Developing a Fluorescence-Based High-Throughput Screening Method for Natural Product Fractions to Identify Inhibitors of AR-V7 for Castration-Resistant Prostate Cancer.

Background/

Objectives

The progression of prostate cancer to castration-resistant prostate cancer (CRPC) is often driven by constitutively active androgen receptor splice variants, such as AR-V7, which evade conventional androgen-deprivation therapies. This study aimed to develop a high-throughput, mechanism-based screening platform to identify natural-product-derived AR-V7 inhibitors from the NCI Program for Natural Product Discovery (NPNPD) library.

Methods

A subset of prefractionated NPNPD samples was screened using a CRISPR-edited 22Rv1 cell line expressing endogenous AR-V7 fused to a HiBiT luminescent tag, enabling quantification of AR-V7 levels. Fractions that reduced the HiBiT signal were further evaluated in 22Rv1 and C4-2B CRPC cells and counter-screened in non-malignant RWPE-1 cells. Compounds demonstrating at least 90% inhibition in CRPC cells with no more than 10% toxicity in RWPE-1 cells underwent dose-response analysis, Western blotting, quantitative PCR, and subfractionation to isolate and characterize active constituents.

Results

Of the 704 prefractionated samples, five fractions met stringent activity and selectivity criteria, with three consistently suppressing full-length androgen receptor (AR), AR-V7, and prostate-specific antigen (PSA). Primary screening results in HiBiT-22Rv1 cells were validated by secondary dose-response and Western blot assays, and hits were further prioritized to ensure toxicity remained at or below 10% in RWPE-1 cells. HiBiT-guided screening of 66 subfractions identified seven that robustly downregulated AR signaling, with reduced AR-V7 levels correlating with decreased CRPC cell viability. Chemical characterization revealed two active butanolides, isolitsealiicolide C and isolinderanolide B, which reduced AR and AR-V7 protein and mRNA levels, decreased PSA, downregulated Bcl-2, and induced cleaved PARP, consistent with apoptotic cell death in AR-positive CRPC models.

Conclusions

This integrated high-throughput workflow efficiently identifies AR-V7-targeted natural products from complex libraries and highlights isolitsealiicolide C and isolinderanolide B as promising scaffolds for overcoming androgen receptor-driven resistance in CRPC.