Tirzepatide research: what the dual-agonist mechanism and the SURPASS trial programme measured

The short version

Tirzepatide is the first approved medicine to activate two distinct hormonal receptors — the GIP receptor and the GLP-1 receptor — with a single molecule. Both receptors are triggered naturally after eating and together tell the pancreas to release insulin, tell the liver and muscle what to do with blood sugar, and signal the brain to reduce hunger. Tirzepatide mimics both signals at once.

In the largest clinical trials, Tirzepatide produced larger reductions in blood sugar (measured as HbA1c) and body weight than any other approved agent in its class. It ranked first for glycemic control among 15 GLP-1-based drugs in a 76-trial network analysis. It cut body weight by up to 20.9% in SURMOUNT-1 versus placebo and by 20.2% versus 13.7% in a head-to-head weight-loss trial. Every number here is tied to the specific study that measured it.

Tirzepatide mechanism of action

Tirzepatide is a 39-amino-acid synthetic peptide based on the native GIP hormone sequence, modified with a C20 fatty-diacid (eicosanedioic acid) arm attached via a glutamic-acid linker. That fatty-diacid arm binds tightly to serum albumin, extending the half-life to approximately five days and enabling once-weekly subcutaneous dosing. Molecular formula: C225H348N48O68; molecular weight: 4,813.53 Da; CAS: 2023788-19-2.

The discovery and proof-of-concept study (Coskun 2018) established LY3298176 as a dual GIP and GLP-1 receptor agonist in vitro, confirmed improved glucose tolerance and greater weight reduction than a selective GLP-1 receptor agonist in mice, and supported once-weekly dosing through a 142-subject phase 1 programme ^[1]. A subsequent in vitro characterisation (Willard 2020) classified the compound as an imbalanced dual agonist — engaging the GIP receptor to a greater degree than the GLP-1 receptor — with biased GLP-1 receptor signalling that favors cAMP generation over beta-arrestin recruitment. In primary islet experiments, beta-arrestin-1 limited the insulin response to GLP-1 but not to GIP or tirzepatide, a finding that may explain the enhanced insulin secretion observed in clinical trials ^[2].

Engaging both receptors simultaneously enhances glucose-dependent insulin secretion, suppresses glucagon (the counter-regulatory hormone that raises blood glucose), slows gastric emptying, and reduces appetite and food intake through central mechanisms. The combined action produces larger glycemic and weight effects than selective GLP-1 agonism alone, as the phase 3 programme demonstrated.

SURPASS phase 3 programme: glycemic efficacy in type 2 diabetes

SURPASS-2 (Frias 2021): the pivotal head-to-head phase 3 trial. An open-label, 40-week RCT in 1,879 adults with type 2 diabetes inadequately controlled on metformin. Once-weekly tirzepatide at 5, 10, and 15 mg reduced HbA1c by 2.01, 2.24, and 2.30 percentage points versus 1.86 pp with semaglutide 1 mg; tirzepatide was noninferior and superior at all three doses. Weight reductions were -1.9, -3.6, and -5.5 kg greater with tirzepatide versus the comparator. The most common adverse events were gastrointestinal and mostly mild to moderate ^[3].

SURPASS-AP-Combo (Gao 2023): a 40-week phase 3 RCT in 917 adults in the Asia-Pacific region (83% from China) with type 2 diabetes uncontrolled on metformin. Tirzepatide at 5, 10, and 15 mg reduced HbA1c by 2.24%, 2.44%, and 2.49% versus 0.95% with insulin glargine. Proportions reaching HbA1c < 7.0% were 75.4%, 86.0%, and 84.4% with tirzepatide versus 23.7% with insulin glargine ^[8].

SURPASS J-combo (Kadowaki 2022): a 52-week multicentre open-label phase 3 trial at 34 sites in Japan in 443 adults with type 2 diabetes on oral monotherapy. Mean HbA1c fell from approximately 8.5–8.6% at baseline to 6.0% (5 mg), 5.6% (10 mg), and 5.6% (15 mg) at week 52. Body weight fell -3.8, -7.5, and -10.2 kg across the three doses ^[10].

SURPASS-CVOT is the dedicated cardiovascular outcomes trial; results have been reported and support the cardiovascular safety of tirzepatide in high-risk type 2 diabetes populations.

Network meta-analysis (Yao 2024): pooling 76 RCTs and 15 GLP-1-based drugs in 39,246 participants with type 2 diabetes, tirzepatide produced the largest HbA1c reduction of all agents studied (mean difference -2.10%, 95% CI -2.47 to -1.74; SUCRA 94.2%; high confidence) and the greatest fasting plasma glucose reduction (-3.12 mmol/L; SUCRA 97.2%; high confidence), ranking first for glycemic control ^[11]. A systematic review and meta-analysis of six trials in 6,579 participants confirmed a dose-response (HbA1c WMD -1.07%, body weight WMD -7.99 kg versus controls) with no increase in hypoglycemia as monotherapy or add-on therapy ^[9].

SURMOUNT programme: weight management and beyond

SURMOUNT-1 (Jastreboff 2022): the pivotal obesity trial. A 72-week double-blind phase 3 RCT in 2,539 adults with obesity (BMI ≥30, or ≥27 with a weight-related condition) without type 2 diabetes. Once-weekly tirzepatide produced mean body weight changes of -15.0% (5 mg), -19.5% (10 mg), and -20.9% (15 mg) versus -3.1% with placebo. The 20-week titration schedule escalated from 2.5 mg, through 5, 7.5, 10, and 12.5 mg, to the maintenance dose. Gastrointestinal events were the most common adverse events, mostly mild to moderate and occurring primarily during dose escalation ^[4].

SURMOUNT-5 (Aronne 2025): a phase 3b open-label head-to-head trial in 751 adults with obesity without type 2 diabetes. Participants were randomised to the maximum tolerated dose of tirzepatide (10 or 15 mg) or the maximum tolerated dose of semaglutide (1.7 or 2.4 mg) over 72 weeks. Mean weight change was -20.2% with tirzepatide versus -13.7% with semaglutide (P < 0.001). Tirzepatide produced greater waist circumference reductions and higher proportions reaching ≥10%, 15%, 20%, and 25% weight loss thresholds ^[5].

A 2025 systematic review and network meta-analysis of 56 RCTs in 60,307 patients across pharmacological obesity treatments found both tirzepatide and semaglutide achieved more than 10% total body weight loss; tirzepatide was additionally effective for remission of obstructive sleep apnea and metabolic dysfunction-associated steatohepatitis ^[6].

Post-hoc SURPASS analyses in older adults (≥65 years with BMI < 30): tirzepatide reduced HbA1c by 1.97–2.10% across doses in this lower-BMI older subgroup, with consistent hypoglycemia incidence regardless of background insulin or sulfonylurea ^[22].

Tirzepatide vs semaglutide

Two head-to-head trials are now available comparing tirzepatide directly against semaglutide.

In type 2 diabetes (SURPASS-2): tirzepatide at 5, 10, and 15 mg was noninferior and superior to semaglutide 1 mg for HbA1c reduction at 40 weeks (-2.01/-2.24/-2.30 pp vs -1.86 pp). Weight reductions with tirzepatide exceeded those with semaglutide by 1.9, 3.6, and 5.5 kg ^[3].

In obesity without type 2 diabetes (SURMOUNT-5): tirzepatide at maximum tolerated dose produced -20.2% versus -13.7% with semaglutide at maximum tolerated dose over 72 weeks (P < 0.001) ^[5].

The network meta-analysis of 76 trials ranked tirzepatide first and semaglutide second for glycemic control among all GLP-1-based drugs, with tirzepatide's mean HbA1c advantage of approximately 0.2 to 0.4 percentage points over semaglutide across doses ^[11].

Both agents share a class of gastrointestinal adverse events. Both agents also have the incretin class's thyroid C-cell rodent signal and gallbladder/biliary class association. The head-to-head safety comparison is less thoroughly characterized than the efficacy comparison.

Tirzepatide results: the headline numbers

Glycemic control (type 2 diabetes):

• SURPASS-2 (40 weeks, vs semaglutide 1 mg): HbA1c -2.01/-2.24/-2.30 pp at 5/10/15 mg versus -1.86 pp ^[3]
• SURPASS-AP-Combo (40 weeks, vs insulin glargine): HbA1c -2.24/-2.44/-2.49 pp; 75–86% reached HbA1c < 7.0% ^[8]
• Network meta-analysis (76 RCTs, 39,246 participants): ranked 1st for HbA1c reduction among 15 GLP-1-based drugs (SUCRA 94.2%) ^[11]

Weight management (obesity, no T2D):

• SURMOUNT-1 (72 weeks, vs placebo): -15.0/-19.5/-20.9% vs -3.1% ^[4]
• SURMOUNT-5 (72 weeks, vs semaglutide at maximum tolerated dose): -20.2% vs -13.7% ^[5]

FDA-approved indications: type 2 diabetes mellitus (May 2022), chronic weight management (November 2023), moderate-to-severe obstructive sleep apnea in adults with obesity ^[7].

These are trial-measured outcomes from specific populations and study designs. Individual results differ.