Precise Enzymatic Cleavage Sites for Improved Bioactivity of siRNA Lipo-Polyplexes

Sequence-defined cationic lipo-oligomers are potent siRNA carriers, forming stable lipo-polyplexes based on both electrostatic and hydrophobic interactions and, after endocytosis and endosomal protonation, facilitating the delivery of siRNA into the cytosol. After completion of the nucleic acid delivery process, carriers should be readily biodegradable to ensure minimum accumulation of amphiphilic molecules that are harmful to lysosomes and other intracellular organelles. Endolysosomal enzymes may degrade a surplus of carrier molecules left over in lysosomes and thereby facilitate the generation and rapid excretion of cleavage products. By solid-phase supported synthesis, a library of sequence-defined lipo-oligomers was generated containing artificial and natural amino acids comprising precise enzymatic cleavage sites. Incorporating either short cleavable l-arginine sequences (RR), noncleavable d-arginine linkers (rr), or varieties of both tailored the degradability of lipo-oligomers, as demonstrated upon incubation with the endolysosomal protease cathepsin B. Cleavage products were identified by MALDI-TOF mass spectrometry. The effect of improved intracellular degradation on cell tolerability was studied by transfecting Huh7-eGFPLuc and DU145-eGFPLuc cells. Positioning of enzymatic cleavage sites between a lipophilic diacyl domain and an ionizable oligocationic siRNA binding unit enabled efficient enzymatic degradation of the carrier and reduced the lytic potential under lysosomal conditions. Highly degradable carriers containing at least one l-arginine dipeptide linker significantly improved the viability of transfected cells without hampering gene silencing activity. Therefore, the precise integration of enzymatic cleavage sites in lipo-oligomers is a promising strategy toward biocompatible nucleic acid carriers.