Tirzepatide as a Dual GIP/GLP-1 Receptor Agonist: What the Published Research Reports

Tirzepatide (originally designated LY3298176) is a fatty-acid-modified synthetic peptide engineered to activate both the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor from a single molecular scaffold [1]. In cell lines expressing recombinant or endogenous incretin receptors, the peptide activated signalling at both receptors, and in mice it produced glucose-dependent insulin secretion and improved glucose tolerance through actions at both receptors; with chronic administration in mice, body weight and food intake decreased more than with a selective GLP-1 receptor agonist comparator [1]. Reviews of the incretin axis describe GIP and GLP-1 as binding distinct cognate receptors expressed on islet beta cells and in other tissues, including brain regions involved in food intake regulation, with divergent reported effects on alpha-cell glucagon output and on adipose lipid handling [2][3]. Detailed in vitro pharmacology has characterised the molecule as an imbalanced and biased co-agonist [4]. Using occupancy calculations for each receptor, investigators reported greater engagement of the GIP receptor than the GLP-1 receptor [4]. Signalling assays in that work indicated that the peptide mimicked native GIP at the GIP receptor but, at the GLP-1 receptor, favoured cAMP generation over β-arrestin recruitment and drove less receptor internalisation than GLP-1 itself [4]. Experiments in primary islets in the same report found that β-arrestin1 limited the insulin response to GLP-1 but not to GIP or to tirzepatide, which the authors interpreted as a mechanistic contributor to the observed insulin secretory profile [4]. Narrative reviews place this work in the context of a broader engineering effort to build multi-incretin receptor agonists [5], and discuss how GIP pharmacology might complement the anorexigenic mechanism attributed to GLP-1 in type 2 diabetes [6]. In humans, a Phase 1 programme in healthy subjects and a Phase 1b proof-of-concept cohort with type 2 diabetes reported a pharmacokinetic profile supporting once-weekly subcutaneous administration, along with reductions in fasting serum glucose versus placebo in the diabetic cohort and reductions in body weight versus placebo in both cohorts [1]. A randomised, double-blind Phase 2 trial in adults with type 2 diabetes compared the dual agonist with placebo and with the selective GLP-1 receptor agonist dulaglutide over 26 weeks and reported dose-dependent reductions in HbA1c that did not plateau, along with changes in body weight, fasting plasma glucose, waist circumference, and triglycerides; gastrointestinal treatment-emergent adverse events were the most common and were dose-related [7]. The Phase 3 monotherapy trial SURPASS-1 randomised 478 adults with type 2 diabetes inadequately controlled by diet and exercise to the dual agonist or placebo for 40 weeks and reported superiority for change in HbA1c, fasting serum glucose, bodyweight, and HbA1c target attainment, with mild-to-moderate transient gastrointestinal events as the most frequent adverse events and no reported severe hypoglycaemia in treated participants [8]. A systematic review and meta-analysis of seven randomised trials totalling 6609 participants with type 2 diabetes reported dose-dependent differences in HbA1c and body weight versus placebo, GLP-1 receptor agonists, and basal insulin, together with higher odds of nausea, vomiting, and diarrhoea versus placebo and higher discontinuation due to adverse events at the highest dose studied [9]. A published overview of the SURPASS programme summarises these glucose and weight findings in human trials alongside an adverse-event profile described as comparable to GLP-1 receptor agonists [10]. Mechanistic post hoc analyses of the Phase 2 human trial examined fasting biomarkers of beta-cell function and insulin resistance, reporting changes in HOMA2-B, proinsulin ratios, fasting insulin, HOMA2-IR, adiponectin, IGFBP-1, and IGFBP-2 relative to placebo and to dulaglutide; multiple linear regression indicated that weight loss statistically explained only a minority of the change in HOMA2-IR at the higher doses studied [11]. A review comparing the dual agonist with selective GLP-1 receptor agonism in humans reports greater improvements in insulin sensitivity and insulin secretory responses than semaglutide alongside lower prandial insulin and glucagon concentrations, while noting that GIP-mediated reductions in food intake demonstrated in rodents have not been demonstrated in humans and that the mechanism of action remains an open question [12]. Liver-related endpoints have been studied in humans and animals. Post hoc analyses of the Phase 2 human trial reported changes from baseline in ALT, AST, keratin-18, procollagen III, and adiponectin, with some comparisons reaching significance versus placebo or dulaglutide [13]. A review restricted to histology-endpoint trials in metabolic dysfunction-associated steatohepatitis describes incretin-based agents, including the dual GIP/GLP-1 agonist, as showing changes in disease activity in Phase 2 human trials, with gastrointestinal effects noted as a limitation on adherence [14]. In diabetic mice, the dual agonist reduced body weight, serum and hepatic lipid levels, and markers of liver injury, and 16S rRNA sequencing with targeted bile-acid metabolomics reported shifts in gut microbial composition and in the ratio of FXR antagonist to agonist bile acids, together with reduced intestinal FXR expression [15]. Separate work in colon cancer models reported inhibition of cell proliferation in vitro and tumour regression in multiple murine models, with spatial metabolomics indicating reduced glucose metabolites and destabilisation of HIF-1α with reduced PFKFB3 and PFK-1 activity [16]. Body composition and cardiovascular endpoints have been analysed across trial datasets. A systematic review and network meta-analysis of 22 randomised controlled trials in 2258 adults with diabetes and/or overweight or obesity reported that GLP-1 receptor agonists and GLP-1/GIP dual agonists reduced total body weight and fat mass and also reduced absolute lean mass, with the dual agonist ranked among the most effective for weight and fat mass reduction and among the least effective for preserving lean mass, while relative lean mass as a percentage of baseline was not significantly changed [17]. A systematic review and meta-analysis of 13 randomised trials in 65,878 participants with type 2 diabetes reported reduced odds of major adverse cardiovascular events, all-cause mortality, and cardiovascular mortality for GLP-1 and GIP/GLP-1 receptor agonists versus placebo without detected differences between the two classes, and concluded that dedicated trials are still needed to evaluate the dual agonist for stroke endpoints specifically [18]. A review of the adjudicated cardiovascular data across the clinical programme reports hazard ratios not exceeding 1.0 versus pooled comparators for the composite and its components, based on low event numbers [12]. Two further human-focused reviews address handling and interaction considerations documented in the literature. A review of gastrointestinal motility effects reports that retained gastric contents at upper gastrointestinal endoscopy occur more frequently with GLP-1 receptor agonists and the dual agonist, that associated pulmonary aspiration is rarely reported, and that periprocedural recommendations are constrained by limited evidence given long half-lives, tachyphylaxis with prolonged treatment, and baseline variability in gastric emptying [19]. A pharmacokinetic review notes that structural modification strategies used for GLP-1 receptor agonists and the dual GLP-1/GIP agonist result in minimal metabolism and renal excretion, that clinically significant enzyme- or transporter-mediated drug interactions have not been reported, and that mechanism-of-action interactions are largely limited to delayed gastric emptying, with a notable exposure change observed for oral contraceptives following dual agonist administration [20]. A pipeline review situates the dual GIP/GLP-1 agonist among GLP-1 mono-agonists, glucagon co-agonists, and triple agonists in obesity development programmes, and identifies maintenance of weight loss after treatment as an outstanding question [21].