Mecasermin
GH Axis / PediatricAlso known as: Increlex, Recombinant IGF-1
Mechanism
Recombinant human IGF-1 — the actual growth factor that GH tells your liver to produce. Used for children with severe primary IGF-1 deficiency (Laron syndrome), where the GH receptor doesn't work so giving GH is useless. These children are extremely short, and mecasermin directly replaces the missing IGF-1. Requires twice-daily injections with meals due to significant hypoglycemia risk.
Technical detail
Recombinant human IGF-1 (rhIGF-1), 70-amino acid single-chain polypeptide (7.6 kDa) with 3 disulfide bonds, produced in E. coli. Identical to endogenous IGF-1. Mechanism: binds IGF-1 receptor (IGF-1R) → tyrosine kinase activation → IRS-1 phosphorylation → PI3K/Akt (growth, survival, glucose uptake) and Ras/MAPK (proliferation, differentiation) pathways. Directly stimulates longitudinal bone growth (chondrocyte proliferation, matrix synthesis at growth plates) independent of GH/GHR pathway. Indicated for severe primary IGF-1 deficiency: GH gene deletion, GHR mutations (Laron syndrome), GH antibodies, or post-GHR signaling defects where serum IGF-1 is low despite normal/elevated GH. Dosing: 0.04-0.12 mg/kg BID SC, administered within 20 min of a meal (mandatory — hypoglycemia is dose-limiting toxicity, occurring in ~42% of patients). Half-life: ~5.8 hours (vs. ~15 hours for IGFBP-3-bound endogenous IGF-1, because exogenous IGF-1 saturates available IGFBP-3). Tonsillar/adenoidal hypertrophy occurs in ~22% (IGF-1 stimulates lymphoid growth). Intracranial hypertension risk. Black box warning: lymphoid tissue hypertrophy.
Effects
ENDOCRINE/GROWTH SYSTEM: Mecasermin (Increlex) is recombinant human insulin-like growth factor-1 (rhIGF-1), a 70-amino acid protein identical to endogenous IGF-1. It is the only FDA-approved therapy for severe primary IGF-1 deficiency (SPIGFD) — conditions where the GH-IGF-1 axis fails at or downstream of the GH receptor, making exogenous GH ineffective. The classic indication is Laron syndrome (GH receptor defect), where patients have high GH but undetectable IGF-1 and are severely growth-impaired despite functional GH secretion. Mecasermin bypasses the GH receptor entirely, directly activating the IGF-1 receptor (IGF-1R) on target tissues to stimulate linear growth, protein synthesis, and metabolic regulation [RCT, clinical studies — Chernausek et al., 2007; Laron, extensive case series]. METABOLIC SYSTEM: IGF-1 has potent insulin-like metabolic effects — it lowers blood glucose via IGF-1R activation on skeletal muscle (GLUT4 translocation) and via cross-reactivity with the insulin receptor at high concentrations. This creates the most significant and dangerous side effect of mecasermin: severe hypoglycemia. Unlike insulin-induced hypoglycemia, IGF-1-induced hypoglycemia can be prolonged and refractory because IGF-1 simultaneously suppresses GH and insulin secretion (counter-regulatory hormone blunting). The hypoglycemia risk is highest in the first hour after injection and is dramatically worsened by missed meals. Lipogenic effects: IGF-1 promotes adipogenesis and can increase body fat percentage, particularly at higher doses. Protein anabolic: stimulates amino acid uptake and protein synthesis in muscle, though less potently than GH-stimulated IGF-1 in the endocrine/paracrine context. TISSUE GROWTH: IGF-1 is a potent mitogen — it stimulates proliferation of virtually all cell types. This drives the therapeutic benefit (chondrocyte proliferation → linear growth) but also the safety concerns. Lymphoid tissue hypertrophy is characteristic: tonsillar and adenoid hypertrophy occurs in 20-30% of patients, occasionally requiring tonsillectomy/adenoidectomy and posing risk for obstructive sleep apnea. Thymic hypertrophy is common. Coarsening of facial features can occur with long-term treatment. Theoretical oncogenic risk: chronic IGF-1R activation could promote tumor growth — epidemiological data on cancer risk in Laron syndrome patients (who are IGF-1 deficient) shows near-zero cancer rates, suggesting IGF-1 is indeed relevant to cancer biology. CARDIOVASCULAR: IGF-1 has cardioprotective effects at physiological levels — anti-apoptotic, anti-inflammatory, and pro-angiogenic in cardiac tissue [animal]. At therapeutic doses in SPIGFD patients, no significant cardiovascular adverse effects, though cardiomegaly has been reported rarely with long-term use.
Practitioner Guide
DOSING: Start at 0.04-0.08 mg/kg (40-80 mcg/kg) SC twice daily. Titrate by 0.04 mg/kg per dose every week as tolerated to a maximum of 0.12 mg/kg (120 mcg/kg) twice daily. CRITICAL: Dose must ALWAYS be administered within 20 minutes of a meal or snack — never on an empty stomach. If the child cannot eat or will not eat, SKIP the dose entirely. This is the single most important safety instruction. The meal must contain adequate carbohydrates and calories. ADMINISTRATION: SC injection into upper arm, thigh, or abdomen. Twice-daily dosing (with breakfast and dinner). Store refrigerated, protect from light. Use within 30 days of first puncture. Do not freeze. HYPOGLYCEMIA MANAGEMENT: Educate families extensively — hypoglycemia is the major risk. Symptoms: shakiness, sweating, pallor, irritability, confusion, seizures, loss of consciousness. Treat with rapid-acting glucose (juice, glucose tablets). Keep glucagon emergency kit available. Monitor blood glucose before each injection during initiation and dose escalation. Overnight hypoglycemia is less common with twice-daily meal-associated dosing but remains a concern — bedtime snack mandatory. Avoid strenuous exercise within 2-3 hours of injection without additional carbohydrate intake. TONSILLAR HYPERTROPHY: Monitor for snoring, mouth breathing, sleep apnea symptoms, and recurrent otitis media. Refer for sleep study if symptoms develop. Some patients require tonsillectomy/adenoidectomy — this is a recognized and expected complication, not a reason to discontinue treatment. PATIENT SELECTION: Mecasermin is ONLY for severe primary IGF-1 deficiency — defined as height SDS ≤-3.0, basal IGF-1 SDS ≤-3.0, and normal or elevated GH levels. This means: Laron syndrome (GH receptor mutations — the classic indication), GH gene deletions with anti-GH antibodies (patients who develop neutralizing antibodies to exogenous GH), and post-GH receptor signaling defects (STAT5b mutations). It is NOT indicated for secondary IGF-1 deficiency (pituitary GHD — use somatropin) or for normal-variant short stature. Off-label use in bodybuilding and anti-aging is dangerous and strongly discouraged due to the hypoglycemia risk and narrow therapeutic window. MONITORING: Height velocity (expect 4-8 cm/year gain, less robust than GH in GHD), IGF-1 levels (target normalization but avoid supraphysiological), blood glucose, tonsil size (examine oropharynx at every visit), signs of intracranial hypertension (papilledema — rare), and fundoscopic exam. CONTRAINDICATIONS: Active or suspected malignancy (IGF-1 is mitogenic), closed epiphyses, IV use (severe hypoglycemia risk). Use with caution in patients with history of sleep apnea.
Research Summary
TIER 1: Chernausek et al., 2007 — largest mecasermin efficacy study: 71 patients with SPIGFD treated up to 12 years. Height velocity improved from 2.8 to 8.0 cm/year in the first year, with sustained gains. Mean height SDS improvement of +1.9 over long-term treatment. FDA approved 2005 for SPIGFD (Increlex, Ipsen). European regulatory approval with similar labeling. Post-marketing safety data from the Increlex Growth Forum Database confirming hypoglycemia as the primary safety concern and lymphoid hypertrophy as the most common anatomical side effect. TIER 2: Laron's original case series spanning decades — foundational clinical observations of GH insensitivity treated with IGF-1 (Laron et al., 1966 — original description; treated patients from the 1990s onward). Systematic reviews of rhIGF-1 therapy in primary IGF-1 deficiency. Pharmacokinetic studies characterizing absorption, distribution, and the role of IGF-binding proteins (IGFBP-3 levels are low in SPIGFD, leading to faster IGF-1 clearance and wider glucose fluctuations). Comparison of mecasermin monotherapy vs. GH+IGF-1 combination approaches (combination may improve efficacy, but mecasermin+GH combination product was withdrawn from market). TIER 3: Endocrinology case reports of atypical SPIGFD presentations. International registry data from rare disease networks. Laron syndrome epidemiological studies (Ecuadorian cohort — Guevara-Aguirre et al., 2011 — showing cancer/diabetes resistance in GH receptor-deficient patients, indirectly informing IGF-1 biology). KEY FINDINGS: Mecasermin is a life-changing therapy for the extremely rare population with SPIGFD — without it, these children reach adult heights of 4-4.5 feet. Efficacy is real but less robust than GH in GHD (growth velocity increase is lower, and final height gain is more modest). The narrow therapeutic window (hypoglycemia vs. efficacy) makes it one of the most demanding peptide therapies to manage. GAPS: Optimal dosing not fully refined. Final adult height data limited by small patient numbers. Long-term cancer surveillance data sparse. No biomarker to predict hypoglycemia susceptibility. ACTIVE TRIALS: Limited due to orphan disease population size. Ongoing post-marketing surveillance.