Why Some Peptides Cross the Blood-Brain Barrier and Others Don't

For research use only. Nothing here is medical advice, and these materials are not approved for human or animal use.

TL;DR: The blood-brain barrier is endothelial cells sealed by tight junctions. Small, less polar molecules slip through more easily than big, charged, water-loving peptides. Nose delivery in animals can bypass the wall. Papers on Semax, Selank, and Dihexa are easy to over-read, and semaglutide in the rodent work stayed mostly on the blood side.

What the blood-brain barrier actually is

The brain does not sit in a puddle of blood. Its smallest vessels are lined by endothelial cells, sealed by tight junctions, with transporters and enzymes in the same cells [3]. Banks adds very little pinocytosis and no leaky windows, so serum proteins do not drift in [1]. Circumventricular organs are the exception. Peptide papers almost always mean this capillary lining [3].

What decides whether a peptide gets through

Passive crossing means the molecule dissolves into the membrane and comes out the other side. Pardridge's review says a useful amount of that kind of crossing usually means under about 400 to 500 daltons and fewer than about 8 to 10 hydrogen bonds with water [2]. Banks agrees that low molecular weight and a more fat-liking character help, and he warns that a hard size cutoff is often a misreading. Some older "too big" examples were pumped out by P-glycoprotein [1]. Charge, shape, and sticking to blood proteins matter too, and enzymes mean a fragment in a brain sample is not the intact chain [1][2].

Specific doors exist. L-DOPA rides an amino-acid carrier [2]. Transferrin crosses in vesicles after binding a receptor, and adsorptive endocytosis can pull in a charged molecule stuck to the cell surface [1][2]. Efflux pumps throw many molecules back into the blood [1].

The nose is a bypass, not a hole in the wall

Lochhead and Thorne reviewed nose-to-brain delivery. From the nasal lining, material can travel along olfactory and trigeminal paths and reach the brain without first crossing capillary endothelium [4]. Banks had already said the amount that arrives is the real question [1]. A nose study does not show the same peptide would cross from blood, and nasal enzymes destroy much of what is placed there [4][5].

Short peptides on this catalogue

These notes are published experiments, not a property of a BlueNex Labs vial. The lot file is the identity record. Start with How do peptides work? and peptide delivery and bioavailability.

Semax is a 7-amino-acid analogue of ACTH(4-10). Read What is Semax?. In rats, a tritium-labelled version in the nose left 0.093% of the radioactivity per gram of brain at two minutes, and about 80% of that was still Semax [5]. A 2019 review put peak brain content near 0.13% for Semax and 0.16% for Selank, after a route that can bypass the wall [6].

Selank is a 7-amino-acid tuftsin analogue. See What is Selank?. An intranasal rat study reported a BDNF change in the hippocampus [7]. That is a brain readout after nose delivery, not a measurement of intact peptide crossing the capillary wall.

Dihexa is a short, modified angiotensin IV analogue. A 2013 rat paper described it as barrier-permeant [8]. The journal attached a notice of concern in 2021, and the paper has not been retracted [9]. A 2014 HGF/c-Met paper was retracted in 2025 [10]. The mechanism is not settled, and none of this describes the vial.

PE-22-28 is also 7 amino acids, cut from spadin. See What Is PE-22-28?. The 2017 mouse paper scored behaviour after the peptide was given into the abdomen. It did not measure brain entry [11]. Short length raises the question. It does not answer it.

Cerebroprotein here is its own 60 mg listing, separate from Semax, Selank, and Dihexa. Cerebrolysin papers describe a pig-brain mixture of short peptides. In one endothelial-cell dish, that mixture and a different cerebroprotein hydrolysate did not act the same way [12]. Those papers do not identify this listing or show that it crosses the barrier.

Peptides that mostly stay on the blood side

GHK-Cu is a copper complex of a 3-amino-acid peptide, so "too big" is the wrong story. The cited work is matrix, skin, and gene-expression models, not a capillary-crossing measurement [13]. Read What is GHK-Cu?.

BPC-157 is 15 amino acids, above the range Pardridge links to useful passive diffusion [2]. A brain-gut review does not report a permeability number for brain capillaries [14]. See What is BPC-157?.

Semaglutide is a long GLP-1 analogue with a fatty-acid tail that sticks to albumin. In rodents, a labelled form did not cross the barrier. It showed up at circumventricular organs and a few sites beside the ventricles, the leakier windows [15]. See GLP-1 peptide research in 2026. It is not a lot file.

Retatrutide (LY3437943) is longer still, with a fatty diacid, studied at GIP, GLP-1, and glucagon receptors [16]. The discovery paper describes a lasting blood profile and does not report a barrier measurement. Largely peripheral fits that paper. It is not a number borrowed from semaglutide.

What a barrier paper does not do

A rat experiment does not authorize a person, a pet, or a catalogue vial. Laboratory listings are Dihexa 10 mg, Selank 10 mg, Semax 10 mg, Cerebroprotein 60 mg, PE-22-28 10 mg, GHK-Cu 50 mg, BPC-157 10 mg, semaglutide, and retatrutide 10 mg. Retatrutide 20 mg and 30 mg are separate fills. Browse research peptides, shipping across Canada, or the general FAQ.

FAQ

What is the blood-brain barrier?

It is the wall of the brain's tiniest blood vessels. Endothelial cells there are sealed by tight junctions, so most substances in the blood do not leak into brain tissue.

Why do many peptides fail to cross it?

Most peptides are large compared with a typical small drug, carry charge, and form many hydrogen bonds with water. Those traits make passive passage through the endothelial membrane unlikely. Enzymes and efflux pumps can remove what does get in.

Can a peptide reach the brain without crossing that wall?

Yes, in research models. Some use a carrier or a receptor on the endothelial cell. Others are studied by the nose, along paths that can bypass the barrier. A bypass is not the same result as diffusion through the wall.

Do Semax and Selank cross the blood-brain barrier?

The clearest Semax and Selank measurements are after nose delivery in rats, which can bypass the barrier rather than cross it. Only a small share of the labelled material was found in brain, and enzymes broke Semax down quickly. That is not proof the intact peptide diffuses through brain capillaries.

Does Dihexa cross the blood-brain barrier?

A 2013 rat paper described Dihexa as able to pass the barrier. The journal later attached a notice of concern to that paper, and it has not been retracted. Read the barrier sentence with that flag in view. It is not a finding about a catalogue vial.

Do BPC-157, semaglutide, and retatrutide cross it?

BPC-157 is a 15-residue peptide, and the brain-gut review does not report a permeability number for brain capillaries. In rodents, labelled semaglutide did not cross the barrier and showed up at leakier windows called circumventricular organs. Retatrutide is a much longer fatty-acid-modified peptide studied for peripheral receptor signalling. That description paper does not report a barrier measurement.

Are these peptides approved for people?

No. BlueNex Labs supplies them for laboratory research only. A paper about a barrier is not permission to use a vial in a person or an animal.

References

1. Banks WA. Characteristics of compounds that cross the blood-brain barrier. BMC Neurol. 2009;9(Suppl 1):S3. https://pubmed.ncbi.nlm.nih.gov/19534732/

2. Pardridge WM. The blood-brain barrier: bottleneck in brain drug development. NeuroRx. 2005;2(1):3-14. https://pubmed.ncbi.nlm.nih.gov/15717053/

3. Abbott NJ, Patabendige AA, Dolman DE, Yusof SR, Begley DJ. Structure and function of the blood-brain barrier. Neurobiol Dis. 2010;37(1):13-25. https://pubmed.ncbi.nlm.nih.gov/19664713/

4. Lochhead JJ, Thorne RG. Intranasal delivery of biologics to the central nervous system. Adv Drug Deliv Rev. 2012;64(7):614-628. https://pubmed.ncbi.nlm.nih.gov/22119441/

5. Shevchenko KV, Nagaev IYu, Alfeeva LYu, et al. Kinetics of Semax penetration into the brain and blood of rats after its intranasal administration. Bioorg Khim. 2006;32(1):64-70. https://pubmed.ncbi.nlm.nih.gov/16523722/

6. Shevchenko KV, Nagaev IYu, Andreeva LA, Shevchenko VP, Myasoedov NF. Prospects for intranasal delivery of neuropeptides to the brain. Pharm Chem J. 2019;53:89-100. https://doi.org/10.1007/s11094-019-01960-x

7. Inozemtseva LS, Karpenko EA, Dolotov OV, et al. Intranasal administration of the peptide Selank regulates BDNF expression in the rat hippocampus in vivo. Dokl Biol Sci. 2008;421:241-243. https://pubmed.ncbi.nlm.nih.gov/18841804/

8. McCoy AT, Benoist CC, Wright JW, et al. Evaluation of metabolically stabilized angiotensin IV analogues as procognitive/antidementia agents. J Pharmacol Exp Ther. 2013;344(1):141-154. https://pubmed.ncbi.nlm.nih.gov/23055539/

9. Notice of concern on McCoy and colleagues, 2013. J Pharmacol Exp Ther. 2021;378(3):313. https://pubmed.ncbi.nlm.nih.gov/34551989/

10. Retraction notice for Benoist and colleagues, 2014, the HGF/c-Met paper on this molecule. J Pharmacol Exp Ther. 2025. https://pubmed.ncbi.nlm.nih.gov/40312093/

11. Djillani A, Pietri M, Moreno S, Heurteaux C, Mazella J, Borsotto M. Shortened spadin analogues display better TREK-1 inhibition, in vivo stability, and antidepressant activity. Front Pharmacol. 2017;8:643. https://doi.org/10.3389/fphar.2017.00643

12. Therapeutic effect of Cerebrolysin on reducing impaired cerebral endothelial cell permeability. 2021. https://pubmed.ncbi.nlm.nih.gov/33661804/

13. Pickart L, Margolina A. Regenerative and protective actions of the GHK-Cu peptide in the light of the new gene data. Int J Mol Sci. 2018;19(7):1987. https://pubmed.ncbi.nlm.nih.gov/29986520/

14. Sikiric P, Seiwerth S, Rucman R, et al. Brain-gut axis and pentadecapeptide BPC 157: theoretical and practical implications. Curr Neuropharmacol. 2016;14(8):857-865. https://pubmed.ncbi.nlm.nih.gov/27138887/

15. Gabery S, Salinas CG, Paulsen SJ, et al. Semaglutide lowers body weight in rodents via distributed neural pathways. JCI Insight. 2020;5(6):e133429. https://pubmed.ncbi.nlm.nih.gov/32213703/

16. Coskun T, Urva S, Roell WC, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: from discovery to clinical proof of concept. Cell Metab. 2022;34(9):1234-1247. https://pubmed.ncbi.nlm.nih.gov/35985340/