A long-acting IGF-1 analog that bypasses the body's regulatory brake
Injection · 503A Compounding
Educational content. This page describes IGF-1 LR3 for informational purposes only and is not medical advice, diagnosis, or treatment. Consult a licensed provider before starting, stopping, or modifying any therapy.
Researched and maintained by the PepHookup team. Regulatory status last verified April 12, 2026.
The Arg3 substitution and 13-amino-acid N-terminal extension sharply reduce affinity for IGF binding proteins. Because free native IGF-1 is cleared within roughly 10 minutes and most of it normally circulates bound to IGFBP-3, this redesign keeps far more of the peptide free and active for far longer.[1][3]
In glucocorticoid-treated rats, LR3 IGF-1 and des(1-3) IGF-1 were roughly 2.5 to 3 times more potent than native IGF-1 at restoring growth, though neither fully reproduced insulin's metabolic actions in liver and muscle.[8]
Continuous Long R3 IGF-1 infusion in atherosclerotic mice reduced lumen stenosis and necrotic core size in early lesions and more than doubled smooth muscle cell content while lowering intraplaque hemorrhage in advanced plaques, a vascular-stabilizing effect in this preclinical model.[11]
IGF-1 LR3 (Long R3 Insulin-Like Growth Factor-1) is an 83-amino-acid synthetic analog of human IGF-1, which is natively 70 amino acids. It carries two engineered changes: a 13-amino-acid extension at the N-terminus and a substitution of arginine for glutamate at position 3. These modifications were developed by the Adelaide, Australia research group that characterized the IGF-1 fusion-protein analogs, primarily to create a more potent supplement for serum-free cell culture rather than as a human therapeutic.
The two modifications together drastically lower the peptide's affinity for IGF binding proteins (IGFBPs). Since roughly 80 to 90 percent of circulating native IGF-1 is normally carried in a ternary complex with IGFBP-3 and the acid-labile subunit, reducing IGFBP binding fundamentally changes the pharmacology: a much larger fraction of administered LR3 stays free and bioactive. The original characterization reported markedly enhanced biological potency versus native IGF-1. IGF-1 LR3 has never been studied in a published human clinical trial, is not FDA-approved for any indication, and is prohibited at all times by the World Anti-Doping Agency.
IGF-1 LR3 binds and activates the Type I IGF receptor (IGF-1R), a transmembrane receptor tyrosine kinase. Ligand binding triggers receptor autophosphorylation, which recruits adaptor proteins: IRS-1 feeds the PI3K/Akt arm, while Shc feeds the Ras/Raf/MEK/ERK arm.
The PI3K/Akt/mTOR pathway is the principal driver of skeletal muscle protein synthesis. Akt activates mTOR and inhibits GSK3, increasing translation, while also suppressing protein breakdown through FOXO inactivation. In cultured myotubes, IGF-1 induces hypertrophy specifically through this PI3K/Akt/mTOR and PI3K/Akt/GSK3 signaling.
The Ras/MAPK/ERK pathway drives cell proliferation and supports satellite cell activity. The defining difference from native IGF-1 is that LR3 largely escapes the IGFBP system, so it is not sequestered and inactivated the way endogenous IGF-1 is. That removes much of the body's natural brake on the intensity and duration of IGF-1R signaling, which is both the appeal and the central safety concern of the molecule.
A long-acting IGF-1 analog that bypasses the body's regulatory brake
IGF-1 LR3 activates the IGF-1 receptor to drive PI3K/Akt/mTOR signaling (protein synthesis, anti-apoptosis, glucose uptake) and Ras/MAPK/ERK signaling (proliferation, satellite cell activity). Its sharply reduced IGFBP binding keeps more of the peptide free and active far longer than native IGF-1, removing much of the natural regulatory brake on IGF-1R signaling.
LR3 IGF-1 and des(1-3) IGF-1 were about 2.5 to 3 times more potent than native IGF-1 at restoring growth. Neither variant fully replaced the insulin-dependent metabolic actions normally seen in liver, muscle, and other tissues.
Tomas FM, Knowles SE, Owens PC, Chandler CS, Francis GL, Read LC, Ballard FJ · Biochemical Journal, 282(Pt 1):91-97 (1992) · PubMed
IGF treatment altered host energy partitioning and reduced carcass fat, but the IGF analogs failed to drive net muscle protein accretion in this catabolic tumor model. Insulin co-administration had additive effects on host weight.
Tomas FM, Chandler CS, Coyle P, Bourgeois CS, Burgoyne JL, Rofe AM · Biochemical Journal, 301(Pt 3):769-775 (1994) · PubMed
Infusion increased the fractional weights of adrenals, gut, kidneys, and spleen but did not stimulate overall body growth or weight gain. Plasma IGF-1, IGF-2, and IGFBPs were all suppressed, indicating negative feedback on the endogenous GH/IGF axis.
Conlon MA, Tomas FM, Owens PC, Wallace JC, Howarth GS, Ballard FJ · Journal of Endocrinology, 146(2):247-253 (1995) · PubMed
Infusion decreased average daily gain and food intake and lowered plasma growth hormone (about 23 percent mean reduction, 60 percent peak-area reduction), IGFBP-3, endogenous IGF-1, and insulin. Exogenous LR3 triggered negative feedback that reduced rather than enhanced growth.
Dunaiski V, Dunshea FR, Walton PE, Goddard C · Journal of Endocrinology, 155(3):559-565 (1997) · PubMed
start low, go slow
Subcutaneous / intramuscular infusion (preclinical only)
Subcutaneous / intramuscular infusion (preclinical only): All dosing figures come from animal studies. No validated human dosing exists, and any human use would be determined only by a licensed provider, not by the protocols below.
There are no published human clinical trials of IGF-1 LR3. Every dosing figure on this page comes from animal research, primarily rats, guinea pigs, and pigs, and does not translate to a safe or established human protocol.
Animal results were directionally inconsistent: growth restoration in deficiency models such as glucocorticoid-treated rats, but growth suppression in healthy pigs and disproportionate organ growth in guinea pigs. More signal did not reliably mean more lean growth.
The closest FDA-approved comparator is mecasermin (Increlex), unmodified recombinant human IGF-1, dosed at 40 to 120 mcg/kg twice daily for a narrow pediatric indication and titrated against meals to manage hypoglycemia. LR3's bypass of IGFBP regulation makes it pharmacologically distinct from that approved therapy.
IGF-1 LR3 was engineered as a cell culture reagent, not as a medicine. Its core design feature, escaping IGFBP regulation, removes much of the body's natural control over IGF-1 signaling. That is simultaneously its pharmacological selling point and its primary safety liability.
The cancer concern is mechanistic and epidemiological rather than proven from LR3 trials. Large prospective analyses, including a pooled study of 17 cohorts for breast cancer and UK Biobank analyses across 30 cancer sites, consistently associate higher circulating IGF-1 with increased risk of breast, prostate, and colorectal cancer.
Animal data undercut the assumption that more IGF-1 simply means more growth. In healthy pigs, exogenous LR3 suppressed the GH/IGF axis and reduced growth, and in guinea pigs it enlarged internal organs without adding body mass. Human safety beyond these signals is unknown because no human trials exist.
| Peptide | Primary Use | Administration | Cycle Length | Key Differentiator |
|---|---|---|---|---|
| IGF-1 LR3 | Muscle growth and recovery (claimed; preclinical only) | Injection | 4 to 7 days (animal studies only) | Sharply reduced IGFBP binding keeps the peptide free and active far longer than native IGF-1, extending bioavailability but also removing the natural brake on IGF-1 signaling |
| Tesamorelin | Visceral Fat & GHRH Signaling | Subcutaneous Injection | Ongoing daily | The only FDA-approved GHRH analog, with Phase III data in over 800 patients, documented liver-fat reduction, and physiologic pulsatile GH release |
| GHK-Cu | Anti-Aging & Skin Regeneration | Topical, Injection | 8-12 weeks | A naturally occurring copper-carrier peptide that rebuilds the skin matrix and, per gene-expression analysis, shifts expression of about a third of human genes toward repair |
| Sermorelin | GH restoration and healthy aging | Subcutaneous injection, daily at bedtime | 3-6 months | The GHRH analog with prior FDA approval and a multi-decade clinical record, preserving natural feedback rather than replacing growth hormone |
| BPC-157 | Recovery & Healing | Injection, Oral | 4–8 weeks | Unusually stable in gastric acid; broad multi-system healing signal, though evidence is largely preclinical |
This substance does not appear on any FDA bulk drug substances list. It is classified as research-use only and cannot be legally compounded for human use.
IGF-1 LR3 does not appear on any FDA 503A or 503B bulk drug substances list. It is not FDA-approved for any human indication. While the FDA Substance Registration System has a UNII (M9L22Y19H9), this indicates only that the substance has been identified, not that it is approved or eligible for compounding. Significant risks include severe hypoglycemia and potential tumor growth acceleration. Available only as a research chemical.
Last verified April 12, 2026. PepHookup tracks public FDA actions. This is not legal or medical advice.
Francis GL, Ross M, Ballard FJ, Milner SJ, Senn C, McNeil KA, Wallace JC, King R, Wells JR “Novel recombinant fusion protein analogues of insulin-like growth factor (IGF)-I indicate the relative importance of IGF-binding protein and receptor binding for enhanced biological potency.” Journal of Molecular Endocrinology, 8(3):213-223 (1992)
Voorhamme D, Yandell CA “LONG R3IGF-I as a more potent alternative to insulin in serum-free culture of HEK293 cells.” Molecular Biotechnology, 34(2):201-204 (2006)
Conlon MA, Tomas FM, Owens PC, Wallace JC, Howarth GS, Ballard FJ “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 pig.” Journal of Endocrinology, 146(2):247-253 (1995)
Endogenous Hormones and Breast Cancer Collaborative Group; Key TJ, Appleby PN, Reeves GK, Roddam AW “Insulin-like growth factor 1 (IGF1), IGF binding protein 3 (IGFBP3), and breast cancer risk: pooled individual data analysis of 17 prospective studies.” Lancet Oncology, 11(6):530-542 (2010)
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