What Is Berobenatide (MET-097i)?

1. How does berobenatide work?

The receptor pharmacology is that of semaglutide: a single agonist at the GLP-1 receptor, producing glucose-dependent insulin release, slower gastric emptying, and reduced appetite. Unlike the dual and triple agonists elsewhere in this library, berobenatide does not add GIP or glucagon activity. Its distinguishing feature is duration. Semaglutide’s half-life of about a week comes from binding albumin; berobenatide is built on Metsera’s HALO platform, described in the company’s own SEC filing as “a novel peptide stabilization and lipidation platform technology that enables peptides to bind simultaneously to albumin and to a drug target, designed to facilitate a half-life approaching that of albumin.”

Albumin itself persists in circulation for roughly three weeks, which is the basis for monthly dosing. Metsera’s 2024 annual report (Form 10-K) discloses a measured figure: single-ascending-dose data showed “dose-proportional PK with an approximately 380-hour half-life with modest variability,” roughly 15 to 16 days.

The practical implication is dosing frequency. Every approved GLP-1 injection is weekly or daily. A monthly injection would change adherence, the missed-dose problem, and the size of the dose per injection, which is why the milligram numbers on this page, 0.4 to 2.4 mg, are not comparable with any other GLP-1 drug’s. In VESPER-3, the monthly maintenance dose is roughly four times the weekly dose that preceded it, reflecting the longer interval rather than a stronger drug.

2. How much weight do people lose on berobenatide?

VESPER-1, the Phase 2b weekly-dosing trial, randomised 239 adults with obesity or overweight and no diabetes to placebo or four doses (roughly 54 per arm) for 28 weeks, without dose titration. The trial population had an average BMI of about 36 and was roughly two-thirds female. Some individual participants lost as much as 26.5% of body weight.

VESPER-3, NCT06973720, is the trial that tests the monthly idea: 268 adults received 12 weekly doses (with or without titration, depending on arm), then switched to monthly maintenance dosed at roughly four times the weekly amount, with five arms including placebo. The two monthly regimens (arms described by Pfizer as the 3.2 mg and 4.8 mg maintenance doses) produced 10.0% and 12.3% placebo-adjusted weight loss under the efficacy estimand, the analysis that counts people only while they stay on the drug, and 8.4% and 10.5% under the treatment-policy estimand, the stricter analysis that counts everyone who was randomised whether or not they stopped.

Both were significant against placebo. The trial started April 7, 2025 and its primary endpoint at week 28 was followed by a longer observation through week 64.

For scale, semaglutide 2.4 mg produced 14.9% at 68 weeks in STEP 1 and tirzepatide 15 mg 20.9% at 72 weeks in SURMOUNT-1, both raw rather than placebo-adjusted and in longer trials. Berobenatide’s 28-week numbers are in semaglutide’s range; the 72-week Phase 3 endpoints will show where it lands.

3. What are the side effects of berobenatide?

Gastrointestinal, and the discontinuation numbers are the lowest in this library.

Pfizer attributes the tolerability to the pharmacokinetics: a very long-acting drug rises to its level slowly, which is the same reason dose escalation reduces nausea with weekly drugs. Whether the profile holds over 72 weeks and in thousands of people is what Phase 3 is for.

Metsera’s own September 2025 disclosure of the VESPER-1 data credited the CMO, Dr. Steve Marso, with describing the results as “dual-agonist-like weight loss of 14.1% at 28 weeks, with only 2.9% study discontinuation,” and UNC’s Dr. John Buse called MET-097i “the first NuSH analog in late-stage development on track to meet” the class’s efficacy and tolerability bar. No peer-reviewed publication exists yet; all figures come from Pfizer’s and Metsera’s releases, an SEC exhibit, and 2026 conference presentations.

4. What is the VESPER Phase 3 program?

Pfizer moved berobenatide into Phase 3 in 2026 with a roster of trials, each testing a different population or regimen on the registry:

VESPER-6 started June 10, 2026, with primary completion estimated for May 2028. VESPER-4 and VESPER-5 are already active and not recruiting, meaning enrollment is complete. SOLIS-1, a Phase 2b trial rather than Phase 3, shows Pfizer also testing berobenatide alongside a second investigational molecule, PF-08653945. No regulatory filing has been made.

5. Why did Pfizer pay $7 billion?

Pfizer completed its acquisition of Metsera on November 13, 2025 at $65.60 per share in cash, an enterprise value of about $7.0 billion, plus contingent value rights worth up to $20.65 per share tied to later development and regulatory milestones, including berobenatide’s progress through Phase 3. Pfizer had no obesity drug after discontinuing danuglipron, its oral GLP-1, in April 2025; Metsera’s portfolio gave it one about to enter Phase 3. After the deal, Pfizer renamed the molecule PF-08653944; the generic name berobenatide followed. Metsera’s earlier-stage assets, not part of the VESPER Phase 3 program, included a monthly amylin-analog combination (MET-233/097), a GIP-receptor combination, and an oral formulation (MET-097o), all Phase 1 or Phase 1/2a at the time of the deal.

Trade press coverage of the February 2026 data noted Pfizer’s plans for roughly 10 Phase 3 studies of PF-08653944 alone, part of a broader 20-plus-trial program, and quoted Leerink Partners analyst David Risinger calling the early results “slightly inferior” to tirzepatide’s roughly 13% placebo-adjusted weight loss at 28 weeks in cross-trial comparison, a comparison with the usual limitations of comparing separate trials.

6. How does berobenatide compare with the approved drugs?

MariTide is the other monthly candidate, built on an antibody rather than an engineered peptide. For a log, an investigational drug is entered like any other: product, date, milligrams, and site, with the interval set to 4 weeks for monthly dosing. How GLP-1 tracking works covers what to record.

Pfizer’s own framing, in trade-press coverage of the February 2026 data, put a rough date on the earliest possible outcome: potential initial approvals “from 2028,” contingent on Phase 3 success across the VESPER-4, VESPER-5, and VESPER-6 populations. That timeline tracks with VESPER-6’s own estimated primary completion of May 2028 on the registry. Until then, berobenatide, MET-097i, and PF-08653944 all describe the same investigational molecule available only to people enrolled in one of Pfizer’s trials.

7. Berobenatide FAQ

• How much weight do people lose on berobenatide?

  In VESPER-1 at 28 weeks, 8.1% to 14.1% placebo-adjusted across four weekly doses, with 14.1% at 1.2 mg. In a 32-week extension, participants who had been on placebo through week 28 then escalated to 2.4 mg weekly and reached 15.9% total (non-placebo-adjusted) weight loss by week 60, with weight still falling; this figure reflects 32 weeks of active treatment, not 60 weeks of continuous dosing. In VESPER-3, switching to monthly injections after 12 weekly doses produced 10.0% and 12.3% placebo-adjusted loss at 28 weeks.

• Is berobenatide a monthly injection?

  That is the design goal. The molecule is engineered to circulate for weeks, and VESPER-3 tested 12 weekly doses followed by monthly maintenance. The Phase 3 trial VESPER-6 tests once-monthly dosing from the start in 954 adults over 72 weeks. Every approved GLP-1 injection is weekly or daily.

• What are the side effects of berobenatide?

  Gastrointestinal, at rates Pfizer describes as low for the class. In VESPER-1, nausea ran from 4% to 23% depending on dose, only 2 of 239 participants stopped for adverse events, and total discontinuation was 2.9%. In VESPER-3, five participants stopped for adverse events during the weekly phase and five during the monthly phase, against none on placebo, with severe nausea or vomiting at most once per dose group.

• What is the difference between MET-097i, PF-08653944, and berobenatide?

  Three names for one molecule. MET-097i was Metsera's code. PF-08653944 is Pfizer's code after the acquisition. Berobenatide is the generic name. Pfizer's own materials also use the shorthand PF'3944.

• When will berobenatide be approved?

  No filing exists. Phase 3 trials started in 2026; VESPER-6 lists primary completion in May 2028, so a filing would follow those readouts and approval would come later. No peer-reviewed publication of the Phase 2 data exists yet; the numbers come from Pfizer's and Metsera's releases, an SEC exhibit, and conference presentations.

• How many Phase 3 trials is Pfizer running for berobenatide?

  Four are registered as Phase 3: VESPER-4 (3,578 adults, weekly, no diabetes), VESPER-5 (1,078 adults, weekly, with type 2 diabetes), VESPER-6 (954 adults, monthly maintenance), and VESPER-JM (197 adults, monthly, in Japan). SOLIS-1, combining PF-08653944 with a second Pfizer molecule, PF-08653945, in 872 adults, is registered as Phase 2b, not Phase 3. Trade press has reported Pfizer's plans for roughly 10 Phase 3 studies of PF-08653944 alone as part of a broader 20-plus-trial obesity program.

8. Sources