IGF-1 LR3

None of the human research connected to IGF-1 LR3 involved giving it to a person. The human data that exists is about two neighbouring things: how the body's own circulating IGF-I relates to cancer risk across several hundred thousand people, and what happened when children with severe IGF-1 deficiency were treated with mecasermin (Increlex), the approved, unmodified form of the hormone. Neither one tests LR3.

LR3 is an engineered analogue of IGF-1 built to bind poorly to the carrier proteins that normally hold the hormone in check. In rats, that made it 1.5 to 2 times more potent than native IGF-1 for body weight gain when infused continuously, with a smaller edge when injected, and almost no edge for reversing muscle loss. It is not an FDA approved drug, and WADA prohibits it at all times under class S2.3.

Insulin-like growth factor 1 is a single chain of 70 amino acids. About 98 percent of it circulates bound to one of six IGF binding proteins, with IGFBP-3 carrying roughly 80 percent of the bound pool, and while bound its activity is suppressed.

IGF-1 LR3 is a modified version with two changes: arginine substituted at position 3, and a 13 residue extension on the N-terminal end. Both serve one purpose, which is to keep the binding proteins from catching it. In the rat work that characterised it, LR3 had very low affinity for those proteins.

It is a different molecule from mecasermin. Increlex has an amino acid sequence identical to the body's own IGF-1, and its approval covers growth failure in children aged 2 and over with severe primary IGF-1 deficiency. That approval does not extend to LR3.

What the Binding Proteins Normally Do

IGF binding proteins hold IGF-1 with equal or greater affinity than the IGF-1 receptor does, so they decide how much of the hormone actually reaches tissue. Binding also prolongs its half-life in circulation and keeps IGF-1 away from the insulin receptor. The Increlex label shows the same relationship from the drug side: clearance of mecasermin is inversely proportional to IGFBP-3 levels.

What Happens at the Receptor

When IGF-1 binds its receptor, the receptor phosphorylates an adaptor protein called IRS-1, which recruits PI3K and leads to Akt phosphorylation. In skeletal muscle the PI3K/Akt/mTOR route raises protein synthesis, and Akt also inhibits the FoxO proteins that drive muscle protein breakdown. PI3K/Akt and MAPK signalling both take part in activating satellite cells, the muscle stem cells behind regeneration. All of that is IGF-1 biology in general, worked out largely in rodents and cultured cells, not in LR3 users.

What LR3 Changes

Escaping the binding proteins cuts both ways. More of the analogue is free to reach receptors, which is the likely source of its extra potency in rats. It also loses the protection binding provides: in the rat study, LR3 was cleared from circulation more quickly than native IGF-1, not more slowly.

Treating the cancer epidemiology as a verdict on LR3. The UK Biobank and pooled prostate findings describe IGF-I varying within the range people produce on their own. Mendelian randomization analyses, cited by the UK Biobank authors, suggest the links with colorectal, breast and prostate cancer may be causal. That is a real signal about the pathway. It is not a measurement of LR3, and no cancer risk figure exists for it. The accurate version is narrower and still uncomfortable: LR3 was built to make more IGF-1 signalling available to tissue, and higher natural levels of that signal track with several cancers in very large populations.

Borrowing the Increlex approval. Mecasermin is approved for growth failure in children aged 2 and over with severe primary IGF-1 deficiency, and the label explicitly excludes secondary forms of deficiency. It is also a different molecule. None of that approval transfers to LR3, and none of it reaches healthy adults.

Quoting the rat potency without the rest of the study. The up to twofold figure comes from continuous infusion and applies to body weight, organ weight and feed efficiency. On carcass muscle in catabolic rats, the analogue was barely equal to native IGF-1.

Assuming it lasts longer. Binding proteins are what extend IGF-1's time in circulation. LR3 escapes them, and in rats it was cleared faster than native IGF-1.

There is no human safety record for LR3. The nearest one belongs to mecasermin, from 71 children with severe primary IGF-1 deficiency treated for a mean of 3.9 years.

Hypoglycemia was reported by 30 of them (42 percent) at least once. Five had severe episodes requiring assistance, and 4 had hypoglycemic seizures or loss of consciousness. Intracranial hypertension occurred in three. Tonsillar hypertrophy appeared in 11 (15 percent) during the first one to two years, and 7 children had a tonsillectomy or adenoidectomy. Kidney and spleen length rose quickly early in treatment, in some children reaching or passing the 95th percentile. A low number of cases of anaphylaxis requiring hospitalization have been reported since marketing.

The label contraindicates mecasermin in children with malignant neoplasia or a history of malignancy. Postmarketing reports describe a variety of cancers in treated children, mostly in those with conditions that already raise cancer risk, and more often at higher than recommended doses or doses that pushed IGF-1 above the normal range. Whether the drug caused them is unknown. Caregivers are told to report any new growth, and treatment is stopped if a malignancy develops.

Every one of those figures came from a molecule that binds the carrier proteins normally, given to children who needed it, with a physician watching.

What the evidence supports: in rats, an IGF-1 analogue that escapes the binding proteins beats native IGF-1 on whole-body growth measures under continuous infusion, with a smaller lead by injection. In people, higher natural IGF-I is associated with higher risk of several common cancers, and the unmodified hormone given as a medicine causes frequent low blood sugar even under supervision.

What it does not support: any statement about muscle gain, recovery, body composition or safety in people taking LR3, because none of those has been measured. The one muscle readout in the cited animal work came out close to a tie.

For anyone in tested sport, S2.3 has already settled the question.

Not Increlex. Mecasermin is sequence-identical to the body's own IGF-1 and binds the carrier proteins normally. LR3 carries a position 3 arginine substitution and a 13 residue extension designed to avoid them.

Not an approved or categorised drug. FDA has not approved it. It does not appear on FDA's Category 1 or Category 2 compounding lists, and FDA has stated it does not intend to categorise substances nominated on or after 7 January 2025, so that absence says nothing about safety or legality. Mechano growth factor and PEG-MGF appear on FDA lists; neither is IGF-1 LR3.

Not permitted in tested sport. WADA's S2.3 prohibits "Insulin-like growth factor 1 (IGF-1, mecasermin) and its analogues" at all times. LR3 is not printed by name, and it is plainly an analogue.

Not a longer-lasting IGF-1. In the rat study, escaping the binding proteins made it clear from the blood faster, not slower.

I would rather be precise about IGF-1 LR3 than loud. The tempting line is that it causes cancer. The sources do not say that, and I am not going to say it either.

What they do say is uncomfortable enough. Across nearly 400,000 people, higher natural IGF-I tracks with several common cancers, genetic analyses point toward that being causal, and the approved form of the hormone carries a malignancy contraindication. LR3 exists to push more of that signal past the body's own brakes. Meanwhile the animal case for it is thinner than the sales copy, with the muscle result in the founding rat study close to a draw.

The gap I care about sits between a molecule engineered to amplify a growth pathway and a complete absence of human testing of that molecule. Until somebody closes it, I treat LR3 as a compound with a rat file and no human record, not as a performance tool.