On this pageThe Activin Type II Receptor Pathway
Antibodies that act on the activin type II receptor (ActRII) pathway are an active area of investigational research. This overview covers the receptor pharmacology behind three of them, bimagrumab, trevogrumab and garetosmab, and summarizes the design of the Phase 2 COURAGE study (sponsor, phase and study arms). It does not report study outcomes.
The Activin Type II Receptor Pathway
Activin type II receptors are single-pass transmembrane serine/threonine kinases. Two genes encode them: ActRIIA (ACVR2A) and ActRIIB (ACVR2B). They bind several TGF-β superfamily ligands, including:
- Activin A and activin B
- Myostatin (GDF-8) and GDF-11
- Certain BMPs, such as BMP-9 and BMP-10, with lower selectivity
A ligand dimer first binds two type II receptors, which then recruit two type I receptors, mainly ALK4 (ACVR1B), ALK5 (TGFBR1) or ALK7 (ACVR1C). The constitutively active type II kinase phosphorylates the type I receptor GS domain, and the type I kinase phosphorylates SMAD2 and SMAD3. These associate with SMAD4 and regulate transcription in the nucleus.
Three Ways to Block ActRII Signaling
Investigational biologics interrupt this pathway at different points, which is what makes their pharmacology worth comparing:
| Strategy | Example | What is bound | Ligands affected |
|---|---|---|---|
| Receptor-blocking antibody | Bimagrumab | ActRIIA and ActRIIB | All ligands that require ActRII |
| Ligand-neutralizing antibody | Trevogrumab (REGN1033) | Myostatin (GDF-8) | Myostatin only |
| Ligand-neutralizing antibody | Garetosmab (REGN2477) | Activin A | Activin A only |
| Soluble receptor (ligand trap) | ActRIIA-Fc / ActRIIB-Fc fusion proteins | Several ligands in solution | Broad, depends on construct |
A receptor antibody produces broad pathway blockade with one molecule. Ligand antibodies are more selective, so combining two of them is a way to approximate broader blockade while keeping each ligand's contribution measurable.
Bimagrumab: Activin Type II Receptor Antibody
Bimagrumab (originally BYM338) is a fully human monoclonal antibody that binds the ligand-binding domains of ActRIIA and ActRIIB, preventing ligands from engaging the receptor. It was originated by Novartis and later developed by Versanis Bio, which was acquired by Eli Lilly and Company in 2023. Bimagrumab remains investigational.
In the laboratory, bimagrumab-type receptor antibodies are characterized by:
- Binding affinity and kinetics against ActRIIA and ActRIIB extracellular domains (SPR or BLI)
- Competition with ligands such as GDF-8, GDF-11 and activin A for receptor binding
- Inhibition of SMAD2/3 phosphorylation and SMAD-responsive reporter activity across a ligand panel
- Receptor internalization and turnover measurements
Note that bimagrumab is not one of the agents in the COURAGE study; it is included here as the reference example of receptor-level ActRII blockade.
COURAGE Study Design: Sponsor, Phase and Arms
COURAGE is a Phase 2 study sponsored by Regeneron Pharmaceuticals and registered as NCT06299098. It evaluates Regeneron's two ligand-neutralizing antibodies, trevogrumab and garetosmab, in combination with the GLP-1 receptor agonist semaglutide.
The study is structured in two consecutive 26-week periods. In the first period, participants were assigned to one of four arms:
- Semaglutide alone
- Semaglutide plus trevogrumab (first trevogrumab arm)
- Semaglutide plus trevogrumab (second trevogrumab arm)
- Semaglutide plus trevogrumab plus garetosmab
The second 26-week period continues the study under a separate arm structure described in the registry entry. Tissue-compartment measurements in the study use DXA (X-ray absorptiometry) scanning. Readers interested in study endpoints and reported data should consult the registry and the sponsor's peer-reviewed publications directly.
Why the COURAGE Arms Map Onto ActRII Pharmacology
From a receptor-pharmacology standpoint, the COURAGE arm structure tests ligand selectivity in sequence: a GLP-1 receptor agonist alone, the same agonist with myostatin-only neutralization, and the same agonist with combined myostatin and activin A neutralization. The triple arm is the ligand-antibody approach to broader ActRII pathway blockade, the same pathway that bimagrumab targets at the receptor level.
For background on the incretin receptor side of the design, see the tirzepatide research overview and retatrutide research overview, which describe GLP-1 receptor pharmacology.
Laboratory Tools for ActRII Pathway Research
Research groups studying the ActRII pathway typically work with:
- Recombinant ligands (activin A, activin B, GDF-8, GDF-11) and their prodomain complexes
- Soluble ActRIIA-Fc and ActRIIB-Fc proteins as broad-spectrum ligand traps
- SMAD-responsive luciferase reporter cell lines (for example CAGA-luc)
- Phospho-SMAD2/3 and total SMAD2/3 antibodies for immunoblotting
- Endogenous antagonists such as follistatin and FSTL3 as comparators
Protein reagents should be accompanied by documentation of identity, purity by SEC-HPLC or SDS-PAGE, and endotoxin results. See endotoxin testing and certificates of analysis.
This overview is provided as a laboratory research reference only.