Samarium-153
A radioactive metal, given as an injection, that homes in on the actively growing edges of bone and gives off radiation there. It is used to ease pain from cancer that has spread to bone.
Overview
Samarium-153 lexidronam (Quadramet, 153Sm-EDTMP) delivers 153Sm chelated to a bone-seeking phosphonate that concentrates roughly five-fold more in osteoblastic lesions than in normal bone. It decays with a physical half-life of 46.3 hours (1.93 days) by emission of three principal beta-particle energies (maxima around 640, 710 and 810 keV; mean beta energy about 233 keV) plus a 103 keV gamma photon (about 28-29% abundance) that permits scintigraphic imaging of biodistribution.
The beta particles have an average range in soft tissue of roughly 0.5-0.8 mm and a maximum of about 3 mm (about 1.7 mm in bone), similar in scale to other bone-palliation radiopharmaceuticals but with a much shorter half-life than strontium-89, giving faster onset and clearance.
153Sm is produced in high yield and purity by neutron irradiation of isotopically enriched samarium-152 oxide (152Sm2O3) in a reactor, 152Sm(n,γ)153Sm; this is a routine, widely available reactor product and not considered supply-constrained, though Quadramet's US manufacturer has changed hands and availability has fluctuated commercially.
- β− maxima 640 keV (30%), 710 keV (50%), 810 keV (20%); mean 233 keV
- γ 103 keV (~28-29%)
- EDTMP
Theranostic partners
- Technetium-99m · The single most widely used medical radioisotope: a metastable metal with a six-hour lifetime, milked on demand from a bedside generator (a 'moly cow') and attached to a cold kit to image bone, heart, kidneys or lymph nodes.
Quadramet (samarium-153 lexidronam) is a radioactive bone-seeking injection that lodges in bone metastases and irradiates them from inside, approved in 1997 to relieve pain from prostate, breast and other cancers spread to bone. Most patients get relief within one to two weeks, but marrow suppression follows at three to five weeks, and radium-223 and PSMA radioligands have largely replaced it.
A radioactive form of strontium is injected into a vein. Because it behaves like calcium, it collects in the damaged, overactive bone around cancer that has spread to bone, and its radiation can relieve bone pain for months.
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