Bone-specific drug delivery systems: approaches via chemical modification of bone-seeking agents.

Despite several decades of progress, bone-specific delivery is still limited by the unique anatomical features of bone, which mainly consists of inorganic hydroxyapatite. A practical approach to this problem is to produce targeted drugs that have a high affinity for hydroxyapatite. Bisphosphonates are a class of synthetic compounds structurally related to pyrophosphate. Bisphosphonates rapidly localise on the bone surface after being administered either intravenously or orally, since the P-C-P portion of the bisphosphonate structure has high affinity for hydroxyapatite. Therefore, bisphosphonate modification might be a promising method for targeting drugs selectively to the bone. Bisphosphonate-conjugated drugs are hydrophilic and highly water-soluble due to the acidic nature of the bisphosphonate moiety at physiological pH, and therefore they hardly permeate through the biological membrane of soft tissues. These physicochemical changes also reduce the intrinsic susceptibility of the drug to metabolism, promoting urinary or biliary excretion as unchanged drug. All these physicochemical and pharmacokinetic alterations contribute to the exceptional skeletal disposition of bisphosphonate-conjugated drugs. Bisphosphonate conjugation is based on chemical modification of the targeting molecule, and therapeutically optimised bisphosphonate derivatives have to be custom-developed on a case-by-case basis. The bisphosphonate moiety is usually coupled with the targeting drug through a specific linkage. The high affinity of bisphosphonate conjugates for the bone is not simply dependent on the bisphosphonate moiety but on the resultant molecule as a whole, including the linker and the linked drug. Lipophilicity (represented as log P) appears to be an appropriate index for predicting the osteotropic properties of bisphosphonate derivatives. Several strategies using bisphosphonate-conjugated drugs have been investigated at a laboratory level with the aim of obtaining therapeutically optimised treatments for conditions such as osteoporosis, osteoarthritis and bone cancer. In each case, the intention is to achieve prolonged local exposure to high concentrations of the targeting drug, thereby improving therapeutic index by enhancing pharmacological efficacy and minimising systemic adverse effects. Although most examples of bone-specific drug delivery via bone-seeking agents still remain in preclinical studies, several phosphonate-coupled radiopharmaceuticals, such as samarium-153 complexed to tetraphosphonate, are expected to be an effective pain palliation therapies for metastatic bone cancer and are currently being developed in clinical trials. Furthermore, recent reports on bisphosphonate-modified proteins have illustrated the feasibility of bone-specific delivery of biologically active protein drugs, such as cytokines and growth factors.