Phosphorothioate-Free and Self-Interaction-Reduced Acyclic Nucleic Acids for Effective Antisense Oligonucleotide

Acyclic artificial nucleic acids, serinol nucleic acid (SNA) and acyclic l-threoninol nucleic acid (L-aTNA), are promising next-generation nucleic acid therapeutics with strong nuclease resistance and stable RNA hybridization, eliminating the need for phosphorothioate (PS) modifications that are associated with toxicities and complicate manufacturing due to diastereomer generation. However, both platforms suffer from self-interactions in self-complementary regions, limiting their therapeutic utility. To overcome this, we incorporated pseudocomplementary bases, 2,6-diaminopurine (D) and 2-thiouracil (sU), into SNA and L-aTNA oligonucleotides. This strategy effectively suppressed self-interactions and enhanced the RNA affinity. As a proof of concept, SNA and L-aTNA oligonucleotides targeting miR-21, which has a self-complementary region, with D and sU substitutions demonstrated significantly improved anti-miR-21 activity in cancer cell lines. Furthermore, PS-free L-aTNA incorporating D and sU effectively suppressed tumor growth with low toxicity in vivo when delivered via unit polyion complexes. This platform offers a safer and more effective strategy for antisense oligonucleotide therapeutics.