The Lean-Mass Anchor
Bimagrumab
Also known as BYM338
At a glance
- What it is
- Investigational ActRII antibody given intravenously every 12 weeks. Not self-administered.
- Evidence
- Clinical trialsHow the tiers workBELIEVE (Nature Medicine, March 2026, NCT05616013, n=507, 48 weeks, nine arms) reported least-squares mean absolute weight change of −9.3 kg on bimagrumab 30 mg/kg, −14.2 kg on semaglutide 2.4 mg, −17.8 kg on the combination and −3.3 kg on placebo, all P<0.001, with improvements continuing to week 72 in an open-label extension. Why this tier
- Half-life
- Not established in available literature
- Routes
- Intravenous
- Category
- Metabolic
- WADA
- We do not state a status
Not schedulable in Claritide. Our sources publish no dosing figure for Bimagrumab, so there is nothing to build a schedule from.
Why this tier
BELIEVE (Nature Medicine, March 2026, NCT05616013, n=507, 48 weeks, nine arms) reported least-squares mean absolute weight change of −9.3 kg on bimagrumab 30 mg/kg, −14.2 kg on semaglutide 2.4 mg, −17.8 kg on the combination and −3.3 kg on placebo, all P<0.001, with improvements continuing to week 72 in an open-label extension. Adverse events were muscle spasms, diarrhoea and acne. Read it against the BMJ network meta-analysis finding that tirzepatide reduced fat mass most (−25.7%) and also lean mass most (−8.3%).
What it is
Bimagrumab is an investigational monoclonal antibody against type II activin receptors. It is not a peptide, and it is given intravenously every 12 weeks, which is not a self-administration route. We carry it as content rather than as something to track: it anchors the question of what happens to lean mass during rapid weight loss.
How it works
Type II activin receptors mediate signalling from myostatin and activin, which restrain muscle growth. Blocking those receptors with an antibody removes the brake, increasing muscle mass and reducing fat mass. That is the opposite direction of travel from an incretin, which reduces both, and it is why the combination arm of the BELIEVE trial is the interesting one.
Commonly researched ranges
- BELIEVE Phase 2 dose30 mg/kg
× body weight in kg
Administered in a clinical setting. Nobody self-administers a quarterly intravenous antibody, which is why this record is not offered as a protocol.
| Phase | Amount | Frequency | Timing | Note |
|---|---|---|---|---|
| BELIEVE Phase 2 dose | 30 mg/kg× body weight in kg | Intravenously every 12 weeks | Not established in available literature | Administered in a clinical setting. Nobody self-administers a quarterly intravenous antibody, which is why this record is not offered as a protocol. |
- Cycle guidance
- Not established in available literature
- Goals
- Muscle, Fat loss
Where this sits with the FDA
An investigational monoclonal antibody with no approval and no compounding pathway. Antibodies are not compoundable products in any case.
Primary sourceWADA status
Not named on the 2026 Prohibited List under this name. Note that agents affecting myostatin signalling are addressed under S4; we could not confirm how bimagrumab is classified and do not state a status.
We looked and could not resolve it. This is not the same as "not prohibited". Treat it as unresolved and check with your federation before competing.
There is nothing to mix
Not applicable. None of the published routes for this compound take a diluent, so there is no concentration to calculate and no beyond-use window to track.
Week by week
Not established in available literature.
Side effects
Common
- Muscle spasms
- Diarrhoea
- Acne
Contraindications and interactions
Do not use if
Not established in available literature.
Interactions
Not established in available literature.
Absence from this list does NOT mean safety. It means lack of research.
Storage and cost
- Storage
- A monoclonal antibody prepared and infused in a clinical setting. There is no home handling protocol.
- Cost
- Not established in available literature
Stacks with
Curated stacks including it
Bimagrumab in full
Limited coverage: content, not a tracker entry
Bimagrumab (BYM338) is an investigational monoclonal antibody against type II activin receptors. It is not a peptide, and it is given intravenously every 12 weeks. Nobody self-administers a quarterly intravenous antibody, and this record is not offered as something to run.
Why it is here
It anchors the lean-mass question that the whole weight-loss category has to answer.
BELIEVE, published in Nature Medicine in March 2026: NCT05616013, 507 participants, 48 weeks, nine arms. Least-squares mean absolute weight change:
- Bimagrumab 30 mg/kg: −9.3 kg
- Semaglutide 2.4 mg: −14.2 kg
- Combination: −17.8 kg
- Placebo: −3.3 kg
All P<0.001, with improvements continuing to week 72 in an open-label extension. Adverse events were muscle spasms, diarrhoea and acne.
Set that against the BMJ network meta-analysis finding that tirzepatide reduced fat mass most (−25.7%) and also lean mass most (−8.3%).
The reason it matters to you even though you will never take it
Scale weight is a bad proxy for body composition, and the drugs that move scale weight fastest also take the most lean mass with them. Bimagrumab exists as a pharmacological demonstration that the two are separable.
The practical version of that demonstration is not an antibody. It is resistance training and adequate protein during a deficit, which is why this library tracks resistance sessions and protein alongside doses.
Sources
- 1Nature Medicine, March 2026 · BELIEVE Phase 2, NCT05616013, n=507
Every record in this library is free to read, and it stays free. Your own schedule, vials and log live in the app: Claritide for iPhone.