Peptides & growth hormone

PEG-MGF

Updated · 18 sources · Editorial policy

PEG-MGF is sold as a “PEGylated” version of mechano growth factor (MGF): the short MGF peptide with a chain of polyethylene glycol (PEG) attached. MGF itself is a peptide copied from the tail of a splice variant of the insulin-like growth factor 1 (IGF-1) gene, covered in detail on the MGF page. The PEG chain is meant to make the peptide last longer in the body.

PEG-MGF has never been approved as a medicine, and no peer-reviewed study has tested it in humans. We also found no published study of the PEG-MGF product itself, in animals or in cells. A 2026 clinical review placed it in its lowest evidence tier, meaning no peer-reviewed human studies exist (Dominikowski et al., Front Endocrinol 2026). Everything said about its effects is borrowed from research on the unmodified or differently stabilized MGF peptide, and that research is itself disputed.

This page summarizes what the MGF research, the general PEGylation literature and the anti-doping rules report, and what they cannot tell us about PEG-MGF. It is for education only and is not medical advice.

Property Detail
Class Synthetic MGF peptide (from the IGF-1Ec E-domain) linked to polyethylene glycol
Approval Not approved for therapeutic use
Human studies None peer-reviewed; lowest evidence tier in a 2026 review
Studies of the product itself None found in PubMed, in humans, animals or cells
Defined structure Not described in the peer-reviewed literature (PEG size, attachment site and MGF form unknown)
Half-life Not reported; no pharmacokinetic data in any species
Purpose of PEGylation To slow breakdown and clearance of a short-lived peptide (general PEGylation principle)
WADA status Prohibited at all times (S2.3, “mechano growth factors”)

What it is

The MGF part

MGF refers to the IGF-1Ec splice variant in humans (IGF-1Eb in rodents) and, more loosely, to a synthetic peptide matching the last 24 amino acids of that variant’s E-domain tail (Matheny et al., Endocrinology 2010). The key facts carry over from the MGF page:

  • No 24-amino-acid MGF peptide has been isolated from the body; whether it exists naturally is disputed.
  • An independent replication at two pharmaceutical companies found that the synthetic peptide had no effect on muscle stem cells (Fornaro et al., Am J Physiol Endocrinol Metab 2014).
  • The short synthetic peptide, and a stabilized analogue, did not activate the IGF-1 receptor in cell assays (Janssen et al., PLoS One 2016). It is therefore not a form of IGF-1, even though it comes from the same gene.

The PEG part

PEGylation means attaching one or more chains of polyethylene glycol, a water-soluble polymer, to a protein or peptide (Veronese & Pasut, Drug Discov Today 2005). It is a well-established technique in licensed drugs. According to a review of the method (Harris & Chess, Nat Rev Drug Discov 2003), many peptide drugs:

  • are broken down by protein-cutting enzymes;
  • are cleared quickly by the kidneys;
  • have a short circulating half-life.

Adding PEG increases the molecule’s size and shields it from those enzymes, which can improve its pharmacokinetics. The trade-off is that PEGylation “frequently comes at the cost of reduced bioactivity” (Braun et al., J Control Release 2018).

Why anyone would PEGylate MGF

MGF researchers have argued that the native E-domain peptides are unstable. A 2012 review by researchers who developed the MGF concept wrote that synthetic E-domain peptides “benefit from chemical stabilisation” and that the natural equivalents are predicted to have a very short half-life (Zabłocka et al., Front Endocrinol 2012). The 2026 review describes the commercial rationale for PEGylation the same way: to increase stability and prolong exposure (Dominikowski et al., 2026).

How research groups have stabilized MGF

The published MGF studies mostly did not use PEGylation. They used other chemical changes, or a PEG-based gel as a slow-release depot. These are different from a PEG-conjugated peptide:

Approach What was done Model Source
D-amino-acid swap Two arginines switched to their mirror-image (D) form to resist enzyme cleavage Human stem cells in culture Collins et al., J Mol Cell Cardiol 2010
D-amino-acid swap plus amidated end Same arginine change plus a chemically capped (amidated) end Rat heart cells; mice after heart attack Mavrommatis et al., Mol Cell Biochem 2013
PEG hydrogel microrods (not PEGylation) Native peptide trapped inside tiny gel rods made of PEG dimethacrylate, released over about 2 weeks Cells in culture Doroudian et al., Biomed Microdevices 2014
PEG hydrogel microrods injected into the heart Same rods injected into heart muscle after an induced heart attack Mice Peña et al., Biomaterials 2015

A 2014 bioengineering paper stated that earlier work had protected MGF from degradation by PEGylation together with a D-arginine swap (Doroudian et al., 2014). We could not confirm the details of that earlier PEGylated construct from the original publication’s abstract, and no study reports its pharmacokinetics.

The hydrogel studies show why stability matters in this field, at least in mice. In the 2015 study, the native peptide released from PEG microrods inside the heart reduced mortality and slowed the decline in heart function after an induced infarct. By contrast, the same peptide injected on its own into muscle showed no significant improvement over untreated mice (Peña et al., 2015). That is a local, slow-release system for an animal heart model. It does not show what a PEG-conjugated peptide injected under the skin would do.

Pharmacokinetics

No study has measured the absorption, half-life, distribution or elimination of PEG-MGF in humans or animals. The 2026 review lists its half-life as “not reported” (Dominikowski et al., 2026). Half-life figures quoted online could not be traced to a primary source.

How much PEGylation extends a half-life depends on the molecule, the size of the PEG chain and where it is attached, so figures cannot be transferred from one PEGylated drug to another. One example shows how large the effect can be when a molecule is properly characterized. In a randomized, double-blind, placebo-controlled first-in-human study of a PEGylated form of full-length IGF-1 (RO5046013) in 62 healthy volunteers, the drug had a half-life of 140–200 h (Kletzl et al., Growth Horm IGF Res 2017). That molecule is a defined pharmaceutical product built on the whole IGF-1 hormone. It is not PEG-MGF, and its numbers say nothing about the products sold under that name.

Regulatory status

PEG-MGF is not approved for any medical use, so there is no labeled dose or approved route of administration. The 2026 review notes that this group of peptides lacks regulatory approval for physique or performance uses, and that no peer-reviewed human study has even reported a route of administration for PEG-MGF (Dominikowski et al., 2026). Unmodified MGF has not been approved for therapeutic use either (Esposito et al., Rapid Commun Mass Spectrom 2012).

What the research shows

Human studies

None. The 2026 review, which searched PubMed, Google Scholar and the Cochrane Library for publications from 1989 to January 2026, found no peer-reviewed human studies of PEG-MGF. It concluded that claims that PEG-MGF reliably speeds recovery or improves body composition in humans “remain unproven” (Dominikowski et al., 2026).

Why the MGF data cannot simply be applied

The review also stressed that PEGylation “fundamentally alters pharmacokinetics and tissue exposure.” Effects seen with native IGF-1Ec or isolated E-domain peptides therefore cannot be assumed to apply to PEG-conjugated versions (Dominikowski et al., 2026). Three gaps stand out:

  • The parent peptide’s effects are contested. Cell and animal studies from groups developing the MGF concept reported effects on muscle stem cells, heart and brain. An industry replication found no effect on muscle cells (Fornaro et al., 2014). Details are on the MGF page.
  • PEG can change activity. Attaching PEG often reduces a molecule’s biological activity (Braun et al., 2018). Whether a PEGylated MGF peptide keeps any activity has not been tested in a published study.
  • Local vs whole-body exposure. MGF is thought to act locally, near the cells that make it, (Zabłocka et al., 2012). A longer-circulating version would, if anything, spread exposure through the body. The 2014 microrod authors cited concerns about side effects from systemic delivery of a stable form and chose local delivery instead (Doroudian et al., 2014).

About “PEG-MGF is a long-acting IGF-1.” The 2026 review groups PEG-MGF with IGF-1 analogues that act through the IGF-1 receptor (Dominikowski et al., 2026). However, the short synthetic MGF peptide did not activate the IGF-1 receptor in a direct assay; only the full-length protein containing IGF-1 did (Janssen et al., 2016). No receptor study of a PEGylated MGF peptide has been published, so which of these it resembles depends on what a given product actually contains.

Side effects and risks

No human safety data

There is no measured rate of any side effect, no known safe dose and no interaction data for PEG-MGF. For the wider class of unregulated growth-hormone and IGF-1-axis peptides, the 2026 review lists these reported adverse effects, while noting there are no human data for PEG-MGF specifically (Dominikowski et al., 2026):

  • low or high blood sugar;
  • fluid retention;
  • muscle and joint pain;
  • injection-site reactions.

Risks linked to PEG itself

PEG is widely used in medicines, but it is not inert in every person. A review of anti-PEG immunity (Yang & Lai, Wiley Interdiscip Rev Nanomed Nanobiotechnol 2015) reports that:

  • the immune system can make antibodies that bind PEG, which can cause “accelerated blood clearance” of PEGylated drugs;
  • anti-PEG antibodies have been found in healthy people never treated with PEGylated drugs; reported rates range from 0.2% in a 1984 study to 27–28% in a later one;
  • in clinical trials, anti-PEG antibodies have been linked to reduced effect of two PEGylated protein drugs, PEG-asparaginase and pegloticase.

How these findings would apply to an unregulated PEG-MGF product is unknown.

Longer exposure and growth signaling

The MGF variant is expressed more strongly in prostate cancer tissue, and in prostate cancer cell lines the synthetic E peptide stimulated growth (Armakolas et al., Prostate 2010). The 2026 review lists this as a theoretical proliferative concern based on preclinical data (Dominikowski et al., 2026). No study shows that MGF or PEG-MGF products cause cancer. A form designed to stay in the body longer would extend any such exposure, but this has not been studied.

What is actually in the vial

We found no peer-reviewed laboratory analysis of products sold as PEG-MGF. Anti-doping labs analyzing products sold as plain MGF have found several different molecules: an MGF analogue with an amidated end (Esposito et al., 2012) and a roughly 12.3 kDa “full-length MGF” protein that contains the IGF-1 hormone (Thevis et al., Growth Horm IGF Res 2014). Since the PEG chain size, attachment site and MGF form of PEG-MGF products are not documented, identity, dose, purity and sterility cannot be assumed.

The WADA 2026 Prohibited List names “mechano growth factors (MGFs)” under S2.3, growth factors and growth factor modulators, prohibited at all times. Section S2 also covers “other substances with similar chemical structure or similar biological effect(s),” and elsewhere in S2 the list names PEG-modified versions of other peptides as examples. Under this wording, attaching PEG does not take an MGF outside the prohibited class. IGF-1 and its analogues, such as IGF-1 LR3 and IGF-1 DES, are banned under the same section.

Key takeaways

  • An MGF peptide with a PEG chain. PEG-MGF is marketed as the synthetic MGF peptide attached to polyethylene glycol, intended to last longer in the body. Its exact structure is not documented in the scientific literature.
  • No studies of the product. No peer-reviewed human study exists, and we found no animal or cell study of PEG-MGF itself; a 2026 review placed it in the lowest evidence tier.
  • Borrowed evidence, and weak at that. Claims rely on research with unmodified or differently stabilized MGF, which an independent industry replication failed to confirm in muscle cells.
  • PEGylation changes the molecule. It can extend half-life but often reduces activity, and anti-PEG antibodies occur even in untreated people. No half-life has been measured for PEG-MGF.
  • Not long-acting IGF-1. The short MGF peptide did not activate the IGF-1 receptor in a direct assay; no receptor study of a PEGylated version exists.
  • Banned in sport. WADA prohibits MGFs, and substances with similar structure or effect, at all times.

Sources

  1. Dominikowski A, Rękoś Z, Olejarz M, et al. The emerging landscape of performance-enhancing peptides modulating GH-IGF1 axis: bridging the gap between clinical evidence and patient self-administration. Front Endocrinol (Lausanne). 2026;17:1822475. PubMed.
  2. Matheny RW Jr, Nindl BC, Adamo ML. Minireview: Mechano-growth factor: a putative product of IGF-I gene expression involved in tissue repair and regeneration. Endocrinology. 2010;151(3):865–875. PubMed.
  3. Fornaro M, Hinken AC, Needle S, et al. Mechano-growth factor peptide, the COOH terminus of unprocessed insulin-like growth factor 1, has no apparent effect on myoblasts or primary muscle stem cells. Am J Physiol Endocrinol Metab. 2014;306(2):E150–E156. PubMed.
  4. Janssen JA, Hofland LJ, Strasburger CJ, van den Dungen ES, Thevis M. Potency of full-length MGF to induce maximal activation of the IGF-I R is similar to recombinant human IGF-I at high equimolar concentrations. PLoS One. 2016;11(3):e0150453. PubMed.
  5. Veronese FM, Pasut G. PEGylation, successful approach to drug delivery. Drug Discov Today. 2005;10(21):1451–1458. PubMed.
  6. Harris JM, Chess RB. Effect of pegylation on pharmaceuticals. Nat Rev Drug Discov. 2003;2(3):214–221. PubMed.
  7. Braun AC, Gutmann M, Mueller TD, Lühmann T, Meinel L. Bioresponsive release of insulin-like growth factor-I from its PEGylated conjugate. J Control Release. 2018;279:17–28. PubMed.
  8. Zabłocka B, Goldspink PH, Goldspink G, Górecki DC. Mechano-Growth Factor: an important cog or a loose screw in the repair machinery? Front Endocrinol (Lausanne). 2012;3:131. PubMed.
  9. Collins JM, Goldspink PH, Russell B. Migration and proliferation of human mesenchymal stem cells is stimulated by different regions of the mechano-growth factor prohormone. J Mol Cell Cardiol. 2010;49(6):1042–1045. PubMed.
  10. Mavrommatis E, Shioura KM, Los T, Goldspink PH. The E-domain region of mechano-growth factor inhibits cellular apoptosis and preserves cardiac function during myocardial infarction. Mol Cell Biochem. 2013;381(1-2):69–83. PubMed.
  11. Doroudian G, Pinney J, Ayala P, et al. Sustained delivery of MGF peptide from microrods attracts stem cells and reduces apoptosis of myocytes. Biomed Microdevices. 2014;16(5):705–715. PubMed.
  12. Peña JR, Pinney JR, Ayala P, Desai TA, Goldspink PH. Localized delivery of mechano-growth factor E-domain peptide via polymeric microstructures improves cardiac function following myocardial infarction. Biomaterials. 2015;46:26–34. PubMed.
  13. Kletzl H, Guenther A, Höflich A, et al. First-in-man study with a novel PEGylated recombinant human insulin-like growth factor-I. Growth Horm IGF Res. 2017;33:9–16. PubMed.
  14. Yang Q, Lai SK. Anti-PEG immunity: emergence, characteristics, and unaddressed questions. Wiley Interdiscip Rev Nanomed Nanobiotechnol. 2015;7(5):655–677. PubMed.
  15. Armakolas A, Philippou A, Panteleakou Z, et al. Preferential expression of IGF-1Ec (MGF) transcript in cancerous tissues of human prostate: evidence for a novel and autonomous growth factor activity of MGF E peptide in human prostate cancer cells. Prostate. 2010;70(11):1233–1242. PubMed.
  16. Esposito S, Deventer K, Van Eenoo P. Characterization and identification of a C-terminal amidated mechano growth factor (MGF) analogue in black market products. Rapid Commun Mass Spectrom. 2012;26(6):686–692. PubMed.
  17. Thevis M, Thomas A, Geyer H, Schänzer W. Mass spectrometric characterization of a biotechnologically produced full-length mechano growth factor (MGF) relevant for doping controls. Growth Horm IGF Res. 2014;24(6):276–280. PubMed.
  18. World Anti-Doping Agency. 2026 Prohibited List. WADA 2026 Prohibited List.