Efficient suppression of secretory clusterin levels by polymer-siRNA nanocomplexes enhances ionizing radiation lethality in human MCF-7 breast cancer cells in vitro*

Damon Sutton^1,†, Saejeong Kim^2,†, Xintao Shuai², Konstantin Leskov¹, Joao T Marques³, Bryan RG Williams³, David A Boothman¹, Jinming Gao¹

¹Simmons Comprehensive Cancer Center, University of Texas Southwestern Medical Center at Dallas, Dallas, TX, USA; ²Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH, USA; ³Department of Cancer Biology, Cleveland Clinic Foundation, Cleveland, OH, USA; ^†These authors contributed equally to the work

Abstract: Small interfering RNA molecules (siRNA) hold great promise to specifically target cytoprotective factors to enhance cancer therapy. Like antisense RNA strategies, however, the use of siRNA is limited because of in vivo instability. As a first step to overcome delivery issues, a series of graft copolymers of polyethylene glycol and polyethylenimine (PEI-g-PEG) were synthesized and investigated as nontoxic carriers for delivery of siRNA targeting the signaling peptide of secretory clusterin (sCLU), a prosurvival factor that protects cells from ionizing radiation (IR) injury, as well as chemotherapeutic agents. Three copolymers with different PEG grafting densities were tested for their abilities to bind and form nanocomplexes with siRNA. A copolymer composed of 10 PEG grafts (2 kDa each) per PEI polymer (2k10 copolymer) gave the highest binding affinity to siRNA by ethidium bromide exclusion assays, and had the smallest nanocomplex size (115 ± 13 nm diameter). In human breast cancer MCF-7 cells, 2k10–siRNAsCLU nanocomplexes suppressed both basal as well as IR-induced sCLU protein expression, which led to an over 3-fold increase in IR-induced lethality over 2k10–siRNA scrambled controls. In summary, this study demonstrates the proof-of-principle in using nanoparticle-mediated delivery of specific siRNAs to enhance the lethality of IR exposure in vitro, opening the door for siRNA-mediated knockdown of specific cytoprotective factors, such as DNA repair, antiapoptotic, free radical scavenging, and many other proteins.

Keywords: siRNA delivery, polyethylenimine, nanomedicine, secretory clusterin, cancer radiotherapy