Cell specific delivery of small interfering ribonucleic acid (siRNA) using well-defined multivalent folate-conjugated block copolymers is reported. Primary amine functional, biocompatible, hydrophilic-block-cationic copolymers were synthesized via aqueous reversible additionfragmentation chain transfer (RAFT) polymerization. N-(2-hydroxypropyl)methacrylamide) (HPMA), a permanently hydrophilic monomer, was copolymerized with a primary amine containing monomer, N-(3-aminopropyl)methacrylamide (APMA). Poly(HPMA) confers biocompatibility while APMA provides amine functionality allowing conjugation of folate derivatives. (HPMA-stat-APMA) was chain extended with a cationic block, poly(N-[3-(dimethylamino)propyl] methacrylamide) in order to promote electrostatic complexation between the copolymer and the negatively charged phosphate backbone of siRNA. Notably, poly(HPMA) stabilizes the neutral complexes in aqueous solution while APMA allows the conjugation of a targeting moiety, thus, dually circumventing problems associated with the delivery of genes via cationically charged complexes (universal transfection). Fluorescence microscopy and gene down-regulation studies indicate that these neutral complexes can be specifically delivered to cancer cells that over-express folate receptors.

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Block ionomer complex (BIC)-siRNA interactions and effectiveness in cell transfection are reported. Aqueous RAFT polymerization was used to prepare a series of hydrophilic-block-cationic copolymers in which the cationic block statistically incorporates increasing amounts of neutral, hydrophilic monomer such that the number of cationic groups remains unchanged but the cationic charge density is diluted along the polymer backbone. Reduced charge density decreases the electrostatic binding strength between copolymers and siRNA with the goal of improving siRNA release after targeted cellular delivery. However, lower binding strength resulted in decreased transfection and RNA interference pathway activation, leading to reduced gene knockdown. Enzymatic siRNA degradation studies with BICs indicated lowered binding strength increases susceptibility to RNases, which is the likely cause for poor gene knockdown.