IGF-1 LR3
Long R3 IGF-1, LR3
Engineered to slip past the binding proteins that normally restrain IGF-1, which extends its half-life roughly a hundredfold and removes the safety mechanism at the same time.
Preclinical only
Not approved anywhere and sold explicitly as a laboratory reagent for cell culture. A different IGF-1 product, mecasermin, is FDA-approved for severe primary IGF-1 deficiency; it is not this molecule.
What it is
IGF-1 is the growth factor through which most of growth hormone's anabolic signalling actually happens. In the body it does not circulate freely: roughly 98% of it is bound to IGF binding proteins, which control how much is available and where.
IGF-1 LR3 is a modified version with an arginine substitution and a thirteen amino acid N-terminal extension. Both changes reduce its affinity for those binding proteins. The result is a molecule that stays free in circulation for twenty to thirty hours instead of minutes.
The modification is the mechanism and also the problem
The binding proteins are not an inefficiency to engineer around. They are a control system. They buffer IGF-1 activity, localise it to tissues, and prevent systemic exposure to a potent mitogen.
LR3 was designed to defeat that system, and it was designed to do so for a specific purpose: keeping cultured cells proliferating in a bioreactor. That is what it is sold as, and the vials genuinely are laboratory reagents rather than a diverted pharmaceutical.
Using it in a person means running an unbuffered mitogen through the whole body for a day at a time, with no human safety data at any dose.
The approved IGF-1 is a different molecule
Mecasermin, sold as Increlex, is recombinant human IGF-1 approved for severe primary IGF-1 deficiency. It exists, it works, and it demonstrates what supervised IGF-1 therapy looks like: careful titration, meals timed around doses, and hypoglycaemia as the well-documented dose-limiting toxicity.
Mecasermin is native-sequence IGF-1 with normal binding protein affinity and a half-life measured in minutes to a few hours. It is not LR3, and its safety record does not transfer.
Practical notes
- Hypoglycaemia is the acute risk, not a theoretical one. IGF-1 has insulin-like activity, which is what the name says. With a molecule that stays active for a day, a hypoglycaemic episode can arrive well after anyone has stopped associating it with the injection.
- The cancer concern is mechanistic, not speculative. IGF-1 signalling drives cell proliferation and suppresses apoptosis. Epidemiological associations between circulating IGF-1 and several cancers exist. Deliberately maximising free IGF-1 exposure is the opposite of cautious.
- Nothing here is monitored the way the approved product is. Mecasermin is dosed with glucose monitoring and meal timing under supervision. Grey-market LR3 has none of that, at a longer duration of action.
Side effects and warning signs
Commonly reported
- Hypoglycaemia, which is the dose-limiting effect and can be abrupt
- Injection-site reactions
- Headache
- Joint pain
- Fluid retention
Stop and get medical help
- Any active or suspected malignancy. IGF-1 is a direct mitogen and this analogue is engineered to circulate freely and persistently
- Sweating, shaking, confusion or loss of consciousness, which is hypoglycaemia and is the acute danger with this compound
- Any use alongside insulin, which compounds the hypoglycaemia risk substantially
Sources
From the community
Discussion for this compound concentrates in r/Peptides.
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