Carrying drug treatments to go just where you want them inside the human body is no easy task, and using high doses of chemicals that are carried by the bloodstream to the wrong tissues or organs can lead to toxic side effects.
Which explains why scientists have been working for years to figure out how to deliver much smaller doses to precise targets, developing chemotherapy drugs that bind only to tumor cells, for instance. Now researchers have developed a new method that traps drugs or other molecules within tiny cages made of DNA, then releases them once they’ve reached the right spot with a quick flash of light.
Xue Han, assistant professor of biomedical engineering at Boston University and author of the new study, says:
“Basically it’s sort of a controlled drug release, and there wasn’t a very good approach [to that] until now. What we did is to put these drugs physically inside a cage.”
Drugs are molecules, many of them big, with different areas on their surfaces that allow them to bind with receptors that have complementary areas, like puzzle pieces fitting together, allowing them to attach to and interact with a cell. The problem is that some cells have receptors that will take up a particular drug, even when they’re not the cells that the drug is supposed to target.
Large Proteins Wearing Small Hats
Scientists have traditionally dealt with this problem by adding small chemical groups to the surface of molecules so the receptor won’t recognize it. Han says it’s like sticking a tiny chemical hat on the drug to disguise it. For some small-molecule drugs, that can work well.
But some drugs, such as proteins, are just so big that a tiny modification won’t disguise it.
“A large protein wearing a small hat still looks like the same protein,” she says. Her idea was to enclose the protein in a cage, hiding it completely from the receptors. “The thing that reacts with this drug will not see the drug,” she says. “It will see a box.”
To make the box, scientists turned to a technique called DNA origami, named for the Japanese art of folding sheets of paper into complex shapes, which scientists have been using for the past decade. Scientists can create a strand of DNA with its nucleic acids arranged in the order they want.
They heat up the strands to near boiling, then let them slowly cool, and the natural attraction and repulsion between the different nucleic acids causes the strands to bend and fold into a desired shape. Han and Richie Kohman, a former postdoctoral fellow in Han’s lab, used DNA origami to create an open-ended barrel about 50 nanometers wide, with a 20-nanometer cavity inside.
“That’s large enough to fit some big proteins inside,” Han says.
To keep their drugs in the enclosure, they left little bits of DNA hanging unattached inside the cage, then added more small molecules to act as tiny chains, binding to the drugs and holding them in place.
The chains were designed so that a small jolt of energy from a beam of light would break them, setting the proteins free. Once the cage is in the right spot, the light snaps the chains and the drugs just drift out, winding up where they’re wanted.
The researchers tested their system using a fluorescent dye called Oregon Green, which is commonly used to tag proteins and other biological molecules. They trapped molecules of the dye inside the cages, then zapped them with low-power beams of ultraviolet light.
After 40 seconds of exposure, almost all the dye had left the cages.
The dye was a small molecule. To see if the scheme would work with larger proteins, the researchers trapped two other kinds of molecules. One was a protein derived from cow’s blood.
The other was streptavidin, a molecule used to bind drugs to cancer cells. After about a minute of low-level light exposure, most of the proteins had escaped the cages.
Finally, to check that the process didn’t repress the biological activity of the molecules, they tried the technique on a small molecule called glutamate that affects the activity of brain cells. They added the caged glutamate to a culture of brain cells, hit them with a brief burst of light, then measured how the flow of calcium in the brain cells had changed.
Just a 1 millisecond flash of light was enough to release the glutamate and increase calcium activity in the cells.
It’s that finding that has Han most excited. She studies neuroscience, and is looking for ways both to understand the activity of brain cells and to deliver drugs that can affect that activity.
For instance, there are certain molecules being studied that might slow the progression of Parkinson’s disease, if they can be delivered to the right spot.
“I’m really interested in all these neuropeptide hormones. There are so many of them in the brain,” Han says. “We know peptide hormones are very important. We just don’t know how they work.”
One difficulty with any drug treatment for brain disorders, including cancer, is getting the drug past the blood-brain barrier.
While that barrier prevents most molecules from moving out of the bloodstream and into brain cells, there are certain ones that pass easily, and it might be possible to use them to carry caged molecules to the brain. That could mean delivering the drug directly to the brain of a Parkinson’s patient without having to stick a needle into it.
One question will be how to get light to these cages if they’re deep in the body. That’s something other people are still working on, Han says.
Though the experiment used ultraviolet light, it might work better to use infrared light, which can penetrate tissue fairly deeply and is less likely to damage cells than ultraviolet. Chemists have already created molecular changes that break under infrared light.
It might also be possible to shine light into certain areas by using an endoscope to carry it to cavities within the body.
Of course, any use in humans would require approval from the US Food and Drug Administration, which would require years of testing. But for lab studies involving cells in petri dishes, the technique could be used almost immediately, Han says.
In fact, the potential uses, from basic research to treating neurological diseases to fighting cancer, could turn out to be many, she says.