Mecasermin (IGF-1) for Severe Growth Failure

Pediatric Growth & Short Stature

8.0 cm/yr

Mean growth velocity in the first year of mecasermin treatment, up from a baseline of 2.8 cm/year in children with severe primary IGF-1 deficiency.

Chernausek et al., Clinical Trial Data

Chernausek et al., Clinical Trial Data

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Mecasermin (brand name Increlex) is recombinant human insulin-like growth factor 1 (rhIGF-1), a peptide hormone that drives skeletal growth downstream of growth hormone. For children whose bodies produce growth hormone normally but cannot convert it into IGF-1, or whose IGF-1 receptors do not respond, mecasermin bypasses the defect entirely by providing the missing growth signal directly. It is the only FDA-approved treatment for severe primary IGF-1 deficiency (SPIGFD), a rare condition in which growth hormone is present but IGF-1 is absent or ineffective. In clinical trials, mecasermin increased growth velocity from a baseline of 2.8 cm/year to 8.0 cm/year in the first year, with children gaining an average of 13.4 cm more in adult height than they would have without treatment. For a broader overview of peptide approaches to childhood growth, see our article on short stature and peptide therapy.

The GH-IGF-1 Axis: Where Mecasermin Fits

Growth hormone does not directly make bones grow. Instead, GH travels from the pituitary to the liver, where it stimulates hepatocytes to produce IGF-1. IGF-1 then circulates to growth plates in long bones and stimulates chondrocyte proliferation and differentiation, which is the actual cellular event that produces linear growth.

This two-step pathway creates multiple potential failure points:

1. Pituitary failure (GH deficiency): The pituitary does not produce enough GH. Treatment: recombinant GH (somatropin).
2. GH receptor failure (GH insensitivity/Laron syndrome): GH is produced normally but the liver's GH receptors are mutated. The liver cannot respond to GH and does not produce IGF-1. Treatment: mecasermin (bypasses GH entirely).
3. Post-receptor signaling failure: GH binds its receptor but downstream signaling to IGF-1 production is impaired. Treatment: mecasermin.
4. GH antibodies: Patients treated with recombinant GH develop neutralizing antibodies that block its action. Treatment: mecasermin (does not require GH signaling).

In all cases where the defect is at or below the GH receptor, growth hormone therapy is ineffective. These patients need the downstream signal, IGF-1, delivered directly. Mecasermin provides that signal.

This is fundamentally different from growth hormone secretagogues like GHRP-2, which stimulate the pituitary to release more GH.^[1] Secretagogues only work when the pituitary can produce GH and the liver can convert GH to IGF-1. In children with Laron syndrome or other forms of SPIGFD, increasing GH release is futile because the pathway is broken downstream. For more on GH-releasing peptides in pediatric growth, see our article on growth hormone deficiency in children.

Diagnosing Severe Primary IGF-1 Deficiency

SPIGFD is defined by four criteria, all of which must be present:

1. Height standard deviation score (SDS) of -3.0 or below at the time of diagnosis
2. Basal IGF-1 levels below the 2.5th percentile for age and sex
3. Normal or elevated growth hormone levels (GH sufficiency confirmed by stimulation testing)
4. Exclusion of secondary causes of low IGF-1, including malnutrition, chronic disease, hypothyroidism, and GH deficiency itself

The key diagnostic distinction is between primary and secondary IGF-1 deficiency. In primary deficiency, the liver cannot produce IGF-1 despite adequate GH stimulation. In secondary deficiency, IGF-1 is low because GH itself is low (which is treated with GH replacement) or because of nutritional or systemic disease.

Laron syndrome, caused by mutations in the GH receptor gene, is the classic form of SPIGFD. Patients with Laron syndrome have extremely high circulating GH levels (because the negative feedback loop from IGF-1 is absent) but undetectable IGF-1. These patients do not respond to exogenous GH treatment at all. A large Ecuadorian cohort of Laron syndrome patients has been followed for decades, providing much of the long-term natural history data for this condition.

GH secretagogue testing can help differentiate pituitary causes from downstream causes. Bercu and colleagues showed that combined GHRH and GHRP-2 testing can evaluate pituitary GH secretory potential and distinguish GH-deficient children from slowly growing non-GH-deficient children.^[2] Children who respond normally to secretagogue stimulation but have low IGF-1 levels are candidates for mecasermin rather than GH therapy.

Clinical Evidence: Growth Outcomes

First-Year Growth Velocity

In 59 patients with pre-treatment height velocity recorded, mecasermin treatment increased mean growth velocity from 2.8 cm/year at baseline to 8.0 cm/year in the first year. This represents a near-tripling of growth rate and is comparable to the first-year growth response seen with recombinant GH in GH-deficient children.

Growth velocity remained significantly above baseline through year six of treatment, though with gradual decline typical of all growth-promoting therapies: 5.8 cm/year in year two, 5.5 cm/year in year three, 4.7 cm/year in years four and five, and 4.8 cm/year in year six.

Adult Height Gains

Twenty-one children treated for an average of 10 years reached adult or near-adult height. They were an average of 13.4 cm taller than predicted without treatment. An independent modeling analysis estimated an average total height gain of 12.5 cm attributable to mecasermin versus no treatment.

These gains are meaningful but modest compared to GH therapy in GH-deficient children (which typically achieves 15 to 20 cm of adult height gain). The difference likely reflects the inherent limitations of replacing a single downstream factor (IGF-1) versus restoring the full hormonal cascade that GH initiates. IGF-1 has important local (paracrine) effects at the growth plate that circulating IGF-1 may not fully replicate.

Limitations of the Evidence

The pivotal clinical data for mecasermin comes from open-label, single-arm studies without randomized control groups. This design was necessary because withholding treatment from children with known SPIGFD would be ethically untenable. The absence of a concurrent control group means that height gains cannot be definitively attributed to mecasermin versus natural growth variability, though the magnitude of change (from 2.8 to 8.0 cm/year) makes spontaneous improvement an unlikely explanation.

Dosing and Administration

Mecasermin is administered by subcutaneous injection twice daily. The recommended starting dose is 0.04 mg/kg per injection, titrated up to a maximum of 0.12 mg/kg per injection based on growth response and tolerability.

Critical administration rules:

• Must be given with meals or a snack. IGF-1 shares structural homology with insulin and can cause hypoglycemia. Food provides glucose to buffer this effect.
• Two daily injections are required because mecasermin has a shorter half-life than endogenous IGF-1 bound to IGFBP-3 and the acid-labile subunit (ALS). Endogenous IGF-1 circulates in a ternary complex that extends its half-life to approximately 16 hours. Exogenous mecasermin lacks this complex formation and clears faster.
• Injection sites include the upper arm, thigh, or abdomen, with systematic rotation to prevent lipohypertrophy.

The twice-daily injection burden is a significant practical consideration for pediatric patients. Several research programs are investigating longer-acting IGF-1 formulations, including IGF-1/IGFBP-3 complex preparations that more closely mimic the endogenous ternary complex and could potentially reduce injection frequency.

Adverse Effects and Safety

Hypoglycemia

The most clinically significant adverse effect is hypoglycemia, reported in clinical trials at rates substantially higher than placebo. The mechanism is direct: IGF-1 has approximately 1/10th the potency of insulin at the insulin receptor, but at therapeutic doses, this cross-reactivity is sufficient to lower blood glucose. The risk is highest when mecasermin is administered without food, during the first weeks of treatment, and in patients with lower body weight.

Tonsillar and Adenoid Hypertrophy

IGF-1 promotes tissue growth broadly, not just at growth plates. Enlargement of the tonsils and adenoids is a recognized adverse effect that can cause snoring, sleep apnea, and in severe cases may require surgical removal. This effect reflects IGF-1's role as a general tissue growth factor.

Intracranial Hypertension

Pseudotumor cerebri (benign intracranial hypertension) has been reported with mecasermin use, as it has with GH therapy. Symptoms include headache, visual changes, and papilledema. This appears to be a class effect of growth-promoting hormones rather than specific to IGF-1.

Theoretical Cancer Risk

IGF-1 promotes cell proliferation and inhibits apoptosis, pathways that overlap with oncogenesis. Elevated endogenous IGF-1 levels have been epidemiologically associated with increased risk of certain cancers (colon, breast, prostate). Whether exogenous IGF-1 replacement at physiological levels in SPIGFD patients increases cancer risk is unknown. Long-term registry data has not identified a clear cancer signal, but the small patient population and limited follow-up duration constrain the statistical power to detect rare events.

A review of GH secretagogue safety by Sigalos and Pastuszak noted similar theoretical concerns about chronic GH/IGF-1 axis stimulation and emphasized the need for long-term safety monitoring in all patients receiving growth-promoting peptide therapies.^[6]

How Mecasermin Compares to Other Growth Peptides

The GH/IGF-1 axis can be stimulated at multiple levels, and each approach has distinct advantages and limitations:

Growth hormone (somatropin): The standard treatment for GH deficiency. Does not work in SPIGFD because the defect is downstream of GH.

GHRH analogs (CJC-1295, sermorelin): Stimulate the pituitary to release more GH. Teichman and colleagues showed CJC-1295 produced sustained, dose-dependent increases in GH and IGF-I levels for 6 to 11 days after a single injection.^[3] These agents require intact pituitary and hepatic GH-to-IGF-1 conversion.

Growth hormone secretagogues (GHRP-2, GHRP-6, ibutamoren): Stimulate GH release through the ghrelin receptor pathway. Pihoker and colleagues demonstrated that intranasal GHRP-2 increased growth velocity from 3.7 to 6.1 cm/year in children with short stature over 6 months.^[1] Codner and colleagues showed oral ibutamoren increased GH, IGF-1, and IGFBP-3 in GH-deficient children.^[4] Like GHRH analogs, these agents cannot help patients with downstream IGF-1 deficiency. GH secretagogue treatment has also been shown to raise IGF-1 levels in adults, with Sigalos and colleagues demonstrating an increase from 159.5 to 239.0 ng/mL with GHRP/sermorelin combination therapy.^[5]

Mecasermin (rhIGF-1): The only option that bypasses the entire GH signaling cascade. Effective specifically when the defect is at or below the GH receptor. Does not require any pituitary or hepatic function.

This hierarchy explains why accurate diagnosis is essential. A child with GH deficiency treated with mecasermin would receive an expensive, twice-daily injection regimen when once-daily GH would be more effective. A child with Laron syndrome treated with GH would receive no benefit at all.

Current Status and Access

Mecasermin remains available but is used by a very small patient population. SPIGFD is rare: estimates suggest fewer than 5,000 patients worldwide meet the strict diagnostic criteria. The drug's cost is substantial, and insurance coverage varies by country and insurer.

The European IGFD Registry and the US National Cooperative Growth Study provide ongoing safety and efficacy monitoring. Near-adult height data from these registries confirms the long-term benefit observed in the pivotal trials, with height gains of 12 to 16 cm over predicted untreated adult height.

Research continues into next-generation IGF-1 therapies: longer-acting formulations, IGF-1/IGFBP-3 complexes that better mimic endogenous physiology, and combination approaches pairing mecasermin with other growth-promoting agents. For a landscape view of all peptide approaches to growth failure, see our article on short stature and peptide therapy.

The Bottom Line

Mecasermin is recombinant IGF-1, the only FDA-approved treatment for severe primary IGF-1 deficiency. It bypasses the growth hormone signaling cascade entirely to deliver the downstream growth signal directly. Clinical evidence shows first-year growth velocity increases from 2.8 to 8.0 cm/year, with adult height gains averaging 13.4 cm over untreated predictions. Key safety concerns include hypoglycemia, tonsillar hypertrophy, and theoretical cancer risk from chronic IGF-1 exposure. The drug fills a niche that GH therapy and GH secretagogues cannot address: conditions where the defect lies at or below the GH receptor.

Frequently Asked Questions

What is mecasermin used for?

Mecasermin (brand name Increlex) is used to treat growth failure in children with severe primary IGF-1 deficiency (SPIGFD). This includes children with Laron syndrome (GH receptor mutations), GH gene deletion with neutralizing antibodies to GH replacement, and other conditions where the body produces growth hormone but cannot convert it to IGF-1. It is not indicated for common growth hormone deficiency, which is treated with recombinant GH instead.

How much does mecasermin increase height?

In clinical trials, mecasermin increased growth velocity from a baseline of 2.8 cm/year to 8.0 cm/year in the first year of treatment. Children treated for approximately 10 years reached adult heights an average of 13.4 cm taller than predicted without treatment. Growth velocity remains elevated but gradually decreases over subsequent years, which is consistent with all growth-promoting therapies as children approach skeletal maturity.

What are the side effects of mecasermin?

The most common adverse effects are hypoglycemia (because IGF-1 cross-reacts with insulin receptors), tonsillar and adenoid hypertrophy (because IGF-1 promotes tissue growth broadly), and injection site reactions. Less common but serious adverse effects include intracranial hypertension (pseudotumor cerebri) and theoretical long-term cancer risk from chronic IGF-1 exposure. Hypoglycemia risk is managed by administering mecasermin with meals.

What is severe primary IGF-1 deficiency?

Severe primary IGF-1 deficiency (SPIGFD) is defined by four criteria: height standard deviation score of -3.0 or below, IGF-1 levels below the 2.5th percentile for age and sex, normal or elevated growth hormone levels (confirming GH sufficiency), and exclusion of secondary causes of low IGF-1 (malnutrition, chronic disease, hypothyroidism). Laron syndrome, caused by GH receptor gene mutations, is the classic form. The condition is rare, with fewer than 5,000 patients estimated worldwide.

Why can't growth hormone treat Laron syndrome?

Laron syndrome is caused by mutations in the growth hormone receptor gene. The liver's GH receptors are nonfunctional, so growth hormone cannot bind and stimulate IGF-1 production regardless of how much GH is present. Patients with Laron syndrome typically have very high circulating GH levels because the normal negative feedback loop (IGF-1 suppresses GH release) is absent. Mecasermin bypasses this defect entirely by providing IGF-1 directly, without requiring any GH receptor function.

How is mecasermin different from growth hormone therapy?

Growth hormone therapy (somatropin) provides recombinant GH, which must be converted to IGF-1 by the liver to promote growth. It works for GH deficiency but fails in conditions where the liver cannot respond to GH (Laron syndrome, GH receptor defects). Mecasermin provides IGF-1 directly, bypassing the GH step entirely. The trade-off: mecasermin requires twice-daily injections (versus once-daily for GH), has a higher hypoglycemia risk (due to IGF-1's insulin-like activity), and costs more per year of treatment.