IGF-1 LR3

IGF-1 LR3 (Long R3 Insulin-like Growth Factor-1) is a synthetic analogue of human IGF-1 that differs from the native hormone in two ways: a 13-amino acid N-terminal extension and an arginine (Arg) substitution at position 3 in place of the native glutamic acid. These modifications dramatically reduce affinity for IGF-binding proteins (IGFBPs) — by at least three orders of magnitude relative to native IGF-1 — and extend the in vivo half-life from under 10 minutes (for free IGF-1) to approximately 20–30 hours. The reduced IGFBP binding means a far greater proportion of administered IGF-1 LR3 remains in free, bioactive form. In research and biotechnology settings, IGF-1 LR3 is a well-established cell culture reagent widely used to support the growth of CHO, HEK293, and other mammalian cell lines in serum-free media, where it can replace insulin at concentrations roughly 200-fold lower. It signals through the same IGF-1 receptor (IGF-1R) as native IGF-1, activating the PI3K/Akt/mTOR and MAPK/ERK pathways that drive protein synthesis, cell survival, and proliferation. IGF-1 LR3 has no approved therapeutic application in humans. It is not an approved drug anywhere in the world. In competitive sport it is prohibited under WADA category S2 (Peptide Hormones, Growth Factors, Related Substances and Mimetics), both in- and out-of-competition. It appears in performance and bodybuilding communities anecdotally as an agent believed to enhance muscle hypertrophy, but no human clinical trials have evaluated it for this or any other therapeutic purpose in humans. All human use occurs outside any regulatory framework, without validated safety or efficacy data, and carries meaningful risks including hypoglycemia and theoretically tumour promotion via IGF-1R. This entry covers what is established in the peer-reviewed scientific literature and is for research reference only.