Direct Co-Targeting of Bcl-xL and Mcl-1 Exhibits Synergistic Effects in AR-V7-Expressing CRPC Models.

There is an unmet need to develop novel treatment options for patients with metastatic castration-resistant prostate cancer (mCRPC). Patients often develop resistance to next-generation hormonal therapies that target the androgen receptor (AR) axis (e.g., abiraterone and enzalutamide). A splice variant of AR, AR-V7, is associated with resistance to these inhibitors as well as mCRPC progression and poor prognoses. We embarked upon a high-throughput screen to identify synergistic combinations of targeted therapies using two CRPC cell lines, LNCaP95 and VCaP-CR. Combinations targeting BCL2L1 (Bcl-xL) (A-1331852 and navitoclax) and MCL1 (S63845) synergistically decreased cell viability and induced apoptotic activity via cleavage of PARP, caspase 3, and caspase 7 across AR-V7-expressing CRPC cell lines (LNCaP95, VCaP-CR, and 22Rv1) and a patient-derived organoid model (LuCaP 167CR). We also explored the use of a Bcl-xL-specific proteolysis-targeting chimera degrader (PROTAC) to minimize platelet toxicity associated with Bcl-xL inhibitors. We showed similar synergistic efficacy with the Bcl-xL-targeting PROTAC in combination with S63845 in the three-dimensional spheroid models. Our findings support further preclinical development of Bcl-xL and Mcl-1 inhibitors for mCRPC. SIGNIFICANCE: Using an unbiased, combinatorial, high-throughput drug screen, we identified the combination of co-targeting Bcl-xL and Mcl-1 to be highly synergistic across AR-V7-expressing CRPC models. We showed efficacy in higher-order models through validation across in vitro models spanning two-dimensional cell culture, three-dimensional cell culture, and a patient-derived organoid model. These findings identify a promising therapeutic strategy for patients with AR-V7-expressing CRPC.