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IGF-1 LR3 vs IGF-1: A Structural and Regulatory Comparison

A structural and regulatory comparison of Long R3 IGF-1, an 83-residue engineered analog with reduced affinity for IGF-binding proteins, and native 70-residue human IGF-1, the sequence reported by Rinderknecht and Humbel in 1978 and the basis of the approved drug mecasermin. Educational reference.

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Introduction

Insulin-like growth factor-1 (IGF-1) is a 70-amino-acid peptide hormone of the insulin superfamily, and IGF-1 LR3 (Long R3 IGF-1) is an 83-residue recombinant analog engineered from it in the early 1990s. The two molecules share a receptor, a fold and most of a sequence, yet they behave very differently in biological systems because of a single design goal: IGF-1 LR3 was built to escape the IGF-binding proteins (IGFBPs) that carry almost all native IGF-1 in circulation. They also occupy opposite ends of the regulatory spectrum. Native-sequence recombinant IGF-1 is an approved drug (mecasermin, Increlex), whereas IGF-1 LR3 has never been approved for any use and exists only as a research reagent.

This article compares the two molecules on structure, origin, pharmacological class, regulatory status and research record. It reports what investigators and regulators have published and draws no conclusion about either compound's suitability for any purpose. Research-grade IGF-1 LR3 is cataloged as a laboratory reference material with per-batch certificates of analysis.

Native Human IGF-1: The 70-Residue Reference Sequence

Rinderknecht and Humbel reported the complete amino-acid sequence of human IGF-1 in the Journal of Biological Chemistry in 1978, identifying a 70-residue single chain of about 7.6 kDa with three intrachain disulfide bonds and noting its structural homology with proinsulin [1]. The molecule is organized into B, C, A and D domains that correspond to the B chain, connecting peptide, A chain and a short C-terminal extension of the insulin precursor. Unlike insulin, IGF-1 retains its C domain in the mature hormone and adds the D domain; unlike insulin, it circulates predominantly bound to carrier proteins.

Those carrier proteins define native IGF-1's pharmacology. Allard and Duan, reviewing the IGF-binding protein family in Frontiers in Endocrinology in 2018, describe six high-affinity IGFBPs (IGFBP-1 through IGFBP-6) that bind IGF-1 with affinities equal to or greater than that of the receptor itself, so that the large majority of circulating IGF-1 exists in binary or ternary complexes rather than as free ligand [6]. The IGFBPs extend the hormone's half-life, restrict its access to tissues and receptors, and in some contexts signal independently of the IGF system.

IGF-1 acts primarily at the type 1 IGF receptor (IGF-1R), a disulfide-linked receptor tyrosine kinase of the insulin receptor family. Li and colleagues reported a cryo-electron microscopy structure of the ligand-bound receptor in Nature Communications in 2019, describing how a single IGF-1 molecule bridges the two receptor halves to bring the kinase domains together for activation [9].

IGF-1 LR3: An 83-Residue Analog With Reduced Binding-Protein Affinity

IGF-1 LR3 (Long Arg3 IGF-1; LR3IGF-I in the primary literature) was developed by Francis, Ballard, Wallace and colleagues at the University of Adelaide and produced as a recombinant protein in Escherichia coli. Francis and colleagues reported in the Journal of Molecular Endocrinology in 1992 that fusion-protein analogs of IGF-1 carrying an N-terminal extension together with an arginine substitution at position 3 showed markedly reduced affinity for IGF-binding proteins while retaining receptor binding, and that the reduction in binding-protein affinity, rather than any change in receptor affinity, accounted for the analogs' greater activity in cell-culture assays [2]. Tomas and colleagues later reported in the Journal of Endocrinology in 1996 that the greater potency of poorly IGFBP-binding analogs observed in rats was maintained across the administration methods the authors compared [3].

The molecule consists of the full 70-residue IGF-1 sequence with two changes: a 13-residue N-terminal extension derived from the fusion-protein expression construct, and substitution of arginine for glutamic acid at position 3 of the native sequence. Its mass is about 9.1 kDa, and it retains the three disulfide bonds of the parent. The Glu3 residue lies within the IGFBP-contact region of the B domain, which is why its replacement disrupts binding-protein recognition without disrupting the receptor interface.

Pharmacokinetic and physiological consequences of those changes were characterized in animal models in the 1990s. Bastian and colleagues reported the plasma clearance and tissue distribution of labeled IGF-1 and LR3IGF-I in pregnant rats in 1993 [4], and Conlon and colleagues reported in 1995 that LR3IGF-I infusion in guinea pigs was associated with changes in organ weights alongside reduced plasma IGF-1, IGF-2 and IGFBP concentrations [5]. Findings from these research models do not establish safety or efficacy in humans. Sparta Labs makes no claims about the use of this compound. The design history is covered in the IGF-1 LR3 history article and receptor-level detail in the IGF-1 LR3 mechanism of action article.

Structural Comparison

  • Chain length. Native IGF-1: 70 residues. IGF-1 LR3: 83 residues (70 native positions plus a 13-residue N-terminal extension).
  • Position 3. Native: glutamic acid (Glu3). LR3: arginine (Arg3). This is the only substitution within the native sequence.
  • Molecular weight. Native: about 7.6 kDa. LR3: about 9.1 kDa.
  • Disulfide bonds and domains. Identical: three disulfides and the B-C-A-D domain organization are preserved in the analog [1, 2].
  • IGF-binding protein affinity. Native IGF-1 is bound with high affinity by IGFBP-1 through IGFBP-6 [6]. Francis and colleagues reported that the LR3 analog binds these proteins with greatly reduced affinity, which the University of Adelaide group placed at several orders of magnitude lower than native IGF-1 in later work [2, 3].
  • Receptor engagement. Both bind IGF-1R; the analog's receptor affinity was reported as somewhat reduced but broadly preserved [2]. At high concentrations both ligands also engage the insulin receptor and IGF-1R/insulin receptor hybrids; Voorhamme and Yandell reported LR3IGF-I activity in HEK293 cell culture in this context in 2006 [7].
  • Origin. Native IGF-1 is an endogenous hormone; recombinant native-sequence IGF-1 is manufactured for pharmaceutical use. IGF-1 LR3 is a laboratory-designed recombinant analog with no endogenous counterpart, first supplied commercially by GroPep Ltd of Adelaide.
  • Behavior in the presence of binding proteins. Prelle and colleagues reported in Endocrinology in 2001 that IGF-1 and LR3IGF-I differed in their effects on bovine embryo development in vitro and on the abundance of IGFBP and IGF-1R messenger RNA, consistent with the analog's altered interaction with the binding-protein system [8].

Pharmacological Class Context

Both molecules belong to the insulin superfamily of peptide growth factors, which includes insulin, IGF-1, IGF-2 and the relaxin-like peptides, and both act at IGF-1R. The IGF-1 system is the distal effector of the growth hormone axis: pituitary growth hormone, itself regulated by GHRH-receptor and ghrelin-receptor inputs, drives hepatic IGF-1 production, and circulating IGF-1 feeds back on the pituitary and hypothalamus. Compounds discussed elsewhere in this library, such as those in the CJC-1295 with DAC research overview, act at the upstream GHRH receptor and were themselves evaluated in part by measuring IGF-1 as a downstream readout. IGF-1 LR3 and native IGF-1 sit at the opposite end of that axis, acting directly at the tissue receptor rather than through the pituitary.

Within the IGF-1 family the analog is classified as a research tool rather than a therapeutic candidate: its purpose is to present the receptor with a ligand that binding proteins cannot sequester, so that receptor-proximal signaling can be studied in isolation from IGFBP effects [2, 6].

Regulatory status

Native-sequence recombinant human IGF-1 is an approved drug. Mecasermin (Increlex) holds FDA approval under NDA 021839, and its prescribing information describes the approved indication of growth failure in children with severe primary IGF-1 deficiency [10]. The approved product does not carry the LR3 modifications and is manufactured, labeled and regulated as a prescription pharmaceutical.

IGF-1 LR3 holds no approval from the FDA or any comparable authority for any use. It is cataloged in the FDA's Unique Ingredient Identifier database (UNII M9L22Y19H9), which reflects chemical characterization only and confers no regulatory review or approval. It is supplied and studied exclusively as a research-use-only reagent for cell culture and animal-model work, a status shared with other engineered growth-factor analogs that never entered pharmaceutical development.

Research record

The two literatures differ in kind. Native IGF-1 has a clinical literature, including the trials that supported mecasermin's approval, in addition to decades of basic biology. IGF-1 LR3's record is preclinical: cell-culture and animal-model studies of receptor signaling, embryo development, fetal physiology and pharmacokinetics [3, 4, 5, 7, 8], with the analog frequently serving as a comparator to native IGF-1 in the same experiment. No published human trial of IGF-1 LR3 exists, and the compounds have never been compared for any clinical outcome. This article accordingly makes no comparative efficacy statement.

References

  1. Rinderknecht E, Humbel RE. The amino acid sequence of human insulin-like growth factor I and its structural homology with proinsulin. J Biol Chem. 1978;253(8):2769-2776. PMID: 632300. PubMed
  2. Francis GL, Ross M, Ballard FJ, Milner SJ, et al. 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. J Mol Endocrinol. 1992;8(3):213-223. PMID: 1378742. DOI: 10.1677/jme.0.0080213
  3. Tomas FM, Lemmey AB, Read LC, Ballard FJ. Superior potency of infused IGF-I analogues which bind poorly to IGF-binding proteins is maintained when administered by injection. J Endocrinol. 1996;150(1):77-84. PMID: 8708565. DOI: 10.1677/joe.0.1500077
  4. Bastian SE, Walton PE, Wallace JC, Ballard FJ. Plasma clearance and tissue distribution of labelled insulin-like growth factor-I (IGF-I) and an analogue LR3IGF-I in pregnant rats. J Endocrinol. 1993;138(2):327-336. PMID: 7693845. DOI: 10.1677/joe.0.1380327
  5. Conlon MA, Tomas FM, Owens PC, Wallace JC, Howarth GS, Ballard FJ. Long R3 insulin-like growth factor-I (IGF-I) infusion stimulates organ growth but reduces plasma IGF-I, IGF-II and IGF binding protein concentrations in the guinea pig. J Endocrinol. 1995;146(2):247-253. PMID: 7561636. DOI: 10.1677/joe.0.1460247
  6. Allard JB, Duan C. IGF-binding proteins: why do they exist and why are there so many? Front Endocrinol (Lausanne). 2018;9:117. PMID: 29686648. DOI: 10.3389/fendo.2018.00117
  7. Voorhamme D, Yandell CA. LONG R3IGF-I as a more potent alternative to insulin in serum-free culture of HEK293 cells. Mol Biotechnol. 2006;34(2):201-204. PMID: 17172665. DOI: 10.1385/MB:34:2:201
  8. Prelle K, Stojkovic M, Boxhammer K, Motlik J, et al. Insulin-like growth factor I (IGF-I) and long R(3)IGF-I differently affect development and messenger ribonucleic acid abundance for IGF-binding proteins and type I IGF receptors in in vitro produced bovine embryos. Endocrinology. 2001;142(3):1309-1316. PMID: 11181549. DOI: 10.1210/endo.142.3.8038
  9. Li J, Choi E, Yu H, Bai XC. Structural basis of the activation of type 1 insulin-like growth factor receptor. Nat Commun. 2019;10(1):4567. PMID: 31594955. DOI: 10.1038/s41467-019-12564-0
  10. US Food and Drug Administration. Increlex (mecasermin) injection: NDA 021839 prescribing information. FDA label

Frequently asked questions

  • What is the difference between IGF-1 LR3 and IGF-1?

    Native human IGF-1 is a 70-amino-acid single-chain peptide with three disulfide bonds, sequenced by Rinderknecht and Humbel in 1978. IGF-1 LR3 is an 83-residue recombinant analog that carries a 13-residue N-terminal extension and substitutes arginine for glutamic acid at position 3. Francis and colleagues reported in 1992 that these changes sharply reduce binding to IGF-binding proteins while preserving engagement of the type 1 IGF receptor.

  • What does LR3 stand for in IGF-1 LR3?

    The L stands for Long, referring to the 13-amino-acid extension added at the N-terminus of the peptide. R3 refers to the arginine (R) residue substituted at position 3 in place of the native glutamic acid. Together the two modifications give the analog its full name, Long Arg3 IGF-1, often written Long R3 IGF-1 or LR3IGF-I in the primary literature.

  • Why was IGF-1 LR3 designed to bind IGF-binding proteins poorly?

    Allard and Duan describe how the large majority of circulating IGF-1 travels bound to one of six high-affinity IGF-binding proteins, which govern its half-life and access to receptors. An analog with reduced binding-protein affinity lets investigators study type 1 IGF receptor signaling with a ligand that is not sequestered by endogenous binding proteins. Francis and colleagues developed the Long R3 analog in the early 1990s for that purpose.

  • Is IGF-1 LR3 FDA approved?

    No. IGF-1 LR3 holds no approval from the FDA or any comparable regulator and is supplied only as a research-use-only reagent. The approved recombinant IGF-1 product is mecasermin (Increlex), which is native-sequence human IGF-1 without the LR3 modifications, approved under NDA 021839 for a specific pediatric indication described in its prescribing information.

  • Do IGF-1 LR3 and native IGF-1 act at the same receptor?

    Both engage the type 1 IGF receptor (IGF-1R), a receptor tyrosine kinase whose activation structure was described by Li and colleagues in 2019. The distinction between the two ligands lies not in receptor selectivity but in how much of each is captured by binding proteins before reaching the receptor. Published animal and cell-culture studies attribute the analog's different behavior to that difference in sequestration.

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