IGF-1 LR3.
IGF-1 LR3 (Long R3 IGF-1) is a synthetic, recombinant analog of human insulin-like growth factor-1.
IGF-1 LR3 (Long R3 IGF-1) is a synthetic, recombinant analog of human insulin-like growth factor-1. IGF-1 LR3 is research / preclinical, and PepCue grades its published evidence F tier (33/100). This is a research reference, not medical or dosing advice.
What it is
IGF-1 LR3 (Long R3 IGF-1) is a synthetic, recombinant analog of human insulin-like growth factor-1. It is an 83-amino-acid polypeptide built from the 70-residue native IGF-1 sequence with two structural changes: a glutamate-to-arginine substitution at position 3 and a 13-residue N-terminal extension peptide. It is produced and sold primarily as a research reagent and as a cell-culture supplement (marketed under names such as LONG R3 IGF-I), not as a licensed human medicine.
How it works
Like native IGF-1, LR3 binds and activates the IGF-1 receptor (IGF-1R), a receptor tyrosine kinase that signals through the PI3K/Akt/mTOR and Ras/MAPK pathways to drive protein synthesis, cell proliferation, and survival. Its distinguishing feature is engineered: the position-3 arginine substitution plus the N-terminal extension dramatically lower its affinity for the six IGF-binding proteins (IGFBPs) that normally sequester circulating IGF-1. Because little of the analog is bound and held by IGFBPs, a much larger fraction remains free to engage IGF-1R, and in animal models its circulating half-life is substantially longer than that of native IGF-1. This same "escape from IGFBP regulation" is why it is favored in mammalian cell culture, where it resists sequestration by cell-secreted binding proteins.
Mechanism pathways
Downstream growth signalling through the IGF-1 receptor and its splice variants.
Insulin-like growth factor 1 is the main mediator of growth hormone's anabolic effects. Growth hormone binds its own receptor on hepatocytes, which dimerises and signals through the JAK2 and STAT5 pathway to drive IGF-1 transcription. Circulating IGF-1 then binds the IGF-1 receptor, a receptor tyrosine kinase closely related to the insulin receptor, which activates the PI3K, Akt, and mTOR cascade alongside the Ras and MAPK cascade. The result is increased protein synthesis, cell proliferation, and suppression of programmed cell death. Muscle also produces IGF-1 locally in response to mechanical loading, and this local supply is thought to matter more for tissue repair than the circulating pool does. A critical regulatory layer is the family of six IGF-binding proteins. The great majority of circulating IGF-1 is bound to these proteins rather than free, which limits how much reaches receptors at any moment and prevents excessive signalling. Several compounds in this group are engineered specifically to evade that control, using amino-acid substitutions and terminal extensions that drop binding-protein affinity dramatically. The consequence is a much larger free fraction and a far longer functional presence than native IGF-1, which is the intended effect and also the source of the main concern about them. A separate branch involves splice variants. Mechanical stress shifts IGF-1 splicing to produce a variant with a distinct carboxy-terminal E-domain, and the isolated E-peptide has been proposed to act on muscle satellite cells through a receptor different from the classical IGF-1 receptor, expanding the stem cell pool before mature IGF-1 drives differentiation. Attaching polyethylene glycol to this peptide is a stability modification rather than a change in mechanism. The honest position differs sharply between branches. Growth hormone and IGF-1 receptor biology is textbook physiology, well established in humans, and recombinant growth hormone is an approved medicine for defined deficiency states. The splice-variant branch is far weaker: independent laboratories have not consistently reproduced the proposed proliferative effect of the isolated E-peptide, and there are essentially no controlled human trials of it. Across the whole pathway, unregulated amplification of a proliferative growth signal raises theoretical concerns about promoting the growth of existing abnormal cells, and these compounds other than approved growth hormone have no established human safety record.
The evidence
Direct evidence for LR3 is overwhelmingly preclinical and in-vitro, not clinical. In a guinea pig study (Conlon et al., J Endocrinol 1995, PMID 7561636), Long R3 IGF-I infusion stimulated organ growth while paradoxically lowering plasma IGF-I, IGF-II, and IGFBP concentrations, illustrating its altered binding-protein behavior in vivo. A mouse study (J Endocrinol 2008, PMID 18577570) reported that long-R3-IGF-I altered mammary signaling and gene expression during prolonged lactation. Analytical work (J Chromatogr B 2003, PMID 12880859) characterized the molecule for bioanalytical detection. There are no controlled human trials demonstrating safety or performance/physique benefits for IGF-1 LR3 specifically; clinical inferences are extrapolated from native IGF-1 (mecasermin) and from receptor pharmacology, which is a meaningful gap because LR3's reduced IGFBP binding changes its tissue exposure relative to the natural hormone.
The evidence, in brief
A synthetic IGF-1 analogue engineered for higher potency and reduced binding-protein affinity, studied almost exclusively in animal and cell models. Essentially no controlled human outcome trials; not an approved human drug.
- Long R3 IGF-I infusion stimulates organ growth in the guinea pigJ Endocrinol, 1995 (PMID 7561636)
Evidence maturity
An evidence-only reading: approval status, human vs preclinical data, mechanism and safety. Popularity never raises it. Research file #032
Mostly preclinical or mechanistic; little human data.
Findings come mainly from animal models, not people.
Claim receipts
Popular claims about IGF-1 LR3, checked against the state of the evidence. The verdict describes evidence maturity, never an invented study result.
No controlled human trial of this analog exists; the claim is extrapolated from native IGF-1 pharmacology.
The approved product is native IGF-1 (mecasermin); LR3 is a structurally modified analog with different binding-protein behavior.
No human safety dataset exists for LR3, and the native IGF-1 label documents hypoglycemia including severe events.
IGF-1 receptor signaling is mitogenic, and the question has never been quantified for this analog in humans.
Safety profile
No human safety dataset exists for IGF-1 LR3 itself; the closest human reference is the FDA-approved native IGF-1 drug mecasermin (Increlex), whose label documents hypoglycemia (including severe, seizure-associated events from its insulin-like action), intracranial hypertension with papilledema, and lymphoid (tonsillar/adenoidal) tissue hypertrophy. Because IGF-1R signaling is mitogenic and anti-apoptotic, a theoretical concern across IGF-1 agonists is the promotion of growth in existing neoplastic tissue, though this has not been quantified for LR3 in humans. LR3's much longer free-ligand exposure could plausibly amplify these effects relative to native IGF-1, but this is unverified. Research-grade material also carries purity, sterility, and mislabeling risks that are not controlled to pharmaceutical standards.
Compound notes
- IGF-1 LR3 (Long-R3 IGF-1) is a modified insulin-like growth factor with an extended half-life and reduced binding to IGF binding proteins, so more stays active.
- It drives growth/anabolic signaling directly downstream of the GH axis.
- Not FDA-approved; research-only and prohibited in sport under WADA.
- Potent growth signaling raises theoretical risks; human safety is not established.
Regulatory status
IGF-1 LR3 is not approved by the FDA or any major regulator for human use; it is sold for laboratory research and cell-culture manufacturing only. The only FDA-approved IGF-1 product is mecasermin (Increlex), recombinant native IGF-1 indicated for severe primary IGF-1 deficiency, which is a different molecule. IGF-1 and its analogues are prohibited in sport at all times under WADA Prohibited List section S2.
Sold research-use-only; human evidence is limited or preclinical.
By the numbers
- 0183-amino-acid IGF-1 analog with a Glu3-to-Arg substitution plus a 13-residue N-terminal extension
- 02Engineered to strongly reduce binding-protein (IGFBP) affinity, increasing the free, receptor-available fraction
- 03Acts as a full agonist at the IGF-1 receptor (IGF-1R) via PI3K/Akt/mTOR signaling
- 04Widely used as a mammalian cell-culture supplement to boost growth and recombinant-protein yield
- 05Evidence base is preclinical/in-vitro; no controlled human trials exist for the LR3 analog
- 06Prohibited in sport at all times by WADA (section S2); not FDA-approved for human use
IGF-1 LR3: research formats
Choose the format you are researching to see route-specific notes.
Requires acetic acid reconstitution; basic residues aggregate at neutral pH.
IGF-1 LR3 is a modified IGF-1 analog with extended half-life (~20–30 h vs 5–10 min for native IGF-1). It is sold as a 1 mg vial and requires 0.6% acetic acid for initial reconstitution because its basic residue composition causes aggregation at neutral pH. After dissolving, dilute with BAC water to the target volume.
At 1,000 mcg/mL and very small target volumes, measuring error is significant. A TB syringe (0.01 mL graduations) is more accurate than a U-100 at sub-10-unit draws.
Subcutaneous injection targets local muscle-group tissue; IM injection is used in some research designs. Route selection affects the delivery pattern.
Sources
Every factual claim above resolves to a real, published source.
- Long R3 insulin-like growth factor-I (IGF-I) infusion stimulates organ growth but reduces plasma IGF-I, IGF-II and IGF binding protein concentrations in the guinea pigJournal of Endocrinology, 1995, PMID 7561636
- Enhancement of maternal lactation performance during prolonged lactation in the mouse by mouse GH and long-R3-IGF-I is linked to changes in mammary signaling and gene expressionJournal of Endocrinology, 2008, PMID 18577570
- Site-specific fluorescent derivatization and liquid chromatographic-mass spectrometric characterization of long R(3) IGF-I for bioanalytical applicationsJournal of Chromatography B, 2003, PMID 12880859
- INCRELEX (mecasermin) injection: FDA prescribing information (native IGF-1 reference for safety pharmacology)DailyMed/FDA, initial U.S. approval 2005
Cite this page
PepCue. “IGF-1 LR3: the evidence.” PepCue, reviewed June 1, 2026. https://www.pepcue.app/p/igf-1-lr3.
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