One of the greatest frustrations for doctors dealing with brain tumors is called the blood brain barrier (BBB). It's the defense our bodies have created to keep toxins from passing from our blood into our brains — but it also stops medicines from making the crossing too. Now that may be about to change.
Today Helen Thompson has a fascinating profile in New Scientist of Toronto medical physicist Kullervo Hynynen, who plans to test a theory he has about how to get cancer medicines out of the bloodstream and into targeted brain tumors. His plan is to to use microbubbles in the blood — basically just really small bubbles of gas — to open up the layer between blood vessels and brain tissue.
And he's going to do it with ultrasound. Writes Thompson:
Hynynen's trial will involve 10 people with a cancerous brain tumour. First, the volunteers will be given a chemotherapy drug that does not usually cross the BBB. They will then receive an injection of microbubbles, which will spread throughout the body, including into the blood vessels that serve the brain.
Next is a treatment called high-intensity focused ultrasound. The volunteers will wear a cap that contains an array of transducers that direct ultrasound waves into their brain. Just as the sun's rays can be focused by a magnifying glass, ultrasound waves can be concentrated inside the body to get the microbubbles to vibrate.
The vibrating bubbles will expand and contract about 200,000 times a second, which will force apart the endothelial cells that form the BBB. The idea is that this will allow the chemotherapy drug in the bloodstream to sneak through the gaps in the barrier and into any nearby tumour cells.
She explains that the ultrasound will only be on for about 2 minutes, and that the BBB will begin to close after it has been turned off (it won't be completely closed for about six hours, however). During the treatment, the patients will be monitored with fMRI brain imaging machines to see whether the drug has crossed the BBB, aided by the vibrating microbubbles. The goal is just to get the drug to nine sites around the brain tumors.
Surgeons will remove the tumors shortly after the treatment, to get more information on whether the microbubble technique has worked. There are no guarantees that it will work, and there are many dangers associated with opening up the BBB. Our bodies evolved the BBB for a reason, which is that there are many chemicals and microbes that are perfectly harmless in our bloodstream — but could become extremely dangerous if unleashed on our gray matter.
The fact that this treatment could leave the brain vulnerable up to six hours is a cause for concern — but being able to deliver medicine to brain tumors may be worth the risk. It could mean a future where we might begin treating brain tumors with drugs, rather than surgery.
Read more in New Scientist