MBX 4291_Dosage, benefits & legal status
Also known as MBX4291, MBX-4291
An experimental once-monthly GLP-1/GIP prodrug in its first human trial.
Common vial size is what vendors typically sell, not a recommendation. Enter your own vial in the calculator.
What is MBX 4291?
Plain language first. The technical label stays, but it never stands alone.Overview
The company is trying to turn a tirzepatide-like drug into a monthly shot. The only human data are blinded results from eight people at eight weeks, two of them on placebo.
MBX 4291 is a prodrug from MBX Biosciences: an inactive form of a GLP-1 and GIP receptor co-agonist that converts to the active peptide at a programmed rate once injected, with a fatty acid chain to slow it further.
What it's used for
An experimental shot for weight loss designed for once-monthly dosing after a few weekly starter doses. First human data are due in full in late 2026.
How does MBX 4291 work?
What it does in the body, and where.MBX 4291 is injected as an inactive prodrug. Chemistry, not enzymes, releases the active GLP-1/GIP peptide over days.
- 01A self-cleaving capA fatty-acylated dipeptide is attached to the active peptide. At body temperature and pH it falls off at a set rate, with no enzyme needed, so the active drug appears gradually rather than all at once.
- 02Albumin bindingThe fatty acid also holds the molecule on albumin in the blood, a second brake on release. Peak levels of the active peptide arrive 13 to 14 days after a dose.
- 03The same two receptors as tirzepatideThe released peptide binds GLP-1 and GIP receptors with activity the company calls similar to tirzepatide. Whether it is tirzepatide itself or a related sequence has not been disclosed.
Hits 2 receptors at once.
What else is it used for?
Each group carries its own evidence level.Approved means regulators have cleared it for that purpose. Investigational means it is in trials. Early research means it is being studied and is not established.
Weight loss
Early research- Human trialEight people, eight weeks, 30 mg weekly for four weeks then one 120 mg dose: mean weight loss 7%, range 0 to 16%. Two of the eight were on placebo and the data are still blinded, so no one knows yet what the six on drug averaged.
- No dataThe company calls this "preliminary blinded data". No placebo-separated result, no 12-week result and no result beyond one cohort has been released.
How long it lasts
Early research- Human trialAfter a single dose the active peptide peaked at 13 to 14 days, against under two days for tirzepatide. After the 120 mg dose, levels took about 26 days to fall by half from the peak.
- No dataThat 26-day figure is time to half of peak, not an elimination half-life, and should not be set beside tirzepatide's five-day half-life as if it were the same measurement.
- Animal studyIn monkeys the active component peaked at 4 to 5 days after a single dose and reached a plateau after four weekly doses.
The 7% weight-loss figure is a blinded mean from eight people, two of them on placebo, at eight weeks. The "26-day half-life" is the time for the active peptide to fall to half its peak, not an elimination half-life; the peak itself comes at 13 to 14 days. "Twelve shots a year" undersells it: every regimen tested starts with four weekly doses. The company has never said the active peptide is tirzepatide, only that it behaves similarly. MBX 2109 (canvuparatide, a parathyroid hormone prodrug) and MBX 1416 (imapextide, a GLP-1 blocker) are different compounds from the same company.
The published record
Papers and trials about MBX 4291
1 papers tagged human · 0 not reviews or lab · 0 clinical trial publications
1 registered trials · none with posted results
Counted from Europe PMC indexing, not read by a person; a human tag can land on reviews, commentary or lab papers; clinical trial publications is the narrower count.
At a glance
Four things to know about MBX 4291
Evidence
Investigational
Category
Weight loss and metabolic
Common vial
Varies
Half-life
Not published (peak at 13 to 14 days)
How much, and do you cycle it?
The range, and whether you take breaks.30 mg weekly for 4 weeks, then 120 to 180 mg monthly (trial regimens), weekly, then monthly (trial regimens). Not cycled in the trial Trial dose
The trial regimens run four weekly starter doses straight into monthly maintenance, with no break built in. Nothing longer than 12 weeks has been tested.
How long does it stay in the body?
The half-life, and where the figure comes from.Not published (peak at 13 to 14 days)Trial protocol
No terminal half-life has been released. The company reports the active peptide peaking 13 to 14 days after a dose and taking about 26 days to fall to half of that peak after the 120 mg dose. These describe slow release, not clearance.
Source: MBX Biosciences, Obesity Day deck, 11 May 2026
You can compare this against the whole library to see where it sits.
How do I dose and price it?
Enter your vial and your dose. Pick a mix. We show the draw and the cost.The vial holds a fixed amount of drug. Adding more water does not make more drug, it just spreads it thinner, so you draw a bigger number on the syringe for the same dose.
Choose your mix
Measuring the water1 mL = one full 100-unit syringe.
How do you store it?
Before mixing, and after.Dry powder in the freezer or fridge. Once mixed, fridge, and use within about a month.
Unmixed lyophilized MBX 4291 keeps for a long time cold. Once you add water the clock starts: most reconstituted vials hold up for roughly 28 to 30 days at fridge temperature. Keep it out of light, and do not freeze it after mixing.
What are the side effects and cautions?
Known cautions and warnings for this compound.Who should avoid it
- A personal or family history of medullary thyroid cancer or MEN 2, excluded from the trial
- A history of pancreatitis
- Type 1 or type 2 diabetes, excluded from the phase 1
- Pregnancy or breastfeeding
- Anyone who wants safety data, because fewer than 40 people have received it and the trial is still blinded
Stop and get help
- Severe stomach pain spreading to your back, a possible sign of pancreatitis
- Vomiting badly enough that you cannot keep fluids down
- Spreading hives, swelling of the face or throat, or trouble breathing
- Anything unusual, since there is no known safety profile to compare it against
Common effects
- Nausea in 7 of 8 people after a single 180 mg dose, the maximum tolerated dose; vomiting in 5 of 8 at that dose
- Only 1 of 8 had any nausea, vomiting or diarrhoea over eight weeks on the 30 mg weekly starter regimen
- Beyond the stomach, unknown. No injection-site, heart-rate or laboratory data have been released
Where this page says nothing is established, that means nobody has studied it, not that a compound is safe.
Is MBX 4291 approved?
And where the numbers on this page come from.No. MBX 4291 is not approved anywhere and is currently in company-run clinical trials. There is no legal route to obtain it outside a trial.
- Approval statusInvestigational
- RouteUnder the skin (subcutaneous)
- Checked10 October 2026
- DoseFrom published clinical trials. Not approved by a regulator for this dose.
- Vial sizeFrom what vendors typically list; not a recommendation.
- SourcesClinicalTrials.gov, MBX 4291 first-in-human phase 1 (NCT07142707)MBX Biosciences, press release and investor deck, Obesity Day (11 May 2026)MBX Biosciences, Form 10-K for 2025MBX Biosciences, Q2 2026 results (6 Aug 2026)MBX Biosciences, Q2 2025 results (7 Aug 2025)FDA GSRS search, no record for MBX 4291
What has happened recently
Not approved anywhere. The FDA cleared the IND in July 2025 and the first-in-human trial (NCT07142707) began in September 2025 at four US sites. No Fast Track or other designation. The company guides to 12-week data from the first monthly-dosing cohort in the fourth quarter of 2026 and has given no phase 2 start date.
You can compare this against the whole library, or read how to check a seller.
What else is like MBX 4291?
Others in GLP-1 agonists, side by side.If you're weighing MBX 4291, it's worth reading Pemvidutide, Ribupatide, and VK2735 in the same class.
An experimental once-monthly GLP-1/GIP prodrug in its first human trial.
- Dose
- 30 mg weekly for 4 weeks, then 120 to 180 mg monthly (trial regimens)
- How often
- Weekly, then monthly (trial regimens)
An experimental weekly shot for weight loss and fatty liver.
- Dose
- 1.2 up to 2.4 mg per week (trial doses)
- How often
- Weekly
An experimental weekly GLP-1/GIP shot from China, now in a 4,900-person phase 3 programme.
- Dose
- 2 up to 10 mg per week (trial doses)
- How often
- Weekly
An experimental weight-loss compound in the tirzepatide family, as a shot or pill.
- Dose
- 2.5 up to 15 mg per week (start low, increase slowly)
- How often
- Weekly
Guides for this compound
- How to Reconstitute a Peptide
Mix peptide powder with bacteriostatic water, step by step.
- How to Read an Insulin Syringe
Read the units, convert to millilitres, and draw the exact dose.
- mcg vs mg: The Peptide Dosing Mistake to Avoid
Your vial says mg and your protocol says mcg. Here is the conversion and how to check it.
- How to Read a Peptide COA
What the purity number does and does not tell you, and how to spot an edited report.
- How to Vet a Peptide Vendor
Independent COAs, HPLC purity, and the red flags that mean walk away.
Sources
- ClinicalTrials.gov, MBX 4291 first-in-human phase 1 (NCT07142707)(2026)Randomised, double-blind, placebo-controlled single- and multiple-ascending-dose study in 124 adults with obesity (BMI 30 to under 50), four US sites, started 3 September 2025, estimated completion October 2026. Primary outcome is adverse events.
- MBX Biosciences, press release and investor deck, Obesity Day (11 May 2026)(2026)Dose-proportional exposure across 15 to 180 mg single doses; peak active peptide at 13 to 14 days; 180 mg reached the maximum tolerated dose with GI events in 7 of 8. First multi-dose cohort (30 mg weekly x4 then 120 mg): mean weight loss 7% (range 0 to 16%) at eight weeks in 8 people including 2 on placebo, blinded; one mild diarrhoea, no nausea or vomiting, no serious adverse events.
- MBX Biosciences, Form 10-K for 2025(2026)Describes the prodrug design: non-enzymatic conversion at a controlled rate plus fatty acylation for albumin binding. In monkeys the active component peaked at 4 to 5 days after a dose versus under 24 hours for tirzepatide. In vitro the active component binds GLP-1 and GIP receptors with activity similar to tirzepatide.
- MBX Biosciences, Q2 2026 results (6 Aug 2026)(2026)First 12-week cohort is testing 30 mg weekly x4, then 120 mg monthly, then 180 mg monthly. Data from it guided for the fourth quarter of 2026.
- MBX Biosciences, Q2 2025 results (7 Aug 2025)(2025)IND submitted June 2025 and cleared to proceed in July 2025.
- FDA GSRS search, no record for MBX 4291(2026)No substance record, no INN and no published sequence or structure exist for this compound.
Beyond these, 1 records are indexed. Browse the records