Introduction

Docetaxel or cabazitaxel-based chemotherapy continues to have a critical role in the treatment of men with metastatic castration-resistant prostate cancer (mCRPC). However, responses are heterogeneous and resistance to therapy is a pressing clinical problem. With the goal of developing liquid biomarkers to aid in treatment selection, we sought to identify genes associated with resistance to chemotherapy using a circulating tumor cell (CTC)-based approach.

Methods

Whole blood (~5mL) was obtained from 25 patients with mCRPC starting docetaxel (n=21) or cabazitaxel (n=4). CTCs were isolated using anti-EpCAM-conjugated magnetic beads, and following cell lysis, mRNA was extracted followed by multiplex qRT-PCR for 44 prostate cancer-related genes plus internal controls. Gene expression was normalized to controls, and samples were considered CTC-positive based on a previously established set of epithelial markers (EpCAM, EGFR, DSG2, KRT8, KRT18 and KRT19). The primary endpoint was PSA progression-free survival (PFS), with PSA progression defined as an increase of 25% or more above the nadir. Univariable Cox regression analyses were performed to assess for genes associated with PFS at false discovery rate (FDR) < 0.20.

Results

Among 25 patients with mCRPC, we identified 84% (21/25) with detectable CTCs. The median age of the cohort was 62 years (IQR 58-70). At a median (IQR) follow up of 5.4 (3.4-9.3) months, 47.6 % (10/21) of patients showed a PSA decrease of at least 30% following treatment initiation. 18/21 patients (85.7%) experienced PSA progression at a median of 2.8 months (IQR 1.7-4.8). In the Cox analysis, KLK2 (HR 2.54, 95%CI 1.24-5.21, p=0.011), GAS6 (HR 3.50, 95%CI 1.30-9.42, p=0.013), and BMP7 (HR 2.01, 95%CI 1.15-3.52; p=0.014) were associated with shorter PFS._x000D_ _x000D_

Conclusions

We have identified three genes associated with progression in mCRPC patients initiating chemotherapy. While these early results need further confirmation, they suggest that CTCs may be utilized to help guide precision-based treatment strategies in patients with mCRPC. Additionally, these results corroborate our recent in vitro and in vivo findings (Lee et al, J Cell Biochem, 2016) indicating that GAS6 protects prostate cancer cells from docetaxel-induced apoptosis.

Funding

This work was supported by the Prostate Cancer Foundation and Department of Defense.