Small Molecule Conjugate-Based Delivery Systems

The molecule Retro-1 became a candidate for conjugation based on a high-throughput screening of small molecules to block the effects of bacterial toxins via inhibition of retrograde toxin transport between early endosomes and the TGN/Golgi complex. This provides the connection to oligonucleotide delivery: internalization of oligonucleotides via any form of endocytosis is followed by participation in cellular traffic of the therapeutic. It was shown that Retro-1 can improve the redistribution of oligonucleotides to the cytosol, leading to an increased concentration of oligonucleotides in the nucleus.

The penetration of biological membranes by highly charged polar oligonucleotide sequences is naturally hindered, which limits the overall activity of such therapeutic agents in in vivo. This limitation stems from hampered cellular uptake (which is dependent on a plethora of pathways) and limited release from endocytic vesicles. To overcome these inherent challenges, it is essential to address the issue of poor cellular delivery. The conjugation of lipophilic small molecules to oligonucleotides is a proven approach to enhance cellular uptake of the resultant conjugate, thus the delivery efficiency of oligonucleotide therapeutics is improved. Common lipophilic small molecules used for conjugating oligonucleotides include cholesterol, α-tocopherol, squalene, and fatty acids.