br TGR agonists current developments and future landscape Th

TGR5 agonists: current developments and future landscape The amphipathic nature of bile acids is crucial in aiding the digestion and Escitalopram Oxalate of vitamins and fats. With the emerging roles of bile acids in the regulation of energy and glucose homeostasis through TGR5, the pharmaceutical industry has focused their efforts towards the discovery and development of potent and selective TGR5 small molecule agonists belonging to bile acid or non-bile acid classes (Figure 4). TGR5 has a binding pocket, which is selective for different bile acid derivatives allowing the pharmacological discrimination over the genomic effects mediated by FXR. The key amino acids involved in the binding of bile acids in the binding pocket of the two receptors are not entirely conserved, which allows the development of different bile acid derivatives with varying degrees of selectivity. Structural features contributing to selectivity include the head piece carboxylic acid binding site. [46]. Screening of various naturally occurring bile acids, semi-synthetic bile acid derivatives and steroid hormones allowed the development of a structure–activity relationship (SAR) that indicated that (i) chain length of the carboxylic acid head piece, (ii) substitution at the alpha position of the carboxylic acid and (iii) rigidity of the cyclic system play crucial roles with respect to potency and selectivity of bile acids at the TGR5 receptor [23]. Shortening the chain length of bile acid derivatives reduces their potency on TGR5, whereas introduction of S methyl group at the α-position of carboxylic acid increases potency and provides highest selectivity over FXR. On the contrary, introduction of a double bond to the cyclic systems of bile acids provides rigidity to the molecular system and, therefore reduces binding to TGR5, suggesting a flexible and spacious binding pocket. Such SAR observations provided important clues for the development of compounds binding selectively at the TGR5 receptor over steroid hormone receptors. The SAR indicated that the TGR5 receptor exhibits highly conserved binding pocket with three hydrogen bond acceptors to anchor three hydroxyl groups of bile acids and a charged surface to bind with carboxylic acid groups. The unique structural features of bile acids, which provide potency and selectivity towards TGR5 can be divided into four important parts (Figure 5). Region A: a large pocket, which consists of negatively charged residues to accommodate oppositely charged functional groups of the bile acids. Neutral groups like alcohols diminish the potency of bile acids binding to the TGR5 receptor. Region B: an accessory-binding pocket, which spans over C-22 and C-23. This pocket is quite large in size in TGR5 receptor compared with FXR and may be a reason why it is possible to achieve good selectivity. Pellicciari et al. have shown that the introduction of a small alkyl group at the C-23 position can be tolerated by TGR5 and provides selectivity against FXR [46]. The pocket is enantiomerically specific for the S-isomer over the R-isomer. Region C: This region is a large hydrophobic pocket, spanning C-6 and C-7 of the bile acids and hence tolerates alkyl (e.g. methyl, isopropyl) and fluoro substitution at C-7. Region D: is a narrow pocket with hydrogen bond acceptor groups, spanning the C-3 region. A hydroxyl (or ketone) group at C-3 anchors bile acids to the receptor through hydrogen bonds; therefore, removing these groups decreases potency at TGR5. In addition, a recent QSAR model shows two more favorable interaction sites of bile acids with TGR5: (i) hydrogen bonding at C-12 and (ii) an electrostatic interaction at the terminal of the carboxylic acid group of bile acids [47]. Takeda performed a high throughput screen in the search for a non-bile acid small molecule agonist for TGR5 [48]. Although the SAR was not entirely clear, a bicyclic system consisting of a charged seven-membered ring coupled with a five or six member aromatic ring appeared to be a unique functional motif, conferring activity at this receptor. Two non-bile acids reported from the screen with activity against the receptor were pyrimidine carboxylic ester 3 and oleanolic acid 4 (Figure 4).