A novel method of treating diseases such as cystic fibrosis or Haemophilia (lat. Hemophilia) by delivering new genes that replace missing or defective versions has been developed by the researchers at MIT. Another promising approach for new therapies is RNA interference, which can be used to turn off overactive genes by blocking them with short strands of RNA known as siRNA. Delivering these types of genetic material into body cells has proven difficult, however, because the body has evolved many defense mechanisms against foreign genetic material such as viruses. 

To help evade these defenses, the team has developed nanoparticles, including many made from fatty molecules called lipids, that protect genetic material and carry it to a particular destination. Many of these particles tend to accumulate in the liver, in part because the liver is responsible for filtering blood, but it has been more difficult to find particles that target other organs. To identify promising candidates, the team generates libraries of thousands of particles, by varying traits such as their size and chemical composition. Researchers then test the particles by placing them on a particular cell type, grown in a lab dish, to see if the particles can get into the cells. The best candidates are then tested in animals. However, this is a slow process and limits the number of particles that can be tried.

To overcome that hurdle, the researchers decided to add 'barcodes,' consisting of a DNA sequence of about 60 nucleotides, to each type of particle. After injecting the particles into an animal, the researchers can retrieve the DNA barcodes from different tissues and then sequence the barcodes to see which particles ended up where. The researchers first tested particles that had been previously shown to target the lungs and the liver and confirmed that they did go where expected. 

Then, the researchers screened 30 different lipid nanoparticles that varied in one key trait - the structure of a component known as polyethylene glycol (PEG), a polymer often added to drugs to increase their longevity in the bloodstream. Lipid nanoparticles can also vary in their size and other aspects of their chemical composition. Each of the particles was also tagged with one of 30 DNA barcodes. By sequencing barcodes that ended up in different parts of the body, the researchers were able to identify particles that targeted the heart, brain, uterus, muscle, kidney, and pancreas, in addition to liver and lung. In future studies, they plan to investigate what makes different particles zero in on different tissues. The researchers also performed further tests on one of the particles, which targets the liver, and found that it could successfully deliver siRNA that turns off the gene for a blood clotting factor.

Finding a good particle is a very rare event, so you need to screen a lot of particles. This approach is faster and can give you a deeper understanding of where particles will go in the body. This type of screen could also be used to test other kinds of nanoparticles such as those made from polymers.