Pyrophosphorylated-Cholesterol-Modified Bone-Targeting Liposome Formulation Procedure

Bone-targeting drug delivery systems have been rapidly developed to increase drug efficacy and safety for musculoskeletal diseases in the past decades. Bone-targeting drug delivery is mainly based on ligands that have hydroxyapatite affinity. We previously reported a pyrophosphorylated cholesterol ligand-based bone-targeting liposome formulation for the treatment of bone fracture delayed union. Different from traditional bone-targeting ligands: bisphosphonates tetracyclines and polyanion peptides. Pyrophosphorylated cholesterol has no intrinsic pharmacological effects and can be naturally degraded into metabolites (both pyrophosphate and cholesterol are substances that naturally exist in the body), leading to minimal safety concerns. Pyrophosphorylated cholesterol is not only biodegradable, but it also provides strong bone affinity, which could target different bone substructures/surfaces, further improving drug delivery efficiency in vivo. Here, we describe the synthesis protocol of pyrophosphorylated cholesterol and a reverse-evaporation-based formulation protocol of pyrophosphorylated-cholesterol-modified bone-targeting liposomes for hydrophilic drug encapsulation. We also provide instructions for the bone-targeting property evaluation of the pyrophosphorylated-cholesterol-modified liposome in vitro and in vivo. Our system has wide applications and has already been used to study drug treatment for fracture delayed union and nonunion. As a promising bone-targeting drug delivery system, our system may be extrapolated to clinical applications of other bone anabolic agents for different bone diseases.