RNA delivery by self-assembling phagocytic-competent protein crystals

The mechanism by which a drug travels following administration (e.g. by intravenous injection) to reach its molecular target is critical to efficacy. Innovations in drug delivery have been fundamental to many medical advances. Most recently, RNA drugs, including widely used vaccines against SARS-CoV-2, have entered the market. RNA drugs required novel mechanisms of drug delivery which overcame specific challenges: Whilst conventional drugs act outside cells or are small enough to permeate through to intracellular targets, RNA drugs are only effective inside cells, and their large size necessitates a specific delivery mechanism to carry them across the cell wall and into the cell cytosol where they act.

Viruses and lipid nanoparticles (LNPs, resembling small hollow spheres of fat) are used to deliver currently approved RNA drugs. Viruses and LNPs envelop and protect the RNA from damage whilst outside the cell and are able to dock or fuse with the cell wall to make the final delivery of RNA. However, these technologies have significant shortcomings including production costs, batch consistency, storage stability, immunogenicity, lack-of-targeting, the efficiency of cell entry, and endosomal escape. Whilst improvements in these technologies are being made, a fresh approach may better address these challenges.

Particles access cells through a variety of passive and active mechanisms. Phagocytosis is a process in which cells actively ingest particles (most efficiently in the 0.3 - 5 microns size range). This process is particularly efficient in professional mononuclear phagocytic (MP) cells. These immune cells present vaccine antigens, ingest debris and eliminate invading pathogens. They also migrate to diseased and injured tissue and are involved in a variety of processes, including the generation of vaccine immunity, autoimmune diseases, wound healing and cancer. Some MPs, such as the liver's Kupfer cells, are not of haemopoietic origin. MPs also harbour many important pathogens. RNA drugs that effectively target MPs have the potential to impact many devastating diseases.

We have engineered microscopic protein crystal particles, called PODS crystals, that contain specific bioactive protein cargos. MPs ingest PODS crystals and secrete their cargo proteins intact. We have also shown that cargo proteins enter the cytosol - the area of the cell where RNA is active - suggesting the possibility to deliver RNA. Here, we plan to generate protein crystal particles to deliver RNA to MP cells. Once this has been demonstrated, we will test RNAs' ability to modify MP cells' behaviour across a range of biomedical applications.