GDF-8 (Myostatin) antibody Apitegromab achieves success in Phase III clinical trial

Scholar Rock’s Apitegromab, a monoclonal antibody targeting Growth Differentiation Factor-8 (GDF-8, also known as myostatin), has achieved success in Phase III clinical trials for spinal muscular atrophy (SMA). This outcome reinforces the therapeutic value of myostatin inhibition in preserving skeletal muscle mass and improving motor function. Supported by validated recombinant GDF-8 proteins from KACTUS Bio, ongoing research continues to refine next-generation therapies for muscle growth regulation and the preservation of healthy muscle tissue.

Abnormal myostatin expression has been observed under various pathological conditions, such as muscular dystrophy, chronic kidney disease, cancer, and liver disorders, often leading to reduced muscle fiber size and impaired regeneration.

Among patients treated with Apitegromab, 30% showed an improvement of 3 points in the HFMSE score, compared to only 12.5% in the placebo group.

Figure 1. Results summary of Apitegromab SAPPHIRE trial to treat spinal muscular atrophy (SMA) [1]. 

Scholar Rock plans to submit a Biologics License Application (BLA) and a Marketing Authorization Application (MAA) filings to the FDA and EMA in the first quarter of 2025. If approved, Apitegromab is expected to become the first approved antibody drug therapy to target human myostatin, marking a milestone in SMA treatment and offering a new therapeutic option to preserve skeletal muscle mass and function in SMA patients. This success marks a significant milestone in the history of SMA treatment. Following this announcement, Scholar Rock's stock price surged by 362% on the same day, bringing its market value to $2.7 billion.

What Is GDF-8 (Myostatin) and How Does It Regulate Muscle Growth?

GDF-8, a growth differentiation factor protein primarily expressed by skeletal muscle cells, stands for Growth/Differentiation Factor 8, also known as Myostatin or MSTN. It belongs to the Transforming Growth Factor-β (TGF-β) superfamily and is a key negative regulator of skeletal muscle growth, acting to regulate muscle mass by inhibiting muscle cell proliferation, thereby reducing muscle mass and regulating skeletal muscle growth. As a result, higher GDF-8 activity can contribute to reduced muscle mass, lower skeletal muscle mass, and impaired muscle development under certain conditions. Under various pathological conditions, such as muscle dystrophy, chronic kidney disease, cancer, liver disease, obesity, and anterior cruciate ligament (ACL) tears, GDF-8 levels may increase, contributing to reduced muscle fiber size and impaired regeneration.

Figure 2. Overview of myostatin (GDF-8) protein structure and maturation [2] 

Maturation Process and Signaling Pathway of GDF-8

Mature GDF-8 is derived from a precursor protein that undergoes a series of cleavage processes: the GDF-8 precursor is synthesized within the cell and first has its signal peptide removed by signal peptidase, resulting in Pro-GDF-8 (also known as precursor GDF-8). Subsequently, the proprotein convertase Furin recognizes and cleaves specific sequences within Pro-GDF-8, forming Latent GDF-8. This step is crucial for myostatin function and myostatin protein maturation. Finally, the metalloproteinases BMP-1 or Tolloid further process Latent GDF-8, which may involve cleavage of the prodomain, thereby activating GDF-8. Mature GDF-8 exists as a homodimer, a structural feature critical for its biological activity and molecular mechanism of action.

Activin type II receptors (ActRII) are the main downstream receptors for GDF-8. When GDF-8 binds to them, it recruits ActRI receptors, subsequently activating intracellular SMAD and AKT signaling pathways. This leads to changes in gene transcription and the degradation of related proteins, ultimately leading to decreased muscle mass.

Figure 3. GDF-8 (myostatin) Signaling Pathway [3]

Development of GDF-8 Targeted Antibody Drugs

As a negative regulator of muscle growth and disease progression, GDF-8 is currently a significant target for treating skeletal muscle diseases, neuromuscular diseases, obesity, and cancer.

Scholar Rock's Apitegromab (SRK-015) is a representative drug in this category. SRK-015 is a fully human monoclonal antibody that binds with high specificity to human Pro-GDF-8 or Latent GDF-8 without binding to mature myostatin (mature GDF-8) and other closely related growth factors. It inhibits GDF-8 before release, offering high selectivity and minimal side effects. SRK-015 is currently in Phase 3 clinical trials (NCT05156320) for spinal muscular atrophy (SMA) and is the first potential muscle-directed therapy for SMA. Roche's RO7204239 (GYM329, RG6237) targets Latent myostatin and is being tested for SMA (NCT05115110) and facioscapulohumeral muscular dystrophy (NCT05548556). Ongoing trials highlight its potential to improve muscle tissue function by limiting pathological myostatin expression.

Figure 4. Design Principle of Apitegromab [4]

Regeneron's REGN-1033 (Trevogrumab) targets the active myostatin form of GDF-8. In collaboration with Eli Lilly, Phase 2 clinical trials are evaluating whether Trevogrumab combined with Semaglutide±Garetosmab (anti-Activin A) can maintain weight loss efficacy by increasing muscle mass and supporting lean mass preservation. Similarly, Keros' KER-065 is designed for GDF-8, being a novel ligand trap drug [4] that can capture GDF-8 or Activin A, treating obesity by increasing muscle hypertrophy potential and reducing fat mass. It can be used as a standalone therapy or combined with GLP-1 receptor agonists to support weight loss and fat loss outcomes.

Figure 5. KER-065 mechanism of action on activin A and myostatin (GDF-8) [6]. 

KACTUS Bio’s Recombinant GDF-8 (Myostatin) Proteins

Because myostatin directly influences muscle growth, research tools that accurately replicate its activity are essential for protein expression studies and preclinical drug screening. GDF-8 has become an important drug target for developing treatments that promote skeletal muscle growth and address muscle atrophy-related conditions. Many drugs target the non-mature form of GDF-8, which helps control the activation process of GDF-8 (myostatin) protein. In contrast, targeting the mature form of GDF-8 can more directly intervene in the regulation of muscle hypertrophy and regulate muscle mass. The most suitable strategy for drug design depends on factors such as the mechanism of action, safety, efficacy, and potential side effects, including the risk of anti-drug antibodies.

To support drug development in various fields, including muscle atrophy, KACTUS has developed high-quality recombinant GDF-8, Latent GDF-8, and related proteins such as Activin RIIA and Activin RIIB. These products cover multiple species and diverse tag designs. They are all rigorously quality tested to ensure activity across increasing concentrations and apply to different research stages, such as drug screening and validation.

Product Validation Data

Figure 6. Bioactivity of Human/Mouse/Rat GDF-8 protein determined by its ability to inhibit the proliferation of MPC-11 cells. The expected ED50 for this effect is <30 ng/ml (QC Test).

Figure 7. Immobilized Human Latent GDF-8, His Tag at 1 ug/ml (100 ul/well) on the plate. Dose response curve for Anti-GDF8 Antibody, hFc Tag with the EC50 of 22.8 ng/ml determined by ELISA (QC Test).

Summary: Myostatin Blockade and the Future of Muscle Therapy

The success of Apitegromab’s Phase III trial highlights how inhibition of GDF-8 expression and signaling can reshape treatment strategies for neuromuscular disease. With advanced recombinant GDF-8 and precursor protein tools, KACTUS Bio continues to empower global research aimed at restoring muscle tissue health, supporting therapeutic discovery across the TGF-β family pathways.

Product List

Catalog Number	Product Information
GDF-HM108	Human Latent GDF-8, His Tag
GDF-HM008	Human/Mouse/Rat GDF-8, No Tag
ACV-HM001	Human Activin A, No Tag
ACV-HM101	Human Latent Activin A, His Tag
ARA-HM12A	Human Activin RIIA, His Tag
ARA-HM22A	Human Activin RIIA, hFc Tag
ARA-HM52AB	Biotinylated Activin RIIA, hFc-Avi Tag
ARA-HM32A	Human Activin RIIA, mFc Tag
ARA-MM12A	Mouse Activin RIIA, His Tag
ARB-HM12B	Human Activin RIIB, His Tag
ARB-HM42BB	Biotinylated Human Activin RIIB, His-Avi Tag
ARB-HM52BB	Biotinylated Human Activin RIIB, hFc-Avi Tag
ARB-HM32B	Human Activin RIIB, mFc Tag
ARB-MM12B	Mouse Activin RIIB, His Tag
TGF-HM6R1	Human TGFBR1, mFc-Avi Tag
TGF-HM6R1B	Biotinylated Human TGFBR1, mFc-Avi Tag
ALK-HM104	Human ACVR1B/ALK-4, His Tag

 

References

[1] https://investors.scholarrock.com/static-files/693dd276-5581-4b1a-9834-b24e16dbe17f 

[2] Hoogaars WMH, Jaspers RT. Past, Present, and Future Perspective of Targeting Myostatin and Related Signaling Pathways to Counteract Muscle Atrophy. Adv Exp Med Biol. 2018;1088:153-206. doi: 10.1007/978-981-13-1435-3_8.

[3] Garber K. No longer going to waste. Nat Biotechnol. 2016 May 6;34(5):458-61. doi: 10.1038/nbt.3557.

[4] https://scholarrock.com

[5] https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=386654

[6] https://ir.kerostx.com/static-files/d666096b-07f9-47ea-b9ad-205f7c89895f