153Sm-lexidronam (Quadramet)

Quadramet (¹⁵³Sm-lexidronam) has a physical half-life of about 46 hours and decays with emissions of both β and γ particles. The β particles have energies of 810 keV (20%), 710 keV (50%), and 640 keV (30%). The γ photon energy is 103 keV, and can be used to image the biodistribution of Quadramet with a gamma camera (see below).

Most of the injected Quadramet is rapidly taken up by osteoblastic bone metastases in proportion to the extent of metastases, causing some interpatient variability. The remainder is rapidly excreted in urine within about 6 hours, which is also subject to interpatient variability. The urinary route of excretion should be considered when prescribing Quadramet to patients with decreased renal function.

Before starting therapy, a routine bone scan must be performed to determine if there will be uptake of the radiopharmaceutical. A bone scan will also define the extent of lesion(s), since one or two lesions are best treated with external beam radiation.

Like other nuclear medicine therapies for painful osseous metastases (strontium-89 and rhenium-186 HEDP), there may be a transient increase in bone pain in the first 2-3 days that lasts for several days. Like strontium-89, approximately 10% of patients will have a painful flare response within 48 hours after initiation of therapy. Symptomatic improvement begins about 1-3 weeks after treatment and often lasts 3-6 months.

Quadramet is limited by its main side effect, bone marrow suppression. Low leukocyte (less than 2,400/μL) or platelet (less than 60,000/μL) counts are contraindications to its use. Leukocyte and platelet counts are obtained every two weeks following therapy until marrow recovery occurs. Leukocyte and platelet counts are reduced by about 50%, with the low-point occurring about a month after therapy, and with recovery occurring after about 6 weeks.

Another contraindication is an allergy to phosphates. Quadramet also should not be administered on the same day as other intravenous bisphosphonates due to competition for the same binding sites.

Finally, Quadramet therapy will not help if the patient's pain is due non-osseous metastases (e.g., an epidural lesion causing nerve root compression) or pathologic fractures.

References

• Mettler FA and Guiberteau MJ. Chapter 9: Skeletal System. In Essentials of Nuclear Medicine Imaging. Fifth Edition. Saunders, Philadelphia. 2006. p 290.
• Serafini AN. Therapy of metastatic bone pain. J Nucl Med. 2001 Jun;42(6):895-906.