Minuscule particles with the ability to cross hard-to-penetrate barriers can be loaded with drug treatments to target intractable diseases.

Scientists have used pig semen to develop eye drops that can stop tumour growth in the retina and preserve vision, a study1 in mice shows. The work is published today in Science Advances.

It is hoped that the drops could be developed to treat children with retinoblastoma, a cancer of the retina. The condition is typically treated with injections of drugs into the eye, chemotherapy or laser therapy, all of which can damage non-cancerous parts of the eye. Yu Zhang and colleagues at Shenyang Pharmaceutical University in China wanted to develop a method that could penetrate the barrier around the retina and deliver the drugs without causing any unnecessary damage.

The eye drops contain exosomes — tiny particles released by almost every type of cell — engineered to carry molecules that kill cancer cells. Mice treated with the eye drops showed signs that the tumours had stopped growing. They also had comparable eyesight to control animals without tumours.

The team showed that it was possible for exosomes to penetrate the retina. The technique could improve drug delivery across other barriers that are similarly difficult to breach, such as the blood–brain barrier — to treat conditions including Alzheimer’s disease — or the mucosal barrier, says Chunxia Zhao, a researcher studying drug delivery and nanomedicine at Adelaide University in Australia.