What Is Cyclic Glycine-Proline? Definition and What Research Reports

Plain definition

Cyclic glycine-proline (cGP) is a small cyclic dipeptide built from two amino acids — glycine and proline — joined head-to-tail so that the molecule forms a closed ring rather than a linear chain. Molecules with this ring structure belong to the chemical class known as 2,5-diketopiperazines. cGP is structurally and metabolically related to glycine-proline-glutamate (GPE), the N-terminal tripeptide cleaved from insulin-like growth factor-1 (IGF-1); the cyclic dipeptide is the form most often measured in plasma and cerebrospinal fluid. Because of that relationship, cGP appears in the literature primarily as a putative endogenous regulator of IGF-1 rather than as a receptor-binding hormone in its own right, and a 2023 review described cGP in terms of normalising IGF-1 function in the ageing brain and in age-related neurological conditions (PMID 36770687).

What class of molecule it is, and where it comes from

cGP is not a large therapeutic peptide like a GLP-1 analogue or a growth hormone secretagogue. It is a two-residue cyclic compound, small enough that researchers typically quantify it by mass spectrometry in biological fluids rather than by immunoassay. Two general origins appear in the literature:

• Endogenous formation. cGP is discussed as a metabolite arising from the IGF-1 N-terminal tripeptide GPE, and a 2014 report described cGP regulating IGF-1 homeostasis by altering the binding of IGFBP-3 to IGF-1 (PMID 24633053).
• Exogenous and natural sources. Diketopiperazines including cGP occur in foods and in microbial metabolites; a 2024 analytical paper reported quantitative monitoring of cyclic glycine-proline in marine mangrove-derived fungal metabolites (PMID 39590698). A separate human study reported that supplementation with blackcurrant anthocyanins increased cGP in the cerebrospinal fluid of Parkinson's patients (PMID 29865234).

How the term is used in peptide research

In the published literature, "cyclic glycine-proline" is used in three fairly distinct ways, and readers encountering the term benefit from noticing which one an article means.

1. As a biomarker or ratio

Several clinical papers treat cGP as something measured, not administered. Researchers reported that the plasma cyclic glycine-proline/IGF-1 ratio predicted clinical outcome and recovery in stroke patients (PMID 31019991). In this usage the ratio is presented as an index of IGF-1 availability rather than as an intervention.

2. As a regulator of IGF-1 bioavailability

The mechanistic framing is that cGP influences how much IGF-1 is free versus bound to its binding proteins. The 2014 Scientific Reports paper reported that cGP regulated IGF-1 homeostasis by altering the binding of IGFBP-3 to IGF-1 (PMID 24633053), and a 2016 study reported that maternally administered cGP increased IGF-1 bioavailability in developing offspring (PMID 27355491). Notably, the direction of effect is described as normalising rather than uniformly stimulating: one 2017 study reported that cGP accelerated mammary involution by promoting apoptosis and inhibiting IGF-1 function (PMID 28063218).

3. As an administered compound in animal models

The third usage is experimental administration in rodents, most often in neurological, developmental or cardiometabolic models. These are preclinical designs, and the endpoints are tissue markers, behaviour or physiology rather than clinical outcomes in people.

What the published literature reports

Brain, memory and synaptic markers (animal)

A 2023 study in an APP/PS1 transgenic mouse model of Alzheimer's disease reported that cyclic glycine-proline improved memory and reduced amyloid plaque load (PMID 36909366). In rats, researchers reported that administration of cGP during infancy improved adult spatial memory, astrocyte plasticity, vascularization and GluR-1 expression (PMID 34565308). An earlier developmental study reported that maternally administered cGP increased novelty recognition in developing offspring alongside increased IGF-1 bioavailability (PMID 27355491).

Metabolic and cardiovascular models (animal)

Two rat studies used high-fat diet-induced obesity. The first reported that cGP administration normalized high-fat diet-induced synaptophysin expression in obese rats (PMID 31146894), and the second reported that cGP normalized systolic blood pressure in high-fat diet-induced obese male rats (PMID 31753784). A separate 2018 article discussed the role of cGP as a biological regulator of IGF-1 in pregnancy-related obesity and weight changes (PMID 29921371).

Human observations

Human data in the verified set are observational or nutritional rather than interventional peptide trials. Researchers reported that the plasma cGP/IGF-1 ratio predicted clinical outcome and recovery after stroke (PMID 31019991), and a separate study reported that blackcurrant anthocyanin supplementation increased cGP concentrations in the cerebrospinal fluid of Parkinson's patients (PMID 29865234). A 2023 review summarised the argument that cGP normalises IGF-1 function, with discussion of clinical significance in the ageing brain (PMID 36770687).

Summary table of cited findings

Safety and Adverse Events: What Studies Report

The publications cited on this page were not designed as safety or tolerability trials, and their abstracts describe efficacy, biomarker and mechanistic endpoints rather than adverse-event tabulations. The closest signal that effects are bidirectional comes from the mammary tissue work, where researchers reported that cGP accelerated involution by promoting apoptosis and inhibiting IGF-1 function (PMID 28063218) — a reminder that a compound described as "normalising" IGF-1 (PMID 36770687) can reduce IGF-1 signalling in some tissues and raise available IGF-1 in others (PMID 27355491). No adverse-event frequency data can be quoted from the verified set here. This page is for educational purposes only and is not medical advice; consult a licensed physician about any health question or any compound discussed in the research literature.

What the cited literature does not establish

• Dosing. The verified papers are summarised here at the level of their titles and abstract-scope findings; no administration schedule is reproduced on this page.
• Human interventional outcomes. The human data cited are observational (PMID 31019991) or nutritional supplementation with an anthocyanin source rather than cGP itself (PMID 29865234).
• Regulatory status. Nothing in the cited set describes an approved cGP drug product; the compounds discussed appear in research and, in the case of dietary diketopiperazines, in food and microbial sources (PMID 39590698).

References

• Cyclic glycine-proline regulates IGF-1 homeostasis by altering the binding of IGFBP-3 to IGF-1 (Scientific Reports, 2014)
• Maternally Administered Cyclic Glycine-Proline Increases Insulin-Like Growth Factor-1 Bioavailability and Novelty Recognition in Developing Offspring (Endocrinology, 2016)
• Cyclic-glycine-proline accelerates mammary involution by promoting apoptosis and inhibiting IGF-1 function (Journal of Cellular Physiology, 2017)
• Supplementation of Blackcurrant Anthocyanins Increased Cyclic Glycine-Proline in the Cerebrospinal Fluid of Parkinson Patients (Nutrients, 2018)
• The role for cyclic Glycine-Proline, a biological regulator of insulin-like growth factor-1 in pregnancy-related obesity and weight changes (Journal of Biological Regulators and Homeostatic Agents, 2018)
• Plasma cyclic glycine proline/IGF-1 ratio predicts clinical outcome and recovery in stroke patients (Annals of Clinical and Translational Neurology, 2019)
• Cyclic glycine-proline administration normalizes high-fat diet-induced synaptophysin expression in obese rats (Neuropeptides, 2019)
• Cyclic glycine-proline normalizes systolic blood pressure in high-fat diet-induced obese male rats (Nutrition, Metabolism and Cardiovascular Diseases, 2020)
• Administration of cyclic glycine-proline during infancy improves adult spatial memory, astrocyte plasticity, vascularization and GluR-1 expression in rats (Nutritional Neuroscience, 2022)
• Cyclic Glycine-Proline (cGP) Normalises Insulin-Like Growth Factor-1 (IGF-1) Function: Clinical Significance in the Ageing Brain and in Age-Related Neurological Conditions (Molecules, 2023)
• Cyclic Glycine-Proline Improves Memory and Reduces Amyloid Plaque Load in APP/PS1 Transgenic Mouse Model of Alzheimer's Disease (International Journal of Alzheimer's Disease, 2023)
• Quantitative Monitoring of Cyclic Glycine-Proline in Marine Mangrove-Derived Fungal Metabolites (Journal of Fungi, 2024)