Background
Tirzepatide is a novel dual GIP/GLP-1 receptor agonist that activates both glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptors. This dual mechanism was hypothesized to produce greater weight loss than GLP-1 agonism alone, given GIP’s role in adipose tissue metabolism and the additive satiety effects.
SURMOUNT-1 was the pivotal phase 3 trial establishing tirzepatide’s efficacy for obesity, setting a new benchmark for pharmacological weight loss.
Methods
Phase 3, randomized, double-blind, placebo-controlled trial at 119 sites in 9 countries. Participants were adults with BMI ≥30 (or ≥27 with weight-related comorbidities) without type 2 diabetes.
Arms: tirzepatide 5 mg, 10 mg, 15 mg weekly, or placebo. All participants received lifestyle counseling.
Primary endpoints:
- Percentage change in body weight from baseline
- Proportion achieving ≥5% weight reduction
Key Findings
| Arm | Mean weight loss | ≥5% | ≥10% | ≥15% | ≥20% |
|---|---|---|---|---|---|
| Tirzepatide 5 mg | −15.0% | 85% | 69% | 50% | 30% |
| Tirzepatide 10 mg | −19.5% | 89% | 79% | 62% | 44% |
| Tirzepatide 15 mg | −20.9% | 91% | 83% | 69% | 57% |
| Placebo | −3.1% | 35% | 12% | 5% | 1.5% |
Weight loss from trial with 72-week endpoint; later 88-week extension data showed up to 22.5% for 15 mg.
Clinical Significance
SURMOUNT-1 demonstrated that tirzepatide produces weight loss exceeding any previously approved anti-obesity medication and approaching the outcomes of sleeve gastrectomy. The 57% of 15 mg participants losing ≥20% of body weight was unprecedented in pharmacological weight management.
Tirzepatide (Zepbound) received FDA approval for chronic weight management in November 2023.
Limitations
- Non-diabetic population only; SURMOUNT-2 addressed T2DM patients
- No active comparator (semaglutide 2.4 mg not compared directly)
- Predominantly white (84%) cohort
- Long-term cardiovascular outcomes not assessed in this trial (SURPASS-CVOT ongoing)