Introduction
Tirzepatide (LY3298176) is a synthetic peptide that represents a novel class of dual incretin receptor agonists, acting simultaneously on both glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptors. Originally developed by Eli Lilly and Company, tirzepatide was engineered as a 39-amino acid linear peptide based on the native GIP sequence, incorporating modifications to enable GLP-1 receptor co-agonism (Coskun et al., 2018). The molecule features a C20 fatty diacid moiety conjugated via a linker to lysine at position 20, facilitating albumin binding and thereby extending the pharmacokinetic half-life to approximately 5 days, which permits once-weekly subcutaneous administration.
The rationale for dual receptor engagement stems from the recognition that both GIP and GLP-1 are key incretin hormones secreted by enteroendocrine cells in response to nutrient ingestion. While selective GLP-1 receptor agonists such as semaglutide and liraglutide have demonstrated substantial clinical efficacy, preclinical and translational research suggested that concurrent GIP receptor activation could yield additive or synergistic effects on glycaemic regulation, body weight reduction, and metabolic parameters (Willard et al., 2020). This hypothesis formed the basis for the extensive SURPASS and SURMOUNT clinical trial programmes that have since generated a substantial body of evidence regarding tirzepatide's pharmacological profile.
Mechanism of Action
Dual Incretin Receptor Agonism
Tirzepatide functions as an imbalanced dual agonist, exhibiting full agonist activity at the GIP receptor with approximately five-fold greater potency relative to native GIP, while demonstrating biased agonism at the GLP-1 receptor with reduced potency compared to native GLP-1 (Willard et al., 2020). At the GLP-1 receptor, tirzepatide preferentially activates cAMP signalling pathways over beta-arrestin recruitment, a pharmacological characteristic that may attenuate receptor desensitisation and contribute to sustained efficacy. This biased signalling profile distinguishes tirzepatide mechanistically from conventional selective GLP-1 receptor agonists.
Pancreatic Effects
At the level of the pancreatic beta cell, engagement of both GIP and GLP-1 receptors results in potentiation of glucose-dependent insulin secretion through complementary intracellular signalling cascades. Both receptors couple to Gs proteins, stimulating adenylyl cyclase and increasing intracellular cAMP concentrations, which in turn activate protein kinase A and Epac2 pathways to enhance insulin exocytosis. Importantly, the insulinotropic effect remains glucose-dependent, preserving the physiological safeguard against hypoglycaemia (Thomas et al., 2021). Clinical data from mechanistic substudies have demonstrated that tirzepatide significantly improves both first- and second-phase insulin secretion, as well as insulin sensitivity, assessed by clamp methodology, suggesting effects beyond simple incretin-mediated insulin release (Thomas et al., 2021).
Extrapancreatic Effects
Beyond glycaemic regulation, tirzepatide exerts effects on multiple organ systems relevant to metabolic homeostasis. GLP-1 receptor activation in the central nervous system, particularly within hypothalamic and hindbrain nuclei, modulates appetite and satiety signalling. GIP receptor agonism appears to contribute additional centrally mediated effects on energy balance, although the precise mechanisms remain an active area of investigation. In adipose tissue, GIP receptor signalling has been implicated in lipid buffering, adipogenesis, and adipokine regulation, potentially influencing systemic insulin sensitivity (Coskun et al., 2018). Preclinical models have demonstrated that the dual agonist approach produces greater reductions in body weight and improvements in metabolic markers than either receptor agonist alone, supporting the concept of complementary incretin pathway engagement.
Key Clinical Research Findings
The SURPASS Programme: Type 2 Diabetes
The SURPASS clinical trial programme comprised a series of phase 3 randomised controlled trials evaluating tirzepatide at doses of 5 mg, 10 mg, and 15 mg once weekly in populations with type 2 diabetes mellitus across varying treatment backgrounds.
In SURPASS-1, tirzepatide was evaluated as monotherapy in treatment-naïve participants. At 40 weeks, HbA1c reductions from baseline ranged from −1.87% to −2.07% across the three dose groups, with 83–92% of participants achieving HbA1c levels below 7.0%. Notably, body weight reductions of 7.0–9.5 kg were observed (Rosenstock et al., 2021). These data established the foundational efficacy profile for tirzepatide monotherapy.
SURPASS-2 directly compared tirzepatide with semaglutide 1 mg weekly in participants receiving metformin. This head-to-head comparison demonstrated the superiority of all three tirzepatide doses over semaglutide 1 mg for HbA1c reduction, with tirzepatide 15 mg producing a mean reduction of −2.46% versus −1.86% with semaglutide. Body weight reductions were also significantly greater with tirzepatide 10 mg and 15 mg (−9.65 kg and −11.20 kg, respectively) compared with semaglutide (−5.67 kg) (Frías et al., 2021). This trial provided pivotal comparative data positioning tirzepatide within the existing GLP-1 receptor agonist landscape.
SURPASS-3 compared tirzepatide with insulin degludec as add-on to metformin, demonstrating superior glycaemic control with all tirzepatide doses. Whereas insulin degludec increased body weight by a mean of 2.3 kg, tirzepatide produced dose-dependent reductions of 7.5–12.9 kg (Ludvik et al., 2021). SURPASS-4 extended these findings to a cardiovascular risk population, comparing tirzepatide with insulin glargine over 52 weeks, confirming superior HbA1c reduction and weight loss with an acceptable cardiovascular safety profile (Del Prato et al., 2021).
Across the SURPASS programme, the most frequently reported adverse events were gastrointestinal in nature, including nausea, diarrhoea, and decreased appetite, consistent with the known class effects of incretin-based therapies. These events were predominantly mild to moderate in severity and tended to diminish with continued treatment (Min and Bain, 2021).
The SURMOUNT Programme: Obesity
The SURMOUNT trials evaluated tirzepatide specifically for weight management in populations with obesity or overweight. SURMOUNT-1 enrolled participants without type 2 diabetes and demonstrated mean body weight reductions of −15.0% (5 mg), −19.5% (10 mg), and −20.9% (15 mg) compared with −3.1% for placebo at 72 weeks. At the highest dose, over one-third of participants achieved weight reductions exceeding 25% (Jastreboff et al., 2022). These results represented a substantial advance in pharmacological weight reduction, exceeding outcomes previously reported with selective GLP-1 receptor agonists in comparable trial designs.
SURMOUNT-2 extended these observations to participants with both obesity and type 2 diabetes, a population historically more resistant to pharmacological weight loss. Mean weight reductions of −12.8% and −14.7% were observed with tirzepatide 10 mg and 15 mg, respectively, compared with −3.2% for placebo, alongside significant improvements in glycaemic control (Garvey et al., 2023).
Pharmacokinetics
The pharmacokinetic profile of tirzepatide is characterised by its C20 fatty diacid modification, which promotes non-covalent binding to serum albumin and thereby substantially extends the circulating half-life. Following subcutaneous administration, tirzepatide exhibits an elimination half-life of approximately 5 days (approximately 113 hours), supporting once-weekly dosing (Coskun et al., 2018). Peak plasma concentrations are attained at 8 to 72 hours post-injection, with steady-state concentrations reached after approximately 4 weeks of weekly administration.
Tirzepatide demonstrates dose-proportional pharmacokinetics across the clinically studied dose range. The molecule is primarily eliminated through proteolytic degradation, with renal clearance of the intact peptide representing a minor elimination pathway. Population pharmacokinetic analyses have indicated that age, sex, race, ethnicity, and body weight do not necessitate dose adjustments, although exposure increases modestly with decreasing body weight. Mild to moderate renal or hepatic impairment does not produce clinically significant alterations in pharmacokinetic parameters.
Current Research Directions
Active research investigations involving tirzepatide extend beyond glycaemic control and weight management into several adjacent domains. Cardiovascular outcome data continue to accrue, with dedicated cardiovascular outcome trials designed to establish whether the metabolic improvements observed with tirzepatide translate into reductions in major adverse cardiovascular events. Preliminary analyses from SURPASS-4 demonstrated non-inferiority to insulin glargine for cardiovascular safety, but definitive superiority data from purpose-designed trials remain forthcoming (Del Prato et al., 2021).
Investigations into the effects of tirzepatide on non-alcoholic steatohepatitis (NASH), subsequently reclassified as metabolic dysfunction-associated steatohepatitis (MASH), represent a significant avenue of translational interest. The dual incretin mechanism, combined with substantial weight reduction, is hypothesised to ameliorate hepatic steatosis, inflammation, and fibrosis through both weight-dependent and weight-independent mechanisms. Early-phase clinical data have demonstrated reductions in liver fat content and improvements in biomarkers of hepatic inflammation with tirzepatide treatment.
Additional research programmes are exploring the potential role of tirzepatide in obstructive sleep apnoea, heart failure with preserved ejection fraction, and chronic kidney disease associated with type 2 diabetes. The breadth of these investigations reflects the recognition that the metabolic improvements conferred by dual incretin receptor agonism may have therapeutic implications across a spectrum of obesity-related and cardiometabolic conditions. Furthermore, structure-activity relationship studies continue to explore modifications to the tirzepatide scaffold that may yield next-generation molecules with enhanced receptor selectivity profiles or altered pharmacokinetic properties (Willard et al., 2020).
The emergence of tirzepatide as the first dual GIP/GLP-1 receptor agonist to undergo extensive clinical evaluation has fundamentally expanded understanding of incretin biology and its therapeutic applications. The consistent demonstration of superior efficacy relative to established comparators across both the SURPASS and SURMOUNT programmes underscores the scientific rationale for poly-receptor agonism as a pharmacological strategy. Ongoing research efforts will further delineate the long-term safety profile, cardiovascular and organ-protective effects, and optimal positioning of this compound within the evolving therapeutic landscape (Frías et al., 2021).