Volume 64, Issue 28 e202506954
Research Article

Thioamides Adjacent to the Ionizable Amine Headgroup in Ionizable Lipids Reduce the pKa of Lipid Nanoparticles and Enhance mRNA Transfection Efficiency in Vitro and in Vivo

Yong Chen

Yong Chen

Department of Pharmaceutics, Ghent University, Ghent, Belgium

Both authors contributed equally to this work.

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Emily De Lombaerde

Emily De Lombaerde

Department of Pharmaceutics, Ghent University, Ghent, Belgium

Both authors contributed equally to this work.

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Aimée Bugler-Lamb

Aimée Bugler-Lamb

Laboratory of Myeloid Cell Biology in Tissue Homeostasis and Regeneration, VIB-UGent Center for Inflammation Research, Ghent University, Ghent, Belgium

Laboratory of Myeloid Cell Biology in Tissue Damage and Inflammation, VIB-UGent Center for Inflammation Research, Ghent University, Ghent, Belgium

Department of Biomedical Molecular Biology, Faculty of Science, Ghent University, Belgium

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Zifu Zhong

Zifu Zhong

Department of Pharmaceutics, Ghent University, Ghent, Belgium

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Martijn J. Schuijs

Martijn J. Schuijs

Department of Internal Medicine and Pediatrics, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium

Laboratory of Mucosal Immunology, VIB-UGent Center for Inflammation Research, Ghent University, Ghent, Belgium

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Claudia M. Brenis Gomez

Claudia M. Brenis Gomez

Department of Internal Medicine and Pediatrics, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium

Laboratory of Mucosal Immunology, VIB-UGent Center for Inflammation Research, Ghent University, Ghent, Belgium

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Jamie De Baere

Jamie De Baere

Department of Pharmaceutics, Ghent University, Ghent, Belgium

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Mark Gontsarik

Mark Gontsarik

Department of Pharmaceutics, Ghent University, Ghent, Belgium

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Heleen Lauwers

Heleen Lauwers

Department of Pharmaceutics, Ghent University, Ghent, Belgium

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Kim Deswarte

Kim Deswarte

Department of Internal Medicine and Pediatrics, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium

Laboratory of Mucosal Immunology, VIB-UGent Center for Inflammation Research, Ghent University, Ghent, Belgium

Department of Respiratory Medicine, Ghent University Hospital, Ghent, Belgium

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Niek N. Sanders

Niek N. Sanders

Laboratory of Gene Therapy, Ghent University, Ghent, 9820 Belgium

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Bart N. Lambrecht

Bart N. Lambrecht

Department of Internal Medicine and Pediatrics, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium

Laboratory of Mucosal Immunology, VIB-UGent Center for Inflammation Research, Ghent University, Ghent, Belgium

Department of Respiratory Medicine, Ghent University Hospital, Ghent, Belgium

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Martin Guilliams

Martin Guilliams

Laboratory of Myeloid Cell Biology in Tissue Homeostasis and Regeneration, VIB-UGent Center for Inflammation Research, Ghent University, Ghent, Belgium

Laboratory of Myeloid Cell Biology in Tissue Damage and Inflammation, VIB-UGent Center for Inflammation Research, Ghent University, Ghent, Belgium

Department of Biomedical Molecular Biology, Faculty of Science, Ghent University, Belgium

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Bruno G. De Geest

Corresponding Author

Bruno G. De Geest

Department of Pharmaceutics, Ghent University, Ghent, Belgium

E-mail: [email protected]

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First published: 01 May 2025

Graphical Abstract

Lipid nanoparticles (LNPs) containing an ionizable lipid with a thioamide adjacent to the tertiary amine have a lower pKa than those containing an analogous lipid with a conventional amide, and exhibit significantly improved mRNA transfection efficiency both in vitro and in vivo.

Abstract

Lipid nanoparticles (LNPs) are currently the most clinically advanced mRNA delivery vectors. However, optimizing LNPs for in vivo applications remains largely empirical. The apparent pKa of LNPs is a predictive factor for in vivo performance, with pKa values between 6 and 7 showing the highest efficacy. Despite this critical role of ionizable lipids in LNPs, the relationship between lipid structure and its influence on LNP pKa remains poorly studied. In this study, we report the design and the synthesis of a novel class of ionizable lipids featuring a thioamide moiety, enabling direct comparison between thioamide-containing (SAM) LNPs and amide-containing (OAM) LNPs. We find that substituting oxygen with sulfur in the amide group significantly decreases the apparent pKa of LNPs, increasing the likelihood of identifying lipids in combinatorial libraries that yield LNPs with a pKa in the desired 6–7 range. The reduction in pKa in LNPs containing SAM lipids, compared with OAM lipids, is attributed to the increased hydrophobicity of the thioamide group. Furthermore, by synthesizing multiple libraries of SAM lipids and varying the ionizable head group, alkyl chains, and linker length, we discovered thioamide lipids with distinct tissue tropism, including lipids that mediate splenic targeting by LNPs.

Conflict of Interests

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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