89Sr-Strontium Chloride has been developed under the name Metastron™. It was first approved in the US in 1993, and is now considered as a generic, available from different additional sources.

89Sr-Strontium Chloride is indicated for the relief of bone pain in patients with painful skeletal metastases that have been confirmed prior to therapy. 89Sr is chemically similar to calcium and the agent follows the biochemical pathways of calcium in the body. Strontium is incorporated into the hydroxyapatite molecule in bone with uptake occurring mainly at sites of active osteoblastic activity.

With these favorable parameters, the drug is expected to react directly with the tumor cells and should reduce the tumor mass. Hence the drug should increase the life expectancy at higher doses, but this has never been clinically demonstrated and the drug is authorized only as a pain palliation agent. At pain palliation doses, there is apparently no effect on survival, only a decrease in biomarkers (e.g. PSA) is observed.

Unfortunately, it appears that the agent is not effective in about 30% of the patients, probably as a consequence of the fast repair mechanism of sub-lethal doses. The low dose rate of 89Sr limits the effectiveness of the agent. Combined with certain platinum-mediated radio-sensitizing chemotherapeutic agents 89Sr-strontium chloride appears to be more efficient.

Target/Mechanism: Bones

Leading Emitter: beta electrons (β)