Cabazitaxel in mCRPC Patients With AR-V7 Positive Circulating Tumor Cells (CTCs)

After failure on docetaxel, which has been the standard first line therapy for patients with metastatic castration-resistant prostate cancer (mCRPC), several treatment options are currently available. Two of the treatment options are directed against the androgen receptor (AR), enzalutamide and abiraterone. A third option is cabazitaxel, a next generation taxane. No head-to-head comparisons have been done for these three therapies in second-line mCRPC and as of yet, the optimal choice is unknown. Resistance to the AR-targeted therapies is at least in part a consequence of signaling through constitutively active AR splice variants (AR-Vs). Because AR splice variants only occur after conversion to a castration-resistant tumor, and can be acquired during systemic therapy for mCRPC, analysis of the castration-naïve primary tumor is not informative in the setting of second-line treatment of mCRPC. Circulating tumor cells (CTCs) can be analyzed repetitively and in real-time. Recently, AR-V7 messenger ribonucleid acid (mRNA) expression in CTCs was shown to be associated with lack of response to AR-targeted therapy (reference 1). AR-V7 mRNA expression does not seem to hinder response to cabazitaxel in our retrospective pilot study (reference 2) nor in two recently published retrospective studies (reference 3 and reference 4). Therefore we hypothesize that the mRNA expression of AR-V7 in CTCs assessed before start of second-line treatment for mCRPC does not affect prostate-specific antigen (PSA) response to cabazitaxel in patients who have progressed to docetaxel. Patients who are eligible to undergo second or third line treatment will be asked to undergo prescreening consisting of a CTC count and, in case ≥3 CTCs are detected, AR V7 determination. Patients with ≥3 CTCs with AR-V7 expression will be asked to sign a second informed consent to enter the treatment study. In this study they will receive Cabazitaxel 25 mg/m² every 3 weeks plus prednisone 10 mg daily, and undergo repeated blood sampling for biomarker sample collection. During the prescreening in all patients, 2 x 10 mL blood will be drawn for enumeration and isolation of CTCs. All patients with ≥3 CTCs with AR-V7 expression will be asked to sign consent for the treatment study. All patients included in the treatment study will be administrated cabazitaxel intravenously at a dose of 25 mg/m², during a one-hour infusion every 3 weeks, as well as continuous treatment with prednisone 5 mg orally twice daily, or 10 mg once daily. In the treatment study patients, an additional 2 x 10 mL blood will be drawn after the third cycle of treatment for CTC enumeration and isolation. An additional 10 mL blood will be drawn for storage of plasma at baseline and before start of every cycle (i.e., every 3 weeks) for analysis of cell-free DNA (cfDNA). Moreover, 4 x 5 mL blood (baseline; end of infusion, 2 and 6 hours after end of the first cabazitaxel infusion) will be drawn for pharmacokinetic studies, in order to explore a cabazitaxel exposure effect relation.