It is well-known that most peptide molecules cannot cross the blood-brain barrier (BBB). This is because the BBB is a highly selective biological barrier composed of specialized types of endothelial cells that form a tightly controlled barrier, protecting the brain from the influence of external substances. The inherent characteristics of peptide molecules largely determine that most of them cannot effectively cross the BBB.
Characteristics that influence the ability of peptides to cross the BBB include:
- Size: The BBB selectively allows molecules of a certain size to pass through, typically only permitting small molecules. Peptide molecules are generally larger, exceeding the pore size limitations of the BBB, making it difficult for them to cross.
- Structure: Peptide molecules are typically composed of amino acids, and their molecular structure is not conducive to crossing the BBB. In contrast, small molecule drugs usually have smaller and simpler molecular structures, making them more permeable through the BBB.
- Charge: Many peptide molecules carry charges, increasing their interaction with the endothelial cells of the BBB, making it harder for them to cross.
What Molecules Can Cross the BBB?
- Small Molecules: Small molecule drugs usually have relatively small sizes and simple chemical structures, making them more permeable through the BBB.
- Non-Polar Molecules: Molecules with low polarity are more likely to cross the BBB because they can more easily dissolve in the lipid bilayer.
- Specific Carrier Proteins: Specialized carrier proteins are responsible for transporting specific substances across the BBB. For example, glucose transporter protein (GLUT1) is responsible for transporting glucose into brain cells.
- Gases: Small molecular gases like oxygen and carbon dioxide can freely cross the BBB to meet the brain’s oxygen demand.
It’s important to note that even if a molecule possesses the above characteristics, it may not necessarily cross the BBB easily, as the BBB is highly selective, allowing only specific molecules to pass. Additionally, most drugs require specialized drug development and design to ensure they are small enough and have a high enough lipid solubility to effectively cross the BBB for the treatment of central nervous system diseases.
The BBB Does Not Reject All Peptides
While most peptides cannot cross the BBB, researchers are exploring ways to overcome this limitation to use peptides for the treatment of central nervous system diseases. These methods include altering the molecular structure of peptides, using brain-targeting techniques, or conjugating peptides with carriers to enhance their permeability across the BBB.
One of the early discoveries in peptide science was that peripheral administration of peptides could influence brain function. This led to the proposition that peptides could cross the BBB. An example of this is the discovery by American scientist Kastin in 1975 that the melanocyte-stimulating hormone (MSH) could cross the BBB. MSH release-inhibiting factor-1 (MIF-1) is a tripeptide from the hypothalamus that controls the pituitary secretion of MSH. Kastin found that peripheral administration of MIF-1 had actions in the pituitary gland that were not mediated by pituitary MSH secretion. These actions included attenuation of amnesia induced by drugs or electroconvulsive shock, hypothermia induced by drugs, modulation of the therapeutic effects of Parkinson’s disease, and antidepressant activity.
However, it is known that in some cases, peripheral substances can influence the brain without crossing the BBB. For example, insulin can alter the transport of tryptophan across the BBB, affecting the synthesis of serotonin in the brain. Determining the site of action for peptides can be challenging, as it is often unclear whether the effect is mediated within the brain or by peripheral tissues. However, with appropriate agonists or antagonists, it is sometimes possible to identify the site of action. For example, peripheral administration of a enkephalin analogue that affected the 1-4 Hz delta band of the electroencephalogram was blocked by naltrexone but not by methylnaltrexone. Since naltrexone can cross the BBB while methylnaltrexone cannot, this suggests that the site of action for the peripherally administered enkephalin analogue is in the brain beyond the BBB. This is an indirect proof that peptides can cross the BBB and act directly in the brain.
In addition to these antagonistic effects, differences in the route of administration leading to differences in efficacy can also indirectly demonstrate the ability of peptide molecules to cross the BBB. For example, the cyclic peptide Cyclo[His-Pro] is known to reverse ethanol-induced anesthesia by acting in the brain. When Cyclo[His-Pro] is administered intracerebroventricularly (ICV), its efficacy is approximately 200 times greater compared to intravenous injection. Cyclo[His-Pro] is highly stable against enzymatic degradation and can slowly enter the brain after intravenous injection. Therefore, the ability of peripherally administered Cyclo[His-Pro] to reverse ethanol anesthesia may depend on its ability to cross the BBB.
Peptides that can cross the BBB include the following examples:
| Name | Description | CAS |
| Angiopep-2 | Angiopep-2 is a peptide interacting with a specific receptor (LRP1). | – |
| Tat Peptide | Tat (Trans-Activator of Transcription) peptide is derived from the human immunodeficiency virus (HIV). | – |
| Insulin | Insulin, a peptide hormone, is responsible for regulating blood glucose levels. | 9004-10-8 |
| Amylin | Amylin is a peptide secreted by pancreatic islet cells and helps regulate blood glucose. | 106602-62-4 |
Peptide Drugs that Can Cross the BBB
- Leuprorelin: Leuprorelin is a GnRH agonist peptide drug used to treat various hormone-related conditions such as prostate cancer and endometriosis. It can cross the BBB and influence pituitary release of gonadotropins, thereby affecting hormone levels.
- Oxytocin: Oxytocin is a peptide hormone important for social behavior and emotional regulation. It can cross the BBB and affect the central nervous system, contributing to the formation of emotional connections.
- GLP-1 Analogs: Glucagon-like peptide-1 (GLP-1) is a class of peptide hormones used to treat type 2 diabetes and obesity. Some GLP-1 analogs can cross the BBB, improving appetite and weight control while potentially offering benefits for neuroprotection.
- Somatostatin Analogs: Somatostatin analogs are peptide drugs used to treat neuroendocrine tumors and other endocrine diseases. They can cross the BBB and suppress the secretion of growth hormone, making them useful in the treatment of pituitary tumors and other conditions.
How do Peptides with Large Structures Cross the BBB?
In a study on whether delta sleep-inducing peptide (DSIP), a nine-peptide hormone produced in the brain with roles in influencing sleep patterns and circadian rhythms, can penetrate the BBB, researchers found that under conditions of peripheral injection of DSIP, it can induce sleep. Advanced radioimmunoassay analysis confirmed that DSIP indeed crosses the BBB. In contrast to the notion that molecules cross via a “leakage passage,” it was found that lipophilicity is a key determinant of entry into the brain. Many peptides can traverse the BBB based on their high lipophilicity.
From this perspective, it seems that Lipinski’s Rule of Five, which was developed based on the solubility and permeability data of small orally administered molecules, might not accurately predict oral bioavailability for peptides. While Lipinski’s Rule of Five suggested that molecules with a molecular weight less than 500 Da have better chances of crossing the BBB than larger molecules, it’s essential to note that Lipinski himself cautioned against applying these rules to certain categories of molecules, such as anti-parasitic drugs. So far, the largest molecule known to cross the BBB through a nonsaturated mechanism (active transport or specific receptor-mediated pathways rather than passive diffusion) is CINC-1, a cytokine involved in regulating inflammation and immune responses, with a molecular weight of 7.8 kDa.
In the realm of type 2 diabetes and obesity, GLP-1 receptor agonists have also provided evidence for the ability of peptides to cross the BBB. There’s growing interest in how these GLP-1 and gastric inhibitory peptide (GIP) affect beta-amyloid levels and insulin resistance in Alzheimer’s disease, making their ability to cross the BBB a subject of intense research.
An important factor in peptide transport across the BBB appears to be absolute charge. These peptide molecules seem to primarily enter the brain through adsorptive transcytosis, where the drug binds to specific receptors or ligands on the cell membrane and then fuses with vesicles or endosomes inside the cell, allowing the molecule or drug to enter the cell.
In Kastin’s study, the transport of Tyr-MIF-1, an analog of MIF-1, was unidirectional, going from the brain to the blood. The transporter protein responsible for Tyr-MIF-1 transport is called Peptide Transport System-1 (PTS-1), which is a protein transport system for small peptides to enter cells. PTS-1 has a strong affinity for small peptides containing N-terminal tyrosine (4-5 amino acids). It exhibits higher affinity for Tyr-MIF-1 and Met-enkephalin but weaker affinity for Leu-enkephalin. PTS-1 does not transport phenylalanine-MIF-1. The sole difference between phenylalanine-MIF-1 and Tyr-MIF-1 is the presence of phenylalanine versus tyrosine (a 4-hydroxyl difference). Even D-Tyr-MIF-1 cannot be transported by PTS-1, underscoring the importance of the N-tyrosine conformation.
References:
- Banks, W. A., Viktor Mutt lecture: Peptides can cross the blood-brain barrier, Peptides, 2023, 169. 171079.
- Sánchez-Navarro, M., and Giralt, E., Peptide Shuttles for Blood–Brain Barrier Drug Delivery, Pharmaceutics, 2022, 14, 1874.