At a glance
| Sequence length | 83 amino acids |
|---|---|
| Modification | 13-residue extension and Arg at IGF-1 position 3 |
| Research use | Cell growth and IGF-binding-protein experiments |
| Clinical evidence | No established human efficacy |
| Safety signal | Accelerated tumor growth in a mouse cachexia model |
Overview
Long R3 IGF-I contains the IGF-1 sequence modified at position three plus a 13-amino-acid extension, producing an 83-residue molecule. It was developed as a research reagent for studying receptor and binding-protein contributions to growth-factor action. The word long describes the extended sequence; it should not be treated as a clinically validated human half-life claim. Its use in cell culture and animal models is much better established than any human therapeutic use. Marketing for muscle gain often omits this distinction and may confuse the analog with approved native-sequence IGF-1 treatment. These are separate molecules with different evidence and regulatory histories.
How it works
LR3 IGF-I can activate growth-factor signaling while binding less strongly to some IGF-binding proteins. That changes the fraction available to interact with receptors. Early experiments showed that its relative potency varied with the cell system, especially whether binding proteins were present. Consequently, reduced binding-protein affinity does not mean greater activity in every tissue or a universally superior biological effect.
Potency depends on the experimental system
The original analog study found stronger activity in certain cells that secreted IGF-binding proteins. In a cell system lacking detectable binding proteins, Long R3 IGF-I was less potent than native IGF-I. That result directly contradicts a simple claim that LR3 is always a fixed number of times stronger.
Supporting sources [1]
Muscle preservation can carry a tradeoff
In a mouse cancer-cachexia study, LR3 IGF-I limited loss of muscle mass but also accelerated tumor growth. This is an animal finding and does not quantify human cancer risk. It does demonstrate why a growth-promoting intervention cannot be assumed to act only on desirable muscle tissue.
Supporting sources [2]
Human risk remains undefined
The cited preclinical studies do not establish human pharmacokinetics, an acceptable exposure range, or long-term metabolic and proliferative safety. IGF-related hypoglycemia and tissue-growth concerns require direct investigation for this molecule. Claims of predictable local action or a standardized long duration in humans exceed the evidence reviewed here.
Does an IGF-1 medicine’s approval cover LR3?
No. Native-sequence recombinant IGF-1 and LR3 are different products. A research analog that changes binding-protein interactions needs its own manufacturing controls, clinical evidence, and regulatory authorization. There is no FDA-approved LR3 indication for muscle gain, rehabilitation, anti-aging, or treatment of cachexia.
Reading this profile
Jurisdiction: United States: experimental analog, not an FDA-approved IGF-1 medicine.. Molecular class: Engineered 83-amino-acid insulin-like growth factor I analog.
This profile was written with AI-assisted research and checked against the linked sources. It has not been represented as a clinician’s review. Evidence summaries describe the cited studies and products; they do not establish that a separately purchased preparation is equivalent.
Sources & references
Follow the original material. Product labels define specific formulations and uses; trial registration describes a study and does not by itself establish benefit.
- Original Long [Arg3]-IGF-I analog characterization ↗Original research · Checked 2026-10-27
- Inhibition of activin-like kinase 4/5 attenuates cancer cachexia associated muscle wasting. ↗Original research · Checked 2026-10-27