IGF-1 LR3 and PEG-MGF are often listed side by side because both trace back to the same gene, the one that makes insulin-like growth factor-1 (IGF-1). But they come from different parts of it. IGF-1 LR3 is an engineered version of the mature IGF-1 hormone itself. PEG-MGF is a stabilized copy of a short tail piece, the “E-peptide,” that is cut off one particular form of IGF-1 before the hormone is released. That difference shapes everything else about them, from how well each is understood to how strong the evidence is. This guide compares them side by side.
All compounds discussed are sold for laboratory research use only. This page summarizes published research and is not guidance for human or veterinary use.
At a glance
| IGF-1 LR3 | PEG-MGF | |
|---|---|---|
| What it is | Engineered analog of mature IGF-1 | PEGylated copy of the MGF E-peptide from the IGF-1Ec splice variant |
| Part of the IGF-1 gene product | The 70-residue hormone shared by every IGF-1 variant | The C-terminal tail unique to one variant, normally cleaved off |
| Size | 83 amino acids, about 9,112 g/mol | A short peptide plus a polyethylene glycol (PEG) chain; total size depends on the PEG used and is listed on the batch COA |
| Key modifications | Arg in place of Glu at position 3, plus a 13-residue N-terminal extension | PEG attached to slow the rapid breakdown of the free peptide |
| Receptor | IGF-1 receptor (well established) | No dedicated receptor identified; some studies find its activity depends on the IGF-1 receptor |
| Why it was made | To activate the IGF-1 receptor without being held back by IGF-binding proteins | To test the idea that the E-peptide has its own signaling role in muscle |
| Strength of evidence | Well characterized; a standard supplement in serum-free cell culture | Contested; laboratories disagree on whether the synthetic peptide has activity of its own |
| Regulatory status | Not FDA approved; WADA S2 | Not FDA approved; WADA S2 |
One gene, two different pieces
The IGF-1 gene is alternatively spliced. Every version produces the same 70-residue mature IGF-1 hormone, but each carries a different tail, called the E-peptide, which is cut away during processing. In humans the three main forms are IGF-1Ea, IGF-1Eb and IGF-1Ec.
- IGF-1 LR3 is built on the shared part: the mature hormone, modified so it acts more freely at the IGF-1 receptor.
- PEG-MGF is built on the tail of the Ec form. Geoffrey Goldspink’s group reported that this splice form rose sharply in muscle after stretch or damage and named its product mechano growth factor, or MGF (Goldspink, 2005).
So while both are “IGF-1 peptides,” they are not two versions of the same molecule. One is the hormone; the other is a fragment that is normally trimmed away from it.
IGF-1 LR3: built to escape the binding proteins
In blood and tissue fluids, almost all native IGF-1 is held by one of six IGF-binding proteins (IGFBPs), which limits how much reaches the receptor at any moment. An Australian group designed LR3 to get around that. Swapping glutamic acid at position 3 for arginine weakens binding-protein recognition, and a 13-residue extension at the N-terminus lowers it further. Comparing a series of analogs, Francis and colleagues found that reduced binding-protein affinity, not stronger receptor binding, explained the analogs’ higher potency in cell assays (Francis et al., 1992).
Across species, LR3 associated far less with plasma binding proteins than native IGF-1 (Lord et al., 1994). In diabetic rats it restored growth more potently than IGF-1 (Tomas et al., 1993), and in fasting lambs it had a stronger and longer-lasting effect on blood glucose (Tomas et al., 1997). Its main research role today is as a defined IGF-1 receptor agonist, including in serum-free cell culture. See the IGF-1 LR3 research overview.
PEG-MGF: a stabilized copy of a contested peptide
The central MGF hypothesis is that the E-peptide has activity of its own, separate from mature IGF-1. In cultured muscle precursor cells, Yang and Goldspink reported that a synthetic MGF peptide increased proliferation but not differentiation, and that its effect was not blocked by an antibody against the IGF-1 receptor (Yang and Goldspink, 2002). Expression studies in people have measured IGF-1Ec transcripts rising in muscle after damaging exercise (Philippou et al., 2009), and a 2010 review summarized reports of MGF in heart, bone and nerve tissue (Matheny et al., 2010).
Other laboratories have not confirmed the core claim. Brisson and Barton found that E-peptide activity depended on the IGF-1 receptor, suggesting E-peptides adjust IGF-1 signaling rather than act through a pathway of their own (Brisson and Barton, 2012). A 2014 editorial titled “The fall of mechanogrowth factor?” discussed findings that synthetic MGF had no apparent effect on muscle cells and reviewed why results in the field have been so inconsistent (Rotwein, 2014).
The free E-domain peptide breaks down quickly, which is why PEGylated versions exist. Peer-reviewed studies on PEG-MGF specifically are very limited, so most of what is said about it is extrapolated from work on unmodified or differently modified MGF peptides. See the PEG-MGF research overview.
Where their research overlaps
- Same gene family: both are studied in the context of IGF-1 biology, especially in muscle cell models.
- The IGF-1 receptor question: LR3 acts through the IGF-1 receptor by design, and some of the strongest MGF studies conclude its activity also runs through that receptor.
- Same anti-doping category: IGF-1 analogs and mechano growth factors are both listed under S2 on the WADA Prohibited List, and LR3 has dedicated detection methods (Mongongu et al., 2021).
How researchers choose between them
- A defined IGF-1 receptor agonist: IGF-1 LR3 is the well-characterized tool, especially where binding proteins in serum or culture media would otherwise blunt native IGF-1.
- Testing the MGF hypothesis: PEG-MGF is used by labs studying whether the E-peptide has independent activity. Given the conflicting literature, careful controls, including an IGF-1 receptor blockade arm, are central to that kind of design.
- Comparing IGF-1 analogs: for how LR3 compares with the naturally truncated des(1-3) form, see IGF-1 LR3 vs IGF-1 DES.
What the research does not show
IGF-1 LR3’s data comes from cell culture and animal models; it has never been developed as a drug, and there are no controlled human studies. For PEG-MGF the gaps are larger: it is not established that a free MGF peptide circulates or acts in the body in the form synthetic versions copy, and the peptide sequence and PEG size vary between sources, which makes studies hard to compare. There are no studies comparing the two directly. Neither compound’s research supports any use outside a laboratory.
Verify what you’re working with
Because PEG-MGF preparations vary in sequence and PEG size, the batch COA matters more than usual: it should state what was actually made. IGF-1 LR3, at about 9,112 g/mol, should match its expected mass on identity testing. Every Rogue batch is tested by an independent, ISO 17025 accredited U.S. lab, and the QR code on each vial opens that batch’s report on the lab’s own website. See how to read a peptide COA or browse the COA Library.
View IGF-1 LR3 on Rogue Compounds
View PEG-MGF on Rogue Compounds
References
- 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
- 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
- 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
- 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
- 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
- Goldspink G. Mechanical signals, IGF-I gene splicing, and muscle adaptation. Physiology (Bethesda). 2005;20:232-238. PMID 16024511
- Yang SY, Goldspink G. Different roles of the IGF-I Ec peptide (MGF) and mature IGF-I in myoblast proliferation and differentiation. FEBS Lett. 2002;522(1-3):156-160. PMID 12095637
- Philippou A, et al. Expression of IGF-1 isoforms after exercise-induced muscle damage in humans: characterization of the MGF E peptide actions in vitro. In Vivo. 2009;23(4):567-575. PMID 19567392
- Matheny RW Jr, Nindl BC, Adamo ML. Minireview: mechano-growth factor: a putative product of IGF-I gene expression involved in tissue repair and regeneration. Endocrinology. 2010;151(3):865-875. PMID 20130113
- Brisson BK, Barton ER. Insulin-like growth factor-I E-peptide activity is dependent on the IGF-I receptor. PLoS One. 2012;7(9):e45588. PMID 23029120
- Rotwein P. Editorial: the fall of mechanogrowth factor? Mol Endocrinol. 2014;28(2):155-156. PMID 24460193