Improved Androgen Receptor Splice Variant 7 Detection Using a Highly Sensitive Assay to Predict Resistance to Abiraterone or Enzalutamide in Metastatic Prostate Cancer Patients

In metastatic castration-resistant prostate cancer (mCRPC), androgen receptor splice variant 7 (AR-V7) expression is associated with a low response to androgen receptor signaling (ARS) inhibitors such as abiraterone or enzalutamide.To perform a highly sensitive assay for detecting AR-V7 (hsAR-V7) in circulating tumor cells (CTCs) and evaluate its ability to predict response to ARS inhibitors.From 41 mCRPC patients, CTCs were prospectively enriched using AdnaTest platform and analyzed for AR-V7 with and without the highly sensitive assay.The first objective of the study was to compare AR-V7 detection rates with and without the highly sensitive assay. Next, we investigated how AR-V7 (detected without the highly sensitive assay) and hsAR-V7 status influenced prostate-specific antigen (PSA) response and long-term clinical outcomes (PSA progression-free survival [PFS] and radiological PFS) after ARS-inhibitor treatment. Finally, discriminatory abilities of the assays were assessed by C-index to compare their impact on long-term clinical outcomes.AR-V7 detection rates increased from 22% to 56% when the highly sensitive assay was used. The discriminatory abilities of hsAR-V7 for PSA PFS (C-index, 0.74; 95% confidence interval [CI], 0.60-0.88) and radiological PFS (0.70; 95% CI, 0.55-0.85) were higher than those of AR-V7 detected without the highly sensitive assay (0.60, 0.51-0.72, and 0.56, 0.44-0.67, respectively). After ARS-inhibitor treatment, PSA response was lower in hsAR-V7+ (53%) than in hsAR-V7- (93%) patients (p = 0.016). AR-V7+ patients had shorter median PSA PFS (3.0 vs 10.6 mo, p = 0.032) and nonsignificantly shorter median radiological PFS (6.0 vs 14.8 mo, p = 0.24) compared with AR-V7- patients. The hsAR-V7+ status was associated with shorter median PSA PFS (3.0 mo vs not reached, p = 0.0001) and radiological PFS (median, 6.0 mo vs not reached, p = 0.0026).The hsAR-V7 assay achieved the highest AR-V7 detection rates among those reported in mCRPC. Discriminatory abilities for long-term clinical outcomes were better with hsAR-V7 assay.We prospectively analyzed circulating tumor cells from men with metastatic castration-resistant prostate cancer for androgen receptor splice variant 7 (AR-V7) status using a highly sensitive assay. It yielded higher AR-V7 detection rates and predicted resistance to androgen receptor signaling inhibitors with better discriminatory abilities for long-term clinical outcomes.