The pentasodium salt of samarium Sm 153 lexidronam, a therapeutic agent consisting of a medium energy beta- and gamma-emitting radioisotope, samarium Sm 153, and a teraphosphonate chelator, ethylenediaminetetramethylene phosphonic acid (EDTMP). The chelator moiety of samarium Sm 153 lexidronam associates with hydroxyapatite crystals concentrated in areas of bone turnover, thereby selectively delivering samarium Sm 153-mediated cytotoxic radiation to osteoblastic bone metastases. Check for active clinical trialsusing this agent. or A drug used to treat bone pain caused by bone cancer and other cancers that have spread to the bone. It contains a radioactive substance called samarium SM 153. Samarium Sm 153 lexidronam pentasodium collects in bone and gives off radiation that may kill cancer cells. It is a type of radiopharmaceutical. Also called Quadramet.