Getting drugs to the brain is difficult. The very thing designed to protect the brain’s environment — the blood-brain barrier (BBB), tightly packed cells that prevent most substances from passing through — is one of the main reasons diseases like Alzheimer’s are so hard to treat.

And even if a drug can cross the BBB, it’s difficult to ensure it reaches specific areas of the brain like the hippocampus, which is located deep within the brain and notoriously difficult to target with conventional drugs.

However, new research shows that novel bioengineered proteins can target neurons in the hippocampus. Using a mouse model, the researchers found that these proteins could be delivered to the hippocampus intranasally — through the nose via a spray.

“This is an urgent topic because many potential therapeutic agents do not readily cross the blood-brain barrier or have limited effects even after intranasal delivery,” said Konrad Talbot, PhD, professor of neurosurgery and pathology at Loma Linda University, Loma Linda, California, who was not involved in the study.

This is the first time a protein drug, which is larger than many drug molecules, has been specifically delivered to the hippocampus, said Noriyasu Kamei, PhD, a professor of pharmaceutical science at Kobe Gakuin University in Kobe, Japan, and lead author of the study.

How Did They Do It?

“Smuggle” may be a flip term, but it’s not inaccurate.

Insulin has the ability to cross the BBB, so the team began with insulin as the vehicle. By attaching other molecules to an insulin fragment, researchers theorized they could create an insulin fusion protein that can be transported across the BBB and into the brain via a process called macropinocytosis.

They executed this technique in mice by fusing florescent proteins to insulin. To treat Alzheimer’s or other diseases, they would want to fuse therapeutic molecules to the insulin for brain delivery — a future step for their research.

Other groups are studying a similar approach using transferrin receptor instead of insulin to shuttle molecules across the BBB. However, the transferrin receptor doesn’t make it to the hippocampus, Kamei said.

A benefit of their system, Kamei pointed out, is that because the method just requires a small piece of insulin to work, it’s straightforward to produce in bacteria. Importantly, he said, the insulin fusion protein should not affect blood glucose levels.

Why Insulin?

Aside from its ability to cross the BBB, the team thought to use insulin as the basis of a fusion protein because of their previous work.

“I found that insulin has the unique characteristics to be accumulated specifically in the hippocampal neuronal layers,” Kamei explained. That potential for accumulation is key, as they can deliver more of a drug that way.

In their past work, Kamei and colleagues also found that it could be delivered from the nose to the brain, indicating that it may be possible to use a simple nasal spray.

“The potential for noninvasive delivery of proteins by intranasal administration to the hippocampal neurons is novel,” said John Varghese, PhD, professor of neurology at University of California Los Angeles (he was not involved in the study). He noted that it’s also possible that this method could be harnessed to treat other brain diseases.

There are other drugs that treat central nervous system diseases, such as desmopressin and buserelin, which are available as nasal sprays. However, these drugs are synthetic hormones, and though relatively small molecules, they do not cross the BBB.

There are also antibody treatments for Alzheimer’s, such as aducanumab (which will soon be discontinued), lecanemab, and donanemab; however, they aren’t always effective and they require an intravenous infusion, and while they cross the BBB to a degree, to bolster delivery to the brain, studies have proposed additional methods like focused ultrasound.

“Neuronal uptake of drugs potentially therapeutic for Alzheimer’s may be significantly enhanced by fusion of those drugs with insulin. This should be a research priority,” said Talbot.

While this is exciting and has potential, such drugs won’t be available anytime soon. Kamei would like to complete the research at a basic level in 5 years, including testing insulin fused with larger proteins such as therapeutic antibodies. If all goes well, they’ll move on to testing insulin fusion drugs in people.