IGF-1 LR3: No Human Trials, No Half-Life

What IGF-1 LR3 actually is

Native human IGF-1 is a 70-residue hormone, and IGF-1 LR3 carries 83 residues. That 13-residue difference is the whole molecule in one number, and it is the cleanest way to understand what the analog is.

The compound came out of a 1992 Australian laboratory effort to build IGF-1 variants with useful research properties. Francis and colleagues described a family of fusion peptides, one of which became the molecule now sold as IGF-1 LR3 (PMID 1378742). They built it as a reagent for investigating IGF-1 action, and the published record has stayed in that territory ever since.

Several things it is not are worth stating plainly. IGF-1 LR3 is not native IGF-1, and it is not IGF-2 LR3. It is not DES(1-3)IGF-1, a different analog from the same research program. Nor is it mecasermin, the recombinant human IGF-1 approved as a prescription medicine. Each of those is a separate molecule with a separate evidence base. Treating any of them as interchangeable with LR3 produces the most common errors in this subject.

Everything below concerns laboratory and in-vitro research. PrimeTime BioLabs supplies this compound for that purpose only. Nothing here describes human use, and the article carries no dosing, preparation or administration guidance of any kind.

What the LR3 designation means

The name encodes two modifications, and both of them do work.

"Long" refers to an extension added to the front of the molecule. Francis and colleagues built it from the first 11 amino acids of methionyl porcine growth hormone, followed by valine and asparagine (PMID 1378742). Eleven plus two gives the 13 residues that take the molecule from 70 to 83. Most pages quote the number without comment. Almost none show where it comes from.

"R3" refers to a single substitution. Arginine replaces the glutamic acid normally found at position 3 of the human IGF-1 sequence. The same paper describes sibling analogs where glycine took that position, so the arginine version is one member of a designed set.

Together the two changes reduce how tightly the molecule binds the IGF-binding proteins. Those are the circulating carrier proteins that hold most IGF-1 in the bloodstream. The reduction is described in the literature as marked. A frequently quoted figure of roughly 1,000-fold rests on a single source, so we use the qualitative description that two independent sources support.

One specification question comes up often enough to answer directly. PrimeTime does not publish a molecular weight, a CAS number or a full sequence for this product. Published values vary between sources in ways we cannot reconcile. The lot-matched certificate of analysis is the specification authority for any given vial. Encyclopedia pages do print those values, and we are deliberately not importing them.

How the receptor signaling works

IGF-1 LR3 binds the IGF-1 receptor and activates it. The receptor is a tyrosine kinase, and its activation feeds two downstream cascades that are well characterized in the IGF system generally. One is the PI3K/Akt/mTOR pathway, associated with protein synthesis and cell survival. The other is the MAPK/ERK pathway, associated with proliferation.

A distinction matters here, and most explainers skip it. Pathway detail of that kind comes from the broader IGF-1 literature, not from experiments on LR3 specifically. What direct LR3 work establishes is receptor engagement and the consequences of weak binding-protein affinity. Everything downstream is inherited from what we know about the receptor, which is reasonable science and still worth labeling honestly.

The reduced binding-protein affinity has a straightforward consequence in a culture dish. Where IGFBPs are present in the medium, they sequester IGF-1 and blunt its action, while they leave LR3 comparatively free. That property is why the analog turns up so often as a supplement in serum-free cell culture and in bioprocessing work.

One mechanistic point deserves precision, because the site's own product catalog gets it wrong. IGF-1 LR3 is a receptor agonist acting downstream. It does not stimulate the pituitary to release growth hormone, which is what a growth hormone secretagogue does. Tesamorelin works upstream at the pituitary and raises IGF-1 indirectly, and our tesamorelin evidence file covers that side of the growth hormone axis. The two compounds travel in opposite directions through the same system.

Why the potency claim depends on the assay

A claim repeated across almost every page on this compound says IGF-1 LR3 is about three times more potent than IGF-1. The primary literature says something more interesting and considerably more useful.

Tomas and colleagues compared IGF-I against its analogs in dexamethasone-treated rats and found the variants roughly 2.5-fold more potent (PMID 1371669). The same paper notes something that rarely travels with the potency claim. LR3IGF-I binds the type 1 IGF receptor about three-fold less well than IGF-I does. The widely quoted three-fold potency figure looks very much like that receptor-binding number with its direction reversed.

Francis and colleagues had already shown why a single potency number cannot be right (PMID 1378742). They ranked the analogs across several cell lines and found the order depended on whether the cells secreted binding proteins. In chicken embryo fibroblasts, a line that does not secrete detectable IGFBPs, Long [Arg3]-IGF-I was less potent than IGF-I.

That result settles the question properly. The potency advantage is a property of the system, not of the molecule. Put the analog where binding proteins compete for IGF-1 and it outperforms the hormone. Take the binding proteins away and the advantage disappears, because the advantage was never about the receptor.

Evidence status: the "three times more potent" figure is unsupported as a general claim. The measured value is approximately 2.5-fold, in one rat model, under conditions with elevated binding proteins.

Where the half-life numbers come from

One encyclopedia page gives two different half-lives for this molecule.

Its infobox states 56 to 72 hours. The body text states about 20 to 30 hours. Nothing on the page reconciles them, and anyone reading quickly will leave with whichever number they happened to see.

We traced both figures to their sources. The infobox figure carries no citation at all, because no reference marker is attached to the field.

That body figure carries a citation, and the citation does not support it. The attached source is von der Thüsen and colleagues, a 2011 study in the American Journal of Pathology (PMID 21281823). It investigates plaque stability in atherosclerosis by examining vascular smooth muscle cell phenotype. The researchers used Long R3 IGF-1 in apolipoprotein E knockout mice because the analog is stable. Their reported outcomes are stenosis, core size, cap-to-core ratio, plaque cell content and intraplaque hemorrhage. No pharmacokinetic measurement appears in the abstract, and the study is not a pharmacokinetic study. We could not retrieve the full text to check whether a half-life figure appears inside it. The limit of what we checked is stated here deliberately.

Measured data for this analog do exist. They come from rats, and they point the other way.

Robertson and colleagues measured clearance from subcutaneously implanted wound chambers in rats in 1999 (PMID 10198302). They reported a half-life of 563 minutes for LR3-IGF-I, against 872 minutes for IGF-I and 861 for IGF-II in the same experiment. That works out to roughly 9.4 hours. The measurement is local wound-fluid clearance and not systemic pharmacokinetics. LR3-IGF-I cleared faster than the hormone it is supposed to outlast.

A 2021 anti-doping laboratory study went further. Rats received 100 micrograms per kilogram intramuscularly, and the intact analog disappeared rapidly after four hours (PMID 33587816). Several degradation products persisted longer than the parent molecule. Those authors also incubated the analog in human whole blood and saw the same degradation products form. They offer that as a basis for extrapolating to humans. An extrapolation is not a measurement, and they present it as one.

The oldest finding in this chain is the most direct. Tomas and colleagues state that LR3IGF-I has very low affinity for rat IGF-binding proteins. It is therefore cleared from the circulation more quickly than IGF-I (PMID 8708565). Binding-protein association is what protects circulating IGF-1 from clearance. A molecule engineered to escape that association should leave the circulation sooner, and in rats it does.

So the popular story has the mechanism backwards. Escaping the binding proteins buys free access to the receptor. It does not buy time in the bloodstream, and the published measurements say it costs time.

Evidence status: no human half-life for IGF-1 LR3 has been measured or published. Both figures in general circulation are unsourced. The only measured values are animal, local or intramuscular, and short. None of this translates into an administration interval, and we give none.

What the published literature contains

Counting this literature is harder than it looks, and the difficulty is itself informative.

The analog goes by at least seven spellings in the indexed record. PubMed treats "IGF-1" and "IGF-I" as different strings, so a search on one misses papers that use the other almost entirely. Individual phrase searches return between 8 and 47 records depending on the spelling chosen. A bare search for "LR3" returns something else again, because LR3 is the standard notation for the Liver-3 acupuncture point.

Our method, run on 16 October 2026: a union of six spellings returned 91 records, and we read every abstract. One mentions the analog only as background rationale for a different compound, giving 90 verified records. A seventh spelling, written as one word, adds 15 more that the six-way union misses, for a de-duplicated total of 106. We publish 90 as the figure we read and verified, and we note the wider set so nobody mistakes it for a census.

Reading all 91 produced one finding that matters more than the count.

Not one record describes administering IGF-1 LR3 to a living human being.

• Non-human in-vitro — Records: 36 — What it can establish: Receptor and pathway behavior in cells
• Animal — Records: 33 — What it can establish: Effects in living sheep, rats, mice, cattle, pigs
• Human cells or tissue — Records: 14 — What it can establish: Cell-level behavior, no whole-person outcome
• Reagent and expression — Records: 4 — What it can establish: Manufacturing and culture-supplement use
• Analytical and doping detection — Records: 5 — What it can establish: Identification in seized products and samples
• Review — Records: 2 — What it can establish: Synthesis of the above
• Human interventional — Records: 0 — What it can establish: Nothing, because none exists

A 2026 narrative review of performance-enhancing peptides places this analog among agents with a complete absence of human studies (PMID 42395176).

The trial registries agree with that finding. Searching ClinicalTrials.gov, the EU Clinical Trials Register and ISRCTN across five spellings returned zero registered studies. The same searches for mecasermin returned 40 and 17 respectively, which confirms the searches work and shows what a genuinely registered IGF-1 product looks like. We could not reach the WHO portal or the EU CTIS database, so we do not claim to have checked them.

One record is worth singling out for what it says about the supply side. Kohler and colleagues identified a His-tagged version of the analog in a confiscated black market product in 2010 (PMID 20675162). Material sold into that channel is not necessarily the molecule on the label.

An indexed record is not a trial. Ninety records represent thirty years of laboratory work on a research reagent, and that is a real body of knowledge. It is not evidence of a human outcome, and no quantity of preclinical records converts into one.

What the evidence says about muscle

Muscle is what most people arrive asking about, so here is the state of it.

In cells, the analog does what an IGF-1 receptor agonist would be expected to do. Francis and colleagues reported stimulation of protein and DNA synthesis, and inhibition of protein breakdown, in rat myoblasts (PMID 1378742). That is a real, replicated, cell-level finding.

Animal results get more complicated and less flattering. Tomas and colleagues found the analog partially reversed a catabolic state in dexamethasone-treated rats, with effects on body weight and nitrogen retention (PMID 1371669). The same group later found LR3IGF-I barely equipotent with IGF-I for reversing carcass muscle loss in those rats. That held despite greater potency on other measures (PMID 8708565). Dunaiski and colleagues found that infusion in pigs reduced growth, along with plasma growth hormone, IGFBP-3 and endogenous IGF-I (PMID 9488001). White and colleagues found no growth benefit in growth-restricted fetal sheep, with circulating amino acids lower (PMID 39679943).

In humans, there is nothing at all. No study has administered this analog to a person and measured muscle.

A 2015 randomized trial is frequently pressed into service to fill that gap, and it cannot. Guha and colleagues at Southampton randomized 56 recreational athletes, 30 men and 26 women, across three arms (PMID 26046967). Participants received placebo, or 30 or 60 milligrams per day of a recombinant human IGF-I complexed with IGF-binding protein 3, for 28 days. Body composition was assessed by DXA. They found no significant change in body fat mass or lean body mass in either sex. Maximal oxygen consumption rose by about 7 percent in the combined treatment groups.

That trial tested a different molecule. A recombinant hormone complexed with its binding protein is close to the opposite of an analog engineered to escape binding proteins. The result is good evidence about what it tested. It establishes nothing about IGF-1 LR3 in either direction, and citing it as LR3 evidence is the single most common error in this subject.

Two further distinctions are worth drawing. Hypertrophy means existing muscle fibers growing larger, and hyperplasia means new fibers forming. Adult skeletal muscle fibers are post-mitotic, so claims about new fiber formation require much stronger evidence than claims about fiber growth. That evidence does not exist for this compound in people.

Fat loss comes up alongside muscle, and the record does not support it as a direct action. Tomas and colleagues found reduced carcass fat, by roughly 30 percent, in tumor-bearing rats also losing muscle protein (PMID 8053901). That is a catabolic context, not a body-composition benefit.

How IGF-1 LR3 compares with four related molecules

Five molecules get confused with each other constantly. Here is the short version, followed by what sits behind each one.

• IGF-1 LR3 — What it is: 83-residue synthetic IGF-1 analog — Direct human evidence for it: None
• Native IGF-1 — What it is: 70-residue endogenous human hormone — Direct human evidence for it: Extensive, as the endogenous hormone
• DES(1-3)IGF-1 — What it is: Truncated IGF-1 analog — Direct human evidence for it: None located
• HGH — What it is: 191-residue pituitary hormone — Direct human evidence for it: Extensive, as an approved medicine
• Mecasermin — What it is: Recombinant human IGF-1, a prescription drug — Direct human evidence for it: Yes, with approved labeling

IGF-1 LR3

Built in 1992 as a research reagent, with a 13-residue N-terminal extension and arginine at position 3. It engages the IGF-1 receptor while binding IGF-binding proteins weakly. Its evidence base is 90 verified preclinical records and zero human administrations. The principal uncertainty is everything about how it behaves in a person, including half-life, safety and effect. It holds no regulatory approval anywhere for any use.

Native IGF-1

The 70-residue hormone the body makes, largely in the liver under growth hormone stimulation, and the molecule every analog on this list derives from. It circulates mostly bound to IGF-binding proteins, which regulate how much reaches receptors. Its biology has been extensively studied. That knowledge does not transfer automatically to an analog built specifically to behave differently.

DES(1-3)IGF-1

A truncated analog from the same research program, missing the first three residues of the native sequence. Like LR3, it binds the binding proteins poorly. Tomas and colleagues found both analogs roughly 2.5-fold more potent than IGF-I in dexamethasone-treated rats (PMID 1371669). Our registry searches located no registered human study of it. Direct comparisons between DES and LR3 in the same experiment are scarce. Claims that one outlasts the other rest on evidence we could not find.

HGH

Human growth hormone, a 191-residue pituitary hormone acting at the growth hormone receptor. It sits upstream of this whole discussion, stimulating IGF-1 production without binding the IGF-1 receptor itself. Recombinant versions are approved medicines for defined indications. Describing HGH and IGF-1 LR3 as alternatives to each other misdescribes both, because they act at different receptors at different points in the same axis.

Mecasermin

Recombinant human IGF-1, marketed as Increlex, and the only molecule on this list with an approved IGF-1 label. It is prescribed for severe primary IGF-1 deficiency. Its label carries real safety data, discussed in the next section, and those data are the closest available proxy for IGF-1 receptor activation in people. Proxy is the operative word. Mecasermin carries the native hormone sequence, and LR3 does not.

A note on MK-677, which appears in comparison searches. It is an orally active growth hormone secretagogue, acting on the ghrelin receptor to raise growth hormone and then IGF-1. Comparing it to LR3 compares a pituitary stimulus with a receptor agonist, and no head-to-head evidence exists.

Safety and what the evidence does not establish about it

Every point in this section is graded. Each one is labeled by what kind of evidence stands behind it. A measured finding and a reasonable worry are not the same thing, and treating them alike produces scare copy.

Accelerated tumor growth. Direct animal evidence, in two independent models. In rats bearing mammary adenocarcinoma, IGF peptides increased tumor growth, and LR3-IGF-I did so more than the others (PMID 8053901). Neither peptide promoted growth of the same adenocarcinoma cells in culture, which makes the in-vivo result harder to dismiss. Levolger and colleagues studied C26 tumor-bearing mice and found LR3 IGF-I limited loss of muscle mass while accelerating tumor growth (PMID 31285507). Both findings point the same way, and the second describes exactly the trade-off that should concern anyone extrapolating muscle findings.

Mechanistic concern about proliferation generally. The IGF-1 receptor is overexpressed on many cancer cell types. One research group exploited that directly, building a methotrexate conjugate on long-R3-IGF-1 to target tumor cells overexpressing the receptor (PMID 19446281). Wetterau and colleagues reported that Long-R3 IGF-I stimulated telomerase activity in prostate cancer cells (PMID 12843187). These are cell-level and targeting findings. They do not establish a cancer risk in people, and the honest framing is that a pathway with known proliferative roles is being activated deliberately.

Disproportionate organ growth. Direct animal evidence, across species. Stremming and colleagues infused the analog into fetal sheep for one week (PMID 33427051). Several organs came out heavier, including heart, adrenal gland and spleen, without a matching increase in fetal weight. Conlon and colleagues infused guinea pigs for seven days and reported stimulated organ growth with body weight gain unaffected (PMID 7561636). Steeb and colleagues found intestinal growth in rats within three days, with crypt length up in different gut segments, independent of food intake (PMID 8549937). Organs growing out of proportion to the animal is a consistent, repeated finding.

Suppression of endogenous insulin, growth hormone and IGF-1. Direct animal evidence. Hammon and Blum gave the analog subcutaneously to neonatal calves and recorded lowered plasma glucose and insulin (PMID 9483305). Dunaiski and colleagues found decreased plasma insulin, IGF-I and IGFBP-3 in pigs, with growth hormone suppressed (PMID 9488001). White and colleagues found lower insulin and glucose in fetal sheep, with glucose-stimulated insulin secretion reduced (PMID 33938236). In that study the defect persisted in isolated islets, which suggests an intrinsic change in the islet itself.

Low blood sugar. Related-compound human evidence, plus animal signal. No study has measured hypoglycemia from this analog in a person, because no study has given it to one. What exists is label data for mecasermin, recombinant human IGF-1. The FDA label reports that 42 percent of 71 subjects with primary IGF-1 deficiency experienced at least one hypoglycemic episode. Mean exposure in that population was 3.9 years. Five subjects experienced severe episodes requiring assistance, and four experienced seizures or loss of consciousness, most frequently in the first month. The EMA product information categorizes hypoglycemia as very common, reporting 28 percent of 413 patients in a broader population.

Those are two different denominators from two different populations, and neither is an IGF-1 LR3 rate. Neither of those labels mentions this analog anywhere. The reasoning that connects them is mechanistic. Weaker binding-protein association implies a larger free fraction reaching receptors, and the animal glucose findings above are consistent with that direction. Reasoning is not measurement, and we present it as reasoning.

Insulin resistance. Not supported. We searched the record for it specifically. No record in the set reports insulin resistance with this analog, and the consistent animal finding runs the other way, toward suppressed circulating insulin. Several widely read pages list insulin resistance as a side effect, but we found nothing behind the claim.

Long-term exposure. Unresolved question. No long-term data exist in any species at exposures resembling how the compound is misused, and none exist in people at all.

Direct human safety data. None. There is no toxicology package, no adverse-event rate, no dose-response relationship and no contraindication list grounded in human experience. Anyone publishing a frequency or a severity grading for this compound in humans has invented it.

One pattern is worth naming, without naming names. The most widely read vendor explainers in this category describe hypoglycemia as common and mild. They omit organ enlargement and insulin changes entirely. Enhanced glucose uptake appears as a straightforward benefit. The published animal record supports none of that reassurance.

Where IGF-1 LR3 stands with regulators and in sport

Six categories get mixed together constantly here, and keeping them apart is most of the work.

FDA approval. None. IGF-1 LR3 is not an approved drug, a dietary supplement or a cosmetic, and no regulator anywhere has evaluated it for human use. The approved IGF-1 product is mecasermin, a different molecule with a different label.

Research-use-only labeling. A labeling statement is not a legal permission. Our guide to peptide legality in the US covers how the FDA's category system works and why presentation matters more than a disclaimer. The short version is that calling something research-use-only describes intended use and settles nothing by itself.

Sport. The 2026 WADA Prohibited List includes section S2.3, covering growth factors and growth factor modulators. One entry in it names insulin-like growth factor 1 and mecasermin, and its analogues. The section also closes with a catch-all for other growth factors and modulators. The covered functions are muscle, tendon or ligament protein synthesis and degradation, vascularization, energy utilization, regenerative capacity and fiber type switching. Everything in S2 is prohibited at all times, in and out of competition.

One point of precision matters here. IGF-1 LR3 is not named on the list. It is captured as an analogue of IGF-1, which is a scope judgment about the entry's wording, not a listing. Anyone claiming WADA names this compound has not read the list. Growth hormone itself sits separately, at S2.2.3.

A sports prohibition settles only sport. It does not determine general law in any jurisdiction, and the two questions should not be answered with one sentence.

Open questions. How any given jurisdiction treats possession, sale or import varies, and nothing in this article is legal advice.

What a certificate of analysis can and cannot establish here

A COA answers a chemistry question. People routinely ask it a biology question, and it has no answer.

Properly produced, a COA establishes three things. Identity comes from mass spectrometry, comparing observed mass against theoretical mass. Purity comes from HPLC, expressed as the main peak's area against total peak area. Traceability comes from a lot number matching the vial, the paperwork and the certificate. Our guide to reading a peptide certificate of analysis walks through each of those. The supplier due-diligence checklist covers how to verify the lab behind one.

Three things that guide does not cover are worth adding for this compound specifically.

Sterility is a separate test with a separate result. A purity figure says nothing about it. If a seller claims sterility testing, the result should appear, not merely the claim.

Net peptide content is not the same as vial content. Our IGF-1 LR3 product page states 1 milligram as the total peptide content of the vial as shipped. The page says plainly that this is not a working concentration. Those are different quantities, and conflating them produces real errors in a laboratory.

A purity percentage establishes nothing about biology. A 99 percent result on a lot-matched certificate describes what is in the vial. It does not establish that the material is safe or biologically active. It does not establish correct folding, or suitability for any use in a person. No analytical result can establish those things, and no supplier can honestly claim otherwise.

One catalog note, in the interest of not repeating an error we can see. This product currently sits in a "Growth Hormone Secretagogue" category on our own site. That classification is mechanistically wrong for a downstream IGF-1 receptor agonist, and correcting it is on our list.

What the evidence does not show

Stating this plainly is the most useful thing this page does.

• No human efficacy for anything. Not muscle growth, not recovery, not fat loss, not anti-aging. Zero studies have administered this analog to a person.
• No human half-life. Published figures in general circulation are unsourced, and the measured values are rat, local or intramuscular, and short.
• No human safety rate. No adverse-event frequency exists for this compound, because no human exposure has been studied.
• No dose-response relationship in any species at exposures resembling misuse.
• No long-term data anywhere.
• No head-to-head evidence against other IGF-1 analogs in the same experiment.
• No support for insulin resistance as a listed effect, despite its frequent appearance.

Two things that absence does not mean. It does not mean the compound is safe, because nobody has looked. Nor does it mean the compound does nothing, because the animal record is substantial and includes findings nobody would want. Absence of evidence is absence of evidence.

The bottom line

IGF-1 LR3 is a well-characterized research reagent with thirty years of preclinical work behind it and no human studies in front of it. The molecular story behind it is solid. Receptor engagement is real and replicated. The potency advantage is real where binding proteins compete, and it reverses where they do not.

What falls apart under inspection is everything layered on top. The half-life figures trace back to nothing. That potency figure appears to be a receptor-binding number pointing the wrong way. The one human trial people cite tested a different molecule. Reassuring safety language contradicts the animal record. That record includes accelerated tumor growth in two cancer models, and organs growing out of proportion to the animal.

Three things would change this picture: a registered human trial, a published pharmacokinetic study in people, and a toxicology package. None of those three exists today. Until they do, this compound belongs in a laboratory, and claims about what it does in a person belong nowhere.

*This article will be reviewed as trial-registry status, the primary literature, official regulatory material and our product specifications change. Last checked 16 October 2026.*