Structure, activity and relationship studies of peptide and non-peptide analogs with secretin receptor : in search of agonist and/or antagonist

Abstract

Class B GPCRs are emerging target in drug research. Currently these receptors serve as drug targets for several drug discovery companies and more than 50 percent of the drugs in the market targets GPCRs. Secretin receptor is found to be expressed in various tissues. Secretin regulates many bodily functions from energy to water homeostasis through both central and peripheral system. Though it holds a history of 100 years, the major drawback is its structural insights. In evidence of its integrated role in physiology as a potential target, the lookout for a novel agonist and / or antagonist for secretin receptor is initiated.

As this target is in the primary state of drug research, it is also necessary to develop the appropriate screening platforms. Due to the lack of experimental structure of secretin receptor-ligand, a 3D virtual homology model is developed using multiple template approach. Besides virtual docking, a non-radioactive FRET competitive binding assay is also developed and substantiated to enable the receptor-ligand interaction studies. Both peptide and non-peptide analogs were screened for virtual docking, in vitro binding and functional response.

For the peptide analogs, the modifications were made either in the N or C terminal portion of the peptide based on the previous findings that C-terminal portion is involved in receptor binding followed by allosteric modifications and N-terminal portion is involved in activation. These peptide analogs exhibited binding affinity in the virtual model. Paradoxically it did not exhibit in vitro binding as predicted. Along with this, the agonistic and antagonistic functional responses of these peptide analogs were also found to be negative.

SPECS natural product library of 500 non-peptide analogs were screened virtually against secretin receptor and 32 hits were identified. Of these hits glycyrrhizin’s functions were comparable to secretin was screened for receptor binding and functional response. These in vitro assays did not exert anything positive; however an IP-GTT on WT, 〖SCT〗^(-/- )and 〖SCTR〗^(-/-) mice with acute treatment of glycyrrhizin at 10 mg/kg and chronic treatment of 5 mg/kg exhibited an interesting profile with negligible effect on 〖SCT〗^(-/- )mice whereas in WT and 〖SCTR〗^(-/-) mice it displayed a better profile with improved glucose tolerance. The chronic study serum analysis on day 28 exhibited substantial reduction in blood glucose while significant increase in serum secretin and insulin levels. As glycyrrhizin promotes secretin secretion, its acute effect on blood pressure in WT mice was also analyzed at 10 mg/kg; remarkably exhibited a significant drop in blood pressure.

In summary modifications in the peptide analogs lead to instability in the receptor-ligand binding complex in the in vitro system leading to loss of binding efficiency. In case of non-peptides, though glycyrrhizin could not exhibit in vitro response, its supplementary mechanism through secretin pathway of increased secretin release is confirmed using the WT, 〖SCT〗^(-/- )and 〖SCTR〗^(-/-) mice. The hypotensive effect with an acute treatment in WT is also revealed. Discovery of this new mechanism of an old drug could broaden the research for a new class of drug, “secretin sensitizers / promoters”.