Tirzepatide Research: How the Dual Agonist Works

Tirzepatide Research: How the Dual Agonist Works
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Science & Medicine Obesity Clinical Trials Pharmacology
Investigative Science Report The Twincretin That Changed the Calculation: Tirzepatide, What It Does, and What It Can't Undo Tirzepatide outperformed semaglutide by 47% in relative weight loss in the first direct head-to-head trial. But its most consequential contribution may be demonstrating that activating two incretin receptors simultaneously does things no single-receptor drug can replicate.
2025 · Science Desk

What It Is: The Architecture of a Dual Agonist Tirzepatide (brand names Mounjaro for diabetes, Zepbound for obesity) is a synthetic 39-amino acid peptide developed by Eli Lilly. Its chemical structure is derived primarily from the native GIP sequence, modified with a C20 fatty diacid chain at position 26 that enables albumin binding and extends the half-life to approximately five days — sufficient for once-weekly subcutaneous injection. What makes tirzepatide structurally distinct — and pharmacologically novel — is its simultaneous agonism at two receptors: the GIP receptor (GIPR) and the GLP-1 receptor (GLP-1R). It is not a balanced dual agonist. Studies established that tirzepatide activates the GIPR with potency equivalent to native GIP, while activating the GLP-1R with approximately five times weaker affinity than native GLP-1. At the GLP-1 receptor, tirzepatide shows "biased agonism," preferentially triggering the cAMP signaling pathway while largely bypassing β-arrestin recruitment — resulting in sustained GLP-1 receptor engagement without the receptor downregulation that limits high-dose pure GLP-1 agonists. In practice, this means tirzepatide can push GLP-1 receptor engagement higher than semaglutide without proportionally escalating gastrointestinal side effects — because much of its efficacy load is carried by the GIPR pathway, which is not associated with nausea and vomiting. GIP receptors are present in adipose tissue, in the brain, and in pancreatic beta cells, where co-stimulation with GLP-1 produces a synergistic — not merely additive — insulin secretion response. What the Trials Show
SURPASS Program — Type 2 Diabetes Five global Phase 3 trials, 10,000+ patients. Tirzepatide reduced HbA1c by 1.7–2.4 percentage points, outperforming semaglutide, dulaglutide, insulin degludec, and insulin glargine. FDA approved for T2D under the name Mounjaro on May 13, 2022 — the first dual GIP/GLP-1 receptor agonist ever approved.
SURMOUNT-1 — Obesity Without Diabetes · NEJM, 2022 2,539 adults, 72 weeks. Average weight loss: 16.0%, 21.4%, and 22.5% at 5, 10, and 15 mg doses vs. 2.4% placebo. At 15 mg, 57% lost ≥20% body weight and 36% lost ≥25% — thresholds historically associated only with bariatric surgery. FDA approved for obesity as Zepbound in November 2023.
Body Composition DXA Substudy · 2025 160 SURMOUNT-1 participants. Total weight loss 21.3%, fat mass loss 33.9%, lean mass loss 10.9%. Approximately 75% of lost weight was fat, 25% lean mass — roughly three times more favorable for fat than muscle.
3-Year Diabetes Prevention · NEJM, 2025 1,032 participants with prediabetes, 176 weeks. Only 1.3% on tirzepatide developed T2D vs. 13.3% on placebo — a 93% relative risk reduction. Seventeen weeks after stopping, 2.4% vs. 13.7% had T2D.
SURMOUNT-5 — Head-to-Head vs. Semaglutide · NEJM, May 2025 751 adults, 72 weeks. Tirzepatide: 20.2% average weight loss. Semaglutide: 13.7%. Proportion achieving ≥25% weight loss: 31.6% vs. 16.1%. GI-related discontinuations: 2.7% (tirzepatide) vs. 5.6% (semaglutide).
SURMOUNT-OSA — Obstructive Sleep Apnea · NEJM, June 2024 Tirzepatide reduced the apnea-hypopnea index by up to 62.8% at 52 weeks. Between 42–51% of participants reached AHI levels at which PAP therapy may no longer be recommended. FDA approved for moderate-to-severe OSA in December 2024 — the first pharmacological OSA treatment ever approved.
SUMMIT — Heart Failure with Preserved Ejection Fraction · 2024 In patients with HFpEF and obesity, tirzepatide reduced the composite cardiovascular endpoint by 38%, with a 56% reduction in worsening heart failure hospitalizations (3.3% vs. 7.1% with placebo).
SYNERGY-NASH — Liver Disease · Phase 2 In patients with MASH and moderate-to-severe liver fibrosis, tirzepatide resolved MASH without worsening fibrosis in 44–62% of participants and improved liver fibrosis without worsening MASH in 51–55%. The first obesity pharmacotherapy to show histological fibrosis improvement.
The Weight Regain Problem: SURMOUNT-4 Participants who achieved ≥10% weight loss after 36 weeks were randomized to continue or switch to placebo for a further 52 weeks. Those who stopped regained an average of 14% of body weight — approximately two-thirds of what they had initially lost.
Key Finding — JAMA Internal Medicine, February 2026 82.5% of those who stopped tirzepatide regained at least 25% of their initial weight loss within one year. Weight regain was accompanied by dose-dependent reversal of cardiometabolic improvements — blood pressure, triglycerides, HbA1c, and insulin resistance all worsened. Participants who regained ≥75% of lost weight essentially returned to baseline cardiometabolic parameters.
Tirzepatide manages obesity — it does not cure it.
Tirzepatide vs. Semaglutide vs. Retatrutide
Semaglutide — Single GLP-1 Agonist ~14.9% weight loss at 68 weeks (STEP 1). Only completed cardiovascular outcomes trial in obesity (SELECT, 20% reduction in major events). Deepest real-world evidence base.
Tirzepatide — Dual GLP-1/GIP Agonist 22.5% at 72 weeks (SURMOUNT-1); 20.2% vs. semaglutide's 13.7% in direct head-to-head. FDA-approved for T2D, obesity, and OSA. 93% relative diabetes prevention at 3 years. Available now.
Retatrutide — Triple GLP-1/GIP/Glucagon Agonist 28.7% at 68 weeks in Phase 3 (TRIUMPH-4, December 2025). Not yet approved. Glucagon receptor engagement produces 82% liver fat reduction in MASLD — a liver-specific effect no dual agonist replicates. FDA submission expected late 2026 or early 2027. New dysesthesia signal (20.9% at 12 mg) requires monitoring.
What Remains Unresolved Cardiovascular outcomes at scale. SURMOUNT-MMO (>15,000 participants, primary endpoint: mortality, MI, stroke, HF) is expected to complete in October 2027. Until then, tirzepatide's cardiovascular outcomes profile in obesity without T2D rests on surrogate markers. Long-term safety. Pharmacovigilance continues to accumulate on gallbladder disease (a known GLP-1 class effect), thyroid cancer (class warning from rodent data, no human signal detected to date), and pancreatitis. The chronic treatment question. Most benefit reverses within a year of stopping. What optimal long-term management looks like for patients who cannot afford or tolerate indefinite treatment — a global majority — remains an open and pressing question. A Drug That Illuminates the Disease What tirzepatide has done for the science is clarify how profoundly multi-pathway metabolic disease actually is. Adding GIP to GLP-1 did not simply boost an existing mechanism — it opened new ones in adipose tissue, the beta cell, and the brain simultaneously. The 93% risk reduction in diabetes progression over three years suggests a partial biological reset that persists even after stopping treatment. The central paradox remains: tirzepatide works almost entirely as long as you take it. Stop, and the weight returns. Which means it has not found a cure for obesity — it has found a way to manage it indefinitely, with remarkable precision. Whether "indefinitely" is pharmacologically, economically, and biologically sustainable for the populations who need it most is a question that clinical trials, by design, cannot answer.
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