Evaluating The Role Of Incretin Based Therapies In The Management Of Type 2 Diabetes PDF Download

Gut peptides including glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-l (GLP-I), and oxyntomodulin (Oxm) act to regulate glucose homeostasis, insulin secretion, bodyweight and satiety. However, clinical development has been hindered by rapid enzymatic degradation followed by renal clearance. This thesis evaluates the biological activities of these peptides, and exploits various chemical modifications / approaches to generate novel mimetics with therapeutic potential for type 2 diabetes and obesity. Acylation of Lys37 in GIP with C-14 fatty acid (myristate) afforded dipeptidylpeptidase-IV (DPP-IV) resistance and significantly enhanced in vitro cAMP production and insulin secretion. Furthermore, administration of N- AcGIP(Lys37 MYR) resulted in markedly improved glucose homeostasis and insulin-release in obese diabetic (ob/ob) mice. Once-daily administration of the DPP- IV resistant GIP analogue, GIP[mPEG], decreased non-fasting plasma glucose concentrations, increased insulin concentrations and improved glycaemic and insulin responses to a glucose load in mice with age-related glucose intolerance. These data clearly demonstrate the utility of stable analogues of GIP for treatment of obesity- diabetes. The GLP-l mimetic, Liraglutide, lacking the y-glutamyl linker (Lira-yGlu) demonstrated equi-potent DPP-IV resistance, cAMP production and insulin secretion compared with Liraglutide. In vivo, Lira-yGlu and Liraglutide significantly lowered plasma glucose in fasted mice. Twice-daily administration of either GLP-l analogue decreased food intake and plasma glucose concentrations, whilst increasing plasma insulin, and improving glucose tolerance and insulin sensitivity. These data indicate lack of requirement of y-glutamyl linker together with acylation for generation of long-acting incretin mimetics. Evaluating the effects of a an overnight preparation of Liraglutide and N- AcGIP(Lys37Myr) (Lira-AcGIP - mixed together and incubated for 12h) revealed improved acute glucose tolerance and insulin responses compared to a simple combination (immediate mixture). Furthermore, daily administration of the Lira- AcGIP preparation lowered bodyweight, decreased food intake, plasma glucose and insulin concentrations in ob/ob mice, as well as enhancing glucose tolerance, insulin response to glucose and insulin content. These data indicate the possibility of using combination therapy with stable GLP-l and GIP mimetics for treatment of type 2 diabetes. Oxm analogues, (0-Ser2)Oxm and (0-Ser2)Oxm[mPEG-PAL] exhibited DPP- IV stability, with equi-potent in vitro cAMP production and insulin secretion. In the presence of specific antagonists, cAMP production was reduced in GLP-l and glucagon receptor transfected cells, consistent with actions of dual receptor agonism. In acute studies, (0-Ser2)Oxm and (0-Ser2)Oxm[mPEG-PAL] exhibited glucoregulatory and anorexigenic properties with (0-Ser2)Oxm[mPEG-PAL] demonstrating more potent actions. Once-daily administration of (0- Ser2)Oxm[mPEG-PAL] decreased food intake, bodyweight and plasma glucose concentrations and increased insulin concentrations in ob/ob mice whilst improving glucose tolerance, insulin response to glucose and plasma lipid profiles. The N-terminal domain of GIP plays an important role in regulating its biological activity. Examining several novel N-terminally truncated forms of GIP revealed that GIP(8-42) exhibited reduced cAMP levels compared to native hormone whilst inhibiting GIP-induced cAMP production. In ob/ob mice, GIP(8-42) increased plasma glucose concentrations compared to the glucose-lowering action of native GIP. When GIP(8-42) was co-administered with GIP it countered the ability of the native hormone to lower plasma glucose and increase insulin concentrations. These data demonstrate the importance of the N-terminal of GIP in regulating bioactivity. Collectively, these data illustrate chemical modifications / approaches to improve the biological efficacy of GIP, GLP-l and Oxm. The use of smaller moieties to offset enzymatic degradation and renal clearance offer improved efficacy potentially due to reduced steric hindrance compared to currently available incretin therapies. In addition, incretin preparations and the dual role of Oxm may offer targeted 'smart therapy' for both obesity and type 2 diabetes. Hopefully this thesis will aid the development of pharmaceutical agents and subsequent clinical studies that will ultimately improve the lives of people suffering from type 2 diabetes and obesity.