Mecasermin is recombinant human insulin-like growth factor-1 (rhIGF-1), sold as Increlex. FDA approved it in 2005 for growth failure in children 2 years and older with severe primary IGF-1 deficiency, or with growth hormone (GH) gene deletion plus neutralizing GH antibodies (FDA Increlex label, 2025). It is not IGF-1 LR3, not IGF-1 DES, and it is not a licensed muscle drug.

TL;DR

  • Sequence: 70 amino acids, three disulfide bridges, 7649 Da, identical to endogenous human IGF-1. Made in E. coli.
  • Indication: severe primary IGFD, defined as height SDS at or below -3.0, basal IGF-1 SDS at or below -3.0, and normal or elevated GH. Not a GH substitute.
  • Pooled pediatric data: 71 subjects on the label. Mean height velocity 2.8 cm/yr before treatment and 8.0 cm/yr in year 1.
  • PK: terminal half-life about 5.8 hours after 0.12 mg/kg subcutaneous in severe IGFD. Time to peak about 2 hours.
  • Not a catalog analog. WADA names IGF-1 (mecasermin) and its analogues under S2.3. A research vial labeled LR3 is not Increlex.

Educational only. Not medical advice, not a protocol, and not a reason to treat a research vial as recombinant IGF-1.

What is mecasermin, and how is it different from IGF-1 LR3?

Mecasermin is native human IGF-1. The label describes a single 70-residue chain with three intramolecular disulfide bridges and a sequence that matches endogenous IGF-1. The protein is produced in bacteria that carry the human IGF-1 gene (FDA Increlex label, 2025).

LR3 is a different molecule. IGF-1 LR3 adds a 13-amino-acid N-terminal extension and an arginine substitution at position 3 so it binds IGF binding proteins more weakly. IGF-1 DES is a truncated analog. Neither is the Increlex sequence.

For the size primer, see Peptides 101.

Why does free IGF-1 disappear so fast?

IGF-1 is the downstream mediator of GH on linear growth. GH hits hepatic GH receptors and raises IGF-1 output. IGF-1 then activates the type 1 IGF receptor, which is homologous to the insulin receptor, and drives the uptake of glucose, fatty acids, and amino acids that growing tissue needs (FDA Increlex label, 2025).

Most circulating IGF-1 is not free. More than 80% rides as a ternary complex with IGF binding protein-3 (IGFBP-3) and an acid-labile subunit. In severe primary IGFD, IGFBP-3 is greatly reduced, so injected IGF-1 is cleared faster than it is in healthy people.

That is why the half-life on the label is short. After a single 0.12 mg/kg subcutaneous dose in children with severe primary IGFD, mean terminal half-life was 5.8 hours, mean Tmax was 2 hours, and mean Cmax was 234 ng/mL (n = 3 for those three values). Apparent clearance falls as IGFBP-3 rises (FDA Increlex label, 2025).

The same IGF-1 receptor also sits on muscle and satellite cells, which is why fitness forums imported the molecule. Muscle IGF-1 signaling runs through PI3K, Akt, and mTORC1 (Yoshida and Delafontaine, 2020). That pathway is real biology. It is not an adult hypertrophy indication.

What did the Increlex studies actually measure?

Five clinical studies, four open-label and one double-blind placebo-controlled, were pooled on the label. 71 pediatric subjects with severe primary IGFD received subcutaneous mecasermin, generally 0.06 to 0.12 mg/kg twice daily.

Mean age was 6.7 years and mean height SDS was -6.7. Of 61 subjects with at least one year of treatment, 53 (87%) had Laron syndrome (FDA Increlex label, 2025).

Year on Increlex N for height velocity Mean height velocity (cm/yr) Change from pretreatment
Pretreatment 58 2.8 baseline
Year 1 58 8.0 +5.2
Year 2 48 5.8 +2.9
Year 3 38 5.5 +2.3
Year 8 13 4.3 +0.7

The year-1 jump was highly significant. Later years stayed above baseline through year 6, then the p-values faded as the remaining N shrank (FDA Increlex label, 2025).

Chernausek and colleagues reported a closely overlapping GH-insensitivity series of 76 children. Height velocity rose from 2.8 to 8.0 cm/yr in year 1 and stayed above baseline for up to 8 years. Hypoglycemia was reported by 49%, lipohypertrophy by 32%, and tonsillar or adenoidal hypertrophy by 22% (Chernausek et al., 2007).

Adult height is not a normal-range rescue. Backeljauw followed 21 IGF-naive children for a mean 10.0 years: year-1 velocity rose from 3.1 to 7.4 cm/yr, height SDS gain was +2, and observed height was 13.4 cm more than expected without treatment. Most patients still did not reach the normal adult range (Backeljauw et al., 2013).

What safety signal shows up first?

Hypoglycemia is the labeled metabolic cost. IGF-1 can act through the insulin receptor. On the Increlex label, 30 of 71 subjects (42%) reported hypoglycemia at least once, including five severe cases and four with seizures or loss of consciousness (FDA Increlex label, 2025).

Almost half of those who reported hypoglycemia had a history of it before treatment.

Other labeled warnings include anaphylaxis, intracranial hypertension, tonsillar hypertrophy, slipped capital femoral epiphysis, scoliosis progression, and malignancy. Increlex is contraindicated with closed epiphyses and with active or prior malignant neoplasia.

This is a pediatric endocrine replacement product for a rare GH-insensitivity state. It is not a body-composition trial in trained adults.

Is a research vial the same as Increlex?

No. Increlex is an FDA-approved, multi-dose solution of native rhIGF-1 with a defined pediatric indication and a 5.8-hour half-life in the labeled IGFD population (FDA Increlex label, 2025).

A catalog vial labeled “IGF-1,” “IGF-1 LR3,” or “DES” is not that product. Sequence, salt, sterility, and dose accuracy are the usual gaps.

WADA lists insulin-like growth factor 1 (IGF-1, mecasermin) and its analogues under S2.3, prohibited at all times, along with mechano growth factors (WADA 2026 Prohibited List). Related analog write-ups: PEG-MGF and peptides versus steroids.

Licensed peptide products, when they are the topic, show up in pharmacy and telehealth channels on the provider directory and the price index. Handling education for research peptides lives on the reconstitution guide and the calculator. Clinician voices on growth-factor biology are collected on experts.

Bottom line: Mecasermin is real, FDA-approved recombinant IGF-1 with pediatric height-velocity data in severe primary IGFD. It is not IGF-1 LR3, and a research vial is not Increlex.

Educational only. Not medical advice. Not a recommendation to use mecasermin, Increlex, IGF-1 LR3, IGF-1 DES, or any research-grade IGF-1 analog.