Radium-223 (Xofigo) is a drug for men with castrate resistant metastatic prostate cancer (mCRCP) who have multiple bone metastases. Radium-223 selectively targets bone metastases by mimicking calcium, which is attracted to the bone, especially to the fast growing areas of the tumor in the bone. It emits an alpha-emitting dose of radiation, which is mostly contained in the tumor due to the low spread of alpha waves).

The current FDA approval for Radium-223 is for one course of 6 injections. Men have been asking if it is possible to have a re-treatment of Radium-223 once the disease has progressed after having had the approved dose. In research discussed at the ASCO GU conference in San Francisco it was disclosed that a second round of Radium-223 is well tolerated and continues to control disease progression in bone.

The conclusion was based on a study of 44 men with mCRPC who completed an initial 6-injection course of treatment (the FDA approved dose) of radium-223. The research team, led by Oliver Sartor, MD, of the Tulane Cancer Center in New Orleans, found that radiographic bone progression with radium-223 re-treatment was rare, with most disease progression occurring in soft tissue (or not in bone).

Of the 44 men, 29 (66%) completed re-treatment with all 6 injections. The median time from the end of initial radium-223 treatment was 6 months. Thirty-two men (73%) had failed prior treatment with newer hormonal agents, such as abiraterone (Zytiga) or enzalutamide (Xtandi).

The median time to radiographic bone progression for the men receiving re-treatment was not reached when this information was made public at the conference. The median radiographic progression-free survival (rPFS) was 9.9 months. Of the 13 patients with rPFS events, 8 had soft tissue tumor progression, 2 had radiographic progression from disposition but not documented in radiographic tumor progression, 2 died, and 1 had confirmed radiographic bone progression.

Radium-223 was approved by he FDA based on the results of the pivotal ALSYMPCA trial. The trial showed that treatment with the drug prolonged overall survival by a median of 3.6 months and significantly reducing the risk of death by 30%. In the trial, the dosing regimen was 1 injection every 4 weeks for a total of 6 injections. Radium-223 treatment beyond 6 injections has not been previously reported.

Dr. Sartor and his colleagues identified no marked alterations in treatment-emergent adverse event (TEAE) incidence compared with the ALSYMPCA trial. Only 2 re-treated patients experienced grade 3 hematologic TEAEs. No patients experienced grade 4 or 5 hematologic TEAEs.

This result makes it possible for the FDA to consider allowing an additional round of Radium-223 in the future.