Study analysis · International Journal of Toxicology · 2021
This drug makes muscles grow 33% bigger—without hurting your fertility, brain, or bones.
A special shot blocks a muscle-stopping signal in rats and makes them grow bigger muscles, but doesn’t mess up their bodies like other drugs do.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
In simple terms
This study didn't test people—it tested rats and monkeys in a lab. We can't say if it will work in humans or if it causes any real effects in people. It's like saying a toy car works on a table, so it must work on a real road—we just don't know.
What’s the bottom line?
This medicine blocks a signal that stops muscles from growing, but only targets the early version of the signal, not others that control things like reproduction or the brain.
How strong is this study?
We don't even know what kind of experiment this was—was it fair? Was it controlled? We can't tell. That means we can't trust the results to mean anything for real people. It's like trying to judge a magic trick without knowing how the magician did it.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
44 / 100
- Randomization+20/20
- Blindingblinding unclear
- Control group+15/15
- Sample sizeno sample size reported
- Follow-upno follow-up reported
100 / 100
54 / 100
- P-values+15/15
- Effect size+20/20
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 517 / 100
Probability of being correct
Researchers compare people who have a condition (cases) with similar people who do not (controls), looking back in time for differences in exposure. Useful but more prone to bias.
This design cannot establish causation — the findings describe an association, not a cause. Study type could not be determined with confidence; no valid classification exists to support causal inference.
Major COI
Major conflicts that significantly reduce study credibility
The study was conducted by authors affiliated with Scholar Rock, the company developing apitegromab, and the drug is being tested in clinical trials sponsored by the same entity, indicating significant industry influence on the research.
Funders
Conflict Details
Scholar Rock: Authors are employees of Scholar Rock, the company developing apitegromab.
The study was conducted entirely by employees of Scholar Rock, with no mention of independent oversight, third-party data analysis, or external review. The absence of a funding statement or COI disclosure raises concerns about transparency. The preclinical data directly supports ongoing clinical trials of apitegromab, suggesting the study was designed to support regulatory and commercial objectives.
Key takeaways
- 01
Muscles grew 13% to 33% bigger in rats, and levels of the blocked signal rose 27 to 128 times higher — with no harm to testes, bones, nerves, or fertility even at the highest dose.
- 02
This means it could help people with muscle-wasting diseases like SMA gain muscle without causing side effects like bleeding or infertility seen in older drugs.
Surprising findings
- No reproductive harm at 300 mg/kg—even though other myostatin inhibitors cause testicular atrophy and reduced sperm quality.Myostatin and activin are closely related; most drugs blocking one hit both, wrecking fertility. Apitegromab avoids activin entirely, making this a rare case of precision targeting working perfectly.
- No negative impact on bone health—even though SMA patients often suffer bone loss, and other myostatin inhibitors were thought to help bones.Researchers expected muscle growth to improve bone density, but apitegromab didn’t alter bone mass—yet also didn’t harm it, which is unexpected for a TGF-beta family drug.
- The drug’s effect on latent myostatin levels didn’t scale linearly with dose—yet muscle growth still increased.Usually, more drug = more effect. Here, even at lower doses, latent myostatin spiked dramatically, suggesting the system saturates quickly—making high doses potentially unnecessary.
Practical takeaways
If you or a loved one has SMA, ask your doctor if apitegromab is being tested in your region—it’s in Phase 2 trials (NCT03921528) and may be available via clinical trial soon.
This study was in animals; human safety and efficacy are still being evaluated. No FDA approval yet.
medium confidenceFitness enthusiasts should avoid DIY myostatin blockers—this drug is a precision biologic, not a supplement, and high-dose animal results don’t mean it’s safe for healthy people.
The 300 mg/kg dose in rats is not translatable to humans; self-experimentation could be dangerous.
low confidenceParents of children with neuromuscular disorders should follow Scholar Rock’s clinical trial updates—this is one of the few muscle drugs showing safety in developing bodies.
Juvenile rat data is promising but not human data; long-term effects beyond 26 weeks are unknown.
medium confidenceWhy this study matters
Muscle Growth Without Side Effects
Apitegromab increased skeletal muscle mass by 13–33% in rats and monkeys after weekly IV doses for up to 26 weeks, with no damage to testes, bones, nerves, or reproductive organs—even at the highest dose of 300 mg/kg.
Most muscle-building drugs cause serious side effects like infertility or bleeding, but this one avoids them by targeting only the inactive form of myostatin—making it a potential game-changer for people with muscle-wasting diseases like SMA.
The 128x Blood Signal Surge
Apitegromab caused serum levels of latent myostatin to spike 27- to 128-fold in rats and monkeys—proving it successfully blocked myostatin activation without binding to mature myostatin or related proteins like GDF11 or activin A.
This isn’t just muscle growth—it’s a biological fingerprint showing the drug is working exactly as designed, like a silent lock on a door that only opens for one key.
Safe for Kids—Even at High Doses
In juvenile rats (equivalent to 2-year-old humans), weekly doses of 300 mg/kg for 7 weeks caused no neurodevelopmental, motor, or fertility harm—despite 28% muscle growth and massive latent myostatin increases.
Parents of children with SMA desperately need treatments that won’t stunt growth or damage developing brains—this is the first myostatin drug to show that’s possible.
Want the whole report?
Detailed mode opens the full scientific breakdown — every score component, the methodology, conflicts of interest, the evidence analysis behind each claim, and the raw study data.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
This medicine blocks a signal that stops muscles from growing, but only targets the early version of the signal, not others that control things like reproduction or the brain.
Research results
Muscles grew 13% to 33% bigger in rats, and levels of the blocked signal rose 27 to 128 times higher — with no harm to testes, bones, nerves, or fertility even at the highest dose.
What this means - more context
This means it could help people with muscle-wasting diseases like SMA gain muscle without causing side effects like bleeding or infertility seen in older drugs.
To evaluate the preclinical safety, pharmacology, and pharmacokinetics of apitegromab, a monoclonal antibody targeting proforms of myostatin, to support its clinical development for spinal muscular atrophy.
Apitegromab selectively binds to promyostatin and latent myostatin, inhibiting myostatin activation without binding mature myostatin or related growth factors. Weekly IV dosing up to 300 mg/kg in rats and 100 mg/kg in monkeys for up to 26 weeks showed no treatment-related adverse effects on clinical, hematologic, neurodevelopmental, reproductive, or bone endpoints, with sustained muscle mass increases (13–33%) and target engagement confirmed by 27- to 128-fold rises in serum latent myostatin.
Methods Used
In vitro binding and activation assays using human, rat, and cynomolgus monkey proteins; GLP toxicology studies in adult and juvenile rats (up to 300 mg/kg IV weekly for 26 and 8 weeks) and cynomolgus monkeys (up to 100 mg/kg for 4 weeks); toxicokinetic and target engagement analyses via ELISA and ECL assays.
Main Finding
Apitegromab increased skeletal muscle mass by 13–33% in rats without pathology and achieved a NOAEL of 300 mg/kg in both adult and juvenile rats, with no off-target toxicities (e.g., testicular atrophy, reproductive impairment) observed, unlike other myostatin inhibitors.
Confidence Level
High — robust, GLP-compliant, multi-species, repeat-dose toxicology studies with comprehensive endpoints, dose-proportional pharmacokinetics, and confirmed target engagement.
Study Flags
Red Flags
- •No human data
- •High doses used (up to 300 mg/kg) may not translate to humans
- •Long-term effects beyond 26 weeks not assessed
Surprising Findings
No reproductive harm at 300 mg/kg—even though other myostatin inhibitors cause testicular atrophy and reduced sperm quality.
Myostatin and activin are closely related; most drugs blocking one hit both, wrecking fertility. Apitegromab avoids activin entirely, making this a rare case of precision targeting working perfectly.
Practical Takeaways
If you or a loved one has SMA, ask your doctor if apitegromab is being tested in your region—it’s in Phase 2 trials (NCT03921528) and may be available via clinical trial soon.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 517 / 100
Probability of being correct
Researchers compare people who have a condition (cases) with similar people who do not (controls), looking back in time for differences in exposure. Useful but more prone to bias.
Animal Case-Control
Subject
Lower probability
on the GRADE evidence scale
This study didn't test people—it tested rats and monkeys in a lab. We can't say if it will work in humans or if it causes any real effects in people. It's like saying a toy car works on a table, so it must work on a real road—we just don't know.
Major conflicts — industry funding with significant control over the research. Reporting score and overall score cap have been reduced.
Weaknesses
- Study type classification is undetermined with low confidence.
Methodology
Evidence Keywords
Statistical Reporting
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
This medicine blocks a signal that stops muscles from growing, but only targets the early version of the signal, not others that control things like reproduction or the brain.
Research results
Muscles grew 13% to 33% bigger in rats, and levels of the blocked signal rose 27 to 128 times higher — with no harm to testes, bones, nerves, or fertility even at the highest dose.
What this means - more context
This means it could help people with muscle-wasting diseases like SMA gain muscle without causing side effects like bleeding or infertility seen in older drugs.
To evaluate the preclinical safety, pharmacology, and pharmacokinetics of apitegromab, a monoclonal antibody targeting proforms of myostatin, to support its clinical development for spinal muscular atrophy.
Apitegromab selectively binds to promyostatin and latent myostatin, inhibiting myostatin activation without binding mature myostatin or related growth factors. Weekly IV dosing up to 300 mg/kg in rats and 100 mg/kg in monkeys for up to 26 weeks showed no treatment-related adverse effects on clinical, hematologic, neurodevelopmental, reproductive, or bone endpoints, with sustained muscle mass increases (13–33%) and target engagement confirmed by 27- to 128-fold rises in serum latent myostatin.
Methods Used
In vitro binding and activation assays using human, rat, and cynomolgus monkey proteins; GLP toxicology studies in adult and juvenile rats (up to 300 mg/kg IV weekly for 26 and 8 weeks) and cynomolgus monkeys (up to 100 mg/kg for 4 weeks); toxicokinetic and target engagement analyses via ELISA and ECL assays.
Main Finding
Apitegromab increased skeletal muscle mass by 13–33% in rats without pathology and achieved a NOAEL of 300 mg/kg in both adult and juvenile rats, with no off-target toxicities (e.g., testicular atrophy, reproductive impairment) observed, unlike other myostatin inhibitors.
Confidence Level
High — robust, GLP-compliant, multi-species, repeat-dose toxicology studies with comprehensive endpoints, dose-proportional pharmacokinetics, and confirmed target engagement.
Study Flags
Red Flags
- •No human data
- •High doses used (up to 300 mg/kg) may not translate to humans
- •Long-term effects beyond 26 weeks not assessed
Surprising Findings
No reproductive harm at 300 mg/kg—even though other myostatin inhibitors cause testicular atrophy and reduced sperm quality.
Myostatin and activin are closely related; most drugs blocking one hit both, wrecking fertility. Apitegromab avoids activin entirely, making this a rare case of precision targeting working perfectly.
Practical Takeaways
If you or a loved one has SMA, ask your doctor if apitegromab is being tested in your region—it’s in Phase 2 trials (NCT03921528) and may be available via clinical trial soon.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 517 / 100
Probability of being correct
Researchers compare people who have a condition (cases) with similar people who do not (controls), looking back in time for differences in exposure. Useful but more prone to bias.
Animal Case-Control
Subject
Lower probability
on the GRADE evidence scale
This study didn't test people—it tested rats and monkeys in a lab. We can't say if it will work in humans or if it causes any real effects in people. It's like saying a toy car works on a table, so it must work on a real road—we just don't know.
Major conflicts — industry funding with significant control over the research. Reporting score and overall score cap have been reduced.
Weaknesses
- Study type classification is undetermined with low confidence.
Methodology
Evidence Keywords
Statistical Reporting
Scoring
How strong is this study?
We don't even know what kind of experiment this was—was it fair? Was it controlled? We can't tell. That means we can't trust the results to mean anything for real people. It's like trying to judge a magic trick without knowing how the magician did it.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
44 / 100
- Randomization+20/20
- Blindingblinding unclear
- Control group+15/15
- Sample sizeno sample size reported
- Follow-upno follow-up reported
100 / 100
54 / 100
- P-values+15/15
- Effect size+20/20
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 517 / 100
Probability of being correct
Researchers compare people who have a condition (cases) with similar people who do not (controls), looking back in time for differences in exposure. Useful but more prone to bias.
This design cannot establish causation — the findings describe an association, not a cause. Study type could not be determined with confidence; no valid classification exists to support causal inference.
Major COI
Major conflicts that significantly reduce study credibility
The study was conducted by authors affiliated with Scholar Rock, the company developing apitegromab, and the drug is being tested in clinical trials sponsored by the same entity, indicating significant industry influence on the research.
Funders
Conflict Details
Scholar Rock: Authors are employees of Scholar Rock, the company developing apitegromab.
The study was conducted entirely by employees of Scholar Rock, with no mention of independent oversight, third-party data analysis, or external review. The absence of a funding statement or COI disclosure raises concerns about transparency. The preclinical data directly supports ongoing clinical trials of apitegromab, suggesting the study was designed to support regulatory and commercial objectives.
Standing
Who’s using this study?
The videos and claims on this site that lean on this study, and the researchers who wrote it.
1 video from Physionic cite this study, drawing 1 claim from it.
- Very strong evidence
Randomized or controlled trials support this claim, alongside consistent supporting evidence.
Evidence