 2). Typically, the synthetic GPR40 agonists mimic the fatty acid structure with an acidic head group and a hydrophobic tail [4]. The replacement of the carboxylic acid with its bioisosteres can mostly retain the agonism activity. The β-position to the carboxylic acid can tolerate suitable small substituents in order to reduce the potential of β-oxidation. And the configuration of the chiral β-substituent remarkably affects the activity. The linker between the phenyl propanoic acid head and the hydrophobic tail is typically 2–4 atoms in length with an ether linkage being favorable. The aromatic fragment is a substituted aryl or biaryl moiety which can be further modified by hydrophilic groups to adjust the overall physicochemical property of the agonist.