Research Only Research & Experimental

IGF-1 LR3

also known as: Long R3 IGF-1, Long Arg3 IGF-1, LR3 IGF-1

A 13-amino-acid N-terminal extension plus Arg3 substitution gives IGF-1 LR3 roughly 2–3× the potency of native IGF-1 in bioassays — a research-reagent favorite that became a bodybuilding staple despite no human approval.

A modified recombinant IGF-1 analog with an N-terminal 13-residue extension and a glutamate-to-arginine substitution at position 3, engineered to reduce binding to IGF-binding proteins (IGFBP-1 through -6) and thereby prolong free-IGF-1 exposure at target tissues.

Mechanism of action

Binds the IGF-1 receptor (IGF-1R) and, with lower affinity, the insulin receptor, activating PI3K/Akt/mTOR and Ras/MAPK signaling. Drives myoblast proliferation and differentiation, protein synthesis, and in vitro cellular hypertrophy. The LR3 modifications preserve receptor binding while reducing IGFBP sequestration, so a given injected dose produces substantially more free, bioavailable IGF-1 than equivalent native IGF-1.

Primary uses

  • Research reagent for IGF-1 signaling studies
  • Cell culture (media supplementation)
  • Community bodybuilding use (unapproved, no safety data)

Typical dosing

research-only varies (subcutaneous (community))

No human clinical dosing standard. Community protocols (20–100 µg/day) carry meaningful hypoglycemia risk and unstudied long-term cancer-promotion concerns given IGF-1's role in tumor biology.

Regulatory status

Not approved for human use in any jurisdiction. Sold as a research reagent. Native recombinant human IGF-1 (mecasermin, Increlex) is FDA-approved for severe primary IGF-1 deficiency — LR3 is not.

References

  1. [pubmed] Francis GL, et al. "Insulin-like growth factor (IGF)-I and IGF-II which partially lack N-terminal amino acids have reduced affinity for IGF-binding proteins." J Mol Endocrinol, 1992;8:213-223.
  2. [pubmed] Tomas FM, et al. "Superior potency of infused IGF-I analogues which bind poorly to IGF-binding proteins is maintained when administered by injection." J Endocrinol, 1993;137:413-421.
  3. [fda-pi] Increlex (mecasermin [rDNA origin]) Prescribing Information. Ipsen Biopharmaceuticals. (Reference: native IGF-1, not LR3.)

Related peptides

IGF-1 DES(1-3)

Deleting the first three amino acids of IGF-1 cripples IGFBP binding while preserving receptor activity — giving DES(1-3) roughly 10× the in vitro potency of native IGF-1 at promoting cell growth in IGFBP-rich environments.

PEG-MGF

A pegylated fragment of a proposed muscle-specific IGF-1 splice variant — the underlying "MGF" biology remains scientifically contested, and PEG-MGF itself has no human clinical data.

MGF

The native, non-pegylated splice variant of IGF-1 produced by skeletal muscle in response to mechanical overload — extremely short-lived in vivo and therefore superseded by PEG-MGF in research-chemical markets.

Somatropin (HGH)

FDA-approved recombinant human growth hormone — the direct hormone replacement, with multiple clinical indications, prescription-only status, and specific federal criminal provisions against off-label distribution.

CJC-1295

A long-acting GHRH analog engineered with a DAC linker that binds serum albumin, extending half-life from minutes to approximately 8 days.

Ipamorelin

A selective ghrelin receptor agonist that stimulates GH release without the cortisol, prolactin, or ACTH spike seen with older GHRPs like GHRP-6.
Disclaimer

This entry is for educational purposes only and does not constitute medical advice. Dosing information reflects published regulatory or research data and is not a recommendation. Many compounds described here are not approved for human use in the United States. Consult a licensed medical professional before considering any peptide therapy.