Circulating tumor cells (CTCs) may be useful as biomarkers of response to radium-223 treatment for metastatic castration-resistant prostate cancer (mCRPC), according to study findings presented at the American Society for Radiation Oncology (ASTRO) 2020 virtual annual meeting.

The study enrolled 22 patients with mCRPC who had a median age of 71 years. Of these, 8 patients (36%) had fewer than 6 metastases, 7 (32%) had 6-20 metastases, and 7 (32%) had more than 20 metastases. All progressed to mCRPC on androgen deprivation therapy, and 5 patients (23%) previously received docetaxel. The median overall survival was 18.4 months, according to Keisuke Otani, MD, PhD, of Massachusetts General Hospital in Boston, Massachusetts, who presented study findings. Investigators measured CTCs using 2 methods: CellSearch to enumerate CTCs and a prostate CTC expression assay based on polymerase chain reaction to perform RNA expression analyses. They used the assay to arrive at a CTC RNA score.

Pretreatment CTC counts of 5 CTCs ore more per 7.5 mL of blood correlated with worse survival compared with less than 5 CTCs per 7.5 mL (median 10.0 vs 29.2 months), Dr Otani and colleagues found. An elevated pretreatment digital CTC RNA score of 20 or higher calculated using the CTC RNA expression assay also was associated with worse survival compared with a score of less than 20 (median 11.6 vs 29.2 months). The pretreatment digital CTC RNA score was significantly higher among patients who had demonstrated disease progression on bone scans at 6 months compared with those with stable or decreased disease burden.

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Pleskow H, Otani K, Kusaka E, et al. Circulating tumor cells and radium-223 response in metastatic castration-resistant prostate cancer. Presented at: ASTRO 2020 virtual annual meeting, October 23-29, 2020. Abstract 3222.