Berobenatide

What is Berobenatide?

Berobenatide is the INN for Pfizer's investigational ultra-long-acting, fully-biased GLP-1 receptor agonist, previously known as PF-08653944 (PF'3944) and, in the Metsera era, MET-097 / MET-097i. The asset originated at Metsera and transferred to Pfizer when Pfizer completed its acquisition of Metsera on November 13, 2025 after a bidding war with Novo Nordisk, paying $65.60 per share in cash (an enterprise value of about $7.0 billion) plus a contingent value right worth up to $20.65 per share; the deal is widely reported as worth up to about $10 billion. "Fully-biased" is the developer's term: in Metsera's cell assays the molecule fully activated cAMP signaling at the GLP-1 receptor while recruiting little β-arrestin. A Metsera-funded study measured β-arrestin-2 recruitment at 17% of the reference agonist exendin-4, but receptor internalization was still 68%, and tirzepatide, retatrutide and orforglipron behaved similarly in the same assays — so whether the bias improves tolerability or durability in people is unproven. Chemically it is a lipidated analog of exendin-4 (the Gila-monster peptide behind exenatide), not of human GLP-1. Its defining clinical feature is dosing cadence: extended half-life supports once-monthly subcutaneous injection (0.5 mL volume) after an initial weekly titration phase, a departure from the weekly cadence that defines current approved GLP-1 therapy. The Phase 2b VESPER program (VESPER-1, -2, -3) delivered the data package presented at ADA 2026 — 15.9% mean weight loss over 32 weeks of open-label berobenatide (escalated to 2.4 mg weekly) in 22 VESPER-1 participants who had received placebo for the first 28 weeks (not placebo-adjusted), HbA1c reduction of 2.2% at week 28 in T2D (VESPER-2, 1.6 mg weekly), and 12.3% placebo-adjusted weight loss at 28 weeks under monthly maintenance dosing (VESPER-3) — and Pfizer has moved into Phase 3. VESPER-4 (3,578 participants) and VESPER-5 (1,078, type 2 diabetes) have finished enrolling, with primary completion expected in the second half of 2027, and VESPER-6 (NCT07595549), the pivotal monthly-maintenance study, is recruiting. Pfizer lists 10 ongoing and planned Phase 3 trials for 2026, including obstructive sleep apnea (VESPER-OSA 1, recruiting since September 2026), knee osteoarthritis, a switch study from semaglutide or tirzepatide, and trials in China and Japan, and is targeting first approvals beginning in 2028.

What Berobenatide Is Investigated For

Berobenatide (PF-08653944) is investigated primarily for obesity and chronic weight management, with the Phase 3 program also extending into type 2 diabetes (VESPER-5), weekly titration followed by monthly maintenance in people with and without T2D (VESPER-6, NCT07595549, pivotal; primary endpoint at week 72 in the non-T2D cohort), and additional indications including knee osteoarthritis and obstructive sleep apnea — Pfizer has stated a 10-trial Phase 3 program for 2026. The strongest current evidence is the Phase 2b VESPER package presented at ADA 2026: in the open-label VESPER-1 extension, 22 people switched from placebo and escalated to 2.4 mg weekly lost a mean 15.9% over 32 weeks with no plateau, while participants continuing berobenatide reached 10.3% to 14.9% loss from baseline at week 60; VESPER-2 in T2D delivered HbA1c -2.2% at week 28 (1.6 mg weekly) versus -0.2% placebo; and VESPER-3 showed 12.3% placebo-adjusted weight loss at 28 weeks using weekly titration followed by monthly maintenance, with continued weight loss through the cadence switch. The strategic claim is not effect-size superiority versus tirzepatide or retatrutide (weight-loss magnitude trails the top of the class) but cadence and tolerability — once-monthly maintenance. Pfizer calls GI tolerability favorable, but in VESPER-3 nausea affected 37.7% and vomiting 23.3% of berobenatide participants (vs 15.1% and 1.9% on placebo), and 8.4% stopped treatment because of GI side effects (vs 0% on placebo). Honest caveats: no Phase 3 readouts yet, no peer-reviewed publication of the VESPER data (press-release and ADA-conference stage), no cardiovascular outcomes data, the "fully-biased GLP-1" tolerability hypothesis is pharmacologically interesting but not yet clinically validated in head-to-head studies, and first approvals are not expected before 2028.

History & Discovery

Berobenatide came from Metsera's MINT library of roughly 20,000 nutrient-stimulated hormone analog peptides, built on more than 20 years of work initially led by Professor Stephen Bloom at Imperial College London. Coded MET-097 (MET-097i for the injectable), it entered a Phase 1/2 trial in April 2024 (NCT06857617), followed by the Phase 2b trials VESPER-1 (October 2024), VESPER-2 (March 2025) and VESPER-3 (April 2025). In Phase 1 it showed a half-life of about 15–16 days and dose-dependent weight loss that was maintained eight weeks after the last of five weekly doses. Pfizer agreed to buy Metsera in September 2025 for $47.50 per share plus a contingent value right. After a competing proposal from Novo Nordisk, which Metsera's board ultimately judged to carry unacceptable legal and regulatory risk, Pfizer raised its offer to $65.60 per share plus up to $20.65 per share and closed the deal on November 13, 2025; $6.40 of that contingent payment is tied to FDA approval of monthly berobenatide monotherapy. The drug was then coded PF-08653944, WHO adopted berobenatide as its Recommended INN in 2026, and Pfizer presented the full Phase 2b package at ADA on June 6, 2026.

How It Works

Berobenatide is a GLP-1 receptor agonist (like Ozempic and Wegovy) with two design twists: it's engineered to last long enough for once-monthly shots after initial titration, and it is designed to favor the receptor's main signaling route (cAMP) over a second route (β-arrestin) thought to pull the receptor inside the cell. Its long action comes from a fatty-acid tail that lets it ride on albumin in the blood. The goal is similar efficacy with fewer injections; better tolerability is still unproven.

Berobenatide is a 41-amino-acid peptide built on the exendin-4 backbone (the Gila-monster peptide behind exenatide), with a C20 fatty diacid attached through a γ-glutamate linker to a C-terminal lysine. This lipidation lets it bind albumin and gives a half-life of about 15–16 days (roughly 360 hours) in Phase 1, long enough to support once-monthly maintenance dosing; in animals its half-life was 24 hours in rats and 99 hours in pigs, versus 7.2 and 46.1 hours for semaglutide. The molecular mechanism is canonical GLP-1 receptor agonism: binding to GLP-1R on pancreatic β-cells potentiates glucose-dependent insulin secretion, suppresses glucagon release, slows gastric emptying, and reduces appetite via hindbrain and hypothalamic circuits. The distinguishing pharmacological feature is biased agonism. In BRET biosensor assays, berobenatide (as MET-097) gave a full cAMP response (115% of exendin-4) while acting as a partial agonist for Gαs recruitment (36%) and recruiting β-arrestin-2 at only 17% of exendin-4. The developers hypothesize that this profile sustains receptor signaling. However, receptor internalization was only partly reduced (68% of exendin-4), out of proportion to the drop in β-arrestin, and tirzepatide, retatrutide, orforglipron and danuglipron showed similar profiles in the same assays, so any GI tolerability benefit has yet to be shown in people. The Phase 2b VESPER program tested weekly titration (to minimize acute nausea) followed by transition to monthly maintenance; in VESPER-3, weight loss continued past the weekly-to-monthly cadence switch, supporting the monthly-dosing thesis pharmacokinetically. Pfizer plans a 0.5 mL subcutaneous autoinjector. A dedicated study of berobenatide's effect on stomach emptying (NCT07508241) is under way.

Evidence Snapshot

Research Gaps & Open Questions

What the current literature has not yet settled about Berobenatide:

• 01No peer-reviewed full publication of any VESPER trial; data come from press releases, ADA slides and ClinicalTrials.gov postings.
• 02Efficacy of the 9.6 mg monthly high dose is model-based only; Pfizer notes that actual trial results may differ.
• 03Whether biased signaling gives any real tolerability advantage: there is no head-to-head trial, and the in vitro bias resembles other incretin drugs.
• 04Durability and long-term safety beyond about 60 weeks: no cardiovascular outcome trial is registered, and pancreatic and gallbladder signals need larger datasets.
• 05Whether people on weekly semaglutide or tirzepatide can keep their weight off after switching to monthly berobenatide (VESPER-SWITCH, not yet recruiting).
• 06Whether adding PF-08653945 (MET-233i) adds weight loss without worsening tolerability in people (SOLIS-1; data expected in the second half of 2026).

Forms & Administration

Investigational subcutaneous injection in a 0.5 mL volume compatible with auto-injector formats. Phase 2b VESPER-3 used 12 weekly doses (with or without titration), then a four-fold higher dose every four weeks; 64-week results are pending. Phase 3 tests two schedules. VESPER-4 and -5 use weekly dosing throughout, rising every four weeks from 0.4 mg to 0.8, 1.2 or 2.4 mg. VESPER-6 uses weekly titration (0.4, 0.8, 1.2 mg) followed by 3.2, 4.8 or 9.6 mg every four weeks. Unlike Phase 2, Phase 3 lets doses be reduced for GI side effects. Not available for general clinical or investigational use outside Pfizer's trial program.

Dosing & Protocols

The ranges below reflect protocols commonly discussed in the literature and by clinicians — not a prescription. Actual dosing for any individual should be determined by a qualified healthcare provider who knows the patient.

Berobenatide is not approved anywhere; these doses come only from Pfizer trial protocols.

Timeline of Effects

Common Questions

Who Berobenatide Is NOT For

Drug & Supplement Interactions

No interaction data have been published. Like other GLP-1 drugs, berobenatide is expected to slow stomach emptying, which can change how oral medicines are absorbed; Pfizer is running a dedicated gastric-emptying study (NCT07508241) and a liver-impairment study. In the type 2 diabetes trials, participants could use metformin and SGLT2 inhibitors, but VESPER-5 excludes insulin, DPP-4 inhibitors and other GLP-1 drugs, so combinations with insulin or sulfonylureas, where hypoglycemia risk rises, are untested. In rats, the amylin analog MET-233 plus berobenatide cut body weight by 31.2% over 28 days versus 17.6% for MET-233 alone, and that combination is now in the Phase 2b SOLIS-1 trial.

Safety Profile

Legal Status

Regulatory status changes over time. Verify current local rules with a qualified professional.

Myths & Misconceptions