Key points
- Mecasermin (recombinant IGF-1) has been FDA-approved since 2005 for growth failure from severe primary IGF-1 deficiency in children aged 2 years and older [1].
- In 76 children treated for up to 12 years, height velocity rose from 2.8 to 8.0 cm/yr in the first year. Hypoglycemia was the most common adverse event (49%) [2].
- IGF-1 LR3 has about 1000-fold lower affinity for IGFBP-3 than IGF-1 and was 5 to 10 times more potent in cultured myoblasts [4].
- In rats, IGF-1 LR3 was cleared from plasma about 11 times faster than IGF-1. Its potency is not explained by a long half-life [5].
- There are no human trials of IGF-1 LR3. Its established use is as a cell-culture media supplement [9][10].
US status (October 2026): Mecasermin is FDA-approved as Increlex [1]. IGF-1 LR3 has no status page yet. See the peptide status tracker.
IGF-1 travels bound to its carriers
Insulin-like growth factor 1 (IGF-1) is the main mediator of growth hormone. In blood it is almost always bound: according to the mecasermin label, more than 80% circulates in a complex with IGFBP-3 and an acid-labile subunit. That complex acts as a reservoir, and IGF-1 clearance is inversely proportional to IGFBP-3 concentration [1]. This biology explains both the approved drug and the lab analog your patients may be asking about.
Mecasermin is the IGF-1 that is approved
Mecasermin (Increlex) is recombinant human IGF-1, with initial FDA approval in 2005. It is indicated for growth failure in pediatric patients aged 2 years and older with severe primary IGF-1 deficiency (height and basal IGF-1 at or below -3.0 standard deviations with normal or elevated GH) or with GH gene deletion and neutralizing antibodies. The label states that it is not a substitute for GH and is not indicated in secondary forms of IGF-1 deficiency [1].
Efficacy evidence is solid within that indication. In 76 children with GH insensitivity treated for up to 12 years, height velocity rose from 2.8 cm/yr to 8.0 cm/yr in the first year (P < 0.0001) and stayed above baseline for up to 8 years [2].
The label demands monitoring
The safety profile is known and manageable. In that cohort, hypoglycemia (also observed before treatment) was reported in 49% of patients, injection-site lipohypertrophy in 32% and tonsillar or adenoidal hypertrophy in 22%. Events rarely required stopping treatment [2]. In the European Eu-IGFD registry (306 patients), the rate was 0.11 hypoglycemia events per patient per treatment year, and 0.01 serious events [3].
The label warns of severe hypoglycemia with seizures and asks for dosing within 20 minutes before or after a meal. It also describes intracranial hypertension, lymphoid tissue hypertrophy, slipped capital femoral epiphysis and scoliosis progression. It is contraindicated in active or prior malignant neoplasia, after postmarketing reports of neoplasms whose relationship to the drug is unknown. In children with primary deficiency, the estimated terminal half-life is 5.8 hours [1].
IGF-1 LR3 was engineered to escape the IGFBPs
IGF-1 LR3 (Long R3 IGF-I) was developed in Adelaide in the early 1990s. It adds the first 11 amino acids of methionyl porcine GH plus the dipeptide Val-Asn (13 residues in total) to human IGF-1 and replaces glutamic acid 3 with arginine, for 83 amino acids [6]. The result has been precisely characterized: about 1000-fold lower affinity than IGF-1 for IGFBP-3 and other IGFBPs, and 5 to 10 times greater potency in L6 myoblasts [4].
The receptor is not more strongly activated. In chicken embryo fibroblasts, which secrete no detectable IGFBPs, Long [Arg3]-IGF-I was less potent than IGF-1. Its advantage appears only where binding proteins could sequester it [6].
- About 1000x: lower affinity of IGF-1 LR3 for IGFBP-3 vs IGF-1 [4]
- About 11x: faster plasma clearance of IGF-1 LR3 in rats [5]
- 5.8 h: terminal half-life of mecasermin in children with primary deficiency [1]
Animal studies show potency, including on glucose
In growing female rats, 14 days of subcutaneous IGF-1 infusion were associated with greater weight gain, better nitrogen retention and better feed efficiency. The highest dose, 278 µg/day, produced 18 to 26% increases. With IGF-1 LR3, 44 µg/day produced similar effects. Human GH at 213 µg/day did not stimulate growth in that model [7]. In the same group’s series of comparisons, the analog was about 6 times more potent than IGF-1 for growth [4].
The potency also shows in glucose. In pigs and marmosets, the low-IGFBP-binding variants were 2 to 3 times more potent than IGF-1 at lowering blood glucose, with 4- to 8-fold greater cumulative hypoglycemia over 4 hours [8]. If a patient tells you they use IGF-1 LR3, hypoglycemia is the first risk to ask about.
The half-life is shorter, not longer
It is common to read that IGF-1 LR3 has a half-life of a day or more. The published data say the opposite. In non-pregnant rats, metabolic clearance was 9.84 ml/min/kg for IGF-1 LR3 vs 0.90 ml/min/kg for IGF-1, about 11 times faster. Most of the analog circulated as free peptide [5]. The authors summarize it this way: “an IGF variant which associates poorly with IGFBPs is removed more rapidly from the blood and is more potent than IGF-I” [4].
An anti-doping study confirmed this by another route. After an intramuscular injection in rats, intact IGF-1 LR3 disappeared from blood after 4 hours, although its degradation fragments were detected up to 16 hours [9]. The prolonged effect that has been described is pharmacodynamic: sustained hypoglycemia was seen “despite their more rapid clearance from the circulation” [8].
Its real place is the bioreactor
IGF-1 LR3 has an established use in biotechnology. In serum-free CHO cell cultures producing recombinant proteins, it sustained viability under production conditions better than insulin, the most widely used growth factor in those media [10]. In humans there is no published trial. A French anti-doping laboratory notes that these analogs “were never approved for use in humans” and circulate on the black market with lower-quality oxidized forms [9]. In another black-market product, IGF-1 LR3 was identified carrying a histidine tag, typical of laboratory reagents [11]. That is the supply chain behind most “research use only” IGF-1 LR3, covered in research-use-only peptides and peptide supplier diligence.
IGF-1 and cancer set the honest limit
IGF-1 stimulates cell proliferation, and epidemiology has studied it for two decades. A meta-analysis of 21 studies (3609 cases and 7137 controls) associated high circulating IGF-1 with more prostate cancer (OR 1.49; 95% CI 1.14 to 1.95) and premenopausal breast cancer (OR 1.65), associations the authors described as modest and varying by site [12]. These are data on the body’s own IGF-1, not on any analog. The secretagogue debate is in CJC-1295, ipamorelin and MK-677.
The mecasermin label reflects the same caution: contraindication in malignant neoplasia and monitoring for new neoplasms [1]. For IGF-1 LR3 there are no long-term safety data in any species that would allow that risk to be quantified.
In sport. WADA’s 2026 Prohibited List includes “Insulin-like growth factor 1 (IGF-1, mecasermin) and its analogues” under S2.3, prohibited at all times and as a non-specified substance [13]. Validated methods exist to detect IGF-1 LR3 and its metabolites [9].
What is proven. IGF-1 is a real therapeutic target: mecasermin changed the height of children who do not respond to GH [2]. IGF-1 LR3 is a well-characterized engineering tool, with greater potency than IGF-1 shown in culture and in several animal species [4][7][8], and a proven role in biotechnological production [10]. What is missing is the step into humans.
References
- U.S. National Library of Medicine. DailyMed: INCRELEX (mecasermin) injection, prescribing information (Initial U.S. Approval: 2005). Link
- Chernausek SD, et al. Long-term treatment with recombinant insulin-like growth factor (IGF)-I in children with severe IGF-I deficiency due to growth hormone insensitivity. J Clin Endocrinol Metab. 2007;92(3):902-10. PMID 17192294. Link
- Bang P, et al. Frequency and predictive factors of hypoglycemia in patients treated with rhIGF-1: data from the Eu-IGFD registry. J Clin Endocrinol Metab. 2023;109(1):46-56. PMID 37579214. Link
- Ballard FJ, et al. Effects of interactions between IGFBPs and IGFs on the plasma clearance and in vivo biological activities of IGFs and IGF analogs. Growth Regul. 1993;3(1):40-4. PMID 7683526. Link
- Bastian SE, et al. Plasma clearance and tissue distribution of labelled insulin-like growth factor-I (IGF-I) and an analogue LR3IGF-I in pregnant rats. J Endocrinol. 1993;138(2):327-36. PMID 7693845. Link
- Francis GL, et al. Novel recombinant fusion protein analogues of insulin-like growth factor (IGF)-I indicate the relative importance of IGF-binding protein and receptor binding for enhanced biological potency. J Mol Endocrinol. 1992;8(3):213-23. PMID 1378742. Link
- Tomas FM, et al. Anabolic effects of insulin-like growth factor-I (IGF-I) and an IGF-I variant in normal female rats. J Endocrinol. 1993;137(3):413-21. PMID 8371075. Link
- Tomas FM, et al. IGF-I variants which bind poorly to IGF-binding proteins show more potent and prolonged hypoglycaemic action than native IGF-I in pigs and marmoset monkeys. J Endocrinol. 1997;155(2):377-86. PMID 9415072. Link
- Mongongu C, et al. Detection of LongR3-IGF-I, Des(1-3)-IGF-I, and R3-IGF-I using immunopurification and high resolution mass spectrometry for antidoping purposes. Drug Test Anal. 2021;13(7):1256-1269. PMID 33587816. Link
- Morris AE, Schmid J. Effects of insulin and LongR3 on serum-free Chinese hamster ovary cell cultures expressing two recombinant proteins. Biotechnol Prog. 2000;16(5):693-7. PMID 11027158. Link
- Kohler M, et al. Detection of His-tagged Long-R3-IGF-I in a black market product. Growth Horm IGF Res. 2010;20(5):386-90. PMID 20675162. Link
- Renehan AG, et al. Insulin-like growth factor (IGF)-I, IGF binding protein-3, and cancer risk: systematic review and meta-regression analysis. Lancet. 2004;363(9418):1346-53. PMID 15110491. Link
- World Anti-Doping Agency. The 2026 Prohibited List (S2.3 Growth factors and growth factor modulators). Link
This is general information, not medical or legal advice. Rules vary by state and change. Confirm your own facts with counsel.