Lipid nanoparticles: A delicate nucleic acid delivery system to be further explored

In recent years, an increasing number of gene therapy and nucleic acid drugs have been approved for clinical use by the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA) for treating various diseases [1], [2], [3], [4]. Unlike small molecules or protein drugs, nucleic acids can prevent or treat diseases by controlling the production of proteins at both the transcriptional and translational levels. Long-lasting, highly specific, and significant therapeutic effects can be obtained by gene therapy. For example, exogenous nucleic acids are introduced into cells to compensate for gene expression defects or silence pathogenic genes. The commonly used nucleic acid drugs include small interfering RNA (siRNA), CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9), plasmid DNA (pDNA), and messenger RNA (mRNA). siRNA can silence abnormally expressed genes, and multiple related drugs have been used in clinical treatment [3]. CRISPR/Cas9 has been regarded as a potential therapeutic approach for genetic diseases and cancer via programmable genome modification. pDNA introduces target genes into cells to function properly. Compared with pDNA, mRNA possesses a lower risk of integration into the human genome and can be translated into functional proteins more simply. In the battle against the global COVID-19 pandemic, mRNA vaccines efficiently prevented infection by delivering antigen-encoding mRNA to cells for intracellular antigen expression to trigger the production of antibodies against the virus [5], [6], [7]. The success of mRNA vaccines has greatly boosted interest in the research and development of mRNA or other nucleic acid drugs [8], [9], [10], [11].

Effective intracellular delivery is one of the main challenges and a prerequisite for the successful function of nucleic acid drugs. Nucleic acids have a high degree of instability and immunogenicity, susceptible to degradation by nucleases and self-hydrolysis, their negative charge will induce the immunoreaction and make it difficult for nucleic acids to pass through the anionic cell membrane [12], [13]. Therefore, the use of efficient carriers is essential to deliver nucleic acids to targeted cells and enable the fulfillment of their intended functions. The ideal carrier of nucleic acids should protect them from degradation, efficiently deliver them to the targeted cell, enable endosomal escape, and release the cargo to the cytoplasm or nucleus. Current state-of-the-art delivery vehicles of nucleic acids can be classified into two main categories: viral vectors and non-viral vectors. Viral vectors mainly include lentiviruses, retroviruses, adenoviruses, and adeno-associated viruses (AAVs). By integrating target sequence into the virus genome, virus vector can carry payloads to various cell types with high transfection rate. However, clinical applications of virus vectors were limited due to their inherent safety risks such as immunogenicity and side effects [14]. Non-viral vectors typically include polymers [15], dendrimers [16], micelles [17], inorganic nanoparticles (metal nanoparticles [18], [19], mesoporous silica [20], [21]), and lipid-based carriers (Fig. 1). These different delivery systems employ a variety of ways to encapsulate and release nucleic acids with unique materials design, which may also be able to avoid rapid clearance by immune system and deliver to the target location via surface modifications [22].

Among non-viral vectors [23], [24], the most successful of the front-runner platforms is lipid nanoparticle (LNP), which has been authorized in the first approved siRNA drug Onpattro and the two Covid-19 mRNA vaccines (Table 1). LNP is considered as a vehicle with low immunogenicity, which attributes to its composition of natural and synthetic biocompatible lipids. By regulating the formulation of LNP, the delivery efficiency and in vivo distribution can be tailored. Various advantages enable LNP to act as the most advanced non-viral nucleic acids carrier, which gains rapid development catalyzed by COVID-19 mRNA vaccines at the forefront of these efforts [25], [26], [27]. Several comprehensive reviews mainly focused on the significant breakthroughs in LNP-mediated nucleic acid delivery [28], [29], [30], [31], [32]. However, LNP is a simple but very delicate system, which is worth summarizing from different aspects for a better understanding of the key parameters that affect the structure-activity relationship. Although decades of extensive efforts lead to the current big success of using LNP in siRNA drugs and mRNA vaccines, the potential for further development and application of LNP remains largely unexplored. For example, most current LNP inherently accumulates in the liver due to the apolipoprotein E (ApoE) adsorption and then be recognized by the low-density lipoprotein receptor (LDLR) in hepatic cells [33], [34]. Beyond the liver, organ specific LNP systems are under extensive exploration with many advances, but there is still plenty of room for expansion to ensure more robust application in lots of diseases scenarios. In this review, we summarize the knowledge about LNP mainly from the perspective of how its compositions control the NPs physicochemical properties which determine the delivery efficiency and specificity, how the delivery mechanism affects the fate of LNP in vivo, and how the recent efforts focused on mRNA delivery with novel design to enable targeted delivery for more specific therapeutic scenarios.