RNAi-based therapeutics-current status, challenges and prospects

Abstract

RNA interference (RNAi) is a collection of small RNA directed mechanisms that result in sequence specific inhibition of gene expression. The notion that RNAi could lead to a new class of therapeutics caught the attention of many investigators soon after its discovery. The field of applied RNAi therapeutics has moved very quickly from lab to bedside. The RNAi approach has been widely used for drug development and several phase I and II clinical trials are under way. However, there are still some concerns and challenges to overcome for therapeutic applications. These include the potential for off-target effects, triggering innate immune responses and most importantly obtaining specific delivery into the cytoplasm of target cells. This review focuses on the current status of RNAi-based therapeutics, the challenges it faces and how to overcome them.

Figure 1. Delivery of therapeutics to patients

1. Systemic delivery: (a) double stranded siRNA is packaged into delivery vehicle (targeted nanoparticles, polymers, liposomes, etc.). (b) It is then given intravenously, via inhalator or direct injection into the target tissue (the eye, tumour, etc.).

2. ex-vivo delivery: (a) cells (dendritic cells, haematopoietic stem cells, etc.) are extracted from the patient and transduced with a virus containing shRNAs. (b) The genetically modified cells are then re-infused into the patient.

Figure 2. Delivery strategies for RNAi

The cell (grey ellipse) contains a nucleus (dark circle) and a cell membrane (dark ellipse). Cell surface molecules such as receptors are present on the cell surface (shown in colour). RNAi therapeutics (mainly siRNA (blue)) can be targeted to the cell surface molecules via different delivery vehicles. They can be conjugated to aptamers (A), which can bind specifically to cell surface molecules and be internalized. siRNAs can also be conjugated to cell specific antibodies (B) and be delivered to the target cells via recognition of cell surface molecules by the specific antibody followed by internalization through endocytosis. Targeted nanoparticles (C) transport RNAi therapeutics to specific cells. The modifications of the nanoparticles (targeting ligand) can interact with receptors on the cell surface and the nanoparticle with its load can be internalized. Cholesterol conjugated siRNAs (D) can be delivered to cells and be internalized by the interaction of the cholesterol with the membrane through hydrophobic interactions, triggering Clathrin-dependent endocytosis. Modified viruses (E) can also be used for cell specific delivery of RNAi therapeutics by cell specific cell surface interactions triggering endocytosis.