Androgen Receptor Burden and Poor Response to Abiraterone or Enzalutamide in TP53 Wild-Type Metastatic Castration-Resistant Prostate Cancer

Abstract

This post hoc analysis of a multi-institution cohort study assesses the association between androgen receptor perturbation and poorer outcome in patients with TP53 wild-type metastatic castration-resistant prostate cancer who are receiving abiraterone or enzalutamide.

Androgen receptor (AR) molecular perturbations in liquid biopsy specimens (circulating tumor cells [CTCs] and circulating tumor DNA [ctDNA]) from patients with metastatic castration-resistant prostate cancer (mCRPC) are associated with poorer outcomes on the AR signaling inhibitors (ARSis) abiraterone or enzalutamide, with contradictory claims regarding the clinical use of AR splice variant 7 (AR-V7) or AR amplification status.^1,2,3,4 By using comprehensive liquid biopsy AR profiling,⁵ we previously demonstrated how AR perturbations lost prognostic value when correcting for tumor burden estimates and clinical variables.⁶ Consistent with others’ findings,⁴ we observed that AR-driven biomarkers were outperformed by TP53 alterations, noting that 71.3% and 97.9% of patients carried relevant AR-driven biomarkers at baseline and progression, respectively.⁶ We therefore speculate that long-term chemical castration, alongside the cancer treatment trajectory, will eventually lead to perturbed AR biomarker output in all patients. In this scenario, multiple biomarker assessments of AR are necessary to identify ARSi-sensitive patients.

Methods

After obtaining approval from the Antwerp University Hospital institutional review board and written patient informed consent, we performed simultaneous profiling of AR splice variant (ARV) expression by targeted RNA sequencing of CellSearch-enriched CTCs and plasma ctDNA profiling via low-pass whole genome sequencing and targeted AR sequencing to infer genomic alterations in patients with mCRPC.⁶ Herein we report a post hoc analysis of the prognostic value of the number of ARSi outcome-associated AR perturbations at baseline with progression-free survival (PFS) in patients with TP53 wild-type mCRPC (n = 109 of 145 evaluable patients) after a median follow-up of 13 months.⁶ The summation of outcome-associated ARVs (AR45, AR-V3, AR-V4, AR-V5, and AR-V7) and AR gene alterations (amplifications and intragenic rearrangements) were correlated with PFS defined according to the Prostate Cancer Clinical Trials Working Group 3 criteria, as described previously.⁶

Results

Comprehensive AR profiles were available for 80 (73.4%) of 109 patients (mean [SD] age at registration, 75.09 [8.2] years) (Table). We detected outcome-associated AR perturbations in 49 (61.3%) of 80 patients at baseline, with 16 (32.7%) of 49, 17 (34.7%) of 49, and 16 (32.7%) of 49 patients having 1, 2, and 3 or more significant events (ie, AR perturbations with a significant association to PFS in univariable survival analysis), respectively. Patients without any outcome-associated AR perturbation had the longest median PFS compared with patients with AR perturbation, who demonstrated a decremental PFS as the number of AR perturbations (0, 1, 2, and ≥3) accumulated (median, 13.7, 10.1, 6.1, and 2.8 months, respectively) (P < .001) (Figure, A). The decremental PFS was present in subanalyses of 57 patients treated with abiraterone who had 0, 1, 2, and ≥3 AR perturbations (median PFS, 13.7, 10.1, 5.3, and 2.3 months, respectively) (log-rank test, P < .001) and 23 patients treated with enzalutamide (median PFS, 13.8, 9.2, 9.6, and 3.4 months, respectively) (log-rank test, P = .02). In the multivariable Cox regression analysis incorporating clinical and molecular characteristics, a perturbed AR status was associated with inferior outcome and had independent prognostic value when 3 or more AR perturbations were detected (hazard ratio, 2.97; 95% CI, 1.16-7.65; P = .02) (Figure, B).

Table. Characteristics and Baseline Blood Chemistry in Patients With TP53 Wild-Type Metastatic Castration-Resistant Prostate Cancer.

Characteristic	Patients, No. (%)a (n = 80)
Age at registration, mean (SD), y	75.09 (8.2)
Tumor stage at diagnosis
T1/2	15 (19)
T3/4	20 (25)
M1	26 (32)
Node positive	7 (9)
Not specified	12 (15)
Gleason score at diagnosis
≤7	28 (35)
8-10	41 (51)
Not specified	11 (14)
Primary treatment
ADT (with or without RT)	38 (48)
Radical Px (with or without RT)	29 (36)
Radical Px with ADT	4 (5)
Other	9 (11)
Previous chemotherapy
Naïve	49 (61)
Pretreated	31 (39)
Previous ARSi for CRPC
No	68 (85)
Yes	12 (15)
Initiating therapy
Abiraterone acetate	57 (71)
Enzalutamide	23 (29)
Metastatic burden at start of therapy
Lymph node only	13 (16)
Bone only	28 (35)
Bone and lymph node	29 (36)
Visceral and bone and/or lymph node	8 (10)
Not specified	2 (3)
Baseline blood chemistry, median (IQR)
Lactate dehydrogenase, U/L (n = 57)	303 (222-632)
Alkaline phosphatase, U/L (n = 61)	99 (69-149)
Prostate-specific antigen, μg/L (n = 79)	29.7 (12.3-96.6)
Baseline circulating tumor burden, median (IQR)
CTC, No./7.5 mL (n = 79)	1 (0-10)
ctDNA, % (n = 80)	6 (0-18.5)

Abbreviations: ADT, androgen deprivation therapy; ARSi, androgen receptor signaling inhibitor; CRPC, castration-resistant prostate cancer; CTC, circulating tumor cells; ctDNA, circulating tumor DNA; IQR, interquartile range; Px, prostatectomy; RT, radiotherapy.

SI conversion factor: To convert lactate dehydrogenase and alkaline phosphatase to μkat/L, multiply by 0.0167.

^aUnless otherwise indicated, n = 80.

Figure. Androgen Receptor Burden in Baseline Liquid Biopsy Specimens From Patients With TP53 Wild-Type Metastatic Castration-Resistant Prostate Cancer and Progression-Free Survival on Androgen Receptor Signaling Inhibitors.

A, Kaplan-Meier analysis of progression-free survival in 80 patients, stratified according to the number of androgen receptor (AR) perturbations at baseline. The P value was calculated via log-rank test. The black dashed lines represent the median progression-free survival time. B, Multivariable Cox regression analysis (hazard ratio and 95% CI) of progression-free survival using baseline characteristics and the number of AR perturbations. The P value was calculated using the Wald test of Z statistic. ARSi indicates AR signaling inhibitor; CT, chemotherapy; ctDNA, circulating tumor DNA; PFS, progression-free survival; PSA, prostate-specific antigen.

^aOne observation was deleted due to missing baseline PSA measurement.

^bVariables were log transformed.

Discussion

The number of AR perturbations remains independently associated with poor prognosis in TP53 wild-type disease, thereby demonstrating the prognostic value of comprehensive AR profiling. However, this study has limitations. First, this post hoc hypothesis-generating analysis warrants independent prospective validation, which will occur in our recently initiated randomized clinical trial ProBio (EudraCT number: 2018-002350-78). Second, our study was not designed to assess quantitative biomarker output such as absolute ARV expression levels or AR copy number, which likely harbors prognostic information in the context of varying tumor burden estimates. Finally, we did not include prognostic variables outside of our standard practice such as performance status, alkaline phosphatase, and lactate dehydrogenase. To our knowledge, the data represent the first demonstration of the clinical validity of “AR burden” contributed to by multiple AR gene body alterations and/or ARV expression occurring during progression of mCRPC under treatment selection pressure. Biomarker output from the AR locus appears to be a ubiquitous property of mCRPC, thereby calling into question the current single AR biomarker dogma for the prognostication of outcome in patients initiating ARSi therapy. We anticipate patient evaluation with a comprehensive AR profile; continued AR blockade could be recommended for those with a sufficiently low AR burden in the context of other prognostic factors and the specific AR alterations present.