Circulating and Imaging Biomarkers of Radium-223 Response in Metastatic Castration-Resistant Prostate Cancer

Abstract

Purpose: Radium-223 improves overall survival (OS) and reduces skeletal events in patients with bone metastatic castration-resistant prostate cancer (CRPC), but relevant biomarkers are lacking. We evaluated automated bone scan index (aBSI) and circulating tumor cell (CTC) analyses as potential biomarkers of prognosis and activity.

Patients and methods: Patients with bone metastatic CRPC were enrolled on a prospective single-arm study of standard radium-223. ^99mTc-MDP bone scan images at baseline, 2 months, and 6 months were quantitated using aBSI. CTCs at baseline, 1 month, and 2 months were enumerated and assessed for RNA expression of prostate cancer-specific genes using microfluidic enrichment followed by droplet digital polymerase chain reaction.

Results: The median OS was 21.3 months in 22 patients. Lower baseline aBSI and minimal change in aBSI (<+0.7) from baseline to 2 months were each associated with better OS (P = .00341 and P = .0139, respectively). The higher baseline CTC count of ≥5 CTC/7.5 mL was associated with worse OS (median, 10.1 v 32.9 months; P = .00568). CTCs declined at 2 months in four of 15 patients with detectable baseline CTCs. Among individual genes in CTCs, baseline expression of the splice variant AR-V7 was significantly associated with worse OS (hazard ratio, 5.20 [95% CI, 1.657 to 16.31]; P = .00195). Baseline detectable AR-V7, higher aBSI, and CTC count ≥5 CTC/7.5 mL continued to have a significant independent negative impact on OS after controlling for prostate-specific antigen or alkaline phosphatase.

Conclusion: Quantitative bone scan assessment with aBSI and CTC analyses are prognostic markers in patients treated with radium-223. AR-V7 expression in CTCs is a particularly promising prognostic biomarker and warrants validation in larger cohorts.

FIG 1.

(A) Study schematic. Serial bone scans were obtained at pretreatment baseline, 2 months, and 6 months. Serial CTC analyses were performed using two different CTC methods at pretreatment baseline, 1 month, and 2 months. A detailed flow diagram of actual patient evaluations collected is shown in Appendix Figure A1. (B) Bar graph in the top panel shows patients in a clinically uniform prospective cohort (N = 22) ordered by OS from left to right (vertical blue bars). Four patients noted with red asterisks were alive at the time of final follow-up. Heatmap shows the level of signal of various assays for individual patients, including CTC expression of AR-V7, TMRPSS2:ERG, and prostate genes, CTC_M score, CellSearch CTC count, and %aBSI. aBSI, automated bone scan index; CTC, circulating tumor cell; NA, not applicable; OS, overall survival; PSA, prostate-specific antigen; tAP, total alkaline phosphatase.

FIG 2.

aBSI is prognostic for OS. (A) Time course of aBSI assessments in a single patient with metastatic prostate cancer at baseline (BSI, 3.8%), 2 months (aBSI, 2.4%), and 6 months (aBSI, 1.6%). (B) Kaplan-Meier estimates of OS by baseline aBSI relative to the optimized cut point of 0.9. (C) Kaplan-Meier estimates of OS by aBSI change of ≥+0.7 versus <+0.7 from baseline to 2 months. aBSI, automated bone scan index; OS, overall survival.

FIG 3.

Higher baseline CellSearch count or CTC_M score was associated with significantly worse OS. Kaplan-Meier estimates of OS by (A) baseline CellSearch CTC counts greater than or less than five CTCs/7.5 mL blood and (B) baseline CTC_M score relative to the optimized cut point of 20. CTC, circulating tumor cell; OS, overall survival.

FIG 4.

AR-V7 expression in CTCs at baseline is independently prognostic. (A) Kaplan-Meier estimates of OS by the AR-V7 signal in CTCs at baseline. (B) Box plot showing baseline CellSearch CTC counts by AR-V7 status. (C) Scatter plot of baseline CellSearch CTC counts versus baseline AR-V7 signal, showing no significant correlation between CTC count and the presence of AR-V7. CTC, circulating tumor cell; OS, overall survival.

FIG A1.

Detailed schematic description of the study. (A) One patient with the bone scan image not analyzable for aBSI; (B) laboratory operating CellSearch CTC enumeration assay closed after recruiting Pt 19; (C) one patient sample had a technical issue during processing; (D) patient decision to discontinue participation; (E) discontinuation from study because of disease progression: one patient discontinued before bone scan, and the other patient discontinued after bone scan; (F) one patient whose baseline image was not analyzable for aBSI, (G) disease progression. aBSI, automated bone scan index; CTC, circulating tumor cell; Pt, patient.

FIG A2.

Plots showing longitudinal time courses of AR-V7, aBSI, PSA, and CellSearch CTC counts for each individual participant. Some PSA values and CellSearch CTC counts with high values were scaled differently and shown in italics. From top to bottom, left to right, these are aligned by the order in Figure 1B (ie, shortest survival to longest survival). aBSI, automated bone scan index; CTC, circulating tumor cell; OS, overall survival; PSA, prostate-specific antigen.

FIG A3.

Kaplan-Meier estimates of overall survival by (A) baseline Global Health Score and (B) baseline Pain Score.

FIG A4.

Kaplan-Meier estimates of overall survival by CTC decline or nondecline. CTC, circulating tumor cell.

FIG A5.

Cox proportional hazards regression analyses of promising radium-223 biomarkers to examine for independent prognostic value when adjusted for PSA or tAP. Each separate multivariable model is limited to two covariates because of limitations of sample size. aBSI, automated bone scan index; HR, hazard ratio; PSA, prostate-specific antigen; tAP, total alkaline phosphatase.

FIG A6.

Plots showing longitudinal time courses of gene expression measured in CTCs for each individual participant. From top to bottom, left to right, these are aligned by the order in Figure 1B (ie, shortest survival to longest survival). CTC, circulating tumor cell; OS, overall survival.