Cellular uptake and intracellular trafficking of oligonucleotides☆
Graphical abstract
Introduction
Realization of the potential oligonucleotides as therapeutic agents began more than three decades ago with the discovery of antisense molecules [1]. The discovery of RNA interference [2], [3] increased enthusiasm and was further reinforced by insights into the complex roles of non-coding RNAs in regulating genome function [4]. Many aspects of oligonucleotide therapeutics are covered in this theme issue. Here we will stress a key basic aspect of oligonucleotide behavior that underlies all potential therapeutic utilization, namely the cellular uptake and intracellular trafficking of these molecules.
Despite FDA approval of the first antisense drug [5] and the advent of multiple clinical trials in cancer and other diseases [6], [7], [8], [9], [10], oligonucleotide therapeutics has progressed slowly. A major issue has been the poor efficacy of oligonucleotides. In large part this is because effective delivery of such large, polar molecules to their sites of action within tissues is a very challenging problem [11], [12], [13]. This review will primarily emphasize the behavior of ‘free’ oligonucleotides and of molecular-scale oligonucleotide conjugates. Oligonucleotides associated with nanocarriers are more fully discussed elsewhere in this theme issue. The emphasis here will be on processes at the cellular and subcellular level rather than on classical pharmacokinetics and biodistribution. A key theme for this article is the observation that the processes that govern intracellular traffic of internalized molecules are enormously complex and that understanding these processes is vital for the future of oligonucleotide therapeutics.
Section snippets
Endocytotic and trafficking pathways
Oligonucleotides usually enter cells via endocytosis; however, it is important to realize the complexities associated with the multiple pathways of internalization and subsequent trafficking. These pathways are regulated by a plethora of unique proteins and lipids that underlie all aspects of internalization and trafficking. An illustration of some of the pathways is given in Fig. 1.
Overview
There are thousands of publications on the pharmacokinetics, biodistribution, cellular uptake and biological effects of oligonucleotides as individual molecules or associated with various carriers. To provide an overview of this complex literature, this theme issue contains timely reviews on oligonucleotide pharmacokinetics and biodistribution by Geary and colleagues, on the behavior of oligonucleotide conjugates by Ming and colleagues, and on delivery using lipoplexes or cell-penetrating
Conclusions
The therapeutic development of antisense, siRNA, SSOs and other types of oligonucleotides has, to some degree, outstripped our fundamental knowledge of how these molecules behave in cells and in the body. Thus although there is much information regarding the overall pharmacokinetics and biodistribution of oligonucleotides, there is not an equivalent depth of knowledge about their behavior at the cellular and intracellular levels. Recent work has uncovered some interesting and surprising
Acknowledgments
This work was supported by NIH grant RO1 CA151964 to RLJ.
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This review is part of the Advanced Drug Delivery Reviews theme issue on “Oligonucleotide therapeutics”.