Bone-Targeting Radiopharmaceuticals Including Radium-223

Bone-seeking radionuclides including samarium-153 ethylene diamine tetramethylene phosphonate and strontium-89 have been used for decades in the palliation of pain from bone metastases especially from prostate cancer. Emerging evidence of improved survival in metastatic castration-resistant prostate cancer (CRPC) with the first-in-class alpha-radionuclide, radium-223 (Ra-223) has rekindled interest in the role of bone-seeking radionuclide therapy. We review the literature for randomized controlled trials of bone-seeking radionuclides and explore some of the issues regarding the optimal use of these agents. In particular, we discuss dose, dose rate, radiobiology, and quality of radiation and postulate on potential future directions in particular combination schedules. beta-Emitting, bone-seeking radionuclides have proven ability to control pain in prostate cancer metastatic to bone with pain response rates in the order of 60% to 70% when used as single agents. Most of the published trials were underpowered to detect differences in survival; however, there is evidence of the potential for disease modification when these agents are used in combination with chemotherapy or in multiple cycles. Data from the recent phase III ALSYMPCA trial that compared Ra-223 to placebo in symptomatic CRPC demonstrate a significant improvement in median overall survival of 3.6 months for patients with symptomatic CRPC metastatic to bone treated with 6 cycles of the alpha-emitting radionuclide Ra-223 compared with placebo. The success of Ra-223 in improving survival in CRPC will lead this agent to become part of the treatment paradigm for this disease, and with such an excellent safety profile, Ra-223 has huge potential in combination strategies as well as for use earlier in the natural history of metastatic prostate cancer.