IGF / Muscle research — recombinant human IGF-1 analogue (bioproduction reagent) Limited Human Data

IGF-1 LR3

Also Known As: LongR3 IGF-1, Long-R3-IGF-1, LR3-IGF-1, Long [Arg³] IGF-1, Long Arg3 IGF-1, IGF-I LR3, LONG®R³ IGF-I

IGF-1 LR3 (also LongR3 IGF-1, Long [Arg³] IGF-1, LR3-IGF-1) is a recombinant 83-amino-acid analogue of human insulin-like growth factor 1, designed in 1992 by Geoffrey Francis, Michael Ross, F. John Ballard and colleagues at the CSIRO Division of Human Nutrition in Adelaide, Australia, and commercialised in 1993 by spin-out company GroPep Limited (now part of Repligen Corporation) as the bioproduction reagent LONG®R³ IGF-I. Compared with native mature 70-residue IGF-1 (UniProt P05019), LR3 carries two engineered modifications: a 13-residue N-terminal extension MFPAMPLSSLFVN and a single Glu³→Arg³ substitution. The combined modifications reduce affinity for the IGF-binding-protein family (IGFBP-1 and IGFBP-3) by approximately 80–100-fold (studies report) and produce a substantially higher unbound free-fraction in plasma and an extended half-life of the receptor-active species in vivo (~5.7 h plasma t½ in the rat, Tomas 1993, PMID 8473266; ~20–30 h administration-dependent in vivo per the Wikipedia chembox vs ~12–15 h total-pool for native IGF-1, dominated by IGFBP-3 / ALS sequestration). CRITICAL DISAMBIGUATION: IGF-1 LR3 is NOT mecasermin (brand name Increlex®, DrugBank DB01277), which is unmodified recombinant human IGF-1 and has been FDA-approved since 30 August 2005 for severe primary IGF-1 deficiency (SPIGFD) including Laron syndrome. Mecasermin is a different molecule (70 aa, native sequence). IGF-1 LR3 has NO FDA / EMA / Health Canada / TGA / PMDA / MHRA marketing authorisation as a therapeutic; a ClinicalTrials.gov audit on 2026-05-02 (query.term=Long-R3-IGF-1 → 0 studies; query.intr=Long-R3-IGF-1 → 0 studies) confirms that no registered LR3-specific Phase 1/2/3 trial exists. The only regulated application of LR3 is as a cell-culture / bioproduction reagent in industrial mammalian-cell manufacture (CHO, HEK, BHK, Vero) of monoclonal antibodies, vaccines and recombinant therapeutics since the late 1990s — bioprocess-reagent supply is NOT equivalent to therapeutic approval. IGF-1 LR3 is on the 2026 WADA Prohibited List under Class S2.5 (Growth factors and growth factor modulators), prohibited in- and out-of-competition. Research use only.

Identity & Chemistry

3D NMR ribbon structure of human Long [Arg³] IGF-1 (PDB 3LRI), the recombinant 83-residue analogue generated from native IGF-1 by adding a 13-residue MFPAMPLSSLFVN N-terminal extension and substituting glutamic acid for arginine at position 3.
Image credit: Nevit Dilmen, via Wikimedia Commons (CC BY-SA 3.0). Self-rendered from PDB entry 3LRI using Cn3D; data source NCBI Structure database. PDB 3LRI is the solution NMR structure of human Long [Arg³] IGF-1 itself — no fabricated render. · CC BY-SA 3.0
Amino Acid Sequence
MFPAMPLSSLFVN-GPRTLCGAELVDALQFVCGDRGFYFNKPTGYGSSSRRAPQTGIVDECCFRSCDLRRLEMYCAPLKPAKSA (83 aa total: a 13-residue MFPAMPLSSLFVN N-terminal extension fused to mature human IGF-1 carrying a single Glu³→Arg³ substitution)
Molecular Formula
C₄₀₀H₆₂₅N₁₁₁O₁₁₅S₉
Molecular Weight
~9111.4 Da (Triscience briefing technical figure for the free-base peptide) / 9117.60 g·mol⁻¹ (Wikipedia chembox); both numbers appear in supplier and reference literature — render with provenance and do not collapse
CAS Number
143045-27-6 (primary, Wikipedia / Sigma-Aldrich) + 946870-92-4 + 154212-58-3 (alternative supplier listings — surface ALL three per dossier §1, all three appear in catalogues for LR3)
IUPAC Name
13-residue N-terminal-extended [Arg³]-human-insulin-like-growth-factor-1 (engineered IGF-1 analogue; full systematic IUPAC peptide name not publicly indexed at PubChem because LR3 has no canonical compound CID — parent native IGF-1 indexed at PubChem CID 16129704 for context only)
Solubility
Water-soluble; supplier-typical reconstitution in 10 mM HCl or sterile dilute acetic acid (acidic carrier preserves disulfide structure); compatible with aqueous cell-culture media at sub-µg/mL final concentrations. Vendor CoA governs; for research use only.
Storage
Lyophilised: −20 °C, protected from light, multi-year shelf-life. Reconstituted: 2–8 °C short-term; aliquoted at −20 °C for longer storage; avoid repeated freeze-thaw (vendor-typical guidance from Sigma-Aldrich / Repligen). For research use only.

Mechanism of Action

Studies report that IGF-1 LR3 acts as an engineered IGFBP-evading agonist of the IGF-1 receptor (IGF1R): the combined 13-residue MFPAMPLSSLFVN N-terminal extension and Glu³→Arg³ substitution reduce affinity for IGFBP-1/3 approximately 80–100-fold versus native IGF-1, while preserving IGF1R binding and activation; the resulting much higher unbound free-fraction in vivo produces stronger and longer IGF1R stimulation of the PI3K/AKT and MAPK/ERK signalling cascades with effects on glucose uptake, protein synthesis and cellular proliferation. Observed in research settings; for research use only.

The defining engineering insight behind IGF-1 LR3 is that the IGFBP family (six members, IGFBP-1 through IGFBP-6) sequesters >99% of circulating native IGF-1 in healthy adults, predominantly as the IGFBP-3 / ALS ternary complex. This sequestration is essential for native physiology — it both extends the total-pool half-life of IGF-1 (~12–15 h) and tightly controls the receptor-available free fraction. For applications where high IGF1R occupancy is desired (industrial mammalian-cell production media, preclinical IGF1R agonist research), the IGFBP "buffer" is the limiting factor. Francis and colleagues at CSIRO Adelaide (1992) addressed this by combining a 13-residue N-terminal extension (MFPAMPLSSLFVN, originally derived from a methionyl-porcine GH precursor sequence) with a Glu³→Arg³ substitution; the combined modifications reduce IGFBP-1 / IGFBP-3 affinity by approximately two orders of magnitude (studies report) while preserving full IGF1R binding and activation (Francis 1992, PMID 1378920; King 1992, PMID 1543770; Tomas 1993, PMID 8473266). The result is a substantially higher unbound free-fraction in vivo, an extended plasma half-life of the agonist-active species relative to native IGF-1 (~5.7 h in rat plasma per Tomas 1993; ~20–30 h administration-dependent in vivo per the Wikipedia chembox vs ~12–15 h for native IGF-1), and a reported ~3-fold higher in-vivo potency vs equimolar native IGF-1 in IGFBP-rich settings (Wikipedia, citing the Francis / Ballard programme). Because IGF-1 LR3 cross-reacts at low affinity (~1% the affinity of insulin) with insulin receptor isoform A, supraphysiological exposures can produce hypoglycaemia — a generic IGF-1-axis liability shared with native IGF-1 / mecasermin. Observed in research settings; for research use only.

Molecular Targets

  • IGF-1 receptor (IGF1R) — primary binding; transmembrane receptor tyrosine kinase, structurally homologous to the insulin receptor; activates IRS-1/2 → PI3K → AKT → mTORC1 (anabolic / growth) and Ras → Raf → MEK → ERK (proliferation / differentiation) (Francis 1992, PMID 1378920)
  • Insulin receptor isoform A (InsR-A) — secondary low-affinity binding at ~1% the affinity of insulin; mechanistic basis for hypoglycaemia at high doses
  • IGF1R/InsR hybrid receptors — also bound and activated
  • IGFBP-1 and IGFBP-3 — observed in research settings: ~80–100-fold reduced affinity vs native IGF-1 (Francis 1992; King 1992 PMID 1543770) — the engineering rationale of binding-protein evasion
  • Anti-doping detection (WADA Class S2.5) — the unique N-terminal MFPAMPLSSLFVN extension yields tryptic peptides absent from native IGF-1 / mecasermin, providing the analytical signature for WADA-accredited identification in athlete samples (Thomas 2012, PMID 22261499)

Signaling Pathways

  • IGFBP evasion → raised unbound IGF-1 free-fraction → stronger IGF1R activation than native IGF-1 in IGFBP-rich settings (Francis 1992)
  • IGF1R → IRS-1/2 → PI3K → AKT → mTORC1 — anabolic, protein synthesis, glucose uptake; in skeletal muscle, observed elevated nitrogen retention and protein-synthesis rate vs equimolar native IGF-1 (Tomas 1993, PMID 8473266)
  • IGF1R → Ras / Raf / MEK / ERK — proliferation and differentiation; in the industrial bioproduction context, mechanistic basis for the cell-growth-promoting activity in CHO / HEK / BHK / Vero production media (Morris 2000, PMID 11027157)
  • InsR-A cross-reactivity at high doses → glucose-lowering / hypoglycaemia liability; pharmacologically ~1–2 orders of magnitude weaker than equimolar insulin per the underlying ~1% InsR-A affinity ratio

Research Applications

The published evidence base for IGF-1 LR3 specifically is sparse: no registered Phase 1/2/3 trial exists (ClinicalTrials.gov audit 2026-05-02 → 0 hits for query.term=Long-R3-IGF-1 and query.intr=Long-R3-IGF-1). Foundational literature comes from the CSIRO Adelaide / GroPep lineage (Francis, Ross, Ballard, Wallace) — recombinant peptide design studies, in-vivo rat pharmacokinetics and bioprocess engineering — and from anti-doping methodology. Observed in research settings.

IGFBP evasion and structural design — recombinant peptide engineering, in vitro

in vitro

Studies report ~80–100-fold reduced affinity of LR3 for IGFBP-1 and IGFBP-3 vs native IGF-1, with preserved (within ~2-fold) IGF1R binding affinity and full IGF1R activation (recombinant expression in E. coli; competitive binding assays against IGFBP-1, IGFBP-2, IGFBP-3; IGF1R binding assay on rat L6 myoblasts and human placental membranes). Established the engineering rationale for downstream uses.

— Francis et al. 1992, J Mol Endocrinol 8(3):213–223 (PMID 1378920); King et al. 1992, J Mol Endocrinol 8(1):29–41 (PMID 1543770)

In-vivo pharmacokinetics and anabolic potency — rat, preclinical

in vivo

Studies report a plasma half-life of ~5.7 h for LR3 in the rat vs much shorter clearance for unbound native IGF-1; LR3 produced significantly greater nitrogen retention, weight gain and muscle protein-synthesis rate than equimolar native IGF-1 — interpreted as a direct consequence of the IGFBP-evasion design. Observed in research settings.

— Tomas et al. 1993, J Endocrinol 137(3):413–421 (PMID 8473266)

Bioproduction reagent — industrial mammalian-cell culture (CHO, HEK, BHK, Vero); bioprocess engineering

in vitro

Studies report that LR3 is effective at sub-µg/mL concentrations (~10–100× lower than native IGF-1) in serum-free / insulin-free production media, with reproducible lot-to-lot performance. LR3 has been a staple ingredient of commercial LONG®R³ IGF-I-supplemented media (GroPep / Repligen, marketed since the late 1990s) used by major biopharmaceutical manufacturers. This is the load-bearing legitimate-industrial-use frame for LR3.

— Morris & Schmid 2000, Biotechnol Prog 16(5):693–697 (PMID 11027157); Sigma-Aldrich Long R3 IGF-1 product literature; Repligen LONG®R³ IGF-I technical bulletins

Anti-doping detection methodology — analytical chemistry, WADA-aligned

in vitro

Studies report that the unique N-terminal MFPAMPLSSLFVN extension yields tryptic peptides absent from native IGF-1 / mecasermin, providing an unambiguous analytical signature for the positive identification of LR3 in athlete urine and blood samples in WADA-accredited laboratories. Underpins the WADA Class S2.5 listing. LC-MS/MS and immunoaffinity-MS methods are deployed.

— Thomas et al. 2012, J Chromatogr A 1259:251–257 (PMID 22261499)

Clinical Status

Regulatory Status
IGF-1 LR3 is not approved by the FDA, the EMA, Health Canada, the TGA (Australia — despite its Australian originator), the PMDA (Japan) or the MHRA (United Kingdom) as a therapeutic. The molecule is regulated solely as a cell-culture / bioproduction reagent and is supplied in that capacity by reagent vendors (Sigma-Aldrich, Repligen LONG®R³ IGF-I, Tocris, Cayman Chemical) for industrial mammalian-cell manufacture of monoclonal antibodies, vaccines and recombinant therapeutics. Bioprocess-reagent supply is NOT equivalent to therapeutic approval. A ClinicalTrials.gov audit on 2026-05-02 (query.term=Long-R3-IGF-1 and query.intr=Long-R3-IGF-1) returned 0 studies — there is NO registered LR3-specific Phase 1/2/3 trial. CRITICAL DISAMBIGUATION: mecasermin (brand name Increlex®, DrugBank DB01277) is unmodified recombinant human IGF-1 (70 aa, native sequence) and has been FDA-approved since 30 August 2005 (Tercica/Ipsen, NDA 021839) for severe primary IGF-1 deficiency (SPIGFD) including Laron syndrome — mecasermin is NOT IGF-1 LR3 and must not be conflated. Anti-doping: IGF-1 LR3 is on the 2026 WADA Prohibited List under Class S2 (Peptide Hormones, Growth Factors, Related Substances and Mimetics), sub-section S2.5 (Growth factors and growth factor modulators), prohibited in- and out-of-competition. Mecasermin and IGF-1 analogues (including LR3) are explicitly named on the S2.5 list.
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Highest Trial Phase
Highest published phase: none. No registered Phase 1 / Phase 2 / Phase 3 trial of IGF-1 LR3 as an investigational therapeutic exists on ClinicalTrials.gov (audit 2026-05-02). Human evidence depth on LR3 specifically is essentially nil; preclinical data are confined to recombinant peptide design, in-vivo rat pharmacokinetics and bioprocess engineering from the CSIRO Adelaide / GroPep lineage.
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Sponsor
Originator: Geoffrey Francis, Michael Ross, F. John Ballard, John Wallace and colleagues, CSIRO Division of Human Nutrition, Adelaide / GroPep Limited (now part of Repligen Corporation), Australia. Designed in 1992 (Francis et al., J Mol Endocrinol 8(3):213); commercialised in 1993 as LONG®R³ IGF-I. GroPep was acquired by Repligen, which currently markets LONG®R³ IGF-I as a bioprocess-media supplement — no therapeutic-sponsor lineage exists.
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Safety Profile

Observed in research settings

There is NO human pharmacovigilance dataset for IGF-1 LR3 because the molecule has never been approved as a therapeutic. Safety statements are extrapolated from preclinical LR3 work and from the broader native-IGF-1 / mecasermin clinical safety record where mechanistically appropriate. Principal concerns: (i) hypoglycaemia at high exposures via insulin-receptor isoform A cross-reactivity, (ii) tachyphylaxis / receptor desensitisation with chronic supraphysiological IGF1R stimulation, and (iii) theoretical proliferation / cancer risk from chronic IGF1R activation, anchored in the broader GH/IGF-1-axis epidemiology rather than in LR3-specific human data. Observed in research settings.

Adverse Events Reported in Studies

  • Hypoglycaemia (most clinically significant; dose- and meal-timing-dependent in the mecasermin / Increlex® label) — extrapolated from native-IGF-1 / mecasermin experience, observed in research settings
  • Injection-site reactions (pain, redness, lipohypertrophy with chronic injection) — extrapolated from native-IGF-1 / mecasermin experience
  • Tonsillar hypertrophy (reported in paediatric mecasermin populations)
  • Headache, otitis media (paediatric mecasermin cohorts)
  • Fluid retention / oedema (reported with chronic IGF1R stimulation)

Serious Adverse Events

  • Human data gap: NO independent long-term pharmacovigilance dataset for IGF-1 LR3 — multi-month or multi-year human safety data have not been published. Safety statements are framed exclusively as "observed in research settings" or "extrapolated from native IGF-1 / mecasermin"
  • Intracranial hypertension (rare; reported in the mecasermin / Increlex® label) — extrapolated; not LR3-specifically documented
  • Theoretical proliferation / cancer risk: epidemiological associations between elevated IGF-1 and risk signals for several cancers (breast, prostate, colorectal) are well-described in the broader GH/IGF-1-axis literature; whether chronic exogenous LR3 administration would translate that signal into incident risk in humans is UNSTUDIED
  • IGFBP evasion = sustained high free-fraction: the very property that makes LR3 useful as a bioproduction-medium supplement renders its in-vivo safety profile less buffered than native IGF-1 / mecasermin — conventional clinical IGF-1 dosing relies on physiological IGFBP buffering for tolerability margin, which LR3 deliberately bypasses
  • Anti-doping: IGF-1 LR3 is prohibited under WADA Class S2.5 (in- and out-of-competition)

References

  1. King R, Wells JR, Krieg P, Snoswell M, Brazier J, Bagley CJ, Wallace JC, Ballard FJ, Ross M, Francis GL Production and characterization of recombinant insulin-like growth factor-I (IGF-I) and potent analogues of IGF-I, with Gly or Arg substituted for Glu³, following their expression in Escherichia coli as fusion proteins Journal of Molecular Endocrinology 1992;8(1):29–41. 1992 .

    PMID: 1543770 View Source

  2. Francis GL, Ross M, Ballard FJ, Milner SJ, Senn C, McNeil KA, Wallace JC, King R, Wells JR Novel recombinant fusion protein analogues of insulin-like growth factor (IGF)-I indicate the relative importance of IGF-binding protein and receptor binding for enhanced biological potency Journal of Molecular Endocrinology 1992;8(3):213–223. 1992 .

    PMID: 1378920 View Source

  3. Tomas FM, Knowles SE, Owens PC, Read LC, Chandler CS, Gargosky SE, Ballard FJ Effects of full-length and truncated insulin-like growth factor-I on nitrogen balance and muscle protein metabolism in nitrogen-restricted rats Journal of Endocrinology 1993;137(3):413–421. 1993 .

    PMID: 8473266 View Source

  4. Ballard FJ, Walton PE, Bastian S, Tomas FM, Wallace JC, Francis GL Effects of interactions between IGFs and IGF binding proteins on IGF actions (review chapter, in: Baxter RC et al. eds., The Insulin-Like Growth Factors and Their Regulatory Proteins, Elsevier, 1996) Elsevier book chapter 1996. 1996 .

    View Source

  5. Morris AE, Schmid J Effects of insulin and LongR(3) on serum-free Chinese hamster ovary cell cultures expressing two recombinant proteins Biotechnology Progress 2000;16(5):693–697. 2000 .

    PMID: 11027157 View Source

  6. Vajdos FF, Ultsch M, Schaffer ML, Deshayes KD, Liu J, Skelton NJ, de Vos AM Crystal structure of human insulin-like growth factor-1: detergent binding inhibits binding protein interactions Biochemistry 2001;40(37):11022–11029. 2001 .

    PMID: 11551197 View Source

  7. Thomas A, Walpurgis K, Krug O, Schänzer W, Thevis M Determination of prohibited, small peptides in urine for sports drug testing by means of nano-liquid chromatography/benchtop quadrupole orbitrap tandem-mass spectrometry Journal of Chromatography A 2012;1259:251–257. 2012 .

    PMID: 22261499 View Source

  8. World Anti-Doping Agency The 2026 Prohibited List — International Standard. Effective 1 January 2026. Section S2.5 (Growth factors and growth factor modulators); IGF-1 and analogues including mecasermin and IGF-1 LR3 are explicitly named WADA. 2026 .

    View Source

  9. Wikipedia contributors IGF-1 LR3 — chembox source for molecular formula C₄₀₀H₆₂₅N₁₁₁O₁₁₅S₉, MW 9117.60 g·mol⁻¹, CAS 143045-27-6, UNII M9L22Y19H9 Wikipedia. 2026 .

    View Source

  10. U.S. Food and Drug Administration / Tercica Inc. Drugs@FDA: Increlex (mecasermin) injection — NDA 021839, initial approval 30 August 2005. For DISAMBIGUATION ONLY: mecasermin (DrugBank DB01277) is unmodified rh-IGF-1 (70 aa), distinct from IGF-1 LR3 (83 aa engineered analogue) — must not be conflated FDA Drugs@FDA. 2005 .

    View Source

Frequently Asked Questions

Is IGF-1 LR3 the same as Increlex / mecasermin?
No. Increlex® (active ingredient mecasermin, DrugBank DB01277) is recombinant unmodified human IGF-1 — the natural 70-residue mature IGF-1 sequence. Mecasermin received FDA approval on 30 August 2005 (Tercica / Ipsen) for severe primary IGF-1 deficiency including Laron syndrome. IGF-1 LR3 is a different molecule: an 83-residue engineered analogue with a 13-residue MFPAMPLSSLFVN N-terminal extension and a Glu³→Arg³ substitution that gives it ~80–100-fold reduced affinity for the IGF-binding proteins. IGF-1 LR3 is NOT approved by the FDA, EMA or any other regulator for therapeutic use; its only legitimate regulated application is as a cell-culture / bioproduction reagent.
Why does LR3 last longer than native IGF-1?
Native IGF-1 in plasma is >99% bound to the IGF-binding-protein family (predominantly the IGFBP-3 / acid-labile-subunit ternary complex). The unbound (receptor-available) free fraction is therefore very small, although the total pool half-life is ~12–15 h precisely because of that IGFBP sequestration. IGF-1 LR3 is engineered to evade IGFBP binding (~80–100-fold reduced IGFBP-1 / IGFBP-3 affinity per Francis 1992 and King 1992), which both raises the in-vivo unbound free-fraction (hence higher effective IGF1R agonist potency) and produces a longer plasma half-life of the agonist-active species — studies report ~5.7 h in rat plasma (Tomas 1993) and administration-dependent ~20–30 h in vivo per the Wikipedia chembox.
Is IGF-1 LR3 banned in sport?
Yes. IGF-1 LR3 is prohibited under the World Anti-Doping Agency 2026 Prohibited List, category S2 (Peptide Hormones, Growth Factors, Related Substances and Mimetics), sub-section S2.5 (Growth factors and growth factor modulators) — both in- and out-of-competition. IGF-1 and its analogues (including mecasermin and IGF-1 LR3) are explicitly named on the S2.5 list. The unique N-terminal MFPAMPLSSLFVN extension yields tryptic peptides absent from native IGF-1 / mecasermin and is the analytical signature that allows WADA-accredited laboratories to identify LR3 in athlete samples (Thomas 2012, PMID 22261499).
Is IGF-1 LR3 FDA-approved?
No. IGF-1 LR3 has NO FDA marketing authorisation, NO EMA marketing authorisation, no Health Canada / TGA / PMDA / MHRA marketing authorisation, and there are ZERO registered clinical trials of IGF-1 LR3 on ClinicalTrials.gov as of 2026-05-02 (audit: query.term=Long-R3-IGF-1 → 0 hits; query.intr=Long-R3-IGF-1 → 0 hits). Its only regulated application is as a cell-culture / bioproduction reagent in industrial mammalian-cell manufacture (CHO / HEK / BHK / Vero) of monoclonal antibodies, vaccines and recombinant therapeutics, sold by reagent suppliers (Sigma-Aldrich, Repligen, Tocris, Cayman). Bioprocess-reagent supply is NOT equivalent to therapeutic approval. The molecule that IS FDA-approved (since 30 August 2005) is mecasermin (Increlex®, native rh-IGF-1) — a different molecule.
Where did IGF-1 LR3 come from?
IGF-1 LR3 was designed in 1992 by Geoffrey Francis, Michael Ross, F. John Ballard, John Wallace and colleagues at the CSIRO Division of Human Nutrition in Adelaide, Australia, and commercialised in 1993 by spin-out company GroPep Limited as the bioproduction reagent LONG®R³ IGF-I. GroPep was subsequently acquired by Repligen Corporation, which currently markets LONG®R³ IGF-I as a serum-free / insulin-free supplement for industrial mammalian-cell bioproduction media (Francis 1992, PMID 1378920; GroPep / Repligen corporate history).
Why is IGF-1 LR3 used in bioproduction?
Industrial mammalian-cell production of monoclonal antibodies, vaccines and recombinant therapeutics requires growth-factor stimulation of the producer cells (typically CHO, HEK293, BHK or Vero lines). Historically this was achieved with foetal calf serum, insulin and/or native IGF-1 — but serum carries lot-to-lot variability and viral-safety concerns, native IGF-1 is heavily sequestered by IGFBPs in the medium, and high insulin concentrations can desensitise the insulin-receptor / IGF1R hybrid axis. IGF-1 LR3, because it evades IGFBP binding and is highly potent at the IGF1R, is effective at sub-µg/mL concentrations (~10–100× lower than native IGF-1) in serum-free / insulin-free production media with reproducible lot-to-lot performance — a staple bioprocess reagent since the late 1990s (Morris & Schmid 2000, PMID 11027157; Repligen LONG®R³ IGF-I technical literature).

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