Rational design and structure-activity relationship of random copolymers for enhanced siRNA delivery

Cationic polymers and their derivatives have garnered significant interest as advanced vectors for siRNA delivery. Recently, we developed a robust diblock copolymer featuring an innovative binding block and a stealth block that work synergistically to facilitate efficient delivery of biotherapeutics. However, the fundamental mechanisms underlying its superior delivery capacity remain to be fully elucidated.

Since the binding block dominantly regulate the delivery performance, we synthesized a series of adapted copolymers, P(AAPBA_m-co-DMAPMA_n), by solely incorporating the key involved units, namely 3-acrylamidophenylboronic acid (AAPBA) and N-(3-dimethylaminopropyl)methacrylamide (DMAPMA). We thoroughly varied the block combinations, sequences and lengths, and investigated their effects on siRNA delivery.

AAPBA and DMAPMA can bound to siRNA through reversible ester bonds and electrostatic interactions, respectively. The former enhanced siRNA release due to its responsive properties, while the cationic DMAPMA promoted endosomal escape of the complexes through its inherent interaction with membrane. Notably, only the rational combination of 20 units of each monomer, defined as copolymer P(AAPBA₂₀-co-DMAPMA₂₀), integrated the multiple yet balanced functions that sequentially promoted siRNA loading, endocytosis, endosome escape, and cytoplasmic release, ultimately leading to superior gene silencing. The clarified structure-activity relationships and revealed principles are valuable for the rational design of novel polymeric vectors to improve siRNA delivery and therapeutic applications.