IGF-1 LR3: Research Overview

IGF-1 LR3, short for Long R3 IGF-1, is an engineered analog of human insulin-like growth factor-1 (IGF-1). It keeps the receptor-binding core of the native 70-residue hormone but adds a 13-amino-acid extension at the N-terminus and swaps the glutamic acid at position 3 for arginine. The result is an 83-residue protein that binds IGF-binding proteins (IGFBPs) very poorly while still activating the IGF-1 receptor.

That combination is the reason it exists. Because so much native IGF-1 is held by binding proteins, researchers wanted a ligand whose activity was not buffered by them. The sections below cover how the analog was designed, what each change does, how it behaves in animal models compared with IGF-1, and where the published evidence stops.

At a glance

• Class: Synthetic IGF-1 analog (recombinant fusion protein); IGF-1 receptor agonist with low IGFBP affinity
• Origin: Designed by an Australian group (Francis, Ballard and colleagues) and characterized in the early 1990s (Francis et al., 1992)
• Structure: 83 amino acids: a 13-residue N-terminal extension plus the IGF-1 sequence with Arg at position 3; C₄₀₀H₆₁₉N₁₁₁O₁₁₅S₉; 9111.6 g/mol (calculated from sequence); CAS 143045-27-6
• Main research areas: IGFBP versus receptor binding, IGF-1 receptor signaling in cell culture, serum-free media, animal models of IGF action, anti-doping detection
• Regulatory status: Not FDA approved for any use; WADA S2 (IGF-1 and its analogues)

Structure and origin

Native IGF-1 is a single-chain protein of 70 residues with three disulfide bonds, closely related in structure to proinsulin. In blood and tissue fluids almost all of it is bound to one of six IGFBPs, which extend its lifetime but also restrict how much can reach the receptor at any moment. By the late 1980s it was known that a naturally truncated form missing the first three residues, des(1-3)IGF-1, bound these proteins weakly and appeared more potent in culture. Glutamic acid at position 3 turned out to be an important contact for binding-protein recognition.

The LR3 design applies that insight in two ways:

• The R3 substitution: replacing Glu3 with Arg reduces IGFBP binding without removing the N-terminal residues entirely. On its own this gives the analog called R3-IGF-1.
• The extension peptide: a 13-residue leader, derived from the N-terminus of porcine growth hormone, is fused ahead of the IGF-1 sequence. It was introduced as part of a fusion-protein expression strategy, and in the finished analog it further lowers binding-protein affinity.

Francis and colleagues compared a series of these fusion analogs, measuring both IGFBP binding and IGF-1 receptor binding against biological potency in cell assays. Their results pointed to reduced binding-protein affinity, rather than changes at the receptor, as the main source of the enhanced potency (Francis et al., 1992). Long R3 IGF-1 was the analog carried forward, and it later became a standard defined supplement in serum-free mammalian cell culture.

Mechanism: what the research shows

IGF-1 LR3 acts through the IGF-1 receptor, a receptor tyrosine kinase. Ligand binding triggers autophosphorylation and downstream PI3K/Akt and MAPK/ERK signaling, the same pathways used by native IGF-1. Its receptor binding is broadly comparable to IGF-1’s and is not what sets it apart; the distinctive behavior comes from escaping the binding proteins.

That matters because cultured cells secrete their own IGFBPs, and serum or plasma contains large amounts of them. In those settings native IGF-1 is partly sequestered, while LR3 stays largely free. Lord and colleagues measured how IGF-1 and its variants associate with plasma binding proteins from rat, sheep, pig, human and chicken, and found that the variants associated far less than IGF-1, with the size of the difference depending on species (Lord et al., 1994). This species comparison is a useful reminder that IGFBP-related effects seen in one model do not transfer automatically to another.

Key areas of study

Separating binding-protein effects from receptor effects

The original design work remains the clearest example. By comparing analogs with graded IGFBP affinity, researchers could attribute potency differences to binding-protein interactions rather than receptor activation (Francis et al., 1992). LR3 is still used alongside native IGF-1 and des(1-3)IGF-1 for the same purpose.

Metabolic responses in animal models

In a fasting animal model, IGF-1 variants that bind poorly to IGFBPs produced a stronger and longer-lasting fall in blood glucose than IGF-1 itself (Tomas et al., 1997). The finding is usually read as evidence that free, unbound IGF-1 drives insulin-like actions and that binding proteins normally damp them.

Organ and endocrine effects of infusion

In guinea pigs, infusion of Long R3 IGF-1 was associated with increased weight of several organs, while plasma concentrations of endogenous IGF-1, IGF-II and IGFBPs fell (Conlon et al., 1995). The study is often cited for that second point: an exogenous analog can lower the animal’s own IGF and binding-protein levels, which complicates interpretation of any infusion experiment.

Diabetic rodent models

Tomas and colleagues compared IGF-1 with more potent variants in diabetic rats, examining growth-related measures alongside insulin-like metabolic markers. The variants restored growth in that model without reproducing every characteristic insulin effect (Tomas et al., 1993), a result used to argue that IGF-1 receptor and insulin receptor actions can be partly separated.

Anti-doping detection

Because IGF-1 analogs are prohibited in sport, anti-doping laboratories need methods that can tell them apart from endogenous IGF-1. Mongongu and colleagues described immunopurification combined with high-resolution mass spectrometry to detect LongR3-IGF-I, R3-IGF-I and des(1-3)-IGF-I (Mongongu et al., 2021).

What the evidence does not show

• The core literature is from the 1990s and is almost entirely in vitro or in animal models such as guinea pigs and rats. There are no controlled clinical studies of this analog.
• Results depend heavily on the IGFBP environment of the model. The species differences in plasma binding (Lord et al., 1994) mean animal findings cannot be assumed to apply elsewhere.
• Infusion studies show that LR3 suppresses endogenous IGF and IGFBP levels (Conlon et al., 1995), so effects in whole animals reflect a changed hormonal background, not just added receptor activity.
• Its pronounced glucose-lowering action in animal models (Tomas et al., 1997) is a known pharmacological property and a reason it is treated as a potent research tool, not a benign one.
• IGF-1 LR3 is not an approved drug and no regulator has evaluated it for any use.

Regulatory status

IGF-1 LR3 is not approved by the FDA for any use. IGF-1 and its analogues are covered by the World Anti-Doping Agency’s Prohibited List under S2 (peptide hormones, growth factors, related substances and mimetics). Rogue Compounds sells it as a research reagent only.

Handling and storage in the lab

Keep the lyophilized protein sealed and refrigerated at 2–8°C (36–46°F), protected from light.

Sourcing and documentation

IGF-1 LR3 is made in small batches for Rogue Compounds. Every vial carries a batch number and a QR code that opens the independent U.S. testing lab’s own report for that batch, and the same COA is posted on the product page and in the COA Library.

View IGF-1 LR3 on Rogue Compounds

References

1. Francis GL, et al. 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. J Mol Endocrinol. 1992;8(3):213-223. PMID 1378742
2. Tomas FM, et al. Insulin-like growth factor-I and more potent variants restore growth of diabetic rats without inducing all characteristic insulin effects. Biochem J. 1993;291(Pt 3):781-786. PMID 7683875
3. Lord AP, et al. Differences in the association of insulin-like growth factor-I (IGF-I) and IGF-I variants with rat, sheep, pig, human and chicken plasma-binding proteins. J Endocrinol. 1994;140(3):475-482. PMID 7514204
4. Conlon MA, et al. 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. J Endocrinol. 1995;146(2):247-253. PMID 7561636
5. Tomas FM, et al. IGF-I variants which bind poorly to IGF-binding proteins show more potent and prolonged hypoglycaemic action than IGF-I in fasting lambs. J Endocrinol. 1997;155(2):377-386. PMID 9415072
6. Mongongu C, et al. Detection of LongR3-IGF-I, Des(1-3)-IGF-I, and R3-IGF-I using immunopurification and high resolution mass spectrometry for antidoping purposes. Drug Test Anal. 2021;13(7):1256-1269. PMID 33587816

Research use only. Rogue Compounds products are sold strictly for laboratory research. They are not for human or veterinary use, and nothing in this article is medical advice. The studies cited here describe published research, not this product or any use of it.