The engineering challenges and opportunities when designing potent ionizable materials for the delivery of ribonucleic acids

ABSTRACT

Introduction

Ionizable lipids are critical components in lipid nanoparticles. These molecules sequester nucleic acids for delivery to cells. However, to build more efficacious delivery molecules, the field must continue to broaden structure–function studies for greater insight. While nucleic acid-binding efficiency, degradability and nanoparticle stability are vitally important, this review offers perspective on additional factors that must be addressed to improve delivery efficiency

Areas covered

We discuss how administration route, cellular heterogeneity, uptake pathway, endosomal escape timing, age, sex, and threshold effects can change depending on the type of LNP ionizable lipid.

Expert opinion

Ionizable lipid structure–function studies often focus on the efficiency of RNA utilization and biodistribution. While these focus areas are critical, they remain high-level observations. As our tools for observation and system interrogation improve, we believe that the field should begin collecting additional data. At the cellular level, this data should include age (dividing or senescent cells), sex and phenotype, cell entry pathway, and endosome type. Additionally, administration route and dose are essential to track. This additional data will allow us to identify and understand heterogeneity in LNP efficacy across patient populations, which will help us provide better ionizable lipid options for different groups.

KEYWORDS:

• Ionizable lipid
• ribonucleic acid
• RNA
• lipid nanoparticle
• LNP
• functional delivery
• uptake mechanism
• endosomal escape
• structure–function
• patient stratification

Article highlights

• LNPs are formulated by combining RNA, ionizable lipids, and helper molecules

• Upon administration, LNP uptake into cells is influenced by cell activation, sex, and phenotype; more research is needed to determine if ionizable lipid structure can equalize uptake across all factors

• The mechanism of LNP entry into cells can depend on the type of ionizable lipid used

• Once LNPs enter the cell, their fate can be determined by the type of endosome they are in

• LNP-containing endosomes can be broadly categorized as early or late stage; the endosome type can impact the efficiency and timing of RNA release into the cytoplasm.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This research is part of the University of Toronto’s Medicine by Design initiative which receives funding from the Canada First Research Excellence Fund (reference # CFREF-2020). Additional funding sources include the Canada Research Chairs Program (reference # CRC-2020-00097), Natural Sciences and Engineering Research Council of Canada (reference # RGPIN-2021-03421), Canadian Institutes of Health Research (reference # PTT-184009), and the Canada Foundation for Innovation John R. Evans Leaders Fund and Ontario Research Fund (reference # 41323). G.T. and Y.M.A.L. thank the Ontario Graduate Scholarship, J.C.-S. thanks the Wildcat Fellows Program, NSERC and B.&F. Milligan. J.P. received funding from the University of Toronto Faculty of Applied Science & Engineering Graduate Student Endowment Fund (GSEF) Award and the Norman F. Moody Award.