The rise of RNA therapeutics in pharma

Danny Galbraith - head of New Services and Technology Management at Merck KGaA, Darmstadt, Germany, explores the innovation bringing RNA Therapeutics to the clinic.

Innovation in the biopharmaceutical industry has always been rapid; however, this pace is accelerating with the global call to action to develop therapies to prevent and treat Covid-19. As the industry continues to look to novel modalities as a solution to Covid-19 and other diseases, a new class of therapeutic molecule is capturing attention -- nucleic acid, particularly Ribonucleic Acid (RNA).

These molecules have demonstrated several clinical mechanisms of action in the treatment of a multitude of illnesses. For example, anti-sense or interfering RNA therapies have been used in oncology treatment and the use of the RNA to act as a means of gene delivery is showing promise.

First described in 1961, RNA is the least developed of the biologically active molecules as a therapeutic. Compared with DNA delivered gene vectors, RNA has the advantage of being biologically active in both dividing and non-dividing cells and as such would be the preferred tool to deliver expression of gene encoding proteins. They also have the advantage of no “foreign” genetic materials such as promoters on many DNA vectors. These molecules can be surprisingly easy to manufacture. Producer cells are transfected with an appropriate plasmid DNA and RNA polymerase and these drive the production of RNA. The purification of the RNA away from the other host and manufacturing materials requires multiple steps but fortunately requires little in the way of the development of novel technologies, allowing a faster path to the clinic. A significant downside to RNA molecules, however, is their fragility in biological systems. The largest challenge with these drugs is the delivery system to achieve adequate survival of the biologically active molecule to the cells where the gene transfer and expression can be delivered. Many innovative compounds are being developed to enable these therapies to move to the clinic safely.

The pharmacokinetics of naked RNA is well understood. With a half-life of around seven hours, this type of molecule is rapidly degraded in the extracellular space by a variety of RNase mediated mechanisms. If the molecule survives to reach a cell it requires to translocate across the plasma membrane into the cytosol where the protein can be translated and initiate its therapeutic target. However, these naked RNA molecules are negatively charged and compose a high molecular weight; both characteristics mean that passive movement across the charged cell membrane is incredibly low. For these reasons naked RNA is no longer considered an option for drug developers.

To combat these challenges, two strategies are used to create RNA therapy and delivery systems for the clinic. Firstly, modifications of the RNA molecule can increase stability during transportation to the cell. These modifications have included changes to the 5’ cap and 5’ and 3’ UTR regions to enhance stability and modification of the Poly-A tail. These modifications can enhance the potency of the molecule as well as reducing degradation. However, modifications in the genomic construct alone are seldom enough to improve the pharmacokinetics of the drug. One of the more interesting approaches has been the use of different formulation strategies to stabilise these products. The challenge of the dense negative charge of these RNA molecules can be overcome with the use of lipids or polymers that are used to encapsulate the RNA strand and mask the charge. Simple or complex polymer compounds such as Diethylaminomethyl dextran or polyethyleneimine have been proposed and some clinical work has been carried out, however safety issues have been described. A great deal of attention has been recently focused on lipid-based compounds such as lipidoids. Cationic lipids would appear to be the ideal polymer as they form a spontaneous encapsulated complex with the RNA, thereby providing a mask to the charge and a protection to the degradative activity during transport to the cells. Cationic lipids are generally less toxic but do have some safety hurdles to overcome as pro-inflammatory responses have been identified. A solution to reduce toxicity is to complex neutral lipids with the cationic lipids, as this will also improve stability. These strategies have already shown promise in clinical trials. The chemical flexibility of lipid compounds has seen innovation with ionizable lipids which switch charge depending on the surrounding pH values. Products are encapsulated at low pH and at physiological pH conditions the compound switches to a neutral charge, thereby reducing toxicity. In combination with glycols these lipid compounds are essential for RNA therapeutics to be viable in clinical trials. These compounds still must be evaluated long term and with multiple dosing regimens with respect to toxicity to understand if these novel lipids may have issues with patient tolerance in a long run. Appropriate testing of other characteristics of these lipids and the compounded end products, such as stability, is also required for safety assurance.

Ultimately the lipids that can target specific cell types would be the ideal carrier. If, for example, T cells or tumour cells were able to be selected and RNA coding proteins expressed this would reduce the amount of materials needed to treat a patient and thereby reduce any potential toxicity. Although at early stages, there are some groups investigating the ability of glycan markers on the cell surface to interact with lipids to enhance delivery to a target cell type.

RNA based therapeutics have the potential to revolutionise how we deliver therapeutics. In the future it is possible that in place of a monotherapy, a population of different RNA substrates will code and cascade several proteins to trigger many pathways to eliminate cancer cells or treat autoimmune diseases. The primary obstacles of how to deliver these molecules to cells has seen great progress recently but with the innovations in lipid chemistry we are likely to see new products which advance this field. Reducing toxicity and better targeting of the RNA to specific areas or even particular cell types could be the next step in this journey. The part these drugs play in the treatment of Covid-19 remains to be seen but the information gained from these molecules used in large scale clinical trials that are going on currently will be invaluable.