Researchers have discovered a new drug delivery mechanism for targeting the oncogenic fusion protein responsible for development of fibrolamellar hepatocellular carcinoma (FLC), an aggressive form of pediatric liver cancer. The therapy involves targeting the mRNA of the fusion protein using a small interfering RNA (siRNA).
As described in Molecular Therapy, researchers at Rockefeller University inserted siRNAs inside FLC cells through a surface receptor found only on hepatocytes. Once inside the tumor cells, the siRNA binds to the mRNA of the fusion protein, not only stopping tumor growth but also killing the cell.
Many childhood tumors are the result of two normal genes fusion together. The problem is that fused proteins and their normal protein forms are nearly identical, so targeting just a fused protein will also interact with the other, producing different adverse effects.
“What this approach does is target right at the junction between the two of them, so it does not produce off-target effects,” says senior author Sanford Simon. His laboratory discovered the FLC-causing fusion protein in 2014.
The team designed the siRNA against the mRNA nucleotide sequence at the region making up the fusion junction. This sequence is not present in normal mRNAs that are present in the cell. In an earlier paper the team showed that making a siRNA that could bind to a complementary region of the fusion protein mRNA, eliminated it.
In this paper, the team focused on methods to deliver the siRNA. Unlike antibodies and small molecules, siRNAs are not taken up by cells automatically as they cannot cross the cell membrane. The Rockefeller team used a ligand called GalNac to attach the siRNA. The ligand binds to a receptor present only on hepatocytes called ASGR1.
“It’s like a shovel,” adds first author Christoph Neumayer, says first author Christoph Neumayer, a PhD student in Simon’s lab. The GalNac ligand acts like a shovel bringing the siRNA to the ASGR where it is brought inside the tumor cells. “We found that this method not only inhibits tumor growth by getting rid of the mRNA that codes for the oncogene, but it kills the cells.”
Normal hepatocytes are spared because they do not contain the oncogenic fusion protein mRNA. Studies in patient-derived xenograft FLC models in mice showed that the normal cells of the mouse livers – and other cells – are not affected, so the treatment appeared to have no off-target toxicity. The treatment appears to be effective not only in primary FLC tumors but also in metastatic tumors.
They also tested the specificity of siRNA by injecting it into tumor cells of another type of liver cancer. It had no toxic impact on them.
The field of siRNA delivery has been limited by the lack of known ligands that can target a given cell. In this case, the GalNac ligand has been widely known, but it affirms the strategy for other tumor types where ligands are known or can be discovered.
“That’s the really exciting part,” says Neumayer. “This is proof of concept that siRNAs can be used for FLC, as well as adult liver cancers, which are much more common, and other tumor types elsewhere in the body.”
The team plans to improve the delivery even further. One area includes combination therapies with small molecules. Another is to innovate their current delivery strategy to deliver more therapeutically effective doses.
“We want to get more into cells if possible, including targeting any hepatocyte in the body,” says Simon. While the strategy is very effective against a large liver tumor, the team wants to be sure they can reach small micro metastases around the body. “If we can target the siRNA in all of them, we can potentially get around the problem of metastases.”
