The role of C-terminal part of ghrelin in pharmacokinetic profile and biological activity in rats

Ghrelin is an endogenous ligand for growth hormone secretagogue receptor 1a (GHS-R1a), and consists of 28 amino acid residues with octanoyl modification at Ser3. The previous studies have revealed that N-terminal part of ghrelin including modified Ser3 is the active core for the activation of GHS-R1a. On the other hand, the role of C-terminal (8–28) region in ghrelin has not been clarified yet. In the present study, we prepared human ghrelin, C-terminal truncated ghrelin derivatives and anamorelin, a small molecular GHS compound which supposedly mimics the N-terminal active core, and examined GHS-R1a agonist activity in vitro, pharmacokinetic (PK) profile and growth hormone (GH) releasing activity in rats. All compounds demonstrated potent GHS-R1a agonist activities in vitro. Although the lack of C-terminal two amino acids did not modify PK profile and GH releasing activity, the deletion of C-terminal 8 and 20 amino acids affected them, and ghrelin(1–7)-Lys-NH2 exhibited very short plasma half-life and low GH releasing activity in vivo. In rat plasma, ghrelin(1–7)-Lys-NH2 was degraded more rapidly than ghrelin, suggesting that C-terminal part of ghrelin protected octanoylation of Ser3 from plasma esterases. Subdiaphragmatic vagotomy significantly attenuated GH response to ghrelin but not to anamorelin. These results suggest that the C-terminal part of ghrelin has an important role in the biological activity in vivo. We also found that ghrelin stimulated GH release mainly via a vagal nerve pathway but anamorelin augmented GH release possibly by directly acting on brain in rats.

Research highlights
► The role of C-terminal part of ghrelin has not been clarified yet.
► The lack of C-terminal 8 and 20 amino acids did not affect agonist activity in vitro.
► C-terminal truncated analogs had short half life and low GH releasing activity in rats.
► The C-terminal part of ghrelin is important for the biological activity in vivo.
► Vagotomy attenuated GH response to ghrelin but not to anamorelin, a GHS compound.