An ionizable lipid toolbox for RNA delivery

Abstract

Recent years have witnessed incredible growth in RNA therapeutics, which has benefited significantly from decades of research on lipid nanoparticles, specifically its key component-the ionizable lipid. This comment discusses the major ionizable lipid types, and provides perspectives for future development. Need for ionizable lipidsBroadly speaking, ribonucleic acid (RNA) therapeutics include antisense oligonucleotides (ASOs), small interfering RNAs (siRNAs), microRNAs (miRNAs), messenger RNAs (mRNAs), and single-guide RNAs (sgRNAs)-mediated CRISPR-Cas9 system, which can manipulate essentially any gene of interest through distinct modes of action 1 . However, RNA therapeutics are susceptible to nucleases and cannot permeate cells due to their large size and negative charge. Delivery of RNAs to target cells by clinically translatable lipid nanoparticles (LNPs) provides vast opportunities to tackle a series of life-threatening diseases including COVID-19 2 . LNPs typically consist of four components-ionizable lipid, phospholipid, cholesterol, and PEGylated lipid, among which, the ionizable lipid plays a major role in protecting RNAs and facilitating their cytosolic transport. Ionizable lipids are positively charged at acidic pH to condense RNAs into LNPs, but are neutral at physiological pH to minimize toxicity. They can be protonated in the acidic endosome after cellular uptake, and interact with anionic endosomal phospholipids to form cone-shaped ion pairs that are not compatible with a bilayer (Fig. 1). These cationicanionic lipid pairs drive the transition from the bilayer structure to the inverted hexagonal H II phase, which facilitates membrane fusion/disruption, endosomal escape and cargo release into the cytosol 3 . Since 2008, ionizable lipids with diverse chemical identities have been created. Systematic categorization of these lipids based on their structures can greatly benefit the field and facilitate the development of next-generation ionizable lipids. Currently, there are five major ionizable lipid types that are widely used for RNA delivery (Fig. 1). Unsaturated ionizable lipidsTail saturation greatly influences the fluidity and delivery efficiency of ionizable lipids. Increasing the tail unsaturation from 0 to 2 cis double bonds correlates with an increased tendency for bilayer lipids to form a nonbilayer phase 4 , leading to enhanced membrane disruption and

Rational design of anti‐inflammatory lipid nanoparticles for mRNA delivery

“…Next, the in vivo transfection of C9D1 LNP encapsulating mRNA encoding for luciferase was investigated. MC3 LNP is a clinically validated non‐viral vector for liver transfection 39,40 . As expected, strong luciferase expression in the liver was observed for both C9D1 LNP‐ and C10D0 LNP‐treated mice (Figure 6b ) .…”

“…MC3 LNP is a clinically validated non-viral vector for liver transfection. 39,40 As expected, strong luciferase expression in the liver was observed for both C9D1 LNP-and C10D0 LNP-treated mice (Figure 6b). Interestingly, quantification of the luminescence signal showed a 1.5-fold increase in C9D1 LNPtreated mice compared to C10D0 LNP-treated mice (Figure 6c).…”

Understanding the Role and Impact of Poly (Ethylene Glycol) (PEG) on Nanoparticle Formulation: Implications for COVID-19 Vaccines

“…, lipids that are positively charged at acidic pH to condense the RNA into LNPs and neutral at physiological pH to minimize toxicity in comparison to permanently charged lipids ( Rietwyk and Peer, 2017 ). Efforts are under way to develop biodegradable ionizable lipids to reduce any long-term effects ( Han et al, 2021 ).…”

In vivo fate and intracellular trafficking of vaccine delivery systems

“…Lipid nanoparticles, which are the most representative lipid-based vaccine delivery system, are already widely used in the clinic [15] . A key component of recent lipid nanoparticles developed for mRNA vaccine delivery is an ionizable lipid that can undergo a pH-dependent charge conversion [16] . An ionizable lipid that has a neutral charge under physiological conditions can take on a positive charge under an acidic condition, such as that found in endosomes.…”

“…Since the addition of a permanent cationic charge can induce cytotoxicity, the temporary charge conversion of a lipid nanoparticle offers advantages in terms of safety [17] . Among the various ionizable lipids, ALC-0315 was clinically used for the Pfizer COVID-19 mRNA vaccines, while SM102 lipid was used for the Moderna COVID-19 mRNA vaccine [16] .…”