Samarium [Sm 153] lexidronam pentasodium

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Quadramet (Australian Nuclear Science and Technology Organisation)

6 GBq/3 mL frozen sterile aqueous solution

Indication: bone metastases

Breast cancer and prostate cancer are two examples of tumours which can spread to bone. Bony metastases can be very painful and palliative treatment is required. Radiotherapy is often used, but has its limitations. This new product aims to deliver a dose of radiation internally.

Samarium lexidronam pentasodium was also known as samarium ethylenediaminetetramethylene phosphoric acid (EDTMP). The EDTMP is a chelating agent that binds to bone and the samarium component is a radioactive metal. Beta and gamma radiation are emitted.

The agent is kept frozen in a lead pot. It should be used within 8 hours of thawing out. The dose is given intravenously over one minute. Samarium is rapidly cleared from the plasma and is taken into bone. The uptake by bone is related to the number of metastatic sites. This delivers a higher dose of radiation to bony metastases than other tissues.

An Australian study1 in the early development of the treatment gave samarium to 23 patients with painful metastases. This produced pain relief in 14 patients with a median duration of 8 weeks. Significant effects are present a week after treatment, but some patients may have increased pain in the first few days after treatment.

The radioactivity affects normal bone so myelosuppression is a problem. The platelet and white blood cell counts reach a nadir 3-5 weeks after treatment. Although most patients in clinical trials will report adverse events, the incidence, apart from myelosuppression, is not very different from that of patients given a placebo.

Samarium 153 is excreted in the urine so the bladder wall is exposed to radiation. Patients are encouraged to drink at least 500 mL of fluid before treatment and to empty the bladder as frequently as possible afterwards. Urinary excretion is complete in approximately 6 hours.

The role of samarium 153 for bony metastases needs to be clarified with respect to other treatments such as strontium 89. It can be considered for pain relief in patients with metastases that appear on radionuclide scans. Caution is needed if there is evidence of an impending problem such as spinal cord compression.