Tiludronic acid is a bisphosphonate characterized by a (4-chlorophenylthio) group on the carbon atom of the basic P-C-P structure common to all bisphosphonates. Tiludronate is a first generation (non-nitrogenous) bisphosphonate in the same family as etidronate and clodronate. Tiludronate affects calcium metabolism and inhibits bone resorption and soft tissue calcification. Of the tiludronate that is resorbed (from oral preparation) or infused (for intravenous drugs), about 50% is excreted unchanged by the kidney. The remainder has a very high affinity for bone tissue, and is rapidly absorbed onto the bone surface. Tiludronic acid is marketed under the tradename Skelid. In vitro studies indicate that tiludronate disodium acts primarily on bone through a mechanism that involves inhibition of osteoclastic activity with a probable reduction in the enzymatic and transport processes that lead to resorption of the mineralized matrix. Bone resorption occurs following recruitment, activation, and polarization of osteoclasts. Tiludronate disodium appears to inhibit osteoclasts through at least two mechanisms: disruption of the cytoskeletal ring structure, possibly by inhibition of protein-tyrosine-phosphatase, thus leading to detachment of osteoclasts from the bone surface and the inhibition of the osteoclastic proton pump. SKELID is indicated for treatment of Paget's disease of bone (osteitis deformans).