AR-V7 and resistance to enzalutamide and abiraterone in prostate cancer

Abstract

Background: The androgen-receptor isoform encoded by splice variant 7 lacks the ligand-binding domain, which is the target of enzalutamide and abiraterone, but remains constitutively active as a transcription factor. We hypothesized that detection of androgen-receptor splice variant 7 messenger RNA (AR-V7) in circulating tumor cells from men with advanced prostate cancer would be associated with resistance to enzalutamide and abiraterone.

Methods: We used a quantitative reverse-transcriptase-polymerase-chain-reaction assay to evaluate AR-V7 in circulating tumor cells from prospectively enrolled patients with metastatic castration-resistant prostate cancer who were initiating treatment with either enzalutamide or abiraterone. We examined associations between AR-V7 status (positive vs. negative) and prostate-specific antigen (PSA) response rates (the primary end point), freedom from PSA progression (PSA progression-free survival), clinical or radiographic progression-free survival, and overall survival.

Results: A total of 31 enzalutamide-treated patients and 31 abiraterone-treated patients were enrolled, of whom 39% and 19%, respectively, had detectable AR-V7 in circulating tumor cells. Among men receiving enzalutamide, AR-V7-positive patients had lower PSA response rates than AR-V7-negative patients (0% vs. 53%, P=0.004) and shorter PSA progression-free survival (median, 1.4 months vs. 6.0 months; P<0.001), clinical or radiographic progression-free survival (median, 2.1 months vs. 6.1 months; P<0.001), and overall survival (median, 5.5 months vs. not reached; P=0.002). Similarly, among men receiving abiraterone, AR-V7-positive patients had lower PSA response rates than AR-V7-negative patients (0% vs. 68%, P=0.004) and shorter PSA progression-free survival (median, 1.3 months vs. not reached; P<0.001), clinical or radiographic progression-free survival (median, 2.3 months vs. not reached; P<0.001), and overall survival (median, 10.6 months vs. not reached, P=0.006). The association between AR-V7 detection and therapeutic resistance was maintained after adjustment for expression of full-length androgen receptor messenger RNA.

Conclusions: Detection of AR-V7 in circulating tumor cells from patients with castration-resistant prostate cancer may be associated with resistance to enzalutamide and abiraterone. These findings require large-scale prospective validation. (Funded by the Prostate Cancer Foundation and others.).

Figure 1. Transcript Levels of Full-Length Androgen Receptor mRNA and AR-V7 in Circulating Tumor Cells from Patients with Castration-Resistant Prostate Cancer

Absolute transcript copy numbers of full-length androgen receptor messenger RNA (mRNA) and androgen-receptor splice variant 7 mRNA (AR-V7) detected in circulating tumor cells are shown for the 6 abiraterone-treated patients (Panel A) and the 12 enzalutamide-treated patients (Panel B) who were positive for AR-V7 at baseline (i.e., in pretreatment samples of circulating tumor cells). Ratios of AR-V7 to full-length androgen receptor mRNA are expressed as percentages. Levels of full-length androgen receptor mRNA are also shown for AR-V7–negative samples. Eight abiraterone-treated patients who were negative for full-length androgen receptor mRNA (not shown) were also negative for AR-V7. One enzalutamide-treated patient was negative for both full-length androgen receptor mRNA and AR-V7 (not shown).

Figure 2. Waterifall Plots of Best Prostate-Specific Antigen (PSA) Responses According to AR-V7 Status

Panel A shows the 31 enzalutamide-treated patients, and Panel B the 31 abiraterone-treated patients. The dotted line shows the threshold for defining a PSA response (≥50% reduction in PSA level from baseline). Asterisks indicate an increase of more than 100% in best PSA response. Daggers indicate patients in the enzalutamide cohort who had previously received abiraterone and patients in the abiraterone cohort who had previously received enzalutamide.

Figure 3. Kaplan–Meier Analysis of PSA Progression–free Survival and Clinical or Radiographic Progression–free Survival According to AR-V7 Status

The median PSA progression–free survival in enzalutamide-treated patients (Panel A) was 1.4 months (95% CI, 0.9 to not reached) in AR-V7–positive patients and 6.0 months (95% CI, 3.8 to not reached) in AR-V7–negative patients (hazard ratio for PSA progression with AR-V7 positivity, 7.4; 95% CI, 2.7 to 20.6; P<0.001 by the log-rank test). The median PSA progression–free survival in abiraterone-treated patients (Panel B) was 1.3 months (95% CI, 0.9 to not reached) in AR-V7–positive patients and more than 5.3 months (95% CI, 5.3 to not reached) in AR-V7–negative patients (hazard ratio for PSA progression with AR-V7 positivity, 16.1; 95% CI, 3.9 to 66.0; P<0.001 by the log-rank test). The median clinical or radiographic progression–free survival in enzalutamide-treated patients (Panel C) was 2.1 months (95% CI, 2.0 to not reached) in AR-V7–positive patients and 6.1 months (95% CI, 4.7 to not reached) in AR-V7–negative patients (hazard ratio for clinical or radiographic progression with AR-V7 positivity, 8.5; 95% CI, 2.8 to 25.5; P<0.001 by the log-rank test). The median clinical or radiographic progression–free survival in abiraterone-treated patients (Panel D) was 2.3 months (95% CI, 1.4 to not reached) in AR-V7–positive patients and more than 6.3 months (95% CI, 6.3 to not reached) in AR-V7–negative patients (hazard ratio for clinical or radiographic progression with AR-V7 positivity, 16.5; 95% CI, 3.3 to 82.9; P<0.001 by the log-rank test).

Figure 4. Detection of Full-Length Androgen Receptor mRNA and AR-V7 in Metastatic Prostate-Cancer Tissue

In situ detection of full-length androgen receptor mRNA and AR-V7 in prostate-cancer tumor specimens was performed with the use of RNA in situ hybridization analysis. The tumor-tissue specimens shown are a radical-prostatectomy (RP) specimen that lacks AR-V7 expression from a patient (not enrolled in this study) who had not received hormonal treatment, an autopsy-derived specimen of a liver metastasis from a patient whose circulating tumor cells were shown to express AR-V7 (Autopsy), and core-needle biopsy specimens from a patient in whom AR-V7 was not detected (Biopsy 1) and a patient in whom AR-V7 was detected (Biopsy 2) in circulating tumor cells. All the tumor specimens show expression of full-length androgen receptor mRNA. The prostate-cancer cell lines that served as positive and negative controls for AR-V7 detection by means of RNA in situ hybridization are shown in Figure S6 in the Supplementary Appendix.