SRK-439 — Safety, Research, Risks & Legal Status

Not medical advice. SRK-439 is discussed here for informational and harm-reduction purposes only. We do not endorse use, and any dosing context is informational, not a protocol.

Also known as	SRK-439; Scholar Rock anti-myostatin antibody
Category	Peptide (research chemical)
Developer	Scholar Rock, Inc. (NASDAQ: SRRK), Cambridge, MA
Drug class	Monoclonal antibody; selective anti-pro and latent myostatin inhibitor
Route of administration	Subcutaneous injection
Development stage	Phase 1 (NCT07444294, SAD/MAD in healthy adults; topline data anticipated H2 2026)
Primary indication targets	Obesity/cardiometabolic disorders (lead); FSHD (Phase 2 FORGE trial planned mid-2026)
Selectivity claim	Preclinical data indicate no cross-reactivity with GDF-11 or Activin-A; human selectivity unconfirmed
US legal status	Investigational drug in the United States. SRK-439 is not approved by the FDA or any other regulatory agency. It is currently undergoing Phase 1 clinical evaluation and is not available for clinical, therapeutic, or non-research use outside of authorized clinical trials.

What is SRK-439?

SRK-439 is a novel investigational monoclonal antibody developed by Scholar Rock, Inc. that selectively binds pro- and latent forms of myostatin, a negative regulator of skeletal muscle mass. It is being developed primarily for cardiometabolic indications - initially obesity, particularly in combination with GLP-1 receptor agonists - and for the rare neuromuscular disease facioscapulohumeral muscular dystrophy (FSHD). As of mid-2026, SRK-439 is in active Phase 1 clinical evaluation; no human efficacy or safety data have been publicly disclosed.

How it works

SRK-439 is a selective anti-pro and latent myostatin monoclonal antibody. Myostatin (GDF-8) is a TGF-beta family member that suppresses skeletal muscle growth; inhibiting it is hypothesized to preserve or increase lean muscle mass. SRK-439 binds pro-myostatin and latent myostatin with sub-nanomolar affinity, preventing activation of the mature form. A key claimed differentiator is high selectivity for myostatin: preclinical data suggest SRK-439 does not cross-react with the structurally related GDF-11 or Activin-A, which has been a concern with less selective myostatin inhibitors. It is administered by subcutaneous injection.

Background & history

Scholar Rock, Inc. (Cambridge, MA) disclosed SRK-439 as a next-generation myostatin inhibitor program building on lessons from their earlier compound apitegromab, which demonstrated a manageable safety profile in Phase 2 trials for spinal muscular atrophy. Preclinical data for SRK-439 in obesity models were presented at major scientific conferences including American Diabetes Association annual meetings (2024-2025) and Keystone Symposia. A Phase 1 single- and multiple-ascending dose study in healthy adults (NCT07444294) was initiated, with topline data anticipated in H2 2026. A Phase 2 FORGE trial in FSHD was planned to begin enrollment in mid-2026.

What the research says

All available efficacy evidence for SRK-439 is preclinical as of the knowledge cutoff. In cynomolgus monkeys, subcutaneous doses of 0.3 to 10 mg/kg produced dose-dependent increases in lean mass. Mouse studies showed preservation of lean mass during GLP-1 receptor agonist (GLP-1RA) treatment, improved grip strength, enhanced fat loss when co-administered with metformin, and attenuation of lean mass loss and fat mass rebound following GLP-1RA withdrawal. Phase 1 clinical data in healthy adults are pending and have not been publicly disclosed. No human data on efficacy, pharmacokinetics, or safety specific to SRK-439 are currently available. The overall clinical evidence base must be characterized as very limited - animal data only.

Reported effects

Dose-dependent increases in lean mass in nonhuman primate preclinical studies (cynomolgus monkey, 0.3-10 mg/kg SC) - animal data only
Lean mass preservation during GLP-1 receptor agonist-induced weight loss in mouse models - animal data only
Enhanced fat mass reduction when combined with metformin in mouse models - animal data only
Attenuation of lean mass loss and fat mass rebound following GLP-1RA withdrawal in animal models - animal data only
No human efficacy data currently available; Phase 1 topline results anticipated H2 2026
Hypothesized utility in preserving muscle-to-fat ratio during pharmacologically induced weight loss - clinical confirmation pending

Dosing & administration (informational)

Dose ranges evaluated in nonhuman primate preclinical studies were 0.3 to 10 mg/kg subcutaneously. Human dosing in the Phase 1 SAD/MAD study (NCT07444294) has not been publicly disclosed. No dosing protocol can be derived from available public data. This entry does not constitute dosing guidance. Consult a licensed clinician and refer to the clinical trial protocol for any clinical context.

This is general research/context information, not medical advice or a recommended protocol.

Safety & side effects

No clinical safety data specific to SRK-439 have been publicly disclosed as of mid-2026; Phase 1 is ongoing
Related Scholar Rock compound apitegromab showed manageable safety in Phase 2 (most common AEs: pyrexia, nasopharyngitis, cough) - cross-compound safety inference is speculative
Class-level concerns with myostatin inhibitors have historically included potential cardiovascular effects and off-target TGF-beta pathway modulation; SRK-439's selectivity profile is intended to reduce off-target risk but this is unconfirmed in humans
Subcutaneous injection carries standard injection-site reaction risk
Long-term effects in humans are entirely unknown
Not for human use outside of authorized clinical trials

Drug & supplement interactions

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Who should avoid it

Use outside of an authorized, IRB-approved clinical trial
Combination with other investigational agents without clinical trial framework
Any attempt to self-administer - route is subcutaneous injection requiring clinical oversight
Inference of safety from apitegromab or other myostatin inhibitor data without clinical confirmation
Extrapolation of animal dose ranges to human use

How it is commonly combined

No evidence base exists for stacking SRK-439 with other compounds outside of the specific preclinical contexts studied (GLP-1RAs, metformin). The preclinical combination data with GLP-1RAs are a primary rationale for its obesity development program, but human combination data do not exist. No stacking guidance can be responsibly provided.

Legal & regulatory. Investigational drug in the United States. SRK-439 is not approved by the FDA or any other regulatory agency. It is currently undergoing Phase 1 clinical evaluation and is not available for clinical, therapeutic, or non-research use outside of authorized clinical trials.

Quality & harm reduction

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Frequently asked questions

Is SRK-439 available to purchase or use outside of a clinical trial?

No. SRK-439 is an investigational monoclonal antibody not approved by the FDA or any regulatory agency. It is not commercially available. Legitimate access is limited to participation in authorized clinical trials such as NCT07444294.

What makes SRK-439 different from earlier myostatin inhibitors?

Scholar Rock positions SRK-439 as highly selective for pro- and latent myostatin forms, with preclinical data suggesting no cross-reactivity with GDF-11 or Activin-A. Prior myostatin inhibitor programs (including those from other companies) were sometimes limited by insufficient selectivity. Whether this selectivity advantage translates to a better human safety or efficacy profile has not yet been established.

What human data exist for SRK-439?

As of mid-2026, no human efficacy or safety data for SRK-439 have been publicly disclosed. A Phase 1 study in healthy adults is ongoing; topline results are anticipated in the second half of 2026.

What is the proposed role of SRK-439 in obesity treatment?

The hypothesis being tested is that inhibiting myostatin during GLP-1 receptor agonist-induced weight loss can preserve lean muscle mass and enhance fat loss, potentially improving body composition outcomes relative to GLP-1RA treatment alone. This is supported by preclinical animal data only; clinical confirmation does not yet exist.

Can I get dosing information for SRK-439?

No dosing protocol is available or appropriate to share. Preclinical ranges in nonhuman primates are not translatable to human use. Clinical dosing in the Phase 1 study has not been publicly disclosed. Anyone seeking information about SRK-439 in a medical context should consult a licensed clinician.

What safety signals have been identified for SRK-439?

No safety data specific to SRK-439 in humans have been published. The Phase 1 study is ongoing. The related Scholar Rock compound apitegromab showed a manageable safety profile in Phase 2 (common AEs included pyrexia, nasopharyngitis, and cough), but inferring SRK-439's safety profile from apitegromab data is speculative.

References & further reading

ClinicalTrials.gov: NCT07444294 SRK-439 Phase 1 SAD MAD healthy adults Scholar Rock
PubMed: myostatin inhibition lean mass preservation GLP-1 receptor agonist body composition
PubMed: pro-myostatin latent myostatin selective antibody GDF8 inhibition selectivity
PubMed: apitegromab myostatin inhibitor Scholar Rock spinal muscular atrophy Phase 2 safety
ClinicalTrials.gov: FORGE trial SRK-439 facioscapulohumeral muscular dystrophy FSHD Scholar Rock