IGF-1 / Mecasermin (Increlex): Clinical Overview for Providers

Insulin-like growth factor 1 (IGF-1) is the mediator through which much of growth hormone's activity is expressed. As a medication it exists in recombinant form as mecasermin, marketed as Increlex, and it is FDA-approved — for a narrowly defined pediatric condition rather than for anything resembling elective use.

This guide situates IGF-1 (Mecasermin) within the broader field of peptide therapy and is written for clinicians. It is clinical education, not medical advice, and nothing here should be read as a treatment recommendation or protocol.

What is IGF-1?

IGF-1 is a 70-amino-acid peptide produced primarily by the liver in response to growth hormone. Its name reflects its structure: it is closely homologous to proinsulin, and that resemblance drives both its biology and its risks.

Most of growth hormone's anabolic and growth-promoting activity is mediated through IGF-1 rather than exerted directly. This is why serum IGF-1 is the standard laboratory proxy for growth hormone status — unlike growth hormone itself, which is pulsatile and uninterpretable on a random draw, IGF-1 is relatively stable through the day.

In circulation, IGF-1 is almost entirely bound to IGF binding proteins, principally IGFBP-3. Only a small unbound fraction is biologically active at any moment. That binding system is a regulatory buffer, and manipulating it is precisely what the unapproved analogs attempt to do.

Mecasermin and its approved indication

Mecasermin is recombinant human IGF-1 approved for severe primary IGF-1 deficiency — a rare condition in which a child produces growth hormone normally but cannot generate IGF-1 in response, typically due to growth hormone receptor or post-receptor signaling defects. It is also indicated in patients who have developed neutralizing antibodies to growth hormone.

The logic is straightforward: where the failure is downstream of growth hormone, giving more growth hormone accomplishes nothing, and replacing the missing mediator directly is the only route that works. This is genuine replacement therapy for a defined deficiency, which is quite different from supplementation in someone whose axis is intact.

Safety considerations

Hypoglycemia is the principal risk, and it follows directly from structure. Because IGF-1 resembles proinsulin, it has meaningful activity at the insulin receptor. Mecasermin must be administered shortly before or after a meal, and doses should be withheld if the patient cannot eat — a rule with no flexibility in it.

Labeling describes additional considerations including tonsillar and adenoidal hypertrophy with associated sleep apnea risk, benign intracranial hypertension, and slipped capital femoral epiphysis. Injection site reactions and lipohypertrophy occur.

Mecasermin is contraindicated in patients with active or suspected malignancy. IGF-1 is a mitogenic and anti-apoptotic signal, and its epidemiologic association with cancer risk is the reason this contraindication exists and the reason sustained elevation in anyone is not a neutral intervention.

Why this matters across the category

IGF-1 sits at the end of the pathway that every growth hormone secretagogue and GHRH analog is designed to stimulate. When a patient reports that a peptide protocol "raised their IGF-1," this is the molecule they are describing, and understanding it clarifies what that result does and does not mean.

A rise in IGF-1 confirms the axis responded. It does not by itself demonstrate a clinical benefit — a distinction borne out repeatedly by trials in this category, including anamorelin and MK-677, both of which moved the biomarker without improving function. It also carries the mitogenic considerations above, which is why targeting supraphysiologic IGF-1 is a decision that deserves more scrutiny than it usually receives.