Atelocollagen-Mediated siRNA Delivery: Future Promise for Therapeutic Application

Drug-delivery systems for nucleic acid-based medicine In recent years, nucleic acid-based medicine has attracted attention for its therapeutic applications to cancers and infectious, hereditary, and other diseases. Moreover, treatment by gene transfer with a viral or nonviral vector has been actively studied. In addition, owing to the development of novel gene-transfer reagents and gene systems, the administration of a gene itself has been tested for potential therapeutic efficacy in gene expression. Most recently, the discovery of RNAi has triggered a global competition among many companies and researchers to find effective delivery systems that capitalize on the clinical benefits of siRNA. For in vivo delivery of nucleic acids including siRNA, research and development have focused on the improvement of delivery efficiency, resulting in a wide use of genedelivery systems based on cationic liposomes or polymers. However, these delivery systems still entail clinically relevant problems such as cellular toxicity and the induction of nonspecific immune responses, as the expression of immune-responsive genes and apoptosisrelated genes is enhanced after administration. Establishment of an ideal delivery system with lower toxicity and higher stability is a big challenge [1–10]. To solve these problems, we and others have pioneered in vivo nucleic acid-delivery systems, such as siRNA systemic delivery mediated by highly bio-compatible and lowimmunogenic atelocollagen, which many researchers have extensively employed in studies of various disease models. We also investigated the combined use of siRNA/ atelocollagen with anticancer agents, targeting the antiapoptotic factor Bcl-xL gene. In brief, an siRNA targeting human Bcl-xL complexed with atelocollagen was injected three-times weekly for a total of 12-times through the caudal vein in mice with a human prostate cancer cell line (PC-3) xenograft already grown subcutaneously. These injections significantly inhibited growth as well as the expression of both Bcl-xL mRNA and its protein in tumors. A combinational therapy of Bcl-xL siRNA/atelocollagen with a low dose of cisplatin (12 mg/kg) provided more intense growth inhibition [11]. Atelocollagen has been developed for artificial organs, its safety having been assured as a top-level bio-compatible material by the US FDA. Atelocollagen possesses basicity (positive charge) under physiological conditions, whereas siRNA and other nucleic acids have acidity (negative charge). Therefore, atelocollagen and siRNA are considered to form a complex by electrostatic binding. The nucleic acid complexed with atelocollagen can avoid degradation by nuclease(s) and can be efficiently transported in the bloodstream to tissues and cells, where the siRNA/ atelocollagen complex may be incorporated through endocytosis. Furthermore, recent studies demonstrated that the complex itself can lower the cellular membrane potential enough to promote endocytosis. In animal model experiments, the siRNA/atelocollagen complex that arrived at a tumor tissue downregulated the target gene expression. Accordingly, expectations are growing for the siRNA/atelocollagen complex as a novel Ichiro Fujimoto Koken Research Institute, Koken Co.