Local delivery of small interfering RNA to inhibit osteoporosis using a combination device strategy

ABSTRACT RNA interference (RNAi) is a powerful tool to knock down specific message RNA expression levels in cells by exploiting a natural intracellular regulatory phenomenon in mammalian species. Gene silencing using short interfering RNAs (siRNAs) has many potential therapeutic applications. Local or topical siRNA therapeutics have been most actively investigated. Osteoporosis is rapidly becoming a global healthcare crisis worldwide. Most osteoporotic fractures require surgical intervention, reduction and stabilization, and various fixation devices areused as implants to improve fracture healing, necessitating the development of new treatment options for this condition. After orthopedic fixation device implantation, large numbers of osteoclastprecursors recruited into the peri-implant tissue can rapidly differentiate into osteoclasts which accelerate bone loss and cause aseptic loosening and finally prosthetic failure. Injectable bone cements are clinically used to improve mechanical properties to improve fixation as bone augmentation biomaterials in osteoporosis, with several FDA-approved calcium phosphate cements (CPC) products in clinical use. Compared to nondegradable polymethylmethacrylate bone cement, CPC has several unique properties for bone, including intrinsic osteoconductivity, osteoinductivity and acceptable biocompatibility. Significantly, calcium-based bone cements in bone augmentation use provide a potential depot for local drug delivery directly to bone sites in osteoporosis. The objective of this dissertation research was to investigate a new strategy to deliver siRNA locally to bone to inhibit osteoclast formation and function in improving osteoporotic bone healing. Our initial siRNA delivery data showed successful use of siRNA against receptor activator of nuclear factor-kappa B (RANK) and farnesyl pyrophosphate synthase (FPPS), resulting in knockdown of RANK in both osteoclast precursor and osteoclast cultures and knockdown of FPPS in both osteoclasts and osteoblasts, supporting a proof-of-concept to deliver siRNA to bone. In order to deliver siRNA to osteoclasts locally, resorbable poly(lactic-co-glycolic acid) (PLGA) microparticles were utilized as protective carriers. Cell-specific phagocytosis was achieved by PLGA particle size-cell phagocyte discrimination and selective uptake by bone phagocytes. PLGA-siRNA particles were dispersed equally into clinical-grade injectable CPC to serve as the delivery carrier and facilitate further active and passive in vivo degradation of cement and potentially for enhancing bone in-growth and healing at surgical sites. This