Overview

Cyclic Glycine-Proline (cGP, also known as cyclo(Gly-Pro) or glycyl-L-proline diketopiperazine) is a naturally occurring diketopiperazine formed by cyclization of the N-terminal Gly-Pro-Glu tripeptide cleaved from insulin-like growth factor 1 (IGF-1). This autocrine/paracrine peptide was identified through research led by Professor Jian Guan at the University of Auckland, who demonstrated that cGP plays a critical role in regulating the bioactivity of IGF-1, a growth factor essential for neuronal survival, synaptic plasticity, myelination, and vascular homeostasis.

The primary mechanism of cGP involves competitive modulation of the binding interaction between IGF-1 and IGF-binding protein 3 (IGFBP-3), the major carrier protein that sequesters approximately 80% of circulating IGF-1 in a ternary complex. By displacing IGF-1 from IGFBP-3, cGP increases the concentration of free (bioavailable) IGF-1 available to activate the IGF-1 receptor. This normalizing function is particularly significant because it operates as a homeostatic buffer—increasing free IGF-1 when levels are low (as in aging or neurodegeneration) while not causing excessive IGF-1 signaling when total IGF-1 is adequate. This distinguishes cGP from direct IGF-1 administration, which carries risks of hypoglycemia and potential tumor promotion.

Preclinical research has demonstrated neuroprotective effects of cGP in models of hypoxic-ischemic brain injury, stroke, Parkinson's disease, and age-related cognitive decline. The peptide crosses the blood-brain barrier, is resistant to enzymatic degradation due to its cyclic structure, and has shown oral bioavailability. Notably, cGP is present in common foods, with particularly high concentrations found in blackcurrants and dairy products such as milk and cheese. Human observational studies have found that plasma cGP/IGF-1 ratios correlate with cognitive function in aging populations and with recovery outcomes after stroke, supporting its role as both a biomarker and potential therapeutic agent for conditions involving impaired IGF-1 signaling.

Mechanism of Action

Endogenous Cyclic Dipeptide — IGF-1 Bioavailability Modulator

Cyclic glycine-proline (cGP, cyclo(Gly-Pro)) is an endogenous diketopiperazine — a cyclic dipeptide formed by the spontaneous cyclization of the N-terminal glycine-proline-glutamate (GPE) tripeptide that is cleaved from insulin-like growth factor 1 (IGF-1) by an acid-labile subunit-mediated process. cGP modulates IGF-1 bioavailability by competing with IGF-1 for binding to insulin-like growth factor binding protein 3 (IGFBP-3), the predominant circulating IGF-1 carrier that sequesters ~75-80% of total IGF-1 in a ternary complex with the acid-labile subunit (ALS). By displacing IGF-1 from IGFBP-3, cGP increases the concentration of free (unbound) IGF-1 available to activate the IGF-1 receptor (IGF-1R) (PMID: 30149045).

IGF-1 Receptor Signaling Normalization

The free IGF-1 released by cGP-mediated IGFBP-3 displacement activates IGF-1R tyrosine kinase autophosphorylation, triggering downstream IRS-1/PI3K/Akt survival signaling and Ras/MAPK/ERK proliferative pathways. Critically, cGP functions as a homeostatic normalizer — in conditions of IGF-1 excess, cGP levels decrease (reducing IGFBP-3 displacement), while in IGF-1 deficiency, cGP increases to maximize free IGF-1 bioavailability. This autocrine/paracrine feedback loop maintains IGF-1R signaling within a physiological range (PMID: 32664463).

Neuroprotective Mechanisms

cGP crosses the blood-brain barrier and enhances neuronal IGF-1R signaling in the hippocampus and cortex. Akt activation by free IGF-1 phosphorylates and inactivates GSK-3beta, reducing tau hyperphosphorylation and promoting neuronal survival. cGP also enhances eNOS-mediated cerebrovascular vasodilation through Akt-dependent eNOS Ser1177 phosphorylation, improving cerebral blood flow. In stroke models, cGP administration reduces infarct volume by 30-50% and improves functional recovery (PMID: 31261732).

Blackcurrant-Derived Source & Clinical Translation

Blackcurrant anthocyanins (particularly cyanidin and delphinidin glycosides) are metabolized to cGP by gut microbiota, providing a dietary source. New Zealand blackcurrant extract supplementation increases plasma cGP/IGF-1 molar ratio and improves cognitive function in healthy older adults (PMID: 34356008).

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Safety Profile

Safety Profile: Cyclic Glycine-Proline (cGP)

Common Side Effects

• Mild gastrointestinal symptoms (nausea, stomach discomfort) at higher doses
• Headache (typically transient, during early supplementation)
• Dizziness or lightheadedness in sensitive individuals
• Generally very well tolerated at physiological and moderate supplemental doses

Serious Adverse Effects

• No serious adverse effects have been documented in published human or animal studies to date
• Theoretical risk of excessive IGF-1 bioavailability with chronic high-dose use, which could promote cell proliferation
• Long-term safety data from large clinical trials are not yet available
• No hepatotoxicity, nephrotoxicity, or cardiotoxicity reported

Contraindications

• Known or suspected IGF-1-sensitive malignancies (breast, prostate, colorectal cancer) — cGP increases IGF-1 bioavailability
• Active cancer of any type (precautionary)
• Pregnancy and breastfeeding (insufficient human safety data)
• Individuals with acromegaly or conditions of IGF-1 excess

Drug Interactions

• May interact with growth hormone (GH) therapy by modifying IGF-1 bioavailability — dose adjustments may be needed
• Theoretical interaction with insulin and insulin sensitizers (IGF-1 pathway overlap with insulin signaling)
• No documented interactions with common medications, but limited interaction studies available
• Caution when combining with other IGF-1 modulators or growth factor-based therapies

Population-Specific Considerations

• Elderly: Primary population of interest for neuroprotection; appears well-tolerated in aging populations
• Children: Not recommended — effects on growth and development not characterized
• Cancer survivors: Avoid until IGF-1 pathway implications are better understood
• Neurological conditions: Promising research in stroke and neurodegeneration; clinical data still emerging
• Blackcurrant-derived cGP: Dietary sources (e.g., blackcurrant extract) may offer a gentler dosing approach

Pharmacokinetic Profile