Skeletal function and structure: Implications for tissue-targeted therapeutics

Osteoporosis, arthritis, and periodontal disease are common diseases of the skeleton, all of which could benefit from new therapeutic strategies, including targeted drug delivery. While bone is a rigid structure, it is not inert, with the cells of the skeleton being able to repair damage and respond to alterations in mechanical stimuli and various endocrine agents. Several important factors related to bone physiology that could influence the success of a pharmacological treatment include heterogeneity in bone remodeling activities throughout the skeleton, differences in blood supply and local vascularization, and the “blood-bone” barrier. The structural qualities of bone, especially the presence of hydroxyapatite crystals in the bone mineral and the established binding of certain molecules to this mineral phase, including tetracyclines, bisphosphonates, and other chelators, provide unique opportunities to treat skeletal diseases using targeted drug delivery. Additional opportunities exist in targeting sites with contrasting bone surface activities, including surfaces that are inactive, forming new bone or being resorbed. The ultimate key to developing new bone-targeted therapies is to understand and exploit the physiological characteristics at the desired target sites.