IGF-1 LR3: a bioreactor workhorse with a misunderstood name

A growth factor built for the bioreactor

IGF-1 LR3 is often discussed in bodybuilding circles, where it is marketed as a performance-enhancing "research compound" ^[8]. Its real working life is far less glamorous: it is a growth supplement for cells grown in the laboratory, including Chinese hamster ovary (CHO) cells engineered to make recombinant proteins ^[7].

Its name also misleads. "Long R3" does not mean long-lasting. "Long" refers to the extra stretch of amino acids added to the front of the molecule, and "R3" to the arginine at position 3 ^[1]. As we will see, in animal studies LR3 actually leaves the bloodstream faster than ordinary IGF-1 ^[3,4].

Adelaide, 1992: designing a decoy-proof IGF-1

IGF-1 is a 70-amino-acid protein that carries out much of growth hormone's work in the body; our sermorelin article explains the full GH to IGF-1 axis ^[15]. In blood and tissue fluid, much of it is bound to a family of IGF-binding proteins (IGFBPs), which regulate how much reaches cells ^[2,5].

In 1992, Francis and colleagues at the CSIRO Division of Human Nutrition in Adelaide reported a set of IGF-1 analogues made in E. coli ^[1]. Each was a fusion: the first 11 amino acids of methionyl porcine growth hormone, then Val-Asn, then the IGF-1 sequence. One version kept natural IGF-1; in the others, the glutamate at position 3 was replaced by glycine or by arginine. The arginine version, Long [Arg3]-IGF-I, is what is now called IGF-1 LR3 ^[1]. The N-terminal extension also helped the proteins fold correctly, which made them easier to produce ^[1].

The resulting 83-residue chain begins Met-Phe-Pro-Ala-Met-Pro-Leu-Ser-Ser-Leu-Phe-Val-Asn-Gly-Pro-Arg-Thr and continues exactly as human IGF-1, including its three disulfide bonds ^[1,15].

The team's key experiment compared cell types. In rat myoblasts and other cells that secrete IGFBPs, LR3 was among the most potent analogues tested, on a par with the truncated form des(1-3)IGF-I. In chicken embryo fibroblasts, which secrete no detectable IGFBPs, it was less potent than ordinary IGF-1 ^[1].

In cells that make no IGF-binding proteins, LR3 was weaker than ordinary IGF-1. Its extra power comes from escaping those proteins, not from a stronger grip on the receptor ^[1,2].

How it works

Think of IGF-binding proteins as safe-deposit boxes. Native IGF-1 spends much of its time locked inside them: kept in reserve, protected and released slowly, but not able to act while it is boxed. In blood, much of it sits in a large 150-kDa complex ^[5]. LR3 has lost most of its affinity for the boxes, so more of it is free to reach the IGF-1 receptor.

That explains a paradox from rat studies. LR3 binds the type 1 IGF receptor about three times less well than IGF-1, yet in rats made catabolic with dexamethasone it was roughly 2.5 times more potent as an anabolic agent ^[2].

The same logic explains why LR3 is not long-lived in blood. The binding proteins also protect IGF-1 from being cleared. Researchers who developed LR3 noted that it "is cleared from the circulation more quickly than is IGF-I" in rats ^[3]. In a radiolabelled tracer study, LR3 left the circulation faster than IGF-1, although more intact LR3 reached tissue fluid ^[4]. In a rat model of kidney failure, LR3's total clearance increased further ^[5].

What the research shows

In cell culture and bioprocessing

In the original work, all the long analogues stimulated protein and DNA synthesis and inhibited protein breakdown in L6 rat myoblasts more potently than IGF-1 ^[1]. This property, full activity even when cells release binding proteins into the medium, is what makes LR3 useful in culture.

A 2000 study compared insulin, the usual growth factor in serum-free CHO cultures, with LR3 in two CHO cell lines making recombinant cytokine receptors ^[7]. One line could not grow without one of the two factors. Under production conditions, LR3 sustained the viability of both lines better than insulin, and the authors named it the preferred growth factor for those lines ^[7].

In animals

In the 1992 dexamethasone study, infused IGF-1 partly reversed weight loss in rats, and LR3 was about 2.5 times more potent ^[2]. The IGF peptides increased gut weight by up to 45%, and the extra carcass weight reflected nitrogen rather than fat ^[2].

A 1996 follow-up found that continuous infusion worked better than injections ^[3]. When infused, LR3 was 1.5 to 2 times more potent than IGF-1 for weight gain and organ weights, and it kept some advantage when injected once daily. For reversing carcass muscle loss, however, it was barely equal to IGF-1 ^[3].

In guinea pigs infused for 7 days, LR3 increased the relative weights of the adrenals, gut, kidneys and spleen without increasing overall growth, and it lowered circulating IGF-I, IGF-II and IGF-binding proteins ^[6]. In these studies, internal organs such as the gut often responded more clearly than skeletal muscle ^[2,3,6].

In humans

No human study of IGF-1 LR3 has been published. A 2026 review of GH-axis peptides placed it in the lowest evidence tier, supported only by preclinical extrapolation and user reports ^[8].

The cancer question

IGF-1 signalling influences cell growth and survival, which is why its link with cancer has been studied for decades ^[9]. A 2004 meta-analysis of 21 studies with 3,609 cases found that people at the 75th percentile of circulating IGF-1, compared with the 25th, had higher odds of prostate cancer (odds ratio 1.49) and premenopausal breast cancer (1.65), with modest associations overall ^[9]. In UK Biobank, with 394,388 cancer-free participants followed for an average of 6.9 years, each 5 nmol/L of higher IGF-1 was associated with higher risks of thyroid (hazard ratio 1.18), breast (1.11), colorectal (1.08) and prostate cancer (1.08), and lower risks of ovarian and liver cancer ^[10]. The authors could not exclude reverse causation ^[10].

The mirror image comes from Ecuador, where people with a genetic defect in the growth hormone receptor have very low IGF-1. Over 22 years of monitoring, they had only one non-lethal cancer, against a 17% cancer prevalence in control subjects ^[11].

These data concern natural, lifelong IGF-1 levels. They do not show that an IGF-1 analogue causes cancer, and no such data exist for LR3. The 2026 review frames the concern as theoretical but clinically relevant ^[8]. European regulators raised the same IGF-1 concern when they assessed tesamorelin.

What we don't know yet

• Everything in humans. Pharmacokinetics, safety, immune responses and effective exposure are all unstudied ^[8].
• Blood sugar. IGF-1 has insulin-like effects, and off-label reports describe symptoms suggesting low blood sugar, but these come mostly from non-clinical sources ^[8].
• Organ growth. Rat and guinea-pig studies showed growth of the gut, kidneys and spleen ^[2,6]. What that would mean in people is unknown.
• Feedback effects. LR3 lowered the animals' own IGF-I, IGF-II and binding proteins ^[6]. The consequences of that suppression have not been studied.
• Cancer relevance. Population data link higher natural IGF-1 with some cancers ^[9,10]; whether an IGFBP-evading analogue changes tumour biology is unknown ^[8].

Regulatory status

• Approval. IGF-1 LR3 has never been approved as a medicine; it circulates outside medicine as an unregulated "research compound" ^[8].
• The approved alternative. Mecasermin, recombinant native IGF-1, was approved by the FDA as Increlex on 30 August 2005 ^[12] and authorised in the EU on 2 August 2007, for children and adolescents with severe primary IGF-1 deficiency ^[13].
• Sport. "Insulin-like growth factor 1 (IGF-1, mecasermin) and its analogues" are prohibited at all times under S2.3 of the WADA Prohibited List ^[14].

In the lab: handling and storage

IGF-1 LR3 is supplied as a lyophilised powder. Common practice is long-term storage at −20 °C, with 2–8 °C acceptable for short periods, protected from light and moisture. Its activity depends on correct folding and on the three disulfide bonds it shares with IGF-1 ^[1,15], so reducing agents should be kept away from it. In culture work, the protein is usually dissolved in a small volume of dilute acid before dilution into medium, and stock solutions are aliquoted to avoid repeated freeze-thaw cycles. When comparing it with native IGF-1, remember that serum and many cell types supply IGF-binding proteins, which blunt native IGF-1 far more than LR3 ^[1].

FAQ