IGF-1 LR3 — Long R3 insulin-like growth factor-1 — is an engineered analogue of human IGF-1 created for a laboratory purpose, not a clinical one. It carries two modifications: arginine replaces glutamate at position 3, and a 13-amino-acid extension is added to the N-terminus, giving an 83-residue protein against native IGF-1's 70. Both changes exist for the same reason: to make the molecule escape regulation by IGF binding proteins.
In the body, more than 95% of circulating IGF-1 is bound to one of six IGF binding proteins, principally IGFBP-3 in a ternary complex with the acid-labile subunit. That binding is not a nuisance to be engineered around — it is the control system. It sets the half-life, restricts which tissues see free IGF-1, and buffers the receptor against sudden changes. IGF-1 LR3 has roughly a hundredfold lower affinity for those binding proteins, which is precisely what makes it useful for keeping cells proliferating in a culture dish and precisely what makes extrapolating from it to a human body a leap rather than a step.
This is the central fact about IGF-1 LR3: it is a cell culture reagent. It is sold by scientific supply companies for use in bioreactors and serum-free media. It has never been developed as a drug, has never been through human dose-finding, and has no published clinical trial of any phase. There is no human safety database for it and no established dose, route, frequency or duration.
Recombinant human IGF-1 is a different matter. Mecasermin is an FDA-approved drug for severe primary IGF-1 deficiency, with a label, a boxed set of warnings and post-marketing experience. It is a different molecule — unmodified IGF-1, still regulated by binding proteins — and its dosing does not transfer to LR3. Where mecasermin data is cited below, it is cited to describe what IGF-1 receptor agonism does in people, not to justify anything about LR3.
| Weeks | Dosage | Syringe units (U-100) |
|---|---|---|
| Weeks 1–2 | 0.02 mg | 6 units0.06 mL |
| Weeks 3–4 | 0.04 mg | 12 units0.12 mL |
| Weeks 5–8 | 0.05 mg | 15 units0.15 mL |
FrequencyOnce daily, subcutaneous.
For review — no human dosing exists. IGF-1 LR3 is a laboratory reagent that has never been through human dose-finding; the rows in the chart follow a circulated research protocol rather than any study. The concentration strip above the chart is calculated from the vial and water volume and is correct.
Unverified — no human study establishes any frequency for IGF-1 LR3.
IGF-1 LR3 is an IGF-1 analogue engineered so that binding proteins cannot regulate it, which is what makes it useful in cell culture and what makes a human schedule an extrapolation rather than a finding — there is no trial, no established amount and no frequency, so the chart is flagged for review. Recombinant IGF-1 is an approved medicine with its own label, but it is a different molecule and its dosing does not transfer here. Athena lists one strength: a 1 mg vial made up with 3 mL comes to about 0.33 mg/mL, so the amounts above are 6, 12 and 15 units and all fit comfortably in a 50-unit syringe. The figures are small enough that a measuring error of one or two units is a large proportional change, which is worth knowing about a chart that has no evidence behind it in the first place.
What this evidence establishes. No human dosing exists for IGF-1 LR3. It is a laboratory reagent — an IGF-1 analogue engineered to escape binding-protein regulation so that it stays active in cell culture — and it has never been through human dose-finding. Recombinant IGF-1 (mecasermin) is a different, approved molecule with its own label; its dosing does not transfer to LR3, and the modification that makes LR3 useful in a dish is the same one that removes the regulatory brake in a body. The table below is reconstitution arithmetic only.
For educational and laboratory research purposes only. It does not provide medical advice, dosing recommendations, or instructions for human or veterinary use. Syringe units assume a U-100 syringe, on which 1 mL is 100 units and a 50-unit syringe holds 0.5 mL.
IGF-1 LR3 signals through the same receptor as native IGF-1. What the engineering changes is not the signal but its regulation — how much reaches the receptor, for how long, and in which tissues.
Binds and activates the IGF-1 receptor, a receptor tyrosine kinase, driving PI3K/Akt/mTOR and MAPK signalling — the canonical anabolic and pro-survival pathways.
The Arg3 substitution and N-terminal extension reduce affinity for IGFBPs by roughly two orders of magnitude, leaving far more of the molecule free and receptor-available.
Removing binding-protein sequestration changes clearance and tissue distribution. Native IGF-1's long circulating half-life comes largely from the binding-protein complex it travels in.
Francis and colleagues characterised these analogues in 1992 specifically to separate the contributions of binding-protein affinity and receptor affinity to biological potency. The application was and remains cell culture.
IGF-1 has measurable affinity for the insulin receptor. Hypoglycaemia is the most common significant adverse effect of the approved IGF-1 drug and is the predictable consequence of this cross-reactivity.
The modification that makes LR3 valuable in a dish removes the physiological restraint that keeps IGF-1 signalling proportionate in a body. That trade-off is the whole compound.
IGF binding proteins do three things at once: they extend the half-life of IGF-1 in circulation, they prevent unrestricted access to the IGF-1 receptor across tissues, and they allow local release where specific proteases act. A molecule engineered to evade all three does not simply become "more potent" — it becomes differently regulated in ways that no human study has characterised. Proliferative signalling that is appropriate when a fibroblast line needs to keep dividing in a flask is not automatically appropriate in a person who has tissues that are not supposed to be dividing.
There is no human evidence to have limitations. That is the finding, and it is unusual even among the less-studied compounds on this site: for most of them, at least a Phase 1 pharmacology study exists. For IGF-1 LR3 there is nothing — no pharmacokinetics, no dose-response, no safety database, no adverse event reporting system, no efficacy signal on any endpoint in any human population.
The dosing protocols that circulate are not derived from evidence. They are typically scaled down from mecasermin's label, from animal work, or from other informal protocols — none of which is a valid basis for a molecule with a different binding-protein profile and therefore a different exposure curve. The doses involved are also small in absolute terms, which means measurement error at the syringe is proportionally large.
Finally, purity and identity cannot be assumed. Research-grade material is not manufactured to injectable pharmaceutical standards, and a peptide sold as IGF-1 LR3 has no independent verification of sequence, purity, endotoxin content or sterility unless a third-party certificate of analysis accompanies it — and even then, certificates are not audited.
IGF-1 LR3 has no approved label and no human safety data of its own. The entries below are derived from the pharmacology of IGF-1 receptor agonism and from the labelled warnings for the approved recombinant IGF-1 product, which shares the receptor but not the binding-protein behaviour.
| Condition / Factor | Risk Level | Rationale |
|---|---|---|
| Active or suspected malignancy | CONTRAINDICATED | IGF-1 receptor signalling is mitogenic and anti-apoptotic. Active or suspected neoplasia is a contraindication on the approved IGF-1 product's label, and higher circulating IGF-1 is associated with increased risk of several common cancers in meta-analysis. |
| Hypoglycaemia risk | HIGH | Hypoglycaemia is the most common significant adverse effect of approved recombinant IGF-1, driven by insulin receptor cross-reactivity. It can be severe. The approved product's label requires food intake shortly before or after each dose. |
| Closed epiphyses in adolescents / active growth | CAUTION | The approved product is indicated in growth failure and is not to be used after epiphyseal closure for its labelled purpose. Slipped capital femoral epiphysis and scoliosis progression are recognised concerns with growth-promoting therapy in children. |
| Tonsillar and lymphoid hypertrophy | MODERATE | Tonsillar hypertrophy with associated sleep-disordered breathing is documented in the approved IGF-1 product's labelling. |
| Diabetes and insulin or sulfonylurea therapy | HIGH CAUTION | Additive hypoglycaemic effect with insulin secretagogues and exogenous insulin. |
| Pregnancy and lactation | NOT ESTABLISHED | No reproductive safety data exist for IGF-1 LR3, and growth-factor signalling in development is not a domain in which absence of data is reassuring. |
| Any human administration | NO EVIDENCE BASE | No human trial of any phase has been published for IGF-1 LR3. There is no established dose, route, frequency, duration or safety profile. |
| Competitive athletes | PROHIBITED | IGF-1 and its analogues are prohibited at all times under WADA Prohibited List section S2. |
FDA: IGF-1 LR3 is not approved for any indication and is not a drug in any jurisdiction. It is sold and catalogued as a laboratory reagent for research use only.
The approved alternative in the class: mecasermin (recombinant human IGF-1) is FDA-approved for severe primary IGF-1 deficiency and for growth hormone gene deletion with neutralising antibodies to GH. It is a prescription product, dosed by body weight, with mandatory monitoring for hypoglycaemia. It is a different molecule and is mentioned here only to show what a regulated IGF-1 product looks like.
WADA: IGF-1 and its analogues and mimetics are prohibited at all times under section S2 of the Prohibited List.
Reading the "research use only" label honestly: that designation is not a formality or a legal workaround for a substance that would otherwise be a medicine. For IGF-1 LR3 it is an accurate description of the compound's entire developmental history. Nothing about it has ever been evaluated for human use, because human use was never its purpose.
Test your understanding of what LR3 was built for, what the binding-protein system does, and how much human evidence exists.
This profile is for educational and research purposes only. It is not medical advice, and nothing on it is a protocol, a recommendation, or an instruction for use in a person or an animal.
Athena Peptides Education does not prescribe, sell, or recommend any compound. Compounds discussed here are for laboratory research only and are not for human consumption. Always consult a qualified physician before making any decision about your health.