Treatment of Type 2 Diabetes and Obesity on the Basis of the Incretin System: The 2021 Banting Medal for Scientific Achievement Award Lecture

Diverting hepatic lipid fluxes with lifestyles revision and pharmacological interventions as a strategy to tackle steatotic liver disease (SLD) and hepatocellular carcinoma (HCC)

Steatotic liver disease (SLD) and Hepatocellular Carcinoma (HCC) are characterised by a substantial rewiring of lipid fluxes caused by systemic metabolic unbalances and/or disrupted intracellular metabolic pathways. SLD is a direct consequence of the interaction between genetic predisposition and a chronic positive energy balance affecting whole-body energy homeostasis and the function of metabolically-competent organs. In this review, we discuss how the impairment of the cross-talk between peripheral organs and the liver stalls glucose and lipid metabolism, leading to unbalances in hepatic lipid fluxes that promote hepatic fat accumulation. We also describe how prolonged metabolic stress builds up toxic lipid species in the liver, and how lipotoxicity and metabolic disturbances drive disease progression by promoting a chronic activation of wound healing, leading to fibrosis and HCC. Last, we provide a critical overview of current state of the art (pre-clinical and clinical evidence) regarding mechanisms of action and therapeutic efficacy of candidate SLD treatment options, and their potential to interfere with SLD/HCC pathophysiology by diverting lipids away from the liver therefore improving metabolic health.

New Developments in Pharmacological Treatment of Obesity and Type 2 Diabetes-Beyond and within GLP-1 Receptor Agonists

Guidelines for the management of obesity and type 2 diabetes (T2DM) emphasize the importance of lifestyle changes, including a reduced-calorie diet and increased physical activity. However, for many people, these changes can be difficult to maintain over the long term. Medication options are already available to treat obesity, which can help reduce appetite and/or reduce caloric intake. Incretin-based peptides exert their effect through G-protein-coupled receptors, the receptors for glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), and glucagon peptide hormones are important regulators of insulin secretion and energy metabolism. Understanding the role of intercellular signaling pathways and inflammatory processes is essential for the development of effective pharmacological agents in obesity. GLP-1 receptor agonists have been successfully used, but it is assumed that their effectiveness may be limited by desensitization and downregulation of the target receptor. A growing number of new agents acting on incretin hormones are becoming available for everyday clinical practice, including oral GLP-1 receptor agonists, the dual GLP-1/GIP receptor agonist tirzepatide, and other dual and triple GLP-1/GIP/glucagon receptor agonists, which may show further significant therapeutic potential. This narrative review summarizes the therapeutic effects of different incretin hormones and presents future prospects in the treatment of T2DM and obesity.

Improving glycemic control: transitioning from dulaglutide to tirzepatide in patients with type 2 diabetes undergoing hemodialysis

Background: Tirzepatide-a dual glucose-dependent insulinotropic peptide and glucagon-like peptide-1 receptor agonist-is used to treat type 2 diabetes. However, the efficacy and safety of tirzepatide in patients undergoing hemodialysis remain unclear. Methods: We conducted a single-center retrospective study of patients with type 2 diabetes undergoing hemodialysis who were transitioned from dulaglutide to tirzepatide. We continuously monitored glucose levels in patients undergoing hemodialysis before and after switching from dulaglutide to tirzepatide. Results: Fourteen patients (mean age: 61.9 ± 9.9 years, male: female = 11:3) were included in this study. After switching to tirzepatide, time in range increased to 50.8% from 42.7% (p = 0.02), time above range decreased to 37.8% from 48.4% (p = 0.02), and mean glucose levels decreased to 137.4 mg/dL from 156.6 mg/dL (p = 0.006). In contrast, there was no significant difference in time below range before and after tirzepatide administration (11.3% and 8.9%) (p = 0.75). Three patients experienced dyspepsia (21.4%), and one patient experienced nausea (7.1%); however, no critical adverse events were reported. Conclusion: Transitioning from dulaglutide to tirzepatide improved glycemic control without increasing hypoglycemia in patients undergoing hemodialysis for type 2 diabetes.

Duodenal enteroendocrine cells and GIP as treatment targets for obesity and type 2 diabetes

The duodenum is an important source of endocrine and paracrine signals controlling digestion and nutrient disposition, notably including the main incretin hormone glucose-dependent insulinotropic polypeptide (GIP). Bariatric procedures that prevent nutrients from contact with the duodenal mucosa are particularly effective interventions to reduce body weight and improve glycaemic control in obesity and type 2 diabetes. These procedures take advantage of increased nutrient delivery to more distal regions of the intestine which enhances secretion of the other incretin hormone glucagon-like peptide-1 (GLP-1). Preclinical experiments have shown that either an increase or a decrease in the secretion or action of GIP can decrease body weight and blood glucose in obesity and non-insulin dependent hyperglycaemia, but clinical studies involving administration of GIP have been inconclusive. However, a synthetic dual agonist peptide (tirzepatide) that exerts agonism at receptors for GIP and GLP-1 has produced marked weight-lowering and glucose-lowering effects in people with obesity and type 2 diabetes. This appears to result from chronic biased agonism in which the novel conformation of the peptide triggers enhanced signalling by the GLP-1 receptor through reduced internalisation while reducing signalling by the GIP receptor directly or via functional antagonism through increased internalisation and degradation.

Recent advances in peptide-based therapies for obesity and type 2 diabetes

Options for the treatment of type 2 diabetes mellitus (T2DM) and obesity have recently been expanded by the results of several large clinical trials with incretin-based peptide therapies. Most of these studies have been conducted with the glucagon-like peptide-1 (GLP-1) receptor agonist semaglutide, which is available as a once weekly subcutaneous injection and once daily tablet, and the once weekly injected dual agonist tirzepatide, which interacts with receptors for GLP-1 and glucose-dependent insulinotropic polypeptide (GIP). In individuals with T2DM these therapies have achieved reductions of glycated haemoglobin (HbA1c) by > 2% and lowered body weight by > 10%. In some studies, these agents tested in non-diabetic, obese individuals at much higher doses have lowered body weight by > 15%. Emerging evidence suggests these agents can also offer cardio-protective and potentially reno-protective effects. Other incretin-based peptide therapies in early clinical development, notably a triple GLP-1/GIP/glucagon receptor agonist (retatrutide) and a combination of semaglutide with the amylin analogue cagrilintide (CagriSema), have shown strong efficacy. Although incretin therapies can incur adverse gastrointestinal effects these are for most patients mild-to-moderate and transient but result in cessation of treatment in some cases. Thus, the efficacy of new incretin-based peptide therapies is enhancing the opportunity to control body weight and blood glucose and improve the treatment of T2DM and obesity.

New therapies for obesity

Obesity is a chronic disease associated with serious complications and increased mortality. Weight loss (WL) through lifestyle changes results in modest WL long-term possibly due to compensatory biological adaptations (increased appetite and reduced energy expenditure) promoting weight gain. Bariatric surgery was until recently the only intervention that consistently resulted in ≥ 15% WL and maintenance. Our better understanding of the endocrine regulation of appetite has led to the development of new medications over the last decade for the treatment of obesity with main target the reduction of appetite. The efficacy of semaglutide 2.4 mg/week-the latest glucagon-like peptide-1 (GLP-1) receptor analogue-on WL for people with obesity suggests that we are entering a new era in obesity pharmacotherapy where ≥15% WL is feasible. Moreover, the WL achieved with the dual agonist tirzepatide (GLP-1/glucose-dependent insulinotropic polypeptide) for people with type 2 diabetes and most recently also obesity, indicate that combining the GLP-1 with other gut hormones may lead to additional WL compared with GLP-1 receptor analogues alone and in the future, multi-agonist molecules may offer the potential to bridge further the efficacy gap between bariatric surgery and the currently available pharmacotherapies.

Incretin hormones and type 2 diabetes

Incretin hormones (glucose-dependent insulinotropic polypeptide [GIP] and glucagon-like peptide-1 [GLP-1]) play a role in the pathophysiology of type 2 diabetes. Along with their derivatives they have shown therapeutic success in type 2 diabetes, with the potential for further improvements in glycaemic, cardiorenal and body weight-related outcomes. In type 2 diabetes, the incretin effect (greater insulin secretory response after oral glucose than with 'isoglycaemic' i.v. glucose, i.e. with an identical glycaemic stimulus) is markedly reduced or absent. This appears to be because of a reduced ability of GIP to stimulate insulin secretion, related either to an overall impairment of beta cell function or to specific defects in the GIP signalling pathway. It is likely that a reduced incretin effect impacts on postprandial glycaemic excursions and, thus, may play a role in the deterioration of glycaemic control. In contrast, the insulinotropic potency of GLP-1 appears to be much less impaired, such that exogenous GLP-1 can stimulate insulin secretion, suppress glucagon secretion and reduce plasma glucose concentrations in the fasting and postprandial states. This has led to the development of incretin-based glucose-lowering medications (selective GLP-1 receptor agonists or, more recently, co-agonists, e.g. that stimulate GIP and GLP-1 receptors). Tirzepatide (a GIP/GLP-1 receptor co-agonist), for example, reduces HbA1c and body weight in individuals with type 2 diabetes more effectively than selective GLP-1 receptor agonists (e.g. semaglutide). The mechanisms by which GIP receptor agonism may contribute to better glycaemic control and weight loss after long-term exposure to tirzepatide are a matter of active research and may change the pessimistic view that developed after the disappointing lack of insulinotropic activity in people with type 2 diabetes when exposed to GIP in short-term experiments. Future medications that stimulate incretin hormone and other receptors simultaneously may have the potential to further increase the ability to control plasma glucose concentrations and induce weight loss.

The future of incretins in the treatment of obesity and non-alcoholic fatty liver disease

In the last few decades, glucagon-like peptide-1 receptor (GLP-1R) agonists have changed current guidelines and improved outcomes for individuals with type 2 diabetes. However, the dual glucose-dependent insulinotropic polypeptide receptor (GIPR)/GLP-1R agonist, tirzepatide, has demonstrated superior efficacy regarding improvements in HbA1c and body weight in people with type 2 diabetes. This has led to increasing scientific interest in incretin hormones and incretin interactions, and several compounds based on dual- and multi-agonists are now being investigated for the treatment of metabolic diseases. Herein, we highlight the key scientific advances in utilising incretins for the treatment of obesity and, potentially, non-alcoholic fatty liver disease (NAFLD). The development of multi-agonists with multi-organ targets may alter the natural history of these diseases.

Glucagon, from past to present: a century of intensive research and controversies

2022 corresponds to the 100th anniversary of the discovery of glucagon. This TimeCapsule aims to recall the main steps leading to the discovery, characterisation, and clinical importance of the so-called second pancreatic hormone. We describe the early historical findings in basic research (ie, discovery, purification, structure, α-cell origin, radioimmunoassay, glucagon gene [GCG], and glucagon receptor [GLR]), in which three future Nobel Prize laureates were actively involved. Considered as an anti-insulin hormone, glucagon was rapidly used to treat insulin-induced hypoglycaemic coma episodes in people with type 1 diabetes. A key step in the story of glucagon was the discovery of its role and the role of α cells in the physiology and pathophysiology (ie, paracrinopathy) of type 2 diabetes. This concept led to the design of different strategies targeting glucagon, among which GLP-1 receptor (GLP1R) agonists were a major breakthrough, and combination of inhibition of glucagon secretion with stimulation of insulin secretion (both in a glucose-dependent manner). Taking advantage of the glucagon-induced increase in energy metabolism, biased coagonists were developed. Besides the GLP-1 receptor, these coagonists also target the glucagon receptor to further promote weight loss. Thus, the 100-year story of glucagon has most probably not come to an end.

Dual GIP/GLP-1 receptor agonists: New advances for treating type-2 diabetes

Glucagon-like peptide-1 (GLP-1) receptor agonists currently occupy a privileged place in the management of type-2 diabetes (T2D). Dual glucose-dependent insulinotropic polypeptides (GIP/GLP-1) have been recently developed. Tirzepatide is the most advanced unimolecular dual GIP/GLP-1 receptor agonist to be used as once weekly subcutaneous injection in T2D and recently received approval by the European Medicines Agency. Because of the complementarity of action of the two incretins, tirzepatide showed better dose-dependent (5, 10 and 15mg) efficacy (greater reduction in HbA1c and body weight) than placebo, basal insulin or two GLP-1 analogues (dulaglutide and semaglutide) in the SURPASS program. Its cardiovascular protective effect is currently being assessed versus dulaglutide in the SURPASS-CVOT study. Finally, studies for the treatment of obesity (SURMOUNT program) and metabolic-associated fatty liver disease (MAFLD) are also ongoing. Gastrointestinal tolerance of tirzepatide appears comparable to that of GLP-1 analogues, except for higher incidence of diarrhea. Other original molecules have been built, including triple GIP/GLP-1/glucagon receptor agonists. The risk/benefit ratio will decide whether dual (or triple) receptor agonists should replace pure GLP-1 receptor agonists for the management of T2D in the near future, with a significant role in the pharmacotherapy of obesity.

Cardiovascular effects of incretins - focus on GLP-1 receptor agonists

GLP-1 receptor agonists (GLP-1 RAs) have been used to treat patients with type 2 diabetes since 2005 and have become popular because of the efficacy and durability in relation to glycaemic control in combination with weight loss in most patients. Today in 2022, seven GLP-1 RAs, including oral semaglutide are available for treatment of type 2 diabetes. Since the efficacy in relation to reduction of HbA1c and body weight as well as tolerability and dosing frequency vary between agents, the GLP-1 RAs cannot be considered equal. The short acting lixisenatide showed no cardiovascular benefits, while once daily liraglutide and the weekly agonists, subcutaneous semaglutide, dulaglutide, and efpeglenatide, all lowered the incidence of cardiovascular events. Liraglutide, oral semaglutide and exenatide once weekly also reduced mortality. GLP-1 RAs reduce the progression of diabetic kidney disease. In the 2019 consensus report from EASD/ADA, GLP-1 RAs with demonstrated cardio-renal benefits (liraglutide, semaglutide and dulaglutide) are recommended after metformin to patients with established cardiovascular diseases or multiple cardiovascular risk factors. European Society of Cardiology (ESC) suggests starting with a SGLT-2 inhibitor or a GLP-1 RA in drug naïve patients with type 2 diabetes and atherosclerotic CVD or high CV Risk. However, the results from cardiovascular outcome trials (CVOT) are very heterogeneous suggesting that some GLP-1RA are more suitable to prevent CVD than others. The CVOTs provide a basis upon which individual treatment decisions for patients with T2D and CVD can be made.