Investigation of the pancreatic safety of proglucagon-derived peptides and the development of novel anti-obesity agents

Abstract

GLP-1 is a gut peptide that promotes satiety and insulin secretion. Its agonists are currently licensed and used as a type 2 diabetes treatment. Glucagon is best known for opposing the actions of insulin to increase blood glucose levels in response to hypoglycaemia. Glucagon also increases energy expenditure, but the mechanism for this is poorly understood. A study in humans suggested that co administration of GLP-1 and glucagon can increase appetite inhibition and energy expenditure to reduce body weight, while avoiding glucose excursions. This observation promoted the idea that co administration of the two peptides could act as an anti-obesity agent. However, concerns regarding the safety of GLP-1 receptor agonists have been raised. Patients taking GLP-1 receptor agonists have reported a higher incidence of pancreatitis and the proposed mechanism is elevated pancreatic ductal replication and increased amylase secretion from acinar cells. However, little is known about the acute effect of GLP-1 and glucagon receptor agonists on the exocrine pancreas. Work in this thesis investigates if GLP-1, glucagon, oxyntomodulin, and exendin-4 could cause pancreatitis. In the studies described, treatment with the peptides did not cause a significant increase in proliferation of biliopancreatic ductal cells or amylase secretion in vitro or in vivo in rats. Results did not support the proposed link of GLP-1 receptor agonists to pancreatitis, in rat. A second aim of this thesis was to develop novel analogues of GLP-1 and glucagon with improved efficacy and with enhanced pharmacokinetic profiles to be co administered as a treatment of obesity. Substitution of 14 amino acids in the GLP-1 peptide resulted in an analogue with similar efficacy as GLP-1, as determined by in vitro bioactivity, yet a terminal half-life of approximately 7 days. Substitution of residues His20 and Leu27 and addition of a histidine tail at the C-terminal of glucagon resulted in an analogue with almost 2-fold higher efficacy than glucagon, and a terminal half-life of 3 days. Co administration of the two peptides in vivo demonstrated a reduction in body weight and increased glucose tolerance, an effect suggested being synergistic when compared to individual agent administration groups.