Ionizable lipids are a class of organic lipid molecules that are neutral at physiological pH and protonated (+) at acidic pH. Ionizable lipids contribute to the structure of lipid nanoparticles (LNPs) along with phospholipids, cholesterol, and PEGylated lipids.
Functioning in LNPs, ionizable lipids protect RNA from degradation by hydrolysis, nucleases, sudden pH changes, and oxidative insults in order to facilitate their cytosolic transport (Figure 1). Essentially, ionizable lipids help facilitate RNA delivery into target cells.
Structure-wise, there are currently five major ionizable lipid types that are widely used for RNA delivery; unsaturated, multi-tail, polymeric, biodegradable, and branched-tail (Table 1).
Unsaturated ionizable lipids enhance membrane disruption, and subsequently payload release, by increasing the tendency of bilayer lipids to form a nonbilayer phase. This transitional tendency of bilayer lipids is the consequence of increasing the tail saturation from 0 to 2 cis double bonds. For example, MC3 (Table 1, Row 1) has two cis double bonds in each tail, giving rise to its unique functionality. MC3 demonstrated the ability of unsaturated ionizable lipids to enhance the capacity of LNPs in delivering RNA to target cells, thereby re-igniting the enthusiasm for RNA delivery in therapeutics, particularly mRNA vaccines.
Multi-tail ionizable lipids enhance endosomal disruption, and subsequently RNA delivery, by producing a cone-shaped LNP structure that increases the cross-sectional area of the tail region. Therefore, optimizing an LNP structure while using such a lipid allows for increased RNA potency. For example, C12-200 (Table 1, Row 2) a multi-tail ionizable lipid, increased mRNA expression 7-fold compared to the standard formulation. This optimized formulation while using multi-tail ionizable lipids was adopted for various mRNA delivery purposes, particulary prenatal protein replacement therapy.
Polymeric ionizable lipids enhance particle formation through hydrophobic aggregation, which subsequently enhances RNA delivery. This enhancement through hydrophobic aggregation is accomplished by substituting free amines onto cationic polymers with alkyl tails. For example, G0-C14 (Table 1, Row 3) demonstrates the promise of LNPs in cancer treatment by conferring a high accumulation/potency and an effective transfection of various RNA therapeutics in tumors.
Biodegradable ionizable lipids reduce continual accumulation and toxicity after successful intracellular RNA delivery. This is especially important for RNA therapeutics requiring repeated dosing. This reduced toxicity is attained by including ester bonds because they are stable at physiological pH, but become hydrolyzed within tissues and cells. For example, L319 (Table 1, Row 4) is made by replacing one of the double bonds in each MC3 tail with an ester bond. This maintains the RNA payload potency while displaying improved tolerability by the host.
Branched-tail ionizable lipids increase RNA delivery potency by enhancing endosomal escape and by increasing the cross-sectional area of lipid tails. Due to this multi-facetted enhancement of RNA delivery, branched-tail ionizable lipids are effective at delivering large mRNA constructs for things such as protein supplementation and base editing therapies. For example, FTT5 (Table 1, Row 5) demonstrates how branched-tail ionizable lipids with ester chains have higher transfection efficiency than their linear analogs. This higher efficiency of branched tails when compared to linear ones is presumably due to the slower degradation rate of the secondary ester present on branched ionizable lipids.
As a worldwide leading lipid supplier, BroadPharm offers a wide variety of ionizable lipids such as ALC-0315 analogs, SM-102 analogs, MC3, C12-200, and many others to empower our clients’ advanced research in nanoparticle drug delivery.
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