Formulating and Characterizing Lipid Nanoparticles for Gene Delivery using a Microfluidic Mixing Platform
Summary
Lipid nanoparticles are developed using a microfluidic mixing platform approach for mRNA and DNA encapsulation.
Abstract
Lipid-based drug carriers have been used for clinically and commercially available delivery systems due to their small size, biocompatibility, and high encapsulation efficiency. Use of lipid nanoparticles (LNPs) to encapsulate nucleic acids is advantageous to protect the RNA or DNA from degradation, while also promoting cellular uptake. LNPs often contain multiple lipid components including an ionizable lipid, helper lipid, cholesterol, and polyethylene glycol (PEG) conjugated lipid. LNPs can readily encapsulate nucleic acids due to the ionizable lipid presence, which at low pH is cationic and allows for complexation with negatively charged RNA or DNA. Here LNPs are formed by encapsulating messenger RNA (mRNA) or plasmid DNA (pDNA) using rapid mixing of the lipid components in an organic phase and the nucleic acid component in an aqueous phase. This mixing is performed using a precise microfluidic mixing platform, allowing for nanoparticle self-assembly while maintaining laminar flow. The hydrodynamic size and polydispersity are measured using dynamic light scattering (DLS). The effective surface charge on the LNP is determined by measuring the zeta potential. The encapsulation efficiency is characterized using a fluorescent dye to quantify entrapped nucleic acid. Representative results demonstrate the reproducibility of this method and the influence that different formulation and process parameters have on the developed LNPs.
Introduction
Drug carriers are used to protect and deliver a therapeutic with typical favorable properties including low cytotoxicity, increased bioavailability, and improved stability1,2,3. Polymeric nanoparticles, micelles, and lipid-based particles have previously been explored for nucleic acid encapsulation and delivery4,5,6,7. Lipids have been used in different types of nanocarrier systems, including liposomes, and lipid nanoparticles, as they are biocompatible with high stability8. LNPs can readily encapsulate nucleic acids for gene delivery9,10. They protect the nucleic acid from degradation by serum proteases during systemic circulation11 and can improve delivery to specific sites, as the surface topography and physical properties of LNPs influence their biodistribution12. LNPs also improve tissue penetration and cellular uptake9. Previous studies have demonstrated the success of siRNA encapsulation within an LNP13, including the first commercially available LNP containing siRNA therapeutic for the treatment polyneuropathy of hereditary transthyretin-mediated amyloidosis14 treatment that was approved by United States Food and Drug Administration (FDA) and European Medicines Agency in 2018. More recently, LNPs are being studied for the delivery of larger nucleic acid moieties, namely mRNA and DNA9. As of 2018, there were ~ 22 lipid-based nucleic acid delivery systems undergoing clinical trials14. Additionally, mRNA containing LNPs are currently leading candidates and have been employed for a COVID-19 vaccine15,16. The potential success for these non-viral gene therapies requires forming small (~100 nm), stable, and uniform particles with high encapsulation of the nucleic acid.
Use of an ionizable lipid as a main component in the LNP formulation has shown advantages for complexation, encapsulation, and delivery effciciency14. Ionizable lipids typically have an acid dissociation constant (pKa) < 7; for example, dilinoleylmethyl-4-dimethylaminobutyrate (D-Lin-MC3-DMA), the ionizable lipid used in the FDA approved LNP formulation, has a pKa of 6.4417. At low pH, the amine groups on the ionizable lipid become protonated and positively charged, allowing for the assembly with negatively charged phosphate groups on mRNA and DNA. The ratio of amine, "N", groups to phosphate, "P", groups is used to optimize the assembly. The N/P ratio is dependent on the lipids and nucleic acids used, which varies depending on the formulation18. After formation, the pH can be adjusted to a neutral or physiological pH to allow for therapeutic administration. At these pH values, the ionizable lipid is also deprotonated which imparts neutral surface charge to the LNP.
The ionizable lipid also aids in endosomal escape19,20. LNPs undergo endocytosis during cellular uptake and must be released from the endosome in order to deliver the mRNA cargo into the cell cytoplasm or DNA cargo to the nucleus21. Inside the endosome is typically a more acidic environment than the extracellular medium, which renders the ionizable lipid positively charged22,23. The positively charged ionizable lipid can interact with negative charges on the endosomal lipid membrane, which can cause destabilization of the endosome allowing for the release of the LNP and nucleic acid. Different ionizable lipids are currently being studied for improving efficacy of both LNP distribution, as well as endosomal escape14.
Other typical components of an LNP include helper lipids, such as a phosphatidylcholine (PC) or phosphoethanolamine (PE) lipid. 1,2-Dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) are commonly used helper lipids24,25. DOPE has been shown to form an inverted hexagonal II (HII) phase and enhance transfection by membrane fusion26, while DSPC has been thought to stabilize LNPs with its cylindrical geometry27. Cholesterol is also incorporated in the formulation in order to increase membrane rigidity, subsequently aiding in the stability of the LNP. Finally, lipid-conjugated polyethylene glycol (PEG) is included in the formulation to provide the necessary steric barrier to aid in particle self-assembly27. PEG also improves the storage stability of LNPs by preventing aggregation. Furthermore, PEG is often used as a stealth component and can increase the circulation time for the LNPs. However, this attribute can also pose challenges for recruitment of LNPs to hepatocytes through an endogenous targeting mechanism driven by apolipoprotein E (ApoE)28. Thus, studies have investigated the acyl chain length for diffusion of PEG from the LNP, finding that short lengths (C8-14) dissociate from the LNP and are more amenable to ApoE recruitment compared to longer acyl lengths28. Further, the degree of saturation of the lipid tail that PEG is conjugated to has been shown to influence the tissue distribution of LNPs29. Recently, Tween 20, which is a commonly used surfactant in biological drug product formulations and has a long unsaturated lipid tail, was shown to have high transfection in draining lymph nodes compared to PEG-DSPE, which largely transfected the muscle at the injection site29. This parameter can be optimized to achieve the desired LNP biodistribution.
Conventional methods of forming LNPs include the thin-film hydration method and ethanol-injection method27. While these are readily available techniques, they are also labor intensive, can result in low encapsulation efficiency, and are challenging to scale up27. Advancements in mixing techniques have resulted in methods more amenable to scale up, while developing more uniform particles27. These methods include T-junction mixing, staggered herringbone mixing, and microfluidic hydrodynamic focusing27. Each method has a unique structure, but all allow for rapid mixing of an aqueous phase containing the nucleic acid with an organic phase containing the lipid components, resulting in high encapsulation of the nucleic acid27. In this protocol, rapid and controlled mixing through a microfluidic cartridge is utilized, which employs the staggered herringbone mixing design. This protocol outlines the preparation, assembly, and characterization of nucleic acid containing LNPs.
Protocol
Representative Results
Discussion
Reproducibility, speed, and low volume screening are significant advantages of using microfluidic mixing to form LNPs compared to other existing methods (e.g., lipid film hydration and ethanol injection). We have demonstrated the reproducibility of this method with no impact on encapsulation efficiency or particle size observed with different LNP batches. This is an essential criterion for any therapeutic, including LNPs, to become clinically available.
The technique described here employs sta…
Declarações
The authors have nothing to disclose.
Acknowledgements
Thank you to Atul Saluja, Yatin Gokarn, Maria-Teresa Peracchia, Walter Schwenger, and Philip Zakas for their guidance and contributions towards LNP development.
Materials
| 1,2-dimyristoyl-rac-glycero-3-methoxypolyethylene glycol-2000 (C-14 PEG) | Avanti Polar Lipids | 880151P | |
| 10 µl Graduated Filter Tips (RNase-,DNase-, DNA-free) | USA Scientific | 1121-3810 | |
| 1000 µl Graduated Filter Tips (RNase-,DNase-, DNA-free) | USA Scientific | 1111-2831 | |
| 20 µl Beveled Filter Tips (RNase-,DNase-, DNA-free) | USA Scientific | 1120-1810 | |
| 200 µl Graudated Filter Tips (RNase-,DNase-, DNA-free) | USA Scientific | 1120-8810 | |
| 3β-Hydroxy-5-cholestene, 5-Cholesten-3β-ol (Cholesterol) | Sigma-Aldrich | C8667 | |
| BD Slip Tip Sterile Syringes (1 ml syringe) | Thermo Fisher Scientific | 14-823-434 | |
| BD Slip Tip Sterile Syringes (3 ml syringe) | Thermo Fisher Scientific | 14-823-436 | |
| BD Vacutainer General Use Syringe Needles (BD Blunt Fill Needle 18G) | Thermo Fisher Scientific | 23-021-020 | |
| Benchtop Centrifuge | Beckman coulter | ||
| Black 96 well plates | Thermo Fisher Scientific | 14-245-177 | |
| BrandTech BRAND BIO-CERT RNase-, DNase-, DNA-free microcentrifuge tubes (1.5mL) | Thermo Fisher Scientific | 14-380-813 | |
| Citric Acid | Fisher Scientific | 02-002-611 | |
| Corning 500ml Vacuum Filter/Storage Bottle System, 0.22 um pore | Corning | 430769 | |
| Disposable folded capillary cells | Malvern | DTS1070 | |
| Ethyl Alcohol, Pure 200 proof | Sigma-Aldrich | 459844 | |
| Fisher Brand Semi-Micro Cuvette | Thermo Fisher Scientific | 14955127 | |
| Invitrogen Conical Tubes (15 mL) (DNase-RNase-free) | Thermo Fisher Scientific | AM12500 | |
| MilliporeSigma Amicon Ultra Centrifugal Filter Units | Thermo Fisher Scientific | UFC901024 | |
| NanoAssemblr Benchtop | Precision Nanyosystems | ||
| Nuclease-free water | Thermo Fisher Scientific | AM9930 | |
| Phosphate Buffered Saline (PBS) | Thermo Fisher Scientific | AM9624 | |
| Quant-iT PicoGreen dsDNA Assay Kit | Thermo Fisher Scientific | P7589 | |
| Quant-iT RiboGreen RNA Assay Kit | Thermo Fisher Scientific | R11490 | |
| Sodium Chloride | Fisher Scientific | 02-004-036 | |
| Sodium Citrate, Dihydrate, granular | Fisher Scientific | 02-004-056 | |
| SpectraMax i3x | Molecular Devices | ||
| Zetasizer Nano | Malvern |
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