What it is
IGF-1 is a 70-amino-acid protein, not a short peptide. The Increlex label describes it as a single chain held in shape by three internal disulfide bridges, weighing 7,649 Da. UniProt (P05019) shows it is cut from a 195-residue precursor: a signal peptide at the front, a short propeptide, the 70-residue hormone, then a 77-residue tail called the E peptide.
It is also called somatomedin C. The "insulin-like" in its name is structural — it resembles insulin — and the label's commonest side effect shows the resemblance is functional too.
How it works
Growth hormone does not drive growth alone: it triggers production of IGF-1, which UniProt describes as related to insulin in structure and function but with much higher growth-promoting activity. In blood, the label states, more than 80% of IGF-1 travels bound to IGF binding protein 3 and a partner called the acid-labile subunit, which slows its clearance.
That is why severe primary IGF-1 deficiency is its own condition. The children have normal or high growth hormone but cannot turn it into IGF-1 — most in the label's trials had Laron syndrome, which UniProt (P10912) describes as a dysfunctional growth-hormone receptor and a failure to generate IGF-1 in response to growth hormone. Giving more growth hormone does nothing; giving IGF-1 bypasses the broken step.
| Measure (Increlex label) | Figure |
|---|---|
| Bioavailability under the skin, healthy adults | Close to 100% |
| Half-life, children with the deficiency (0.12 mg/kg) | 5.8 hours |
| Where it is broken down | Liver and kidney |
The trial record
The label pools five studies — four open-label, one placebo-controlled — in 71 children with severe primary IGF-1 deficiency. At the start they averaged 6.7 years old, 84.8 cm tall and 6.7 standard deviations below average height. Of the 61 with at least a year of treatment, 53 had Laron syndrome, 7 a growth-hormone gene deletion and 1 neutralising antibodies.
| Year of treatment | Children | Height velocity (cm/year) | Change from pre-treatment | p |
|---|---|---|---|---|
| Before | 58 | 2.8 | — | — |
| 1 | 58 | 8.0 | +5.2 | < 0.0001 |
| 2 | 48 | 5.8 | +2.9 | < 0.0001 |
| 4 | 23 | 4.7 | +1.5 | 0.0045 |
| 8 | 13 | 4.3 | +0.7 | 0.31 |
Two limits sit in that table: the comparison is each child against their own past, not against an untreated group, and the number followed shrinks from 58 to 13. Height stayed far below average — the mean standard-deviation score moved from −6.7 to −5.2 by year 8.
Safety
The label's warnings come from IGF-1 acting like insulin and like a growth signal:
- Low blood sugar — 30 of 71 children (42%) at least once; 5 needed help, 4 had seizures or lost consciousness. It was most frequent in the first month and in younger children.
- Tonsil and adenoid growth — 11 children (15%); 7 had them removed.
- Raised pressure inside the skull in three children, slipped capital femoral epiphysis, worsening scoliosis.
- Malignant tumours reported after marketing. The label says a link is unknown, that most cases were in children with cancer-predisposing conditions, and that they were seen more often at doses lifting IGF-1 above the normal range.
- Benzyl alcohol in the vial, which the label ties to fatal "gasping syndrome" in infants.
Where it stands
Mecasermin is approved in the US for the one paediatric indication above. Drugs@FDA lists Increlex (BLA 021839, 2005-08-30) as a prescription product; a second form, Iplex (mecasermin rinfabate, BLA 021884, 2005-12-12), is listed as discontinued (openFDA, 2026-10-05). The label requires supervision by a physician experienced in these children; dose is set by weight and adjusted against blood sugar. PubMed indexes 103 records for mecasermin, 5 tagged as randomised controlled trials (2026-10-05).
No adult use is approved. Claims made for IGF-1 in muscle building or anti-ageing rest on no trial in this label.
Related reading
The modified research compound sold under a similar name has its own entry as IGF-1 LR3. The growth-hormone releasers tesamorelin, sermorelin and ipamorelin raise IGF-1 indirectly, by raising growth hormone first, while drugs such as lanreotide are judged partly by how far they push it down in acromegaly.