IGF-1 LR3
Medium EvidenceA modified form of IGF-1 with extended half-life, studied for muscle growth and tissue development.
What It Is
IGF-1 LR3 (Long R3 Insulin-like Growth Factor-1) is a synthetic 83-amino acid analog of human IGF-1 with two key modifications: an arginine substitution at position 3 and a 13-amino acid N-terminal extension peptide. These modifications reduce binding affinity to IGF-binding proteins (IGFBPs), resulting in a longer biological half-life and greater bioavailability compared to native IGF-1. While endogenous IGF-1 circulates largely bound to IGFBP-3 and is tightly regulated, IGF-1 LR3 remains predominantly in its free, biologically active form. IGF-1 signals through the IGF-1 receptor (IGF-1R), activating the PI3K/AKT and MAPK/ERK pathways to promote cell proliferation, protein synthesis, and glucose uptake in skeletal muscle. In cell culture models, IGF-1 LR3 is approximately 2–3× more potent than native IGF-1 at stimulating cell proliferation due to its reduced IGFBP sequestration. The same enhanced potency raises safety concerns: prolonged, unregulated IGF-1R activation is associated with increased risk of cellular hyperproliferation, and epidemiological data links elevated circulating IGF-1 levels to higher risk of certain cancers (colorectal, prostate, breast). IGF-1 LR3 is classified as a prohibited substance by WADA and is not FDA-approved for any indication. Native IGF-1 (mecasermin/Increlex) is FDA-approved only for severe primary IGF-1 deficiency.
Why Researchers Study It
IGF-1 LR3 has a significantly longer half-life than native IGF-1 due to its reduced affinity for IGF-binding proteins. This makes it a preferred research tool for studying sustained IGF-1 receptor activation, muscle hypertrophy pathways, and satellite cell-mediated tissue repair in preclinical models.
Proposed Mechanisms
- Binds IGF-1 receptors with similar affinity to native IGF-1 but resists binding protein sequestration
- Promotes satellite cell activation and myoblast proliferation
- Stimulates protein synthesis through PI3K/Akt/mTOR signaling cascade
- Extended half-life allows sustained anabolic signaling in muscle tissue
Evidence Snapshot
Commonly Discussed Benefits
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Safety & Cautions
- Potent growth factor with theoretical cancer risk concerns
- May cause hypoglycemia
- Not selective — promotes growth in all tissues
- Not FDA-approved; requires careful medical consideration
Comparisons
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Citations
- [1] Francis GL. et al. — Insulin-like growth factors 1 and 2 in bovine colostrum. Biochem J. 1988 PubMed
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