Bone metastases occur in up to 85% of patients who have breast, lung, or prostate cancer. Bone-seeking radiopharmaceuticals have pharmaco-
kinetic properties similar to either calcium or phosphate. Strontium-89 (89Sr) is a calcium analogue. 32P, 86Re, HEDP, and 153Sm are all phosphate analogues.
Application of 32P-orthophosphate for the treatment of bone pain was effective, but bone marrow toxicity limited its widespread use. 89Sr was the first radioisotope employed as a systemic treatment for bone metastases in prostate cancer. After IV administration of l50 MBq of 89Sr the radiopharmaceutical is avidly accumulated in areas of high bone turnover, such as reactive bone surrounding a metastasis. A transient leucopaenia can be expected after 6 weeks. After a single administration of 89Sr, in 75-80% of patients pain is promptly relieved and progression of further bone disease is delayed, with the response lasting 1-6 months.
Rhenium-186HEDP is a new radiopharmaceutical with similar chemistry and biodistribution to 99mTc-labelled diphosphonates. There is a tendency for palliative response to decrease approximately 7 weeks following injection in some patients. Haematopoietic toxicity is minimal. Samarium-153-EDTMP preferentially localizes in bone metastases and is rapidly cleared from the blood by the kidneys.
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