Within minutes after administration, most nanoparticles that were intended to deliver drugs to specific tissues will have disappeared from the blood. Jeroen Bussmann showed that they are destroyed by specialised scavenger cells in liver blood vessel walls, and found a way to prevent this. His publication in ACS Nano in February 2018 was elected that month’s best paper in the journals of the American Chemical Society.
Improving the design of drug-delivering nanoparticles
Small, nano-sized vesicles such as liposomes are increasingly used to deliver drugs at specific sites of the body, for instance in tumours. ‘As the blood vessels that supply tumours are relatively leaky, after injection the nanoparticles can leave the blood stream there and disintegrate, releasing the drugs to attack the cancer cells. Most other tissues would not have such leaky vessels, so the drugs could be applied with less serious side effects’, Jeroen Bussmann explains.
However, there is a problem: most nanoparticles never arrive at the target. Within minutes after injection, they are removed from the blood stream by the liver. Bussmann: ‘It was assumed that they are taken up by macrophages – immune cells – in liver vessels that scan the blood for the presence of foreign elements, such as bacteria, and engulf and digest them. However, until now, nobody investigated what really happens.’
He decided to fill this gap, using embryonic zebrafish as a research model. The embryos have no liver yet; their blood is purified in the caudal vein instead. Nanoparticles labelled with a fluorescent dye were administered, and as the embryos are transparent, the fate of the nanoparticles could be followed under a microscope.
As expected, some nanoparticles were engulfed by macrophages in the caudal vein; however, this was only a minority. Most particles, especially the smaller ones, were taken up by specialised scavenger endothelial cells that line the interior surface of blood vessels. These cells have the task to remove macromolecules from the blood. ‘To us, this was a surprising and puzzling result’, Bussmann says. ‘We wondered if something similar happens in adult mammals.’
Together with colleagues, he set out to see how nanoparticles are cleared from the blood in a more advanced model. Strikingly, it turned out that while some particles are engulfed by macrophages, most particles in that model are taken up by endothelial cells of certain blood vessels: the hepatic sinusoids. ‘These endothelial cells were already known to remove macromolecules and some viruses, just like scavenger endothelial cells in the caudal vein of zebrafish embryos. Now we showed that these cells play a major role in removing nanoparticles from the blood as well, as is the case in the caudal vein in zebrafish embryos. This means that the caudal vein in zebrafish embryos is fully comparable to hepatic sinusoids in adult mammals, with macrophages and endothelial cells performing exactly the same functions.’
The next question was what receptor (surface protein) of these scavenger endothelial cells is able to bind nanoparticles. By eliminating genes that code for receptors one by one using DNA editing technique CRISPR-cas9, he showed that stabilin-2 was the major responsible receptor for nanoparticles with a negative charge. By blocking this receptor, it is possible to inhibit clearance of these nanoparticles and extend the time that they circulate in the blood and may reach targeted tissues dramatically.
‘The use of nanoparticles to deliver drugs to special sites is promising, but still limited. Now that we better understand what happens to them after injection, it is possible to design better methods to apply them and to increase their potential.’
Jeroen Bussmann (1981) studied BioProcess Engineering at Wageningen University. He likes to combine disciplines such as chemistry, genetics and pharmacology. He studied zebrafish as from his internships; at the Massachusetts Institute of Technology in Boston, he studied their immune system. At the Hubrecht Institute in Utrecht, he dedicated his PhD to the development of the blood and lymphatic vessels of zebrafish, and he continued this research at the Max Planck Institute for Molecular Biomedicine in Münster. In 2013, he came to Leiden with a NWO Veni grant, and currently he is an assistant professor.