The Challenge of Genetic Therapy Delivery

Cystic fibrosis occurs when the cystic fibrosis transmembrane conductance regulator (CFTR) protein is either not made correctly or not made at all. The goal of genetic therapies — including RNA therapy, gene therapy, and gene editing — is to give the cells of people with CF the ability to make normal CFTR proteins.

For RNA therapy, gene therapy, or gene editing to work, the therapy must get inside the cells of a person with CF. These genetic therapies are made up of engineered molecules of DNA or RNA, depending on the method used.

Right now, CF scientists are focusing on the challenge of delivering genetic therapies to the cells of the lung, because that is one of the organs most severely affected by the disease. Getting DNA or RNA molecules into lung cells is a big challenge because of the body's natural defenses to block germs and other foreign invaders from entering cells. There are several barriers that make it difficult to deliver genetic therapies to lung cells: 

• Mucus: Even in healthy lungs, lung cells are underneath a protective layer of mucus that coats the entire airway. The mucus is intended to trap anything that is inhaled into the lung to make it more difficult to get to the cells. In the lungs of people with CF, these cells are under a much thicker layer of mucus and are even more difficult to reach.
• Cell membranes: All cells are surrounded by membranes that keep the contents of the cell inside and everything else outside. That means that even if the genetic therapy gets through the mucus, it still needs a way to get through the membranes to be released into the cell. A genetic therapy can only get through the membranes and into the cell if it is coated with a special packaging material, called a vector, which scientists are still trying to develop.
• Nuclear import: Once inside of the cell, some genetic therapies face an additional hurdle — they must cross into the nucleus where our own DNA is located.  

It is important to keep in mind that genetic treatments would work only in the cells that receive the therapy. For example, if cells in the lung receive gene therapy and produce functional CFTR proteins, cells in other parts of the body still would not be able to produce CFTR. Different delivery methods would be needed to get genetic therapies to other organs affected by CF, such as the intestine or pancreas. Although the lung is considered a priority for delivery, the Foundation also is considering approaches to deliver genetic therapies to other affected organs.

Treating a complex disease like CF requires collaboration across genetic research fields. Researchers who are developing RNA therapies, gene therapies, and gene editing tools need to work closely with those developing technologies for delivery. By working together, they can create the most effective genetic therapies for CF.

Chemical Coatings for Genetic Therapy Delivery

If a genetic therapy is inside a chemical package, it can enter cells much more easily. For example, DNA can be packaged inside a lipid coating to form a liposome. A liposome is a membrane that is similar to the membrane that surrounds the cell. The liposome can fuse with the membrane of the cell — like two water droplets merging into one bigger drop — allowing DNA from the liposome to transfer to the inside of the cell.

In 2015, a clinical trial in England tested whether DNA packaged into liposomes could deliver a gene therapy for CF. The liposomes with the normal CFTR DNA were put in a nebulizer, and people with CF inhaled them once per month for a year. The research indicated that the liposomes were safe and did not cause many side effects. The participants who inhaled the liposomes also had a small improvement in forced expiratory volume (FEV₁). This small change means that using liposomes to deliver a gene replacement therapy for CF is promising, but a more efficient gene delivery system will be necessary to achieve a bigger, longer-lasting improvement in lung function.

Scientists are working on developing new coatings to make delivery more effective. They are studying how to make better lipid-based particles and exploring other chemical coatings called polymers. Scientists are also figuring out how to compress DNA into small, dense structures called nanoparticles that can then be coated with lipids or other chemicals. Because nanoparticles are smaller and can be more completely coated with lipid than traditional liposomes, they may deliver DNA more effectively to cells. Multiple clinical trials are testing lipid-based systems to deliver an RNA therapy to lung cells in people with CF.

Viruses for Genetic Therapy Delivery

Researchers also have experimented with using viruses to deliver DNA into cells. Viruses are tiny packages of DNA inside a protein coating. The protein coating can allow the DNA to get inside a cell. There are thousands of different types of viruses. Each type can deliver its DNA to only a few kinds of human cells, based on the properties of its protein coating. For example, some viruses can deliver DNA only to liver cells, and others only to white blood cells.

The genetic material that is normally found inside viruses can be harmful and cause illness. If the harmful DNA inside the virus is replaced by helpful DNA, such as a corrected gene, the virus can be used as a vehicle to get the helpful DNA into cells. Viruses that only contain helpful DNA should not cause illness.

Gene therapies that use a virus to deliver DNA to cells have been tested in people with hemophilia, muscular dystrophy, metabolic disorders, and a specific type of blindness. These therapies were found to be safe and effective. Viruses have also been successfully used to deliver DNA for a treatment for leukemia, called CAR-T therapy.

The main difficulty with using viruses to deliver DNA for a CF therapy is that very few viruses preferentially deliver DNA to the cells in the lung. Scientists are engineering viruses with unique protein and lipid coatings that will allow them to deliver DNA to lung cells. Multiple clinical trials are testing viral vectors to deliver the CFTR gene to lung cells in people with CF.

To see a list of Foundation-supported genetic therapy programs, including viral and non-viral delivery methods, visit the Drug Development Pipeline.