Bone-targeted radiopharmaceuticals
Some radioactive drugs are built to home in on damaged bone rather than the tumour itself, easing bone pain from cancer that has spread to the skeleton and, in one case, helping patients live longer.
Overview
Bone-targeted radiopharmaceuticals exploit the fact that metastatic bone lesions have abnormally high bone turnover. Radium-223 dichloride behaves as a calcium mimetic, concentrating in newly formed bone matrix next to osteoblastic metastases, where its alpha-particle emissions cause localised DNA damage. It is the only bone-targeted radiopharmaceutical shown to extend overall survival: the phase 3 ALSYMPCA trial (921 patients with symptomatic, bone-predominant, castration-resistant prostate cancer without known visceral metastases) showed improved overall survival and delayed skeletal-related events against placebo plus best supportive care. Older agents — samarium-153 lexidronam and phosphorus-32 — and strontium-89 (not in wide current use) give palliative pain relief from bone metastases but have not shown a survival benefit.
Radium-223's use has narrowed rather than widened since approval. The ERA-223 trial found that combining it with abiraterone and prednisone/prednisolone raised fracture incidence sharply (28.6% vs 11.4%) with no offsetting clinical benefit, so that combination is now contraindicated, and current guidance restricts radium-223 to bone-predominant mCRPC without visceral metastases, typically after at least two prior systemic therapies or in patients unfit for them. A phase 2 trial reported in 2026 (RADICAL, Alliance A031801, JCO) tested adding radium-223 to cabozantinib in bone-metastatic renal cell carcinoma and found no improvement in symptomatic skeletal event-free survival, progression-free survival or overall survival, and closed early for futility — a reminder that radium-223's prostate-cancer benefit does not automatically generalise to other bone-metastatic cancers.
What remains unsolved: assessing whether radium-223 is actually working is harder than for most cancer drugs, because it does not shrink lesions in the way conventional RECIST-based imaging expects and its target is bone remodelling rather than the tumour directly; a 2026 review in Seminars in Nuclear Medicine surveys the resulting patchwork of serum biomarkers, liquid biopsy markers and imaging approaches (18F-fluoride PET/CT, whole-body diffusion MRI, PSMA PET/CT) used to judge response, without a single validated standard emerging.
Notes
top- Radium-223 plus abiraterone/prednisone is contraindicated after ERA-223 found a sharply higher fracture rate (28.6% vs 11.4%) with no clinical benefit.
- The 2026 RADICAL trial found no benefit from adding radium-223 to cabozantinib in bone-metastatic renal cell carcinoma, closing early for futility — radium-223's prostate-cancer survival benefit has not generalised to other tumour types tested so far.
- There is no single validated method for assessing radium-223 response; conventional RECIST imaging is a poor fit because the drug targets bone remodelling rather than shrinking the tumour directly.