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Research profile · Protein

IGF-1 LR3

Also known as Long R3 IGF-I, Long [Arg3] IGF-I

Animal — evidence level 2 of 4: Animal evidenceExperimental — UK regulatory status: Experimental — not authorised for human use in the UK

Overview

A laboratory-engineered analogue of insulin-like growth factor 1, used as a cell-culture reagent. Not the same molecule as recombinant human IGF-1.

Category
Protein
Highest evidence level identified
Animal evidence
Last scientifically reviewed
20 August 2026
Last regulatory review
20 August 2026

Mechanism under investigation

IGF-1 LR3 is a laboratory-engineered analogue of insulin-like growth factor 1, carrying an N-terminal peptide extension together with a single amino-acid substitution. These modifications were designed to reduce affinity for the IGF binding proteins while retaining activity at the type 1 IGF receptor, so that a greater proportion remains unbound and available to engage the receptor. It is a research reagent, and it is a structurally different molecule from recombinant human IGF-1.

Research areas

  • Cell culture reagents
  • Growth factor signalling

These are fields in which the compound has been investigated. Listing a field is not a statement that anything was demonstrated within it.

Current evidence

In-vitro evidence

Cells, tissues or biochemical systems outside a living organism.

Present, and this is where the compound is genuinely established — as a laboratory reagent rather than a therapy. It is used as a cell-culture supplement and in receptor-activation assays. The founding characterisation paper found the substituted variants showed superior potency in cells secreting IGF binding proteins, but reduced potency in cells that do not secrete them.

Animal evidence

Studies in living non-human animals.

Present, and notably mixed rather than uniformly positive. Infusion in guinea pigs increased the fractional weight of several organs but did not stimulate overall body growth. In pigs it decreased daily weight gain and food intake and suppressed endogenous growth hormone, IGF-I and binding protein concentrations. A 2025 study in growth-restricted fetal sheep found it did not promote growth while reducing circulating amino acids.

Human observational evidence

Studies observing people without assigning an intervention.

Absent. No cohort, case-series or case-report data were located. The only human-adjacent literature is anti-doping analytical chemistry, in which the analogue is among the peptides screened for in doping control blood samples.

Clinical-trial evidence

Studies assigning an intervention to human participants.

Absent. No clinical trial in humans was found, and no registered studies exist under any of the compound’s names. A 2026 review independently assigns it the lowest evidence tier and states that no peer-reviewed human studies exist, with claims resting entirely on preclinical extrapolation.

Study limitations

  • There are no human studies of any design — no trials, no observational data, no case reports. Every claim about effects in people is extrapolation from animals or cell culture.
  • Animal findings are inconsistent and include outright negative results: reduced weight gain and food intake in pigs, no body growth in guinea pigs, and no growth benefit in growth-restricted fetal sheep.
  • Several animal studies show the analogue suppresses endogenous growth hormone, IGF-I and binding protein concentrations, so the net effect on the hormonal axis may run opposite to the intuitive expectation.
  • Much of the in vitro literature uses the compound as a culture reagent under conditions chosen to grow cells, which bear no relationship to systemic exposure in an intact organism.
  • It is frequently conflated with mecasermin, recombinant human IGF-1, which is a different molecule and does hold a UK marketing authorisation. Evidence for one is not evidence for the other.

UK regulatory context

Experimental — not authorised for human use in the UK

A compound appearing in research literature that holds no UK authorisation for human use in any indication.

Source: The Human Medicines Regulations 2012, regulation 46 — prohibition on sale or supply of an unauthorised medicinal product (opens legislation.gov.uk in a new tab) (legislation.gov.uk)

Regulatory classification is jurisdiction-specific. A compound authorised elsewhere is not thereby authorised in the UK, and a supplier’s description of a substance does not determine how it is classified in law. See safety and regulation for the framework and the authoritative sources.

Safety and interpretation

Because no human study has been published, there is no direct evidence about safety in people, and the absence of reported harm reflects the absence of research rather than any demonstration of safety. A 2026 review identifies a theoretical mitogenic concern grounded in IGF-1 receptor signalling biology, and animal data show effects on insulin secretion and suppression of the endogenous growth hormone axis.

References