IGF-1 LR3

01 · What it is

The evidence for IGF-1 LR3 is substantial, thirty years old, and almost entirely about cattle, pigs and cell-culture dishes — and where it does address growth in a living animal, it sometimes points the wrong way.

IGF-1 LR3, written in the literature as Long R3 IGF-I or LR3IGF-I, is an 83-residue analogue of human insulin-like growth factor 1. Two modifications separate it from the parent molecule: glutamate at position 3 is replaced by arginine, and a thirteen-residue extension is fused to the N-terminus. Both changes serve one purpose. They collapse the molecule’s affinity for the six insulin-like growth factor binding proteins while leaving its affinity for the type 1 IGF receptor essentially intact.

That purpose is not incidental — it is the entire design. Endogenous IGF-1 circulates almost entirely bound, most of it in a 140 kDa ternary complex with binding protein 3 and an acid-labile subunit, which controls how much free ligand reaches a receptor at any moment. LR3 was built to bypass that control. A group working on fetal sheep described the consequence without euphemism: LR3’s low binding-protein affinity reduces the physiologic regulation of IGF-1 bioavailability, and that group constructed a species-matched native IGF-1 specifically to avoid the problem ²².

The molecule was made as a laboratory tool, and the indexed literature treats it as one. In a substantial fraction of the papers that mention LR3, it appears not as the object of study but as the instrument — the reagent used to demonstrate that some effect is binding-protein-independent. When a group wanted to show that NKX3.1’s influence on IGF-I signalling ran through binding protein 3, they showed that the effect disappeared with LR3, described in that paper as an IGF-I variant peptide that does not bind IGFBP-3 ¹⁹. When another group needed a radioligand to characterise receptor binding in fibroblasts from patients with chromosome 15 deletions, they iodinated LR3 ³.

The second body of work is livestock and rodent infusion science from the 1990s, and it is not uniformly favourable. It is discussed in Section 4.

No human study of IGF-1 LR3 appears in PubMed. Not a pharmacokinetic study, not a tolerability study, not a trial. A 2026 narrative review of compounds modulating the growth hormone–IGF-1 axis names IGF-1 LR3 among the unregulated agents encountered in clinical practice and stratifies the field into tiers running from randomised trial data down to a complete absence of human studies; LR3 sits in the lowest tier ²⁴.

The parent molecule, by contrast, is an approved drug. Recombinant human IGF-1 holds marketing authorisation for severe primary IGF-1 deficiency ¹⁸, which makes the comparison in Section 7 unusually clean: the regulated form of this biology exists, has been trialled, and has a documented efficacy ceiling ²⁰.

02 · Evidence at a glance

Evidence grade

Preclinical — no human study of any kind is indexed

Structure

83 residues; human IGF-1 with Glu3→Arg and a 13-residue N-terminal extension

Design intent

Reduced affinity for IGF binding proteins; IGF-1 receptor binding retained

Primary target

Type 1 IGF receptor

Indexed records for the compound

67

Human trials

None

Human pharmacokinetic data

None

Species studied

Rat, mouse, guinea pig, pig, calf, cow, beef heifer, sheep

Effect on endogenous axis

Suppressed plasma IGF-I, IGF-II and binding proteins in guinea pig ¹ and pig ⁹

A negative growth result

Reduced growth and plasma growth hormone in finisher pigs ⁹

Mitogenic signals in cell culture

Telomerase activation in prostate cancer cells ¹⁷; breast cancer cell proliferation ⁵

Binding-protein escape

Not absolute — IGFBP-3 still inhibited LR3-induced receptor phosphorylation ¹⁵

Product identity finding

His-tagged LR3, a form made for biochemical studies, identified in a black-market vial ²¹

Approval status

None, any jurisdiction

03 · Mechanism of action

The receptor

LR3 acts at the type 1 IGF receptor, a transmembrane tyrosine kinase of the insulin receptor family. Ligand binding to the extracellular α-subunits triggers autophosphorylation of the intracellular β-subunit kinase domains, recruitment of insulin receptor substrate proteins, and divergence into two principal cascades — PI3K/Akt and Ras/MAPK. In prostate cancer cells the telomerase response to both IGF-I and Long-R3-IGF-I was abolished by the PI3K inhibitor wortmannin but survived blockade of the MAPK arm, which places that particular readout on the Akt limb ¹⁷.

The receptor is not modified in LR3. What is modified is everything upstream of it.

The binding proteins, and why the design targets them

Six high-affinity binding proteins govern IGF-1. They extend its half-life, restrict its access to receptors, and in several cases exert receptor-independent effects of their own. The Glu3→Arg substitution and the N-terminal extension together reduce LR3’s affinity for these proteins by orders of magnitude relative to native IGF-1 — which is why LR3 was adopted so widely as a reagent ^3,19, and why it is more potent than IGF-1 in any system where binding proteins are present.

Two findings complicate the simple story. First, the escape is not complete: IGFBP-3 inhibited Long(R3)IGF-I-induced receptor phosphorylation in a dose-dependent manner over a concentration range similar to that for native IGF-I ¹⁵. Second, escaping regulation is not the same as being unregulated in a useful direction — see the next subsection.

Feedback onto the endogenous axis

A molecule that signals at the IGF-1 receptor without binding-protein restraint also engages the negative feedback that IGF-1 exerts on pituitary growth hormone secretion. Two infusion studies found exactly that. In guinea pigs, seven days of LR3IGF-I at 120 µg/day stimulated organ growth but lowered plasma IGF-I, IGF-II and binding protein concentrations ¹. In finisher pigs, a four-day infusion reduced growth, plasma growth hormone, binding protein 3 and endogenous IGF-I ⁹. In neonatal calves, the somatotropic axis was modulated by Long-R3-IGF-I administration, with plasma concentrations rising after subcutaneous but not oral dosing ^8,10.

The direction of these results matters more than their magnitude. The compound suppresses the system it is intended to amplify, and in the one study whose title reports a growth endpoint, growth went down ⁹.

Clearance

LR3 is cleared differently from IGF-1 because it is not carried in the ternary complex. Clearance, degradation and organ distribution were compared for IGF-I, des-(1-3)IGF-I and LR3IGF-I in a rat model of chronic renal failure ⁶, and pepsin-resistant point mutants were designed after the kinetics of pepsin cleavage in LR3 were mapped ⁴ — work that exists because the unmodified analogue is susceptible.

04 · Key research findings

Gastrointestinal growth. The most reproducible effect in the animal literature is on gut. Three days of IGF-I peptide administration stimulated proliferation of the small intestinal epithelium in rats ². Dietary long arginine-3 IGF-I promoted intestinal growth in artificially reared newborn rat pups ¹². In eight-day-old calves, small intestinal morphology responded to Long-R3-IGF-I given subcutaneously or orally ¹¹. But the functional consequence lagged the structural one: effects on intestinal absorption of 3-O-methyl-D-glucose were less pronounced than the mucosal growth responses in the same animals ¹⁶.

The gut result is the family’s most consistent, and it is also the clearest instance of tissue mass changing more than tissue function.

Whole-animal protein metabolism in livestock. Long(R3)-IGF-1 was tested on protein metabolism in beef heifers ¹³ and, in combination with porcine growth hormone, on growth characteristics in finisher pigs, where it reduced growth ⁹. Intramammary infusion of insulin or Long R3 IGF-I did not increase milk protein yield in dairy cows ¹⁴. Endocrine and metabolic responses were characterised in neonatal calves under varying colostrum intake ^8,10.

Four species, three decades, and no consistent anabolic outcome at the whole-animal level.

Neurological work. The single most recent animal study gave intranasal LR3-IGF-1 to male 5XFAD mice, a model of Alzheimer’s pathology. Amyloid plaque was remodelled in cerebral cortex. Cognitive function was not preserved ²³. This is the biomarker-versus-outcome dissociation that recurs throughout this field, reported cleanly in the paper’s own title.

A structural target moved and the functional endpoint did not.

Proliferative signals in transformed cells. Long-R3-IGF-I produced a three-fold enhancement of telomerase activity in prostate cancer cells, P < 0.01, matching native IGF-I and dependent on Akt signalling ¹⁷. It stimulated MCF-7 breast cancer cell growth similarly to insulin, in an experiment designed to avoid binding-protein interference ⁵. It stimulated DNA synthesis in rat ovarian theca-interstitial cells more than IGF-I did ⁷.

These are reagent experiments, not toxicology — but they are the only data on what unrestrained IGF-1 receptor signalling does to a dividing cell, and they point one way.

Product identity. Analytical chemists identified His-tagged Long-R3-IGF-I in a black-market injection vial, characterising it by immunoaffinity purification, nano-UPLC and high-resolution mass spectrometry ²¹. A histidine tag is a purification handle attached to proteins produced for biochemical studies. Its presence means the contents were research-grade protein, not material made to any pharmaceutical specification.

This is a reproducibility finding before it is anything else: two vials labelled the same way may not contain the same molecule.

05 · Evidence overview

06 · Safety profile

Animal data. The infusion studies report physiological rather than toxicological endpoints, and the recurring finding is endocrine suppression: reduced plasma growth hormone, reduced binding protein 3, reduced endogenous IGF-I ^1,9. Hypoglycaemia is the mechanistically expected risk for any molecule with unrestrained IGF-1 receptor agonism and structural similarity to insulin, and it is the reason PEGylation of IGF-1 was pursued in a different programme [see the PEG-MGF guide]. None of the LR3 animal studies was designed to detect it.

Human data. None exists. There is no reported human exposure to IGF-1 LR3 in the peer-reviewed literature at any dose, by any route, for any duration.

What is genuinely unknown. Everything that would normally be established before a first human dose. There is no single-dose tolerability study, no dose-ranging, no repeat-dose toxicology in any species, no genotoxicity or carcinogenicity programme, no reproductive toxicology, and no immunogenicity assessment — the last of which matters particularly because the N-terminal extension is a non-human sequence and the black-market material carries a histidine tag ²¹, both of which are plausible epitopes. Whether sustained binding-protein-independent receptor agonism promotes growth of existing neoplastic tissue is untested in a living animal, while the cell-culture signals that raise the question are unambiguous ^5,7,17. The consequences of suppressing the endogenous somatotropic axis over months, observed over days in pigs and guinea pigs ^1,9, have never been examined.

07 · US regulatory status

Current as of 6 September 2026. IGF-1 LR3 is not approved as a drug in the United States or any other jurisdiction and is not a controlled substance.

The parent molecule is approved. Recombinant human IGF-1 is marketed for growth failure in children with severe primary IGF-1 deficiency, and a second product complexing recombinant IGF-1 with recombinant binding protein 3 was developed and subsequently ceased to be available in the United States and Europe for short stature ¹⁸. The independent drug bulletin Prescrire assessed the approved single-agent product under the heading “Insufficient improvement in statural growth” ²⁰. Both facts belong in a guide to an unapproved analogue: the regulated version of this biology exists, was trialled, and produced a modest result.

Under the World Anti-Doping Code, insulin-like growth factors and their analogues fall within the prohibited peptide hormones and growth factors class, and detection methodology for LR3 specifically has been published ²¹. Competitors should consult the current Prohibited List directly rather than rely on secondary summaries, including this one.

08 · Limitations of the evidence

1. There is no human evidence of any kind. Not one indexed study reports administration of IGF-1 LR3 to a person. Every statement in this guide about what the compound does is a statement about rodents, livestock, or cells in a dish.
2. The animal literature is agricultural, and its endpoints are agricultural. Carcass composition, milk protein yield, gut mucosal mass and feed conversion ^{11,12,13,14,16} were chosen to answer questions about food production. They were not chosen to predict anything about a human, and the doses, durations and infusion routes reflect that.
3. Animal-to-human translation faces a specific obstacle here, not a generic one. The compound’s defining property is escape from binding-protein regulation, and binding-protein biology differs between species in ligand affinity, protease activity and acid-labile subunit handling. A group working in sheep judged the mismatch severe enough to build a species-matched native IGF-1 rather than use LR3 ²².
4. A whole-animal growth endpoint went the wrong way. In pigs, LR3 reduced growth, growth hormone, binding protein 3 and endogenous IGF-I ⁹. This result is thirty years old, has not been refuted, and is rarely mentioned alongside the compound.
5. Structural and functional endpoints dissociate wherever both were measured. Gut mass rose more than glucose absorption ¹⁶; cortical amyloid was remodelled while cognition was not preserved ²³. Both are single studies, and both point the same way.
6. The proliferation signals have never been followed up in vivo. Telomerase activation in prostate cancer cells ¹⁷ and growth stimulation in MCF-7 cells ⁵ were incidental to the aims of those papers. No animal study has asked whether sustained LR3 exposure affects tumour initiation or progression. Absence of evidence here is absence of investigation.
7. Research is concentrated in one era and a small number of groups. Sixty-seven records, the great majority published between 1995 and 2003, with the Adelaide and CSIRO groups and a small number of animal-science laboratories accounting for a large share. Publication bias in a corpus this small and this old cannot be estimated, and the file-drawer contents of 1990s livestock trials are unrecoverable.
8. Material identity is not established for anything sold under this name. The one analytical examination of a product found a His-tagged protein made for biochemical studies ²¹. Nine of the references in this guide describe experiments with characterised research reagent. None of them describes what is in a vial.
9. This guide asserts absence from the indexed record, which is not the same as absence. No human study of IGF-1 LR3 appears in PubMed. Unindexed work, conference abstracts and unpublished industry data cannot be ruled out and cannot be cited.

09 · References

1. 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. J Endocrinol. 1995 Aug;146(2):247–253.

   PMID 7561636 ↗

2. Steeb CB, Trahair JF, Read LC. Administration of insulin-like growth factor-I (IGF-I) peptides for three days stimulates proliferation of the small intestinal epithelium in rats. Gut. 1995 Nov;37(5):630–638.

   PMID 8549937 ↗

3. Siebler T, Lopaczynski W, Terry CL, et al. Insulin-like growth factor I receptor expression and function in fibroblasts from two patients with deletion of the distal long arm of chromosome 15. J Clin Endocrinol Metab. 1995 Dec;80(12):3447–3457.

   PMID 8530582 ↗

4. Bryant KJ, Read LC, Forsberg G, Wallace JC. Design and characterisation of long-R3-insulin-like growth factor-I muteins which show resistance to pepsin digestion. Growth Factors. 1996;13(3–4):261–272.

   PMID 8919033 ↗

5. Vink-van Wijngaarden T, Pols HA, Buurman CJ, Birkenhäger JC, van Leeuwen JP. Inhibition of insulin- and insulin-like growth factor-I-stimulated growth of human breast cancer cells by 1,25-dihydroxyvitamin D3 and the vitamin D3 analogue EB1089. Eur J Cancer. 1996 May;32A(5):842–848.

   PMID 9081364 ↗

6. Gillespie CM, Hazel SJ, Walton PE, Martin AA. Effects of chronic renal failure on plasma clearance of insulin-like growth factor I, des-(1-3)IGF-I, and LR3IGF-I. Am J Physiol. 1996 Oct;271(4 Pt 1):E649–E657.

   PMID 8897852 ↗

7. Duleba AJ, Spaczynski RZ, Olive DL, Behrman HR. Effects of insulin and insulin-like growth factors on proliferation of rat ovarian theca-interstitial cells. Biol Reprod. 1997 Apr;56(4):891–897.

   PMID 9096870 ↗

8. Hammon H, Blum JW. The somatotropic axis in neonatal calves can be modulated by nutrition, growth hormone, and Long-R3-IGF-I. Am J Physiol. 1997 Jul;273(1 Pt 1):E130–E138.

   PMID 9252489 ↗

9. Dunaiski V, Dunshea FR, Walton PE, Goddard C. Long [R3] insulin-like growth factor-I reduces growth, plasma growth hormone, IGF binding protein-3 and endogenous IGF-I concentrations in pigs. J Endocrinol. 1997 Dec;155(3):559–565.

   PMID 9488001 ↗

10. Hammon H, Blum JW. Endocrine and metabolic changes in neonatal calves in response to growth hormone and long-R3-insulin-like growth factor-I administration. Biol Neonate. 1998;73(2):121–128.

    PMID 9483305 ↗

11. Bühler C, Hammon H, Rossi GL, Blum JW. Small intestinal morphology in eight-day-old calves fed colostrum for different durations or only milk replacer and treated with long-R3-insulin-like growth factor I and growth hormone. J Anim Sci. 1998 Mar;76(3):758–765.

    PMID 9535335 ↗

12. Staley MD, Gibson CA, Herbein JF, Grosvenor CE, Baumrucker CR. Rat milk and dietary long arginine3 insulin-like growth factor I promote intestinal growth of newborn rat pups. Pediatr Res. 1998 Oct;44(4):512–518.

    PMID 9773839 ↗

13. Hill RA, Hunter RA, Lindsay DB, Owens PC. Action of long(R3)-insulin-like growth factor-1 on protein metabolism in beef heifers. Domest Anim Endocrinol. 1999 May;16(4):219–229.

    PMID 10370861 ↗

14. Mackle TR, Dwyer DA, Bauman DE. Intramammary infusion of insulin or long R3 insulin-like growth factor-I did not increase milk protein yield in dairy cows. J Dairy Sci. 2000 Aug;83(8):1740–1749.

    PMID 10984150 ↗

15. Devi GR, Graham DL, Oh Y, Rosenfeld RG. Effect of IGFBP-3 on IGF- and IGF-analogue-induced insulin-like growth factor-I receptor (IGFIR) signalling. Growth Horm IGF Res. 2001 Aug;11(4):231–239.

    PMID 11735239 ↗

16. Garnaut SM, Howarth GS, Read LC. Effects of insulin-like growth factor-I and its analogue, long-R3-IGF-I, on intestinal absorption of 3-O-methyl-D-glucose are less pronounced than gut mucosal growth responses. Growth Factors. 2002 Mar;20(1):17–25.

    PMID 11999215 ↗

17. Wetterau LA, Francis MJ, Ma L, Cohen P. Insulin-like growth factor I stimulates telomerase activity in prostate cancer cells. J Clin Endocrinol Metab. 2003 Jul;88(7):3354–3359.

    PMID 12843187 ↗

18. Kemp SF. Insulin-like growth factor-I deficiency in children with growth hormone insensitivity: current and future treatment options. BioDrugs. 2009;23(3):155–163.

    PMID 19627167 ↗

19. Muhlbradt E, Asatiani E, Ortner E, Wang A, Gelmann EP. NKX3.1 activates expression of insulin-like growth factor binding protein-3 to mediate insulin-like growth factor-I signaling and cell proliferation. Cancer Res. 2009 Mar 15;69(6):2615–2622.

    PMID 19258508 ↗

20. Mecasermin: new drug. Insufficient improvement in statural growth. Prescrire Int. 2009 Jun;18(101):111–113.

    PMID 19637420 ↗

21. Kohler M, Thomas A, Walpurgis K, Terlouw K, Schänzer W, Thevis M. Detection of His-tagged Long-R3-IGF-I in a black market product. Growth Horm IGF Res. 2010 Oct;20(5):386–390.

    PMID 20675162 ↗

22. Stremming J, White A, Donthi A, et al. Sheep recombinant IGF-1 promotes organ-specific growth in fetal sheep. Front Physiol. 2022 Aug 25;13:954948.

    PMID 36091374 ↗

23. Engel MG, Narayan S, Cui MH, et al. Intranasal long R3 insulin-like growth factor-1 treatment promotes amyloid plaque remodeling in cerebral cortex but fails to preserve cognitive function in male 5XFAD mice. J Alzheimers Dis. 2025 Jan;103(1):113–126.

    PMID 39610283 ↗

24. Dominikowski A, Rękoś K, Olejarz M, Szczepanek-Parulska E, Domin R, Ruchała M. The emerging landscape of performance-enhancing peptides modulating GH-IGF1 axis: bridging the gap between clinical evidence and patient self-administration. Front Endocrinol (Lausanne). 2026.

    PMID 42395176 ↗