Mecasermin

What is Mecasermin?

Mecasermin is recombinant human insulin-like growth factor 1 — a 70-amino-acid single-chain protein with three intramolecular disulfide bridges, molecular weight 7,649 Da, whose sequence is identical to the IGF-1 the human body produces. It is made in genetically modified E. coli and sold as Increlex. Its approved use is narrow and specific: growth failure in children aged two and over with severe primary IGF-1 deficiency, or with growth hormone gene deletion who have developed neutralizing antibodies to GH. These are children who make growth hormone but cannot respond to it, so giving them more GH does nothing — the pathway is broken downstream, and replacing the missing IGF-1 directly is the only option. For anyone arriving from the performance-peptide side of the subject, mecasermin is the reference point: it is the real, regulated, unmodified IGF-1 that the research-chemical analogues such as IGF-1 LR3 and IGF-1 DES are engineered variations on, and its label documents in detail what happens when you actually give humans IGF-1 — most notably that roughly half the children in the pivotal trial experienced hypoglycaemia.

What Mecasermin Is Investigated For

Mecasermin exists for one group of patients and the evidence for them is strong. Severe primary IGF-1 deficiency is defined on the label by three simultaneous criteria — height standard deviation score at or below -3.0, basal IGF-1 SDS at or below -3.0, and normal or elevated growth hormone — a biochemical signature of a child whose GH signal cannot be converted into IGF-1. The pivotal long-term study treated 76 such children for up to 12 years and reported mean height velocity rising from 2.8 cm/year at baseline to 8.0 cm/year in the first year of treatment, with velocities remaining above baseline for up to eight years. A smaller cohort of 21 patients followed to adult or near-adult height gained a mean 13.4 cm more than predicted, with a cumulative height SDS change of about +2 — real and substantial, though the authors note plainly that most patients still did not reach the normal adult height range. Real-world registry data reinforce the pattern and add nuance: among 102 registry patients reaching near-adult height, treatment-naive prepubertal patients gained 1.4 height SDS and almost half ended up within the normal range, while only 10.5% of patients with Laron syndrome did. Starting earlier matters more than anything else. Outside this indication, mecasermin has been tested in Rett syndrome and Duchenne muscular dystrophy without reaching approval in either, and there is no evidence base, approval, or legitimate supply route for its use in healthy adults seeking muscle growth.

History & Discovery

The clinical case for recombinant IGF-1 was made by Zvi Laron, whose work defined the disease mecasermin treats. From 1966 onward Laron described a group of patients in Israel with the physical phenotype of severe growth hormone deficiency but high circulating growth hormone — a paradox that resolved only when it became clear they were insensitive rather than deficient, carrying mutations in the growth hormone receptor that left them unable to generate IGF-1 in response to GH. Laron syndrome became the textbook demonstration that growth hormone works through IGF-1 rather than directly, and it defined a patient group for whom GH therapy, the obvious treatment, was useless. The first IGF-1 treatment studies followed in the early 1990s, once recombinant IGF-1 could be produced. Klinger and Laron reported three years of IGF-I treatment in children with Laron syndrome in 1995; Backeljauw and Underwood published prolonged treatment data in growth hormone insensitivity syndrome in 1996 and extended it to six and a half to seven and a half years in 2001; a European multicentre study reported its long-term results in 1999. These were small cohorts — the disease is very rare — and the programs ran for years at academic centres before any commercial sponsor took the drug forward. Two companies eventually did, and they ended up in court. Tercica developed mecasermin as Increlex and won FDA approval on 30 August 2005 under BLA 021839. Insmed developed mecasermin rinfabate — rhIGF-1 complexed with its natural carrier protein rhIGFBP-3, intended to extend the half-life and allow once-daily dosing — and won approval for iPLEX on 12 December 2005 under BLA 021884. Patent litigation over the IGF-1 deficiency indication followed, Insmed withdrew iPLEX from the US market, and the product is listed as discontinued. Tercica was acquired by Ipsen in 2008, which has marketed Increlex since. The approval rested on a pooled long-term program whose central component treated 76 children with GH insensitivity for up to 12 years, reporting height velocity rising from 2.8 to 8.0 cm/year in the first year. Post-approval, two surveillance registries — the European Increlex Growth Forum Database and the Global IGFD Registry, sharing the registration NCT00903110 — have accumulated the real-world data that now shapes clinical understanding, particularly the finding that patients typically start treatment far too late and that outcome tracks closely with how early they start. Attempts to broaden mecasermin beyond growth failure have not succeeded. A Rett syndrome program ran from a serious preclinical hypothesis through early-phase trials without reaching approval; that indication was ultimately filled in 2023 by trofinetide, a synthetic analogue of the IGF-1-derived tripeptide glycine-proline-glutamate rather than IGF-1 itself. A Duchenne muscular dystrophy trial improved growth but not motor function. Interest in neonatal applications, particularly retinopathy of prematurity, continues and remains unproven.

How It Works

Growth hormone does not make children grow directly. It travels to the liver and other tissues and tells them to make IGF-1, and IGF-1 is what actually drives the growth plate. Some children make plenty of growth hormone but cannot make IGF-1 in response to it — the receptor or the signalling pathway is broken. Giving them more growth hormone achieves nothing, because the step that is missing is downstream of it. Mecasermin skips the broken step by supplying the IGF-1 itself.

Mecasermin is recombinant human IGF-1: 70 amino acids in a single chain with three intramolecular disulfide bridges, molecular weight 7,649 Da, sequence-identical to endogenous human IGF-1 and expressed in E. coli carrying the human IGF1 gene. It acts at the IGF-1 receptor, a transmembrane receptor tyrosine kinase structurally homologous to the insulin receptor, triggering PI3K/Akt and MAPK signalling that drives chondrocyte proliferation at the growth plate along with broader anabolic and anti-apoptotic effects. The therapeutic logic rests on the GH-IGF-1 axis: growth hormone acts on the GH receptor to induce hepatic and local IGF-1 production, and IGF-1 is the proximate mediator of linear growth. In primary IGF-1 deficiency — classically Laron syndrome, caused by GH receptor mutations, but also post-receptor signalling defects — GH is normal or elevated while IGF-1 is profoundly low, so exogenous GH is futile and exogenous IGF-1 is the only way to restore the signal. Two pharmacological consequences follow from mecasermin being native rather than engineered IGF-1. First, it binds the IGF binding proteins normally, and circulating endogenous IGF-1 travels largely as a ternary complex with IGFBP-3 and the acid-labile subunit that gives it a long half-life. Patients with GH receptor defects also have low IGFBP-3 and low acid-labile subunit, since both are GH-dependent, so administered rhIGF-1 is cleared faster than it would be in a normal carrier environment — which is the pharmacologic basis for twice-daily subcutaneous dosing and part of why the IGF-1/IGFBP-3 complex product was developed as an alternative. Second, the structural homology between IGF-1 and insulin means meaningful cross-reactivity at the insulin receptor and genuine insulin-like metabolic activity, producing the dose-limiting hypoglycaemia that dominates the drug's safety profile and dictates that it be given with food.

Evidence Snapshot

Research Gaps & Open Questions

What the current literature has not yet settled about Mecasermin:

• 01Whether decades of supraphysiologic IGF-1 exposure in childhood carries any long-term cancer risk. The malignancy contraindication reflects a theoretical concern grounded in IGF-1 biology; registry surveillance has not demonstrated an excess, but the treated population is small and follow-up into middle age is limited.
• 02How much of the shortfall in final height is attributable to late diagnosis rather than to a ceiling on what IGF-1 replacement can achieve. Registry data showing a mean treatment start at 11.8 years, and markedly better outcomes in treatment-naive prepubertal patients, suggest the answer is mostly late diagnosis — but no trial has tested very early initiation prospectively.
• 03Whether an IGF-1/IGFBP-3 complex really would have been better. The rationale for iPLEX — longer half-life, once-daily dosing, less hypoglycaemia — was never settled on clinical merit, since the product was withdrawn for commercial and legal reasons rather than failing a trial.
• 04Why mecasermin improved growth but not motor function in Duchenne muscular dystrophy, and whether that dissociation generalizes — the finding bears directly on the widespread assumption that IGF-1 is a muscle-building agent.
• 05Whether IGF-1 supplementation has any genuine role in neurodevelopmental disorders. The Rett syndrome program produced interesting early-phase signals and no approval, and the indication was ultimately filled by a GPE analogue rather than IGF-1 itself.
• 06Adult safety and pharmacology, which are essentially undocumented because adults are not treated — a gap worth stating explicitly, since it is the population most likely to encounter IGF-1 outside medical supervision.

Forms & Administration

Subcutaneous injection only. Increlex is supplied as a sterile, clear, colourless aqueous solution in a 4 mL multiple-dose vial containing 40 mg of mecasermin at 10 mg/mL, with benzyl alcohol, glacial acetic acid, polysorbate 20, sodium acetate and sodium chloride at approximately pH 5.4. The recommended starting dose is 0.04 to 0.08 mg/kg twice daily; if tolerated for at least one week the dose may be increased by 0.04 mg/kg per dose to a maximum of 0.12 mg/kg twice daily. Each dose must be given shortly before or after a meal or substantial snack, and the dose should be withheld rather than given if the child cannot eat. Injection sites — upper arm, thigh, abdomen, buttock — must be rotated to avoid lipohypertrophy, which affected roughly a third of children in the pivotal trial. Treatment should be supervised by a physician experienced in diagnosing and managing short stature associated with severe primary IGF-1 deficiency. All injectable peptides should only be administered under the guidance of a qualified healthcare provider.

Dosing & Protocols

The ranges below reflect protocols commonly discussed in the literature and by clinicians — not a prescription. Actual dosing for any individual should be determined by a qualified healthcare provider who knows the patient.

Mecasermin is approved only for growth failure in children aged 2 and over with severe primary IGF-1 deficiency or with GH gene deletion and neutralizing antibodies to GH. It is not a substitute for growth hormone in approved GH indications, is not indicated for secondary IGF-1 deficiency, and has no approved or evidenced adult use. It is contraindicated in closed epiphyses and in active or previous malignancy.

Timeline of Effects

Monitoring & Measurement

Common Questions

Who Mecasermin Is NOT For

Drug & Supplement Interactions

The interactions that matter with mecasermin are metabolic rather than hepatic. Because rhIGF-1 lowers blood glucose through IGF-1 receptor signalling and cross-reactivity at the insulin receptor, any concurrent glucose-lowering therapy compounds the risk — in a child with coexisting diabetes, insulin or oral hypoglycaemic doses typically need to be reduced when mecasermin is started or escalated, and glucose monitored closely. Pharmacologic corticosteroids work in the opposite direction, suppressing the GH-IGF-1 axis and blunting growth response, and chronic corticosteroid therapy is in fact one of the causes of secondary IGF-1 deficiency for which mecasermin is explicitly not indicated. Growth hormone is not a substitute and not a partner in the approved population, since the whole premise of primary IGF-1 deficiency is that GH cannot be converted into IGF-1; co-administration has been studied in a different population — short children with low IGF-1 but sufficient GH — and is not part of the approved use. Beta-blockers can mask the adrenergic warning symptoms of hypoglycaemia, which is a meaningful concern in a drug whose principal toxicity is hypoglycaemia. Mecasermin is a protein cleared by proteolysis rather than cytochrome P450 metabolism, so classical CYP-mediated drug interactions are not expected.

Safety Profile

Legal Status

Regulatory status changes over time. Verify current local rules with a qualified professional.

Myths & Misconceptions