Cancer that spreads to the central nervous system is notoriously difficult to treat, but a new nanocapsule may be small enough to do just that.

When it comes to treating some cancers, often our own body causes us the most difficulty. While between 15pc and 40pc of all cancers spread to the central nervous system, there are very few treatment options and, even then, they only work on a small number of patients.

This is because of the blood-brain barrier, a natural defence system that prevents harmful agents from entering the brain, including many drugs.

Now, however, researchers from the University of California have found a way to send a nanocapsule containing drugs that can break through the barrier and treat the central nervous system.

In a paper published to Nature Biomedical Engineering, the scientists said they had tested the nanocapsule, which contained a single dose of the cancer-fighting drug rituximab, in mice. In tests, it showed to have eliminated all B-cell lymphoma that had metastasised to the animals’ central nervous systems.

To safely travel through the blood-brain barrier, they had to engineer a capsule measuring just one nanometre across, or 100,000 times thinner than a sheet of paper. The capsule was then coated with a substance called 2-methacryloyloxyethyl phosphorylcholine, which they hypothesised would be unlikely to be blocked by the blood-brain barrier. Once past this barrier, it would release the antibodies when it comes into close proximity to cancerous cells.

While additional research is needed – including testing on human subjects – the researchers said the capsule could be used to carry drugs approved by regulators in the near future.

The approach could be useful not only for cancers that metastasise to the central nervous system – such as breast cancer, small-cell lung cancer and melanoma – but also for primary brain tumours or other brain diseases.