--- title: "Apitegromab | PepTracker Pro" url: https://peptrackerpro.com/peptides/apitegromab description: "Apitegromab (SRK-015) is an investigational, fully human IgG4 monoclonal antibody developed by Scholar Rock that inhibits myostatin - the muscle-produced growth factor that limits skeletal-muscle mass - through a novel, muscle-selective mechanism. Rather than neutralizing mature, active myostatin (the approach of earlier failed inhibitors), apitegromab binds only the inactive precursor forms of the protein - promyostatin and latent myostatin - stored locally in muscle, and blocks their proteolytic conversion into the active ligand. Because it spares the closely related growth factors activin A, BMP9/10, and TGF-beta1, it aims to avoid the off-target effects (such as bleeding and vascular changes) that sank broad ActRII-pathway drugs. Apitegromab is the first muscle-targeted therapy to succeed in a pivotal Phase 3 trial in spinal muscular atrophy (SMA): in the 156-patient SAPPHIRE study of nonambulatory Type 2/3 patients aged 2-12 already on an SMN-directed therapy (nusinersen or risdiplam), adding apitegromab (10 or 20 mg/kg IV every 4 weeks) improved motor function by a pooled 1.8 points on the Hammersmith Functional Motor Scale Expanded (HFMSE) versus placebo at 12 months (p=0.019). Scholar Rock's initial Biologics License Application received an FDA Complete Response Letter in September 2025 tied solely to a third-party (Catalent Indiana) fill-finish manufacturing inspection - not to the drug's efficacy or safety - and the company resubmitted the BLA on March 31, 2026, with a PDUFA target action date of September 30, 2026. In parallel, apitegromab produced positive Phase 2 obesity data (EMBRAZE): added to tirzepatide over 24 weeks it preserved about 55% more lean mass (roughly 1.9 kg / 4.2 lb) than tirzepatide alone, positioning myostatin inhibition as a potential lean-mass-sparing partner for GLP-1-based weight loss." lang: en --- Skip to main content **Educational information only.** This site does not provide medical advice. Read full disclaimer (https://peptrackerpro.com/disclaimer) **Research-only content.** This page is for educational purposes and does not constitute medical advice. Read full disclaimer → (https://peptrackerpro.com/research-disclaimer) # Apitegromab Medium Evidence Apitegromab (SRK-015) is an investigational, fully human IgG4 monoclonal antibody developed by Scholar Rock that inhibits myostatin - the muscle-produced growth factor that limits skeletal-muscle mass - through a novel, muscle-selective mechanism. Rather than neutralizing mature, active myostatin (the approach of earlier failed inhibitors), apitegromab binds only the inactive precursor forms of the protein - promyostatin and latent myostatin - stored locally in muscle, and blocks their proteolytic conversion into the active ligand. Because it spares the closely related growth factors activin A, BMP9/10, and TGF-beta1, it aims to avoid the off-target effects (such as bleeding and vascular changes) that sank broad ActRII-pathway drugs. Apitegromab is the first muscle-targeted therapy to succeed in a pivotal Phase 3 trial in spinal muscular atrophy (SMA): in the 156-patient SAPPHIRE study of nonambulatory Type 2/3 patients aged 2-12 already on an SMN-directed therapy (nusinersen or risdiplam), adding apitegromab (10 or 20 mg/kg IV every 4 weeks) improved motor function by a pooled 1.8 points on the Hammersmith Functional Motor Scale Expanded (HFMSE) versus placebo at 12 months (p=0.019). Scholar Rock's initial Biologics License Application received an FDA Complete Response Letter in September 2025 tied solely to a third-party (Catalent Indiana) fill-finish manufacturing inspection - not to the drug's efficacy or safety - and the company resubmitted the BLA on March 31, 2026, with a PDUFA target action date of September 30, 2026. In parallel, apitegromab produced positive Phase 2 obesity data (EMBRAZE): added to tirzepatide over 24 weeks it preserved about 55% more lean mass (roughly 1.9 kg / 4.2 lb) than tirzepatide alone, positioning myostatin inhibition as a potential lean-mass-sparing partner for GLP-1-based weight loss. Aliases Apitegromab +4 more Evidence Medium Evidence Last Updated 2026-07-17 Reading Time 6 min ## What It Is Apitegromab (development code SRK-015) is an investigational biologic - a fully human IgG4 monoclonal antibody - from Scholar Rock that targets myostatin (also called GDF-8), the body's built-in brake on skeletal-muscle growth. Myostatin is made and stored in muscle as inactive precursors (promyostatin and latent myostatin) that are later cleaved into the active hormone; active myostatin then signals through the ActRII receptors to restrain muscle size. Earlier myostatin-pathway drugs tried to block the active ligand or its receptor broadly, which also hit related TGF-beta-family proteins (activin A, BMP9/10) and produced dose-limiting side effects and clinical failures. Apitegromab takes a more selective route: it binds the pro- and latent forms of myostatin inside muscle and prevents their activation, so it dials down myostatin signaling without disturbing the neighboring growth factors. This 'muscle-directed' selectivity is the molecule's core differentiator from broad inhibitors such as the ActRII-blocking antibody bimagrumab and the myostatin/activin decoy-receptor taldefgrobep alfa. Its lead indication is spinal muscular atrophy, a genetic motor-neuron disease in which the existing SMN-restoring drugs (nusinersen/Spinraza, risdiplam/Evrysdi, and the gene therapy onasemnogene abeparvovec/Zolgensma) preserve motor neurons but leave residual muscle weakness; apitegromab is designed to strengthen the muscle itself on top of that background therapy. After the positive Phase 2 TOPAZ study, the Phase 3 SAPPHIRE trial made apitegromab the first muscle-targeted candidate to hit its primary motor-function endpoint in SMA. A separate franchise is emerging in obesity: because GLP-1 and GLP-1/GIP weight-loss drugs strip away a substantial fraction of lean muscle along with fat, apitegromab is being studied to protect muscle during that weight loss, with positive Phase 2 EMBRAZE data alongside tirzepatide. Scholar Rock is also running the Phase 2 OPAL study in infants and toddlers under two with SMA, advancing apitegromab into facioscapulohumeral muscular dystrophy (FSHD), and developing a subcutaneous follow-on (SRK-439) for obesity. Also known as: Apitegromab, SRK-015, Scholar Rock myostatin inhibitor, anti-promyostatin antibody, pro/latent myostatin inhibitor ## Regulatory Status Investigational (not approved by any regulator). Developed by Scholar Rock (Nasdaq: SRRK). The initial U.S. BLA for SMA received an FDA Complete Response Letter in September 2025 tied solely to a routine inspection of a third-party fill-finish facility (Catalent Indiana), not to efficacy or safety; Scholar Rock resubmitted the BLA on March 31, 2026 (now including a second U.S. fill-finish site), and the FDA set a PDUFA target action date of September 30, 2026. In Europe, an EMA decision is anticipated around mid-2026 with a first launch expected in the second half of 2026 (Germany first). ## Why Researchers Study It Apitegromab is the first myostatin/muscle-directed therapy to succeed in a pivotal Phase 3 trial in a neuromuscular disease, validating the long-pursued idea that inhibiting myostatin can meaningfully improve muscle function in patients - here, on top of SMN-restoring SMA drugs that fix the nerve but not the muscle. Its precursor-selective mechanism is studied as a template for hitting myostatin hard while avoiding the off-target liabilities that doomed earlier inhibitors. The molecule has also become a focal point of the obesity field: as GLP-1-class drugs drive dramatic weight loss but sacrifice lean muscle, apitegromab's positive EMBRAZE data make it a leading test of whether myostatin inhibition can preserve muscle quality during pharmacologic weight loss, potentially as a combination partner for incretin therapies. ## Proposed Mechanisms - Precursor-selective myostatin inhibition: apitegromab binds the inactive precursor forms of myostatin - promyostatin and latent myostatin - stored in skeletal muscle, blocking their proteolytic activation into mature, signaling myostatin (GDF-8). - Reduced ActRII signaling: by lowering the pool of active myostatin, it decreases activation of the activin type II receptors and downstream SMAD2/3 signaling that restrains muscle growth, favoring muscle hypertrophy and function. - Growth-factor selectivity: it spares the closely related ligands activin A, BMP9/10, and TGF-beta1, an intended contrast to broad ActRII-pathway inhibitors and a strategy to avoid off-target bleeding and vascular effects. - Complementary to SMN-directed therapy in SMA: acting on muscle rather than the motor neuron, it is designed to add motor-function benefit on top of nusinersen, risdiplam, or gene therapy that preserve or restore the neuron. - Lean-mass preservation during weight loss: sustained muscle-directed myostatin inhibition is proposed to protect skeletal muscle while GLP-1/GIP agonists (e.g., tirzepatide) drive fat and overall weight loss. ## Evidence Snapshot Medium Evidence Low Medium High | Study Type | Model | Outcome | Link | | --- | --- | --- | --- | | RCT (human, Phase 3 - SAPPHIRE, 12 months) | 156 nonambulatory Type 2/3 SMA patients aged 2-12 on background SMN therapy (nusinersen or risdiplam); apitegromab 10 or 20 mg/kg IV every 4 weeks vs placebo (1:1:1) | Met primary endpoint: pooled mean HFMSE improvement of ~1.8 points vs placebo at 12 months (p=0.019); the 10 mg/kg arm showed a ~2.2-point difference (nominal p=0.0121). First muscle-targeted therapy to succeed in a pivotal SMA trial; generally well tolerated. | Source: https://investors.scholarrock.com/news-releases/news-release-details/scholar-rock-reports-apitegromab-meets-primary-endpoint-phase-3 | | RCT (human, Phase 2 - EMBRAZE, 24 weeks, obesity) | Adults with obesity receiving tirzepatide; apitegromab 10 mg/kg added vs tirzepatide alone | Statistically significant preservation of lean mass: apitegromab preserved ~54.9% more lean mass (about 1.9 kg / 4.2 lb) than tirzepatide alone over 24 weeks, supporting myostatin inhibition as a lean-mass-sparing partner for GLP-1/GIP weight loss. | Source: https://investors.scholarrock.com/news-releases/news-release-details/scholar-rock-reports-positive-phase-2-embraze-trial-results | | Human (Phase 2 - TOPAZ / open-label extensions and OPAL, ongoing) | Earlier Phase 2 TOPAZ study in Type 2/3 SMA (proof-of-concept for HFMSE gains) and the ongoing Phase 2 OPAL study in infants/toddlers under 2 with SMA on SMN-directed therapy or gene therapy | TOPAZ demonstrated motor-function improvements that supported SAPPHIRE; OPAL extends evaluation to the youngest SMA patients, with dosing underway. Long-term extension data continue to accrue. | Source: https://clinicaltrials.gov/study/NCT03921528 | ## Commonly Discussed Benefits Metabolism: https://peptrackerpro.com/benefits/metabolism Researching Apitegromab? Track it, set reminders, and keep notes in the free app. Track in App (https://app.peptrackerpro.com/?add=apitegromab) ## Safety & Cautions - Not approved by any regulator; the initial U.S. BLA received a Complete Response Letter in September 2025 (manufacturing/fill-finish inspection related), and approval now depends on the resubmitted BLA and a September 30, 2026 PDUFA decision. - Efficacy is established in SMA when added to SMN-directed therapy in nonambulatory Type 2/3 patients; benefit in other SMA populations, in obesity, or in FSHD is not yet proven by pivotal data. - Administered by intravenous infusion every 4 weeks (a subcutaneous follow-on, SRK-439, is separate and earlier-stage). - Long-term effects of sustained myostatin inhibition are unknown; earlier myostatin-pathway drugs failed or showed off-target effects in other diseases. - Obesity (EMBRAZE) data are Phase 2 and short-term; durability, functional/strength outcomes, and safety of long-term use with GLP-1 drugs remain to be established. ## Comparisons See how Apitegromab compares to related peptides: Apitegromab vs Bimagrumab: https://peptrackerpro.com/compare/apitegromab-vs-bimagrumab Apitegromab vs Garetosmab: https://peptrackerpro.com/compare/apitegromab-vs-garetosmab Apitegromab vs Semaglutide: https://peptrackerpro.com/compare/apitegromab-vs-semaglutide Apitegromab vs Sotatercept: https://peptrackerpro.com/compare/apitegromab-vs-sotatercept Apitegromab vs Taldefgrobep Alfa: https://peptrackerpro.com/compare/apitegromab-vs-taldefgrobep Apitegromab vs Tirzepatide: https://peptrackerpro.com/compare/apitegromab-vs-tirzepatide Apitegromab vs Trevogrumab: https://peptrackerpro.com/compare/apitegromab-vs-trevogrumab Apitegromab vs Zilucoplan: https://peptrackerpro.com/compare/apitegromab-vs-zilucoplan ## Calculator Tools Use our research tools to explore dosing and reconstitution data: Reconstitution Calculator: https://peptrackerpro.com/calculators ## Citations 1. [1] Scholar Rock Reports Apitegromab Meets Primary Endpoint in Phase 3 SAPPHIRE Study in SMA - Scholar Rock (2024) PubMed (https://investors.scholarrock.com/news-releases/news-release-details/scholar-rock-reports-apitegromab-meets-primary-endpoint-phase-3) 2. [2] FDA Issues Complete Response Letter for Apitegromab Solely Related to Observations at Catalent Indiana Fill-Finish Facility - Scholar Rock (Sept 2025) PubMed (https://investors.scholarrock.com/news-releases/news-release-details/fda-issues-complete-response-letter-crl-apitegromab-treatment) 3. [3] Scholar Rock Resubmits BLA to FDA for Apitegromab for Children and Adults with SMA (March 31, 2026) PubMed (https://investors.scholarrock.com/news-releases/news-release-details/scholar-rock-resubmits-biologics-license-application-bla-fda) 4. [4] Scholar Rock Reports Positive Phase 2 EMBRAZE Trial Results Demonstrating Preservation of Lean Mass with Apitegromab During Tirzepatide-Induced Weight Loss - Scholar Rock PubMed (https://investors.scholarrock.com/news-releases/news-release-details/scholar-rock-reports-positive-phase-2-embraze-trial-results) 5. [5] Safety and Efficacy of Apitegromab in Patients With Spinal Muscular Atrophy Types 2 and 3 (TOPAZ) - Neurology PubMed (https://www.neurology.org/doi/10.1212/WNL.0000000000209151) ### Keep researching in the app - Log Apitegromab to your private tracker - Set a dosing reminder - Compare it side-by-side with your stack Open PepTracker Pro Free: https://app.peptrackerpro.com/?add=apitegromab Browse more peptides: https://peptrackerpro.com/peptides ## Related Peptides ### Bimagrumab Medium Evidence An anti-activin type II receptor antibody that promotes fat loss while preserving lean muscle mass, studied in combination with GLP-1 agonists for obesity. Fat Loss (https://peptrackerpro.com/benefits/fat-loss)Muscle Preservation (https://peptrackerpro.com/benefits/muscle-preservation)Weight Management (https://peptrackerpro.com/benefits/weight-management)Body Composition (https://peptrackerpro.com/benefits/body-composition)+1 more View Details: https://peptrackerpro.com/peptides/bimagrumab \+ Compare: https://peptrackerpro.com/compare?select=bimagrumab Track in App: https://app.peptrackerpro.com/?add=bimagrumab ### Garetosmab Medium Evidence Garetosmab (REGN2477) is Regeneron's investigational fully human anti-activin A monoclonal antibody. In the Phase 3 OPTIMA trial in adults with the ultra-rare bone disease fibrodysplasia ossificans progressiva (FOP), it cut new heterotopic bone lesions by ~90-94% and reduced new-lesion volume by >99% versus placebo; its BLA was accepted for FDA Priority Review with an August 2026 target action date. The same activin A blockade also makes garetosmab the muscle-preserving partner to trevogrumab and semaglutide in the Phase 2 COURAGE obesity program. Bone Growth (https://peptrackerpro.com/benefits/bone-growth)Rare Disease Treatment (https://peptrackerpro.com/benefits/rare-disease-treatment)Muscle Preservation (https://peptrackerpro.com/benefits/muscle-preservation)Body Composition (https://peptrackerpro.com/benefits/body-composition)+1 more View Details: https://peptrackerpro.com/peptides/garetosmab \+ Compare: https://peptrackerpro.com/compare?select=garetosmab Track in App: https://app.peptrackerpro.com/?add=garetosmab ### Semaglutide High Evidence A GLP-1 receptor agonist FDA-approved for type 2 diabetes and chronic weight management. Weight Management (https://peptrackerpro.com/benefits/weight-management)Appetite Regulation (https://peptrackerpro.com/benefits/appetite-regulation)Metabolism (https://peptrackerpro.com/benefits/metabolism)Fat Loss (https://peptrackerpro.com/benefits/fat-loss)+3 more View Details: https://peptrackerpro.com/peptides/semaglutide \+ Compare: https://peptrackerpro.com/compare?select=semaglutide Track in App: https://app.peptrackerpro.com/?add=semaglutide ### Sotatercept High Evidence Sotatercept (Winrevair, sotatercept-csrk) is Merck's first-in-class activin signaling inhibitor - a recombinant ActRIIA-Fc fusion protein that acts as a ligand trap for activin A, activin B, GDF-8 (myostatin) and GDF-11. It is FDA-approved for adults with pulmonary arterial hypertension (PAH), where it improved 6-minute walk distance by 40.8 meters in the Phase 3 STELLAR trial and, in the Phase 3 ZENITH trial in high-risk patients, cut the composite risk of death, lung transplantation and PAH hospitalization by 76%. An expanded U.S. label reflecting ZENITH was approved on October 27, 2025. Cardiovascular: https://peptrackerpro.com/benefits/cardiovascular Energy: https://peptrackerpro.com/benefits/energy Rare Disease Treatment: https://peptrackerpro.com/benefits/rare-disease-treatment Disease Modification: https://peptrackerpro.com/benefits/disease-modification View Details: https://peptrackerpro.com/peptides/sotatercept \+ Compare: https://peptrackerpro.com/compare?select=sotatercept Track in App: https://app.peptrackerpro.com/?add=sotatercept ### Taldefgrobep Alfa Low Evidence A novel myostatin-activin pathway inhibitor targeting fat reduction and lean mass gain, with Phase 2 results in obesity expected H2 2026. 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Weight Management (https://peptrackerpro.com/benefits/weight-management)Appetite Regulation (https://peptrackerpro.com/benefits/appetite-regulation)Metabolism (https://peptrackerpro.com/benefits/metabolism)Fat Loss (https://peptrackerpro.com/benefits/fat-loss)+1 more View Details: https://peptrackerpro.com/peptides/tirzepatide \+ Compare: https://peptrackerpro.com/compare?select=tirzepatide Track in App: https://app.peptrackerpro.com/?add=tirzepatide ### Trevogrumab Medium Evidence Trevogrumab (REGN1033) is an investigational fully human anti-myostatin (anti-GDF8) monoclonal antibody from Regeneron being tested as a lean-mass-sparing add-on to semaglutide in the Phase 2 COURAGE obesity trial; adding it to semaglutide prevented roughly half of the ~33% of GLP-1-induced weight loss that normally comes from muscle, and the semaglutide + trevogrumab + garetosmab triplet reached 13.4% weight loss at 26 weeks with only ~7.4% of that loss from lean mass. Muscle Preservation: https://peptrackerpro.com/benefits/muscle-preservation Body Composition: https://peptrackerpro.com/benefits/body-composition Fat Loss: https://peptrackerpro.com/benefits/fat-loss Weight Management: https://peptrackerpro.com/benefits/weight-management View Details: https://peptrackerpro.com/peptides/trevogrumab \+ Compare: https://peptrackerpro.com/compare?select=trevogrumab Track in App: https://app.peptrackerpro.com/?add=trevogrumab ### Zilucoplan High Evidence A self-administered, once-daily subcutaneous macrocyclic peptide that inhibits complement component 5 (C5), FDA- and EMA-approved for anti-AChR-positive generalized myasthenia gravis. Immune Modulation: https://peptrackerpro.com/benefits/immune-modulation Neuromuscular Function: https://peptrackerpro.com/benefits/neuromuscular-function View Details: https://peptrackerpro.com/peptides/zilucoplan \+ Compare: https://peptrackerpro.com/compare?select=zilucoplan Track in App: https://app.peptrackerpro.com/?add=zilucoplan ## Structured data ```json [ { "@context": "https://schema.org", "@type": "Organization", "name": "PepTracker Pro", "url": "https://peptrackerpro.com", "logo": "https://peptrackerpro.com/favicon.png", "description": "Evidence-based educational resource dedicated to improving peptide research literacy.", "contactPoint": { "@type": "ContactPoint", "email": "hello@peptrackerpro.com", "contactType": "customer support" }, "sameAs": [] }, { "@context": "https://schema.org", "@type": "MedicalWebPage", "name": "Apitegromab", "description": "Apitegromab (SRK-015) is an investigational, fully human IgG4 monoclonal antibody developed by Scholar Rock that inhibits myostatin - the muscle-produced growth factor that limits skeletal-muscle mass - through a novel, muscle-selective mechanism. Rather than neutralizing mature, active myostatin (the approach of earlier failed inhibitors), apitegromab binds only the inactive precursor forms of the protein - promyostatin and latent myostatin - stored locally in muscle, and blocks their proteolytic conversion into the active ligand. Because it spares the closely related growth factors activin A, BMP9/10, and TGF-beta1, it aims to avoid the off-target effects (such as bleeding and vascular changes) that sank broad ActRII-pathway drugs. Apitegromab is the first muscle-targeted therapy to succeed in a pivotal Phase 3 trial in spinal muscular atrophy (SMA): in the 156-patient SAPPHIRE study of nonambulatory Type 2/3 patients aged 2-12 already on an SMN-directed therapy (nusinersen or risdiplam), adding apitegromab (10 or 20 mg/kg IV every 4 weeks) improved motor function by a pooled 1.8 points on the Hammersmith Functional Motor Scale Expanded (HFMSE) versus placebo at 12 months (p=0.019). Scholar Rock's initial Biologics License Application received an FDA Complete Response Letter in September 2025 tied solely to a third-party (Catalent Indiana) fill-finish manufacturing inspection - not to the drug's efficacy or safety - and the company resubmitted the BLA on March 31, 2026, with a PDUFA target action date of September 30, 2026. In parallel, apitegromab produced positive Phase 2 obesity data (EMBRAZE): added to tirzepatide over 24 weeks it preserved about 55% more lean mass (roughly 1.9 kg / 4.2 lb) than tirzepatide alone, positioning myostatin inhibition as a potential lean-mass-sparing partner for GLP-1-based weight loss.", "url": "https://peptrackerpro.com/peptides/apitegromab", "lastReviewed": "2026-07-17", "reviewedBy": { "@type": "Organization", "name": "PepTracker Pro Research Team", "url": "https://peptrackerpro.com" }, "isPartOf": { "@type": "WebSite", "name": "PepTracker Pro", "url": "https://peptrackerpro.com" } }, { "@context": "https://schema.org", "@type": "BreadcrumbList", "itemListElement": [ { "@type": "ListItem", "position": 1, "name": "Home", "item": "https://peptrackerpro.com" }, { "@type": "ListItem", "position": 2, "name": "Peptides", "item": "https://peptrackerpro.com/peptides" }, { "@type": "ListItem", "position": 3, "name": "Apitegromab", "item": "https://peptrackerpro.com/peptides/apitegromab" } ] }, { "@context": "https://schema.org", "@type": "FAQPage", "mainEntity": [ { "@type": "Question", "name": "People with SMA and their caregivers tracking add-on, muscle-directed therapies that build on SMN drugs (nusinersen, risdiplam) or gene therapy.?", "acceptedAnswer": { "@type": "Answer", "text": "Apitegromab is a research peptide studied in preclinical models. Apitegromab (SRK-015) is an investigational, fully human IgG4 monoclonal antibody developed by Scholar Rock that inhibits myostatin - the muscle-produced growth factor that limits skeletal-muscle mass - through a novel, muscle-selective mechanism. Rather than neutralizing mature, active myostatin (the approach of earlier failed inhibitors), apitegromab binds only the inactive precursor forms of the protein - promyostatin and latent myostatin - stored locally in muscle, and blocks their proteolytic conversion into the active ligand. Because it spares the closely related growth factors activin A, BMP9/10, and TGF-beta1, it aims to avoid the off-target effects (such as bleeding and vascular changes) that sank broad ActRII-pathway drugs. Apitegromab is the first muscle-targeted therapy to succeed in a pivotal Phase 3 trial in spinal muscular atrophy (SMA): in the 156-patient SAPPHIRE study of nonambulatory Type 2/3 patients aged 2-12 already on an SMN-directed therapy (nusinersen or risdiplam), adding apitegromab (10 or 20 mg/kg IV every 4 weeks) improved motor function by a pooled 1.8 points on the Hammersmith Functional Motor Scale Expanded (HFMSE) versus placebo at 12 months (p=0.019). Scholar Rock's initial Biologics License Application received an FDA Complete Response Letter in September 2025 tied solely to a third-party (Catalent Indiana) fill-finish manufacturing inspection - not to the drug's efficacy or safety - and the company resubmitted the BLA on March 31, 2026, with a PDUFA target action date of September 30, 2026. In parallel, apitegromab produced positive Phase 2 obesity data (EMBRAZE): added to tirzepatide over 24 weeks it preserved about 55% more lean mass (roughly 1.9 kg / 4.2 lb) than tirzepatide alone, positioning myostatin inhibition as a potential lean-mass-sparing partner for GLP-1-based weight loss." } }, { "@type": "Question", "name": "Clinicians and researchers comparing myostatin-pathway approaches (apitegromab vs bimagrumab vs taldefgrobep alfa) and their selectivity and safety profiles.?", "acceptedAnswer": { "@type": "Answer", "text": "Apitegromab is a research peptide studied in preclinical models. Apitegromab (SRK-015) is an investigational, fully human IgG4 monoclonal antibody developed by Scholar Rock that inhibits myostatin - the muscle-produced growth factor that limits skeletal-muscle mass - through a novel, muscle-selective mechanism. Rather than neutralizing mature, active myostatin (the approach of earlier failed inhibitors), apitegromab binds only the inactive precursor forms of the protein - promyostatin and latent myostatin - stored locally in muscle, and blocks their proteolytic conversion into the active ligand. Because it spares the closely related growth factors activin A, BMP9/10, and TGF-beta1, it aims to avoid the off-target effects (such as bleeding and vascular changes) that sank broad ActRII-pathway drugs. Apitegromab is the first muscle-targeted therapy to succeed in a pivotal Phase 3 trial in spinal muscular atrophy (SMA): in the 156-patient SAPPHIRE study of nonambulatory Type 2/3 patients aged 2-12 already on an SMN-directed therapy (nusinersen or risdiplam), adding apitegromab (10 or 20 mg/kg IV every 4 weeks) improved motor function by a pooled 1.8 points on the Hammersmith Functional Motor Scale Expanded (HFMSE) versus placebo at 12 months (p=0.019). Scholar Rock's initial Biologics License Application received an FDA Complete Response Letter in September 2025 tied solely to a third-party (Catalent Indiana) fill-finish manufacturing inspection - not to the drug's efficacy or safety - and the company resubmitted the BLA on March 31, 2026, with a PDUFA target action date of September 30, 2026. In parallel, apitegromab produced positive Phase 2 obesity data (EMBRAZE): added to tirzepatide over 24 weeks it preserved about 55% more lean mass (roughly 1.9 kg / 4.2 lb) than tirzepatide alone, positioning myostatin inhibition as a potential lean-mass-sparing partner for GLP-1-based weight loss." } }, { "@type": "Question", "name": "People following muscle-preserving weight-loss research who want to know whether myostatin inhibition can protect lean mass during GLP-1/GIP (e.g., tirzepatide) treatment.?", "acceptedAnswer": { "@type": "Answer", "text": "Apitegromab is a research peptide studied in preclinical models. Apitegromab (SRK-015) is an investigational, fully human IgG4 monoclonal antibody developed by Scholar Rock that inhibits myostatin - the muscle-produced growth factor that limits skeletal-muscle mass - through a novel, muscle-selective mechanism. Rather than neutralizing mature, active myostatin (the approach of earlier failed inhibitors), apitegromab binds only the inactive precursor forms of the protein - promyostatin and latent myostatin - stored locally in muscle, and blocks their proteolytic conversion into the active ligand. Because it spares the closely related growth factors activin A, BMP9/10, and TGF-beta1, it aims to avoid the off-target effects (such as bleeding and vascular changes) that sank broad ActRII-pathway drugs. Apitegromab is the first muscle-targeted therapy to succeed in a pivotal Phase 3 trial in spinal muscular atrophy (SMA): in the 156-patient SAPPHIRE study of nonambulatory Type 2/3 patients aged 2-12 already on an SMN-directed therapy (nusinersen or risdiplam), adding apitegromab (10 or 20 mg/kg IV every 4 weeks) improved motor function by a pooled 1.8 points on the Hammersmith Functional Motor Scale Expanded (HFMSE) versus placebo at 12 months (p=0.019). Scholar Rock's initial Biologics License Application received an FDA Complete Response Letter in September 2025 tied solely to a third-party (Catalent Indiana) fill-finish manufacturing inspection - not to the drug's efficacy or safety - and the company resubmitted the BLA on March 31, 2026, with a PDUFA target action date of September 30, 2026. In parallel, apitegromab produced positive Phase 2 obesity data (EMBRAZE): added to tirzepatide over 24 weeks it preserved about 55% more lean mass (roughly 1.9 kg / 4.2 lb) than tirzepatide alone, positioning myostatin inhibition as a potential lean-mass-sparing partner for GLP-1-based weight loss." } }, { "@type": "Question", "name": "Investors and industry watchers following Scholar Rock (SRRK), the apitegromab BLA resubmission and September 30, 2026 PDUFA date, and the EMBRAZE obesity program.?", "acceptedAnswer": { "@type": "Answer", "text": "Apitegromab is a research peptide studied in preclinical models. Apitegromab (SRK-015) is an investigational, fully human IgG4 monoclonal antibody developed by Scholar Rock that inhibits myostatin - the muscle-produced growth factor that limits skeletal-muscle mass - through a novel, muscle-selective mechanism. Rather than neutralizing mature, active myostatin (the approach of earlier failed inhibitors), apitegromab binds only the inactive precursor forms of the protein - promyostatin and latent myostatin - stored locally in muscle, and blocks their proteolytic conversion into the active ligand. Because it spares the closely related growth factors activin A, BMP9/10, and TGF-beta1, it aims to avoid the off-target effects (such as bleeding and vascular changes) that sank broad ActRII-pathway drugs. Apitegromab is the first muscle-targeted therapy to succeed in a pivotal Phase 3 trial in spinal muscular atrophy (SMA): in the 156-patient SAPPHIRE study of nonambulatory Type 2/3 patients aged 2-12 already on an SMN-directed therapy (nusinersen or risdiplam), adding apitegromab (10 or 20 mg/kg IV every 4 weeks) improved motor function by a pooled 1.8 points on the Hammersmith Functional Motor Scale Expanded (HFMSE) versus placebo at 12 months (p=0.019). Scholar Rock's initial Biologics License Application received an FDA Complete Response Letter in September 2025 tied solely to a third-party (Catalent Indiana) fill-finish manufacturing inspection - not to the drug's efficacy or safety - and the company resubmitted the BLA on March 31, 2026, with a PDUFA target action date of September 30, 2026. In parallel, apitegromab produced positive Phase 2 obesity data (EMBRAZE): added to tirzepatide over 24 weeks it preserved about 55% more lean mass (roughly 1.9 kg / 4.2 lb) than tirzepatide alone, positioning myostatin inhibition as a potential lean-mass-sparing partner for GLP-1-based weight loss." } } ] } ] ```