A phase II study evaluating the efficacy of enzalutamide and the role of liquid biopsy for evaluation of ARv7 in mCRPC patients with measurable metastases including visceral disease (Excalibur study).

Background: Up to 30% of patients with metastatic castration-resistant prostate cancer (mCRPC) develop visceral metastases, which are associated with a poor prognosis.

Objectives: Efficacy of enzalutamide in mCRPC patients with measurable metastases, including visceral and/or extra-regional lymph nodes.

Methods: In this phase II multicenter study, patients with mCRPC and measurable metastases received enzalutamide as the first line. Primary endpoint: 3-month (mo) disease control rate (DCR) defined as the proportion of patients with complete (CR) or partial response (PR) or stable disease (SD) as per Response Evaluation Criteria in Solid Tumors 1.1. Secondary endpoint: safety. Exploratory endpoint: the association between ARv7 splicing variants in basal circulating tumor cell (CTC)-enriched blood samples and treatment response/resistance using the AdnaTest ProstateCancerSelect kit and the AdnaTest ProstateCancer Panel AR-V7.

Results: From March 2017 to January 2021, 68 patients were enrolled. One patient never started treatment. Median age: 72 years. A total of 52 patients (78%) received enzalutamide as a first line for mCRPC. The median follow-up was 32 months. At the 3-month assessment, 24 patients presented an SD, 1 patient achieved a CR, and 23 patients had a PR (3-mo-DCR of 72%). Discontinuations due to adverse events (AEs), disease-related death, or disease progression occurred in 9%, 6%, and 48% of patients. All patients reported at least one grade (G) 1-2 AE: the most common were fatigue (49%) and hypertension (33%). Six G3 AEs were reported: two hypertension, one seizure, one fatigue, one diarrhea, and one headache. Basal detection of ARv7 was significantly associated with poor treatment response (p = 0.034) and a nonsignificant association (p = 0.15) was observed between ARv7 detection and response assessments. At month 3, ARv7 was detected in 57%, 25%, and 15% of patients undergoing progressive disease, SD, and PR, respectively.

Conclusion: The study met its primary endpoint, showing the efficacy of enzalutamide in men with mCRPC and measurable metastatic lesions in visceral and/or lymph node sites.

Trial Registration: ClinicalTrials.gov Identifier: NCT03103724. First Posted: 6 April 2017. First patient enrollment: 19 April 2017.