An engineered IGF-1 analog built for cell-culture labs that dodges IGF binding proteins. No published human studies of any kind.
Also called Long R3 IGF-1, LR3-IGF-I, Long Arg3 IGF-I, [Arg3]-IGF-I with N-terminal extension, LongR3-IGF-I
IGF-1 LR3 is a lab-made version of insulin-like growth factor 1, the hormone the liver releases in response to growth hormone. It has two changes: the glutamic acid at position 3 is swapped for arginine, and a 13-amino-acid tail is added to the front. Together these make it bind very weakly to the IGF binding proteins that normally hold IGF-1 in check in the blood, so more of it is free to hit the IGF-1 receptor. It was developed in Australia in the early 1990s and is sold mainly as a supplement for growing cells in bioreactors.
Every efficacy finding comes from rats and cell cultures. In rats it was 1.5 to 3 times more potent than native IGF-1 at driving weight gain, gut growth and protein synthesis, but in steroid-treated rats it was no better than IGF-1 at preventing muscle loss. No human trial, pharmacokinetic study or safety study of LR3 has been published.
What is known about IGF-1 in humans comes from mecasermin (Increlex), recombinant native IGF-1 approved for children with severe IGF-1 deficiency. Its label warns about low blood sugar severe enough to cause seizures, swelling of the tonsils and adenoids, raised pressure in the skull, and cases of cancer in treated children. A one-year trial of IGF-1 injections in healthy older women found no change in muscle, fat, bone or strength. IGF-1 and its analogs are banned in sport at all times.
What it binds to, and what happens inside the body after that, step by step.
LR3 activates the same receptor as native IGF-1, the type 1 IGF receptor, which switches on the PI3K-Akt and MAPK pathways that drive protein synthesis, cell division and cell survival. Its extra potency comes from barely binding IGF binding proteins, not from binding the receptor more tightly; its receptor affinity is slightly lower than IGF-1. Because it escapes binding proteins, it is also cleared from the blood faster than IGF-1 in rats.
Human results and animal results are kept apart below. A result in rats tells you a mechanism is plausible. It does not tell you what happens in a person.
There are no published human studies of IGF-1 LR3: no trials, no pharmacokinetics, no safety data. Rat studies show it is 1.5 to 3 times more potent than IGF-1 for weight gain and gut growth but not better at preventing muscle loss. Human data on native IGF-1 show a real risk of low blood sugar and, in healthy older adults, no gain in muscle or strength over a year.
16 healthy women over 60 (native IGF-1, not LR3)
Blood IGF-1 rose about fivefold, yet lean mass, fat mass, bone density and strength did not change compared with placebo.
Read the studySeized injection vial analyzed by mass spectrometry
The vial contained a His-tagged lab reagent version of LR3, a form made for biochemistry work whose effects in humans are unknown.
Read the studyRat L6 myoblasts
Arg3 IGF-I bound very poorly to IGF binding protein-2 and was more potent than IGF-I at stimulating protein synthesis, showing the potency comes from escaping binding proteins.
Read the studyDiabetic rats, 7-day infusion
LR3-IGF-I was 2.5 to 3 times more potent than IGF-I at restoring growth, and the IGF peptides raised muscle protein synthesis by up to 50%; unlike insulin, none reduced sugar loss in the urine.
Read the studyNormal and dexamethasone-treated rats, 7 days
When infused continuously, LR3 was 1.5 to 2 times more potent than IGF-I for weight gain and organ growth and kept some of that edge with daily injections, but it barely matched IGF-I at preventing muscle loss and cleared from blood faster.
Read the studyFemale rats, 3-day infusion
LR3 increased small and large intestine weight by 20% and crypt length by up to 22%; native IGF-I did not.
Read the studyThese are questions the published research has not answered.
No product containing IGF-1 LR3 is approved as a medicine by the FDA.
Sold legally only as a cell-culture reagent. Never developed as a human drug. The related molecule mecasermin (recombinant native IGF-1, Increlex) is FDA-approved for severe primary IGF-1 deficiency in children; LR3 is a different molecule. WADA lists IGF-1 and its analogues under S2.3, prohibited in and out of competition, and anti-doping labs have validated tests that detect LongR3-IGF-I in plasma.
No human half-life has been published for IGF-1 LR3, so we do not draw a clearance curve. Half-lives quoted online for it are usually estimates from animal data.
No human study or approved label has published a dose for IGF-1 LR3, so there is no table to show.
No dose of IGF-1 LR3 has been tested in a published human study or approved on a label. Numbers that circulate online come from animal studies scaled up by guesswork or from sellers. We do not reproduce them.
Which symptoms are expected and which mean stop and get help: side effects and red flags.
Tap a claim to see what the studies found.
No human study has looked. Cell and rat work shows more protein synthesis and myoblast division, but muscle fiber hyperplasia in adult humans has never been shown for any IGF-1 form.
No human pharmacokinetic study exists. In rats LR3 actually cleared from the blood faster than native IGF-1, because it does not ride on binding proteins.
There is no human safety data at all. Native IGF-1 carries label warnings for hypoglycemic seizures, intracranial hypertension and malignancy.
A placebo-controlled year of native IGF-1 in women over 60 raised blood IGF-1 about fivefold with no change in lean mass, strength or bone density.
IGF-1 receptors are on almost every tissue. In rats the most striking effect was growth of the intestine, along with other organs.
These pairings come up often. Almost none have been tested together in a trial, and the evidence note on each says so plainly.
Bodybuilding forums pair a systemic IGF-1 analog with an IGF-1 splice-variant peptide, claiming one drives growth and the other repair.
EvidenceNeither has any human data, and no animal study has combined them.
Sold as raising the body's own GH and IGF-1 while adding an analog on top.
EvidenceNo study has tested this combination. It stacks IGF-1 receptor stimulation from two directions with no safety data.
Reagent suppliers ship LR3 freeze-dried for cell culture. There are no published stability data for injection use.
No published stability data for any solution intended for injection.
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