Molecular characterization of enzalutamide-treated bone metastatic castration-resistant prostate cancer

Abstract

Background: Enzalutamide is a novel antiandrogen with proven efficacy in metastatic castration-resistant prostate cancer (mCRPC).

Objective: To evaluate enzalutamide's effects on cancer and on androgens in blood and bone marrow, and associate these with clinical observations.

Design, setting, and participants: In this prospective phase 2 study, 60 patients with bone mCRPC received enzalutamide 160mg orally daily and had transilial bone marrow biopsies before treatment and at 8 wk of treatment.

Outcome measurements and statistical analysis: Androgen signaling components (androgen receptor [AR], AR splice variant 7 (ARV7), v-ets avian erythroblastosis virus E26 oncogene homolog [ERG], cytochrome P450, family 17, subfamily A, polypeptide 1 [CYP17]) and molecules implicated in mCRPC progression (phospho-Met, phospho-Src, glucocorticoid receptor, Ki67) were assessed by immunohistochemistry; testosterone, cortisol, and androstenedione concentrations were assessed by liquid chromatography-tandem mass spectrometry; AR copy number was assessed by real-time polymerase chain reaction. Descriptive statistics were applied.

Results and limitations: Median time to treatment discontinuation was 22 wk (95% confidence interval, 19.9-29.6). Twenty-two (37%) patients exhibited primary resistance to enzalutamide, discontinuing treatment within 4 mo. Maximal prostate-specific antigen (PSA) decline ≥ 50% and ≥ 90% occurred in 27 (45%) and 13 (22%) patients, respectively. Following 8 wk of treatment, bone marrow and circulating testosterone levels increased. Pretreatment tumor nuclear AR overexpression (> 75%) and CYP17 (> 10%) expression were associated with benefit (p = 0.018). AR subcellular localization shift from the nucleus was confirmed in eight paired samples (with PSA decline) of 23 evaluable paired samples. Presence of an ARV7 variant was associated with primary resistance to enzalutamide (p = 0.018). Limited patient numbers warrant further validation.

Conclusions: The observed subcellular shift of AR from the nucleus and increased testosterone concentration provide the first evidence in humans that enzalutamide suppresses AR signaling while inducing an adaptive feedback. Persistent androgen signaling in mCRPC was predictive of benefit and ARV7 was associated with primary resistance.

Patient summary: We report a first bone biopsy study in metastatic prostate cancer in humans that searched for predictors of outcome of enzalutamide therapy. Benefit is linked to a pretreatment androgen-signaling signature.

Trial registration: ClinicalTrials.gov identifier NCT01091103.

Keywords: Adaptive feedback mechanism; Androgen receptor; Androgen signaling inhibition; Bone metastasis; Bone tumor microenvironment; Castration-resistant prostate cancer; Enzalutamide; Predictors of outcome; Primary resistance to enzalutamide; Tissue-based research.

Fig. 1

Proportion of patients progressing to treatment discontinuation over time.

Fig. 2

Androgen receptor subcellular localization at pretreatment and following 8 wk of treatment in four patients (paired specimens).

Fig. 3

(a, b) Nuclear ARV7 expression in bone marrow-infiltrating tumor cells, with corresponding hematoxylin and eosin (H&E) staining, primarily resistant to enzalutamide versus (c, d) absence of ARV7 expression in bone marrow–infiltrating tumor cells, with corresponding H&E staining, sensitive to enzalutamide treatment.

Fig. 4

Changes in blood and bone marrow aspirate (BMA) following 8 wk of enzalutamide treatment: (a) blood cortisol (n = 48); (b) BMA cortisol (n = 44); (c) blood androstenedione (n = 51); (d) BMA androstenedione (n = 43); (e) blood testosterone (n = 51), increase observed in 40 of 51 samples (78%); (f) BMA testosterone (n = 44), increase observed in 34 of 44 (77%) paired samples.