Things to Consider When Choosing a Cationic Lipid for Your Formulation

Cationic lipids are a key component in lipid nanoparticle (LNP) formulations for nucleic acid delivery. Nucleic acids are anionic (negatively charged) molecules and thus require a positively charged (cationic) component to sequester them for encapsulation in delivery vehicles. Cationic lipids are used to meet this requirement. They complex with negatively charged nucleic acids to facilitate encapsulation, as well as help deliver the nucleic acid to the cell by interacting with the cell membrane. Avanti offers several types of cationic lipids - fixed, ionizable, and multivalent. Here are a few things to consider about each type of cationic lipid when choosing the right one for your formulation:

Fixed Cationic Lipids

Fixed cationic lipids have a single positive charge (shown in green) that does not change based on the biological environment.

Fixed cationic lipids are permanently positively charged regardless of the pH of its biological environment. Cationic lipids are similar to other natural lipids except for the replacement of a typical headgroup with a cationic headgroup.

Fixed cationic lipids have been proven to be good transfection agents in cationic liposomes (CLs) and LNPs. Generally considered non-toxic at lower concentrations, these lipids do present some toxicity concerns when used in higher concentrations due to the tetrasubstituted ammonium moiety.

Fixed cationic lipids are generally cheaper, more readily available alternatives to ionizable lipids. Historically, fixed cationic lipids have been more widely studied and therefore a large amount of data has been generated to support their use. They are commonly used for the delivery of DNA, as well as the delivery of siRNA and saRNA where a lesser payload is needed. In the cases of siRNA and saRNA, reducing the payload also reduces the required amount of cationic lipid and the potential for toxic side effects.

Avanti offers a wide range of proven fixed cationic lipids such as DOTMA or DOTAP, but new, innovative cationic lipids are constantly being added. Check out our fixed cationic lipid offerings and fuel your lipid-based nucleic acid delivery research today!

Multivalent Cationic Lipids

In the case of MVL5, there are five positive charges (shown in green)

Multivalent cationic lipids are similar to fixed cationic lipids in some aspects, but their structural difference is the number of positive charges in the molecule. Avanti's fixed cationic lipids have a single positive charge while multivalent cationic lipids have multiple positive charges throughout the molecule. Multivalent cationic lipids such as MVL-5 (five positive charges) do offer some advantages over fixed cationic lipids particularly in applications involving the delivery of siRNA. In some cases, the amount of multivalent cationic lipid needed for a formulation is one-third to one-fifth the amount of the fixed cationic lipid that would be needed. Since high amounts of fixed cationic lipids can be toxic, the lower amount of multivalent cationic lipid required generally results in a less toxic formulation. Multivalent cationic lipids also increase the encapsulation efficiency of siRNA compared to fixed cationic lipid formulations. Click HERE to see our full offering of multivalent cationic lipids!

Ionizable Lipids

The ionizable center (shown in green) will be either neutral or positively charged based on the biological environment and its pH.

To efficiently play their role in an LNP, cationic ionizable lipids must be engineered with a suitable apparent acid dissociation constant (pKa). The apparent pKa of a cationic ionizable lipid is the likely pKa at the LNP surface. Currently, the cationic ionizable lipids in FDA-approved therapeutics all have an apparent pKa between 6-7. This is crucial for the cationic ionizable lipid to maintain a neutral charge while in systemic circulation (pH above the pKa of the lipid, pH ~7.5), as well as its ability to become positively charged in the endosome (pH ~6.5) and facilitate membrane fusion and subsequent cytosolic release.

Not only is the pKa of the ionizable lipid important, but the molecular shape of the lipid is also of importance. A cone-shaped ionizable lipid offers advantages over a cylindrically shaped ionizable lipid. A cone-shape is less compatible with the lipid bilayer and destabilizes the endosomal membrane to further facilitate cytosolic release of the nucleic acid cargo.

Ionizable lipids are generally considered less toxic than fixed cationic lipids and are used in applications where the delivery of a higher payload is required.

Avanti offers several ionizable lipids. Click HERE to learn more about each individual offering!