Tirzepatide: Dual GIP/GLP-1 Receptor Agonism and the SURMOUNT Trials
A research review of tirzepatide (Mounjaro, Zepbound), the first FDA-approved dual GIP and GLP-1 receptor agonist, covering its mechanism, the SURPASS and SURMOUNT trial programs, and how dual agonism may differ from GLP-1 alone.
Tirzepatide is a 39-amino acid synthetic peptide that acts as a dual receptor agonist at both the glucose-dependent insulinotropic polypeptide receptor (GIPR) and the GLP-1 receptor. It was developed by Eli Lilly and Company and received FDA approval in May 2022 under the brand name Mounjaro for type 2 diabetes mellitus, and in November 2023 under the brand name Zepbound for chronic weight management.
Tirzepatide represents a departure from the GLP-1-only pharmacology of semaglutide and earlier drugs in the class. Its development was predicated on evidence that GIP receptor activation might complement GLP-1 receptor activation for both glycemic control and weight loss — though the mechanistic basis for the additive or synergistic effect remains an active area of research.
Molecular Design
Tirzepatide's 39-amino acid sequence is based on the native GIP sequence, modified to confer GLP-1 receptor activity and extended half-life. The key structural features:
Dual receptor activity: The peptide backbone was engineered to bind both GIPR and GLP-1R with approximately equal potency, unlike most GLP-1 agonists which have negligible GIPR activity. The exact design decisions that achieve this balanced dual binding involved extensive medicinal chemistry optimization.
C20 fatty diacid modification: Similar to semaglutide's albumin-binding strategy, tirzepatide is conjugated to a C20 fatty diacid via a hydrophilic linker that enables reversible albumin binding. This modification produces a plasma half-life of approximately 5 days, supporting once-weekly subcutaneous dosing.
Aib substitution at position 2: Replaces alanine to confer DPP-4 resistance, preventing the rapid enzymatic degradation that limits native GIP and GLP-1.
The GIP Receptor: Why It Matters
To understand what makes tirzepatide distinct, it is necessary to understand GIP (glucose-dependent insulinotropic polypeptide) biology. GIP is the other major incretin hormone, secreted from K-cells in the proximal small intestine in response to nutrient ingestion.
Like GLP-1, GIP potentiates insulin secretion in a glucose-dependent manner. However, the role of GIPR activation in body weight regulation is more complex than GLP-1R activation:
- In lean rodent models, GIP receptor agonism has been reported to increase fat storage, which initially made GIPR an unlikely target for weight loss therapy
- In the context of caloric restriction or combined with GLP-1R agonism, GIPR activation may instead promote fat mobilization from adipose tissue
- GIPR is also expressed in bone, brain, and the cardiovascular system — with potential effects on these tissues that are not yet well characterized
The net effect of adding GIPR agonism to GLP-1R agonism in humans appears to produce greater weight loss than GLP-1R agonism alone, as demonstrated in comparative trial data — but the mechanism is not fully resolved.
The SURPASS Trial Program (Type 2 Diabetes)
Lilly conducted the SURPASS phase 3 program across five major trials comparing tirzepatide to placebo and active comparators in type 2 diabetes.
SURPASS-2 — the most clinically relevant comparator trial — enrolled 1,879 people with type 2 diabetes inadequately controlled on metformin and compared tirzepatide 5mg, 10mg, and 15mg versus once-weekly semaglutide 1mg over 40 weeks.[]
HbA1c reductions with tirzepatide 15mg were −2.46 percentage points versus −1.86 for semaglutide 1mg. Weight reductions with tirzepatide 15mg were −11.2kg versus −5.7kg for semaglutide. These differences were statistically significant and represented a clinically meaningful advantage for tirzepatide in both glycemic control and weight reduction at the doses studied.
SURPASS-3, 4, and 5 compared tirzepatide to insulin degludec, insulin glargine, and basal insulin plus liraglutide, respectively, consistently demonstrating superior HbA1c reduction and weight loss.[]
The SURMOUNT Trial Program (Obesity)
The SURMOUNT program evaluated tirzepatide specifically for weight management, using doses up to 15mg weekly in people with obesity.
SURMOUNT-1 enrolled 2,539 adults with BMI ≥30 (or ≥27 with a weight-related comorbidity) without type 2 diabetes over 72 weeks.[]
Weight reductions at maximum dose (15mg):
- Mean percentage weight loss: −20.9% from baseline
- Mean absolute weight loss: approximately 22.5kg
- Proportion achieving ≥20% weight loss: 57% with tirzepatide 15mg vs 3% with placebo
These results were substantially larger than those observed with semaglutide 2.4mg (≈15% in STEP 1), though direct head-to-head comparison was not performed in SURMOUNT-1.
SURMOUNT-2 replicated similar findings in people with type 2 diabetes, where weight loss was approximately 15.7% with 15mg dose.[]
Comparison with Semaglutide
The key question following tirzepatide's approval was whether the greater observed weight loss versus semaglutide in individual trials reflected a genuine pharmacological advantage or differences in patient populations and doses studied.
The available data suggests tirzepatide at maximum dose (15mg) produces greater weight loss than semaglutide at maximum dose (2.4mg). The SURMOUNT-5 trial — a direct head-to-head comparison of tirzepatide 10mg/15mg versus semaglutide 2.4mg in people with obesity — was designed to address this question directly.
Whether the advantage is attributable to dual vs. single receptor agonism, to the specific doses studied, or to other pharmacological differences remains an active area of investigation. See the retatrutide article for how triple agonism may produce even greater effects.
Safety Profile
Tirzepatide's adverse event profile is largely similar to GLP-1 agonist class effects:
Common: Nausea, vomiting, diarrhea, constipation (most common during dose escalation; a slow titration schedule is specified in the label to reduce these)
Black box warning: Thyroid C-cell tumors in rodents, same class warning as semaglutide. Contraindicated in patients with medullary thyroid carcinoma history or MEN2.
Hypoglycemia: Risk is low when used without insulin or sulfonylureas; glucose-dependent mechanism provides inherent safety.
For a framework on interpreting what approved drug status means for evidence quality, see Understanding Clinical Trials for Peptide Drugs.
References
- 1.Frias JP, Davies MJ, Rosenstock J, Perez Manghi FC, Fernandez Lando L, Bergman BK, Liu B, Cui X, Brown K; SURPASS-2 Investigators. “Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes.” New England Journal of Medicine. 2021;385(6):503-515. doi:10.1056/NEJMoa2107519 [PubMed]
- 2.Jastreboff AM, Aronne LJ, Ahmad NN, Wharton S, Connery L, Alves B, Kiyosue A, Zhang S, Liu B, Bunck MC, Stefanski A; SURMOUNT-1 Investigators. “Tirzepatide Once Weekly for the Treatment of Obesity.” New England Journal of Medicine. 2022;387(3):205-216. doi:10.1056/NEJMoa2206038 [PubMed]
- 3.Ludvik B, Giorgino F, Jodar E, Frias JP, Fernandez Lando L, Brown K, Gilder K, Rosenstock J. “Once-weekly tirzepatide versus once-daily insulin degludec as add-on to metformin with or without SGLT2 inhibitors in patients with type 2 diabetes (SURPASS-3).” Lancet. 2021;398(10300):583-598. doi:10.1016/S0140-6736(21)01443-4 [PubMed]
- 4.U.S. Food and Drug Administration. “FDA approves novel, dual-targeted treatment for type 2 diabetes.” 2022. [Link]
- 5.Jastreboff AM, Kaplan LM, Frías JP, Zhang S, Liu B, Bunck MC, Stefanski A; SURMOUNT-2 Investigators. “Tirzepatide once weekly for the treatment of obesity in people with type 2 diabetes (SURMOUNT-2).” Lancet. 2023;402(10402):613-626. doi:10.1016/S0140-6736(23)01200-X [PubMed]