Also known as: Long R3 IGF-1, LR3-IGF-1, Long Arg3 IGF-1
Systemic muscle growth, recovery, and hyperplasia via extended IGF-1 receptor activation (research only)
Amount
20-80 mcg daily
Frequency
Once daily
Duration
4-6 weeks on, 3-6 weeks off
Route
SCSchedule
Once daily
Timing
Morning or post-workout; subcutaneous for systemic effect
Duration
4-6 weeks on, 3-6 weeks off
Rest Period
6 weeks off between cycles
Repeatable
Yes
Diluent: Bacteriostatic water
Storage: Lyophilized powder: Store at -20 to -80 degrees C, protected from moisture. Reconstituted stock solution: Aliquot and store at -20 degrees C; avoid repeated freeze-thaw cycles. Working dilutions in culture media should be prepared fresh. Reconstituted solutions stored at 2-8 degrees C should be used within 1 week.
IGF-1
When: Baseline
Why: Baseline growth factor levels
Fasting glucose and fasting insulin
When: Baseline
Why: IGF-1 LR3 carries significant hypoglycemia risk
HbA1c
When: Baseline
Why: Baseline glycemic status
CBC
When: Baseline
Why: IGF-1 affects hematopoiesis
Fasting glucose
When: Weekly during use
Why: Monitor for hypoglycemia (>40% incidence reported)
IGF-1
When: 2-3 weeks
Why: Assess degree of IGF-1 pathway activation
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IGF-1 LR3 (Long R3 Insulin-like Growth Factor-1) is a synthetic analog of human insulin-like growth factor-1 (IGF-1) that has been engineered for enhanced biological potency. The native human IGF-1 protein is a 70-amino acid single-chain polypeptide with significant structural homology to insulin. It is one of the most important mediators of growth hormone (GH) signaling, responsible for many of the anabolic, proliferative, and metabolic effects attributed to GH.
IGF-1 LR3 incorporates two specific structural modifications compared to wild-type IGF-1. First, the glutamic acid residue at position 3 of native IGF-1 is substituted with arginine (the "R3" modification). Second, a 13-amino acid extension peptide is added to the N-terminus of the protein (the "Long" modification). Together, these changes increase the total length of the protein from 70 to 83 amino acids.
The combined effect of these modifications is a dramatic reduction in binding affinity for IGF binding proteins (IGFBPs), the family of six circulating proteins that normally sequester approximately 98% of circulating IGF-1 and regulate its bioavailability. Because IGFBPs normally limit the free concentration of IGF-1 available to interact with the IGF-1 receptor (IGF-1R), the reduced IGFBP binding of IGF-1 LR3 results in a much higher effective concentration of active growth factor compared to an equivalent dose of native IGF-1.
This property has made IGF-1 LR3 the preferred form of IGF-1 for research applications, particularly in cell culture and bioprocessing, where it provides significantly greater potency per unit mass than native IGF-1.
To understand the mechanism of IGF-1 LR3, it is necessary to appreciate the complexity of the IGF-1 signaling system. Under physiological conditions, IGF-1 is primarily synthesized by the liver in response to growth hormone stimulation via the GH/IGF-1 axis. However, IGF-1 is also produced locally in virtually every tissue, where it acts in autocrine and paracrine fashions.
Circulating IGF-1 levels are tightly regulated, with approximately 98-99% bound to one of six IGF binding proteins (IGFBP-1 through IGFBP-6). The majority of circulating IGF-1 is carried in a ternary complex with IGFBP-3 (or IGFBP-5) and the acid-labile subunit (ALS), which extends the half-life of IGF-1 from approximately 10-12 minutes (free) to approximately 12-16 hours (in the ternary complex).
IGFBPs serve multiple functions: they extend the circulating half-life of IGF-1, transport IGF-1 to target tissues, and critically regulate the amount of free (bioactive) IGF-1 available to interact with the IGF-1 receptor. Specific proteases (including PAPP-A and other metalloproteinases) cleave IGFBPs at target tissues, releasing free IGF-1 locally to activate the IGF-1R. This system provides exquisite spatial and temporal control over IGF-1 signaling.
The two structural modifications in IGF-1 LR3 specifically target the IGF-1/IGFBP interaction interface. X-ray crystallographic and NMR studies of IGF-1 in complex with various IGFBPs have revealed that the N-terminal region of IGF-1, including residue position 3, is directly involved in IGFBP binding contacts. The glutamic acid-to-arginine substitution at position 3 disrupts key electrostatic interactions at this binding interface.
The 13-amino acid N-terminal extension peptide (with the sequence Met-Phe-Pro-Ala-Met-Pro-Leu-Ser-Ser-Leu-Phe-Val-Asn) further reduces IGFBP binding through steric interference with the IGFBP binding site. The combined effect of both modifications reduces IGFBP binding affinity by more than 100-fold compared to native IGF-1, effectively rendering IGF-1 LR3 largely resistant to IGFBP sequestration.
Crucially, these modifications do not significantly impair binding to the IGF-1 receptor (IGF-1R). The IGF-1R binding site on IGF-1 involves residues in the B domain and portions of the A domain that are distinct from the IGFBP interaction surface. As a result, IGF-1 LR3 retains full ability to activate the IGF-1R and its downstream signaling pathways.
Upon binding to the IGF-1 receptor, IGF-1 LR3 activates the same intracellular signaling cascades as native IGF-1. The IGF-1R is a receptor tyrosine kinase that, upon ligand binding, undergoes autophosphorylation and recruits adaptor proteins including insulin receptor substrate-1 (IRS-1) and Shc. This triggers two major downstream signaling pathways.
The PI3K/Akt/mTOR pathway is the primary mediator of the anabolic and anti-apoptotic effects of IGF-1 signaling. Activation of Akt promotes protein synthesis through mTORC1 activation, inhibits protein degradation by suppressing the FoxO transcription factors and the ubiquitin-proteasome system, promotes glucose uptake through GLUT4 translocation, and suppresses apoptosis through phosphorylation and inactivation of pro-apoptotic proteins including Bad and caspase-9.
The Ras/MAPK/ERK pathway mediates primarily the proliferative and differentiation effects of IGF-1 signaling. Activation of ERK1/2 promotes cell cycle progression, stimulates gene expression involved in cellular differentiation, and contributes to the mitogenic effects of IGF-1.
The practical consequence of reduced IGFBP binding is that IGF-1 LR3 is substantially more potent than native IGF-1 in biological systems containing IGFBPs. In serum-containing cell culture media, where IGFBPs are present, IGF-1 LR3 is typically 2-3 fold more potent than native IGF-1 on a molar basis. In systems with higher IGFBP concentrations, the potency advantage can be even greater.
In serum-free conditions (where IGFBPs are absent), the potency of IGF-1 LR3 and native IGF-1 are comparable, confirming that the enhanced activity of IGF-1 LR3 is attributable to its escape from IGFBP sequestration rather than enhanced receptor activation.
The primary commercial application of IGF-1 LR3 is as a cell culture supplement and bioprocessing reagent. In biopharmaceutical manufacturing, mammalian cell lines (such as CHO cells) are cultured in large bioreactors to produce recombinant proteins. IGF-1 LR3 is added to serum-free culture media as a growth factor supplement to promote cell proliferation and viability, reducing the need for fetal bovine serum.
Its enhanced potency compared to native IGF-1 means that lower concentrations are required, reducing production costs. The consistency of its activity (due to reduced interference by variable IGFBP levels in different serum batches) makes it preferable to native IGF-1 for bioprocessing applications.
IGF-1 signaling plays a central role in skeletal muscle biology, including muscle development (myogenesis), hypertrophy, and regeneration after injury. IGF-1 LR3 is widely used in muscle biology research to study these processes.
In cultured myoblasts and myotubes, IGF-1 LR3 promotes proliferation, differentiation, and protein synthesis through the PI3K/Akt/mTOR pathway. In ex vivo and in vivo muscle preparations, IGF-1 LR3 has been used to study the mechanisms of muscle hypertrophy, the role of satellite cells in muscle regeneration, and the interaction between IGF-1 signaling and mechanical loading.
As a tool compound, IGF-1 LR3 is valuable for dissecting the contribution of IGFBP-dependent versus IGFBP-independent regulation of IGF-1 signaling. By comparing the effects of native IGF-1 (subject to IGFBP regulation) with IGF-1 LR3 (largely free from IGFBP regulation), researchers can determine the extent to which IGFBPs modulate IGF-1 action in specific cellular contexts.
This approach has been used to study IGFBP function in cancer biology, where IGFBPs can either inhibit or potentiate IGF-1 signaling depending on the specific IGFBP and cellular context.
IGF-1 shares structural homology with insulin and can activate the insulin receptor (particularly the IR-A isoform) at high concentrations, in addition to its primary target the IGF-1R. IGF-1 LR3 is used in metabolic research to study the insulin-like metabolic effects of IGF-1 signaling, including glucose uptake, lipogenesis, and gluconeogenesis suppression.
It is important to note that IGF-1 LR3 itself is not approved for any clinical indication and has not been evaluated in formal human clinical trials. Recombinant native IGF-1 (mecasermin, marketed as Increlex) is FDA-approved for the treatment of severe primary IGF-1 deficiency (previously known as Laron syndrome) in pediatric patients. This approval provides clinical validation of the IGF-1 signaling axis as a therapeutic target, but the properties of mecasermin differ substantially from those of IGF-1 LR3.
Mecasermin (native IGF-1) requires twice-daily subcutaneous injection and is subject to normal IGFBP regulation, resulting in a pharmacokinetic profile that partially mimics endogenous IGF-1. IGF-1 LR3, with its dramatically reduced IGFBP binding, would be expected to have a very different pharmacokinetic and pharmacodynamic profile, with higher peak free IGF-1R activity and potentially different tissue distribution patterns.
The GH/IGF-1 axis has also been the subject of extensive clinical research in the context of growth disorders, metabolic syndrome, and aging. Understanding the biology of this axis and the tools available for its modulation remains an active area of investigation.
The most fundamental limitation of IGF-1 LR3 from a therapeutic perspective is the complete absence of human clinical data. All information regarding its biological activity comes from in vitro cell culture studies and animal research. The pharmacokinetics, pharmacodynamics, tolerability, and safety profile of IGF-1 LR3 in humans are entirely unknown.
The enhanced potency of IGF-1 LR3 relative to native IGF-1 raises potential safety concerns. The IGF-1 signaling axis is a potent mitogenic pathway, and epidemiological studies have consistently demonstrated associations between elevated circulating IGF-1 levels and increased risk of several cancers, including prostate, breast, and colorectal cancer. A molecule that bypasses the IGFBP regulatory system, which normally serves as a brake on IGF-1 signaling, could theoretically amplify these oncogenic risks.
The metabolic effects of IGF-1 LR3, including insulin-like glucose-lowering activity, represent another potential safety concern. Hypoglycemia is a known adverse effect of mecasermin (native IGF-1), and the enhanced free-fraction bioavailability of IGF-1 LR3 could increase the risk and severity of hypoglycemic episodes.
From a research perspective, the molecular formula and full amino acid sequence of IGF-1 LR3 are not provided in most commercial listings due to the complexity of the 83-amino acid protein. The three-dimensional structure of IGF-1 LR3, including the conformation of the N-terminal extension peptide, has not been determined by X-ray crystallography or cryo-electron microscopy, which limits detailed understanding of its structural basis for reduced IGFBP binding.
The specificity of IGF-1 LR3 for the IGF-1R versus other receptors in the insulin/IGF receptor family (insulin receptor isoforms IR-A and IR-B, and the IGF-2R) at the concentrations used in typical experiments is not fully characterized. Cross-reactivity with insulin receptor isoforms at high concentrations could confound interpretation of experimental results attributed to IGF-1R signaling.
Additionally, IGF-1 LR3 has become widely available through unregulated online peptide vendors for non-research purposes. Self-administration of IGF-1 LR3 outside of clinical supervision carries significant risks, including hypoglycemia, potential promotion of occult neoplasms, and unknown effects on normal GH/IGF-1 axis feedback regulation. The absence of pharmaceutical-grade quality control for products sold through these channels introduces further risks related to product purity, sterility, and accurate dosing.
Finally, the patent landscape and regulatory classification of IGF-1 LR3 as a research reagent rather than a pharmaceutical candidate have limited investment in the formal preclinical and clinical development studies that would be necessary to evaluate its therapeutic potential in a rigorous manner. As a result, while IGF-1 LR3 is among the most widely used research tools in cell biology and bioprocessing, its clinical development prospects remain uncertain.
Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) potentiates the growth-promoting activity of des(1-3)IGF-I and LR3IGF-I in vitro, published in Journal of Endocrinology (Francis GL et al., 1992):
Early characterization study demonstrating the reduced IGFBP binding and enhanced biological potency of IGF-1 LR3 compared to native IGF-1. This work by the group at the Cooperative Research Centre for Tissue Growth and Repair (Adelaide, Australia) was foundational in establishing IGF-1 LR3 as a research tool with enhanced bioavailability.
Long R3 IGF-I as a more potent alternative to native IGF-I for promoting cell growth in culture, published in In Vitro Cellular and Developmental Biology - Animal (Francis GL et al., 1993):
Systematic comparison of IGF-1 LR3 with native IGF-1 across multiple cell types in culture, establishing the standard cell culture concentrations and demonstrating the practical utility of IGF-1 LR3 as a cell culture supplement.
Cell culture applications of IGF-1 LR3 in biopharmaceutical manufacturing, published in Various industry publications (Various industry groups, 2000):
Collective body of industry literature documenting the use of IGF-1 LR3 in serum-free media formulations for CHO cell culture and other biopharmaceutical production systems. IGF-1 LR3 became a standard component of commercial serum-free media formulations during the late 1990s and 2000s.
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