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IGF-1 LR3 Explained: Why a Modified Analogue Behaves Differently From IGF-1

IGF-1 LR3 (Long R3 IGF-1) is an 83-amino-acid IGF-1 analogue with a 13-residue N-terminal extension and arginine in place of glutamate at position 3. Those changes cut its grip by IGF-binding proteins, so in cell cultures that secrete those proteins it is more potent than IGF-1. In rats it cleared faster than IGF-1, not slower , and in one food-restriction study it did not protect muscle protein. We found no published human trial of the analogue itself. It is research material only.

Potency in a dish is not a statement about people. The studies below are cell-culture and rat experiments from a 1990s–2000s research programme. Nothing here is medical or dosing guidance, and these papers report faster clearance than IGF-1, not a longer half-life.

At a glance

IGF-1 LR3 Full name Long Arginine-3 insulin-like growth factor 1 Length 83 amino acids (native IGF-1 has 70) Changes from IGF-1 A 13-residue N-terminal extension, and arginine in place of glutamate at position 3 Molecular weight About 9.1 kDa Developed by A CSIRO-affiliated group in Adelaide, described in 1992 as a research reagent Approval None. The native molecule, mecasermin, is authorised for severe primary IGF-1 deficiency in children Status in sport IGF-1 and its analogues are prohibited by WADA (class S2)

What the name means

“Long” is the extension: the first 11 residues of methionyl porcine growth hormone followed by Val-Asn, 13 residues in all. “R3” is arginine at position 3, where human IGF-1 has glutamate. Francis and colleagues built the family as fusion proteins in E. coli and described them as “very useful reagents in the investigation of IGF-I action” ( J Mol Endocrinol , 1992). The hydrophobic extension also appeared to help the protein fold correctly during production.

The same paper compared the analogues with native IGF-1 and with des(1-3)IGF-1, the truncated form covered in IGF-1 DES vs IGF-1 LR3.

Potency is about binding proteins, not the receptor

In the body, most IGF-1 is held by IGF-binding proteins (IGFBPs), which buffer how much is free to act. The 1992 results show that LR3’s advantage comes from escaping them:

In L6 rat myoblasts every analogue was more potent than authentic IGF-1 at stimulating protein and DNA synthesis and at inhibiting protein breakdown. Across cell lines that secrete IGFBPs the order of potency was Long [Arg3]-IGF-1 and des(1-3)IGF-1 first, then Long [Gly3]-IGF-1, then Long IGF-1, then IGF-1.

In chicken embryo fibroblasts, a cell line with no detectable secreted IGFBPs, Long [Arg3]-IGF-1 was less potent than IGF-1.

The authors concluded that the N-terminal analogues are more potent because of changes in their IGFBP interactions. That is a statement about where the molecule is held, not about a stronger signal at the receptor, and it means potency depends on the system: remove the binding proteins and the advantage disappears.

A 1993 rat study from the same group put IGF-1’s affinity for IGFBP-3, IGFBP-4 and total rat plasma binding proteins at about 1,000-fold that of LR3, and found LR3 5–10-fold more potent in cultured L6 myoblasts (Ballard et al., 1993).

The half-life figure that is hard to source

Supplier and encyclopedia pages quote LR3 half-lives of 20–30 hours, and some quote far more. The rat work points the other way. Ballard and colleagues compared the clearance of the two peptides directly and found that LR3 IGF-1 “was removed from the plasma much more rapidly than was IGF-I,” a difference they tied to its poor association with plasma binding proteins; in pregnant rats, where binding-protein levels are low, IGF-1 was cleared almost as fast ( Growth Regulation , 1993). Tomas and colleagues ( J Endocrinol , 1996) restated it: LR3 IGF-1 “has very low affinity for the IGF-binding proteins in the rat and hence is cleared from the circulation more quickly than is IGF-I.”

Faster clearance did not make it weaker. In the 14-day rat experiments LR3 was about 6-fold more potent than IGF-1 for growth measurements (Ballard et al.), while Tomas et al. found it generally 1.5- to 2-fold more potent on continuous infusion for body weight gain, organ weights and feed-use efficiency, and still more potent for several effects when given by injection, although potency by injection was much lower than by continuous infusion. We could not trace the longer quoted half-lives to a published pharmacokinetic study, so treat them as unsourced until one is cited.

What the rat studies did not show

LR3 is often discussed in connection with muscle. The same research group tested that directly:

Dexamethasone-treated rats (1996). Given by injection, LR3 reduced a marker of muscle protein breakdown to a threefold greater extent than IGF-1, yet was “barely equipotent with IGF-I for reversal of carcass muscle loss.”

Food-restricted rats (Tomas, 2001). Infused LR3 kept body weight about 3–8% higher and improved nitrogen retention than vehicle, but muscle protein was not conserved . In adult rats it worsened the effects of food restriction through more protein breakdown and less protein synthesis.

These are rodent experiments under specific stress conditions, so they do not settle what LR3 does in any other setting. They do show that strong activity in cells and a headline-friendly body-weight effect are not the same as a muscle effect.

Status, evidence and identity

We found no published human trial of LR3 IGF-1 itself. A 2026 endocrinology review places IGF-1 LR3 and PEG-MGF among the GH–IGF-1 axis compounds sold as research chemicals, stratifies the class from regulatory-grade trial data down to a complete absence of human studies, and flags biologically plausible but unproven mitogenic concerns (Dominikowski et al., 2026).

Identity matters for a protein of this size. A 2010 case report from the German Sport University Cologne identified the contents of an injection vial from the black market as LR3 IGF-1 with a His 6 -tag attached, a construct made for biochemical studies, and noted that its effects in humans have not been described (Kohler et al., 2010). Intact-mass confirmation is how a difference like that is found. See how to read a certificate of analysis.

Research takeaway

LR3 is an 83-residue IGF-1 analogue whose extra potency in cells comes from reduced IGF-binding-protein interaction, not stronger receptor activation.

In rats it cleared faster than IGF-1; the longer half-lives quoted online are unsourced.

Rat studies did not show that it protects muscle protein under food restriction.

There is no published human trial of the analogue, no approval, and a prohibited status in sport.

Primary sources

Journal of Molecular Endocrinology — Novel recombinant fusion protein analogues of IGF-I indicate the relative importance of IGF-binding protein and receptor binding for enhanced biological potency (Francis et al., 1992)

Growth Regulation — Effects of interactions between IGFBPs and IGFs on the plasma clearance and in vivo biological activities of IGFs and IGF analogs (Ballard et al., 1993)

Journal of Endocrinology — Superior potency of infused IGF-I analogues which bind poorly to IGF-binding proteins is maintained when administered by injection (Tomas et al., 1996)

Growth Hormone & IGF Research — IGF-I analogue LR3IGF-I ameliorates the loss of body weight but not of skeletal muscle during food restriction (Tomas, 2001)

Growth Hormone & IGF Research — Detection of His-tagged Long-R3-IGF-I in a black market product (Kohler et al., 2010)

Frontiers in Endocrinology — The emerging landscape of performance-enhancing peptides modulating the GH-IGF1 axis (Dominikowski et al., 2026)

Background

World Anti-Doping Agency — Prohibited List (class S2: peptide hormones, growth factors, related substances and mimetics)

IGF-1 LR3 as laboratory research material

New-U lists IGF-1 LR3 as laboratory research material. The studies above describe published cell-culture and animal work, not this material. Research use only – all claims made on this site are for testing and research use only.

Frequently asked questions

What does LR3 stand for in IGF-1 LR3?

L is for the long 13-residue N-terminal extension and R3 is for arginine at position 3, where native human IGF-1 has glutamate. The result is an 83-amino-acid analogue instead of the native 70.

Is IGF-1 LR3 the same as IGF-1?

No. The extension and the position-3 change greatly reduce binding to IGF-binding proteins, which makes LR3 more potent than IGF-1 in cell cultures that secrete those proteins. In a cell line that does not secrete them, LR3 was less potent than IGF-1.

Does IGF-1 LR3 last longer in the body?

Figures of 20 hours or more are quoted online, but rat studies that measured clearance found LR3 was removed from the plasma more rapidly than IGF-1. We could not trace the longer figures to a published pharmacokinetic study.

Is IGF-1 LR3 approved?

No. The native molecule, mecasermin, is authorised for severe primary IGF-1 deficiency in children, but LR3 has no approved use. IGF-1 and its analogues are prohibited in sport under WADA class S2.

Is there human evidence for IGF-1 LR3?

We found no published human trial of the analogue itself. The published work is cell-culture and rodent research, and one rat study found LR3 did not conserve muscle protein during food restriction.

External links are provided for research reference only; New-U is not affiliated with the cited organisations or authors. All figures are published study results shown for research context, not dosing guidance. New-U Research Compounds supplies research compounds strictly for laboratory research use only — not for human consumption .

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Research peptides at >99% HPLC-verified purity, third-party tested by Janoshik Analytical & Freedom Diagnostics, with Certificates of Analysis published per released batch. Supplied strictly for laboratory research use.

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