Species specificity of small molecular
Species specificity of small molecular GPR40 agonists have been reported by Takeda scientist. In the binding pocket of TM5, a Leu186 in human GPR40/FFA1 is replaced with Phe in rat, resulting in dramatic inter-species GPR40 activity discrepancy in certain scaffolds of small molecular GPR40 agonists.
To further understand the species difference, we also built an in house homology model of GPR40 as shown in , using the similar procedure. There is a loop spanning across the possible binding site of TAK-875 and was highlighted in blue. For convenience, we will call it the Blue Loop in the rest of the article.
The residue similarity for the Blue Loop (between a.a. 121 and 180) among human, rat, mouse and monkey was compared. (b) Although whole sequence similarity among these species are all more than 90%, the blue Loop among human, rat and mouse are quite different. At the same time, the blue loops between human and monkey are almost identical with only one exception at residue 143.
In our GPR40 homology model, the residue 143 (shown in blue stick) was found quite far away from the putative ligand binding site (), In summary, both sequence and structural evidences suggested that monkey being a better animal model for the translation between in vitro and in vivo activity.
The conjecture of species specificity of compounds , , and were confirmed by a rat GSIS INS-1 assay. The results were summarized in . TAK-875 was used as a reference olopatadine hcl in this assay. Although exhibited comparable EC value in this assay (77nM vs 93nM), its maximum efficacy was only 40% of TAK-875, indicating partial agonistic activity of to rat GPR40. Similarly, none of compounds , and exhibited over 50% of maximum efficacy in this assay.
Considering the high probability of species specificity and the potential of monkey as a reasonable in vivo model, compounds and were then tested in an obese type 2 diabetes rhesus monkey IVGTT model (Oral administration of (6mg/kg), (6 and 20mg/kg) and TAK-875 (20mg/kg) in high fat feasted male monkeys 1 or 2h prior to dextrose challenge in an intravenous glucose tolerance test). Both and intensively reduced the blood glucose excursion during the test, confirming our conjecture of GPR40 species specificity. The detailed results were summarized in .
GPR40 belongs to a family of FFAs binding GPCRs, which includes GPR40, GPR41, GPR43, and GPR120., While GPR41 and GPR43 are activated by short-chain FFAs, GPR40 and GPR120 are activated by medium- to long-chain FFAs and some eicosanoids. Compounds and were tested against GPR41, GPR43 and GPR120 and none of them were active up to 10μM concentration, indicating high GPR40 selectivity of this new scaffold.
Both compounds and have a molecular weight of less than 400, relatively low log (<3) and only 3 rotatable bonds, which is quite ideal in the sense of drug ability. Compounds and exhibited no hERG inhibition in a patch-clamp assay (IC >30μM), devoid of potential cardiovascular liability. The ICs of compounds and from common Drug Drug Interaction assays of CYP450 subtype were all above 50μM, indicating minimal liabilities in potential drug combination use.
In recent years the number of diabetic patients has been increasing all over the world, thus the efficient and suitable treatment for each patient is in high demand. Insulin secretagogues such as sulfonylureas are widely used for patients with a moderate degree of β-cell dysfunction. Sulfonylureas secrete insulin independently of glucose levels, so they may cause hypoglycemia. Their long term therapy also often leads to the gradual diminution of islets activity., , There are only a few choices of insulin secretagogues with low risk of hypoglycemia such as DPP-4 inhibitors and GLP-1 agonists. Therefore, the novel orally available insulin secretagogues with glucose dependency and strong glucose lowering effects are still in demand. The G-protein coupled receptor GPR40, highly expressed in human and rodent pancreatic islets,, , is found as an attractive target for new therapy of type 2 diabetes., This receptor is activated by medium and long-chain free fatty acids (e.g., palmitic and linolenic acids), and sends signals to downstream pathways resulting in enhancement of insulin secretion by production of inositol triphosphate and release of intracellular Ca from endoplasmic reticulum., Compared to other mechanisms, there are several advantages of GPR40 as a target for the treatment of type 2 diabetes. The most attractive point is that GPR40 induces insulin secretion depending on the concentration of glucose, indicating that the selective agonist has low risk of hypoglycemia. In addition, the distribution of GPR40 has been limited (mainly in islets), in that side effects associated with GPR40 activation in other tissues rarely occur. Many groups reported a variety of synthetic GPR40 agonists., , , , , , , , , , , , Most of GPR40 agonists have the structure of 3-phenylpropanoic acid mimicking medium or long-chain free fatty acids (). Among these compounds, we chose compound as a starting structure to explore novel GPR40 agonists, because it has very strong in vitro agonistic activity against GPR40 (reported EC=8.8nM; in house data EC=6.7nM) and a simple structure that is easy to modify. On the other hand, it is reported that bioavailability of compound was too poor (=0.9% in rats) for oral active agents. In this article, we will show the synthetic efforts to obtain bioavailable compounds starting from compound .