Purpose: Systemic siRNA therapeutics are limited by poor cell membrane permeability, rapid nuclease degradation, and renal clearance. A carrier with long circulatory half-life is needed for effective RNAi and potential tumor targeting via the enhanced permeation and retention (EPR) effect. Existing hydrophilic polymer-modified polyplexes can destabilize complexes, especially for oligonucleotides such as siRNA.
Hypothesis: If cationic comb-type copolymers (CCCs) with a low polycation content (<30 wt%) and abundant water-soluble side chains (>70 wt%) are used as siRNA carriers, then the dense polymer brush will reinforce interpolyelectrolyte complexation with siRNA, protect siRNA from nucleases and plasma components, and prolong blood circulation without chemical conjugation or cross-linking.
Aims: Prepare a series of CCCs composed of poly(L-lysine) (PLL) backbones grafted with dextran (Dex) or poly(ethylene glycol) (PEG) side chains. - Evaluate siRNA binding affinity, nuclease resistance, and plasma stability in vitro. - Assess blood circulation half-life of siRNA/CCC complexes in mice. - Test whether pre-administered CCC can capture subsequently injected naked siRNA in the bloodstream. - Compare with naked siRNA and jetPEI™.
Delivery system:
Component: Polymer class; Details: Cationic comb-type copolymers (CCCs)
Component: Backbone; Details: Poly(L-lysine) (PLL); Mₙ = 7,000 or 28,000
Component: Side chains; Details: Dextran (Mₙ = 6,000) or PEG (Mₙ = 2,000 or 5,000)
Component: Compositions; Details: PLL-g-Dex: 10 wt% PLL/90 wt% Dex (7K6D90, 28K6D90); PLL-g-PEG: 7K2P90, 28K2P90, 7K5P90, 28K5P90, 7K5P70, 28K5P70
Component: Payload; Details: Unmodified 21-nt siRNA; 27-bp blunt-ended dsRNA; Alexa Fluor 546-labeled siRNA/dsRNA
Component: Complexation; Details: Electrostatic interaction at N/P ratio 4
Component: Targeting ligand; Details: None
Component: Key feature; Details: High-density water-soluble polymer brush for steric protection and prolonged circulation; no chemical conjugation of siRNA
Approach: In vitro: Fluorescence polarization (FIDA-PO) for binding; RNase A resistance assay; 90% fresh mouse plasma stability. - In vivo: Male ICR mice (6 weeks old); i.v. administration of siRNA/CCC mixtures; blood sampling at timed intervals; siRNA extraction and polyacrylamide gel electrophoresis. - Additional in vivo: Pre-injection of CCC alone, followed 20 min later by naked siRNA. - Controls: Naked siRNA; jetPEI™ (linear PEI). - No disease model, no tumor model, no therapeutic gene silencing, no cellular uptake studies.
Key methods: Binding affinity: Fluorescence intensity distribution analysis-polarization (FIDA-PO). - Stability: RNase A and 90% mouse plasma incubation; gel electrophoresis. - Blood circulation: Fluorescence gel images of extracted siRNA; time course of % injected dose in blood; AUC estimation. - Comparison: jetPEI™ and naked siRNA.
Key results: Binding: CCC with higher side-chain content (28K5P90; 10 wt% PLL, 90 wt% PEG) showed stronger siRNA binding than lower side-chain content (28K5P70; 30 wt% PLL, 70 wt% PEG). - Stability: siRNA complexed with CCC was resistant to nucleases in 90% plasma for 24 h in vitro. jetPEI™ showed little protection. - Circulation: CCC 28K5P90 increased siRNA circulation time in mouse blood by 100-fold. Naked siRNA cleared within 5 min; with 28K5P90, significant intact siRNA was detected at 2 h. - dsRNA length effect: 27-bp dsRNA/28K5P90 complex showed ~35% initial dose still circulating at 4 h, whereas 21-nt siRNA was nearly gone at 2 h. - Pre-injection: When CCC was injected 20 min before naked siRNA, siRNA half-life increased by >60-fold. - Order of efficacy: 28K5P90 > 28K5P70 > 28K6D90 >> 2P90; higher PLL molecular weight improved circulation.
Interpretation: The authors claim that cationic carriers with water-soluble polymer brushes are a novel strategy for oligonucleotide delivery. The dense PEG brush shields positive charges, reduces nonspecific interactions with blood components, and preserves selective binding to siRNA, enabling simple and effective long-circulating siRNA carriers. This is reported as the first example of a polyplex carrier that prolongs blood circulation of unmodified siRNA without resource-consuming preparation.
Limitations: No therapeutic efficacy: No gene silencing, target tissue delivery, or tumor model. - No biodistribution: Organ accumulation and clearance pathways not assessed. - No cellular uptake or intracellular trafficking data. - No toxicity or immunogenicity assessment. - No long-term safety or repeated-dose data. - Complex size/zeta potential not characterized in this study. - Small animal only: Mice; no large-animal or human validation. - No comparison with other long-circulating nanocarriers beyond jetPEI™. - No targeting ligand: Delivery relies on passive circulation and EPR effect. - DOI/link not provided in the supplied file.
