Samarium lexidronam (153Sm-EDTMP): skeletal radiation for osteoblastic bone metastases and osteosarcoma

Figure 1. β- and γ-decay profile of ¹⁵³ Sm-EDTMP. Therapeutic β-particle radiation is spent after 1-2 weeks (<6% remaining after 1 week and <1% after 2 weeks). Sensitive detection of trace quantities of the γ-emitting ¹⁵⁴ Eu isotope can occur years after Sm administration but are in such minute quantities that they cannot be associated with clinical significance or effects. The neutron capture occurs only during the manufacturing process (i.e., when a ¹⁵² Sm oxide capsule is placed inside a nuclear reactor).
(Figure omitted. See article PDF.)

Figure 2. Medium energy electrons and low energy γ-rays (photons) are produced in the bone environment by the deposited ¹⁵³ Sm-EDTMP as it decays to ¹⁵³ Eu-EDTMP. The therapeutic β-particles have maximal energies and abundance (810 keV: 20%; 710 keV: 50%; 640 keV: 30%); the β-emissions penetrate approximately 1-2 mm of tissue. The g-photon is useful for imaging in a manner similar to a bone scan.
(Figure omitted. See article PDF.)

Figure 3. ¹⁵³ Sm-EDTMP (A) is a methylene tetraphosphonate, ^99m Tc-MDP (B) is a methylene diphosphonate and zoledronate (C) is a nitrogen-containing bisphosphonate. All bind hydroxyapatite with high affinity and are associated with new bone formation.
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Figure 4. Similar bone-specific distribution of radioactivity after ^99m Tc-MDP (white background) and ¹⁵³ Sm-EDTMP (gray background). After rapid blood and urinary clearance, ¹⁵³ Sm-EDTMP has a skeletal and osteoblastic metastasis distribution. The ^99m Tc-MDP images have some soft tissue distribution discernable. This patient has osteosarcoma painfully involving ribs and pleura; he experienced complete pain relief within 1 week after ¹⁵³ Sm-EDTMP 1 mCi/kg, low-dose cisplatin for radiosensitization and 3 Gy × 5 electron beam irradiation to the chest wall.
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Up to 80% of breast, prostate and lung cancer patients will develop skeletal metastases [1]. Bone metastases are a very common and often painful problem experienced by many cancer patients. Successful palliation of osseous metastases can be rewarding for clinicians and improve quality of life and function in those afflicted with numerous metastases and/or bone pain. Although radiation including the use of bone-seeking isotopes is an effective treatment for skeletal metastases [2], it is often underutilized in relation to the high incidence of bone metastases and cancer pain [3-6]. Because...