Everything you need to know about the blood-brain barrier

3 min read

Quick facts

  • The blood-brain barrier (BBB) is the name given to the collection of unique features in and around the blood vessels that supply the brain
  • The BBB protects the brain from external toxins and diseases by restricting what can, and cannot, enter the brain from the body’s blood supply
  • This means the BBB also restricts the movement of the majority of medicines into the brain, making it very difficult to treat certain conditions and diseases
  • The BBB presents a challenge for researchers of many different neurological conditions, including brain tumours, motor neurone disease and Parkinson’s disease

What is the blood-brain barrier?

The blood-brain barrier is designed to protect the brain from damage and disease.

For organs and tissues outside of the brain, nutrients are delivered via the circulatory system. Oxygen, glucose and other useful substances easily move out of capillaries into the fluid around the cells, delivering the resources needed to function.

The capillaries that supply these body systems are ‘leaky’, which means that they have gaps which larger substances such as immune cells and proteins can squeeze through.

The benefit of this is that immune cells travelling through the blood can get into diseased tissues to help fight off infection or clear debris. However, the risk of these gaps is that bacteria, viruses and toxins can move from the tissues back into the bloodstream, potentially spreading infections or toxins throughout the body and causing disease in new areas.  

Because the brain is so sensitive, it needs to be protected from these toxic or infectious agents. Therefore, the capillaries that deliver resources to the brain are structured slightly differently. They have a variety of features in place to prevent the movement of substances into the brain. Oxygen and glucose - the brain’s primary energy sources - are able to move through into the brain freely, however larger substances, like cells and proteins, cannot. It is a very effective method that helps prevent the brain from developing potentially life-threatening infections.

The problem with this blood-brain barrier is that by preventing larger substances from entering, it also prevents useful molecules such as medications from passing into the brain as well, making it very difficult to treat certain medical conditions, such as brain tumours and other neurological disorders.

How does the BBB impact brain tumour treatment?

Over the last 30 years, there have been major breakthroughs in the treatment of a variety of solid-state cancers (cancers with solid tumour masses) such as breast cancer and prostate cancer.

A recent study funded by Cancer Research UK has shown more than 95% of women diagnosed with early-stage breast cancer will survive the disease for five years or more. The picture is not the same for brain tumour patients, with just 13% of high-grade brain tumour patients surviving the disease for five years or more, a prognosis that has remained unchanged for decades. 

There are a number of reasons for this, one of which is the blood-brain barrier. Effective chemotherapy agents such as cisplatin, which demonstrate great responses in the laboratory tests for glioblastoma (GBM) brain tumours, show limited efficacy in human patients. This is because they cannot easily cross the BBB and do not accumulate at concentrations high enough to demonstrate a therapeutic response. In fact, research suggests that 98% of currently-approved drugs do not pass the BBB.

What research is being done to get past the BBB?

Researchers are investigating a variety of techniques, drugs and devices to bypass the BBB and get effective anti-cancer drugs to where they are needed. These include:

  • Coating drugs in membranes similar to the cells in the brain
  • Combining drugs with molecules that already have a route across into the brain, allowing these drugs to ‘piggyback’ their way across the BBB
  • Temporarily disrupting the BBB to create spaces large enough to allow molecules through
  • Placing the drugs directly into the brain cavity

It is unlikely that a one-size-fits-all approach will work for all the different compounds required for brain tumour treatment, and that treatment-specific techniques will be needed.

Our Centre of Excellence at The Institute of Cancer Research, Sutton Campus, is working with the Non-invasive Surgery and Biopsy Laboratory at Imperial College, London, using focused ultrasound to temporarily disrupt and open the BBB in paediatric-type diffuse high-grade gliomas.

What is Brain Tumour Research doing address the challenges presented by the BBB?

The BBB is a challenge that does not solely affect brain tumour patients. Treatments for other neurological conditions such as Alzheimer's, Parkinson's and motor neurone disease also are impeded by the BBB.

On 28th February 2023 the All-Party Parliamentary Group on Brain Tumours (APPGBT), for which Brain Tumour Research provides the secretariat launched its landmark inquiry report demanding action to make brain tumour research a “critical priority”.

Included in the report Pathway to a Cure – breaking down the barriers was the recommendation to: “Make the blood- brain barrier a strategic priority, encouraging investment in cutting-edge research, which could yield ‘game-changing’ results in the treatment of brain tumours and other neurological diseases”.

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