Abstract 3342: Galiellalactone derivative targets stem cell population in ENZ-resistant prostate cancer through inhibition of STAT3
Authors: Vahid Sepideh , Thaper Daksh , Davies Alistair , Van Rensburg Micaela Janse , Gibson Kate , Ketola Krisi , Johansson Martin , Bishop Jennifer L. , Zoubeidi Amina 
Published: July 2016

Background: Androgen ablation remains the most effective therapy for patients with advanced disease. Unfortunately, most patients progress to castrate resistant prostate cancer (CRPC) characterized with hyper-activation of the androgen receptor (AR). Enzalutamide, a potent AR inhibitor showed efficacy by prolonging survival in CRPC patients. However, ENZ resistance (ENZR) rapidly occurs in patients and in our pre-clinical model targeting AR in ENZ resistant tumors with a 3rd generation AR inhibitor is short lived. The role of signal transducer and activator of transcription (STAT) 3 in the progression of prostate cancer is well established and has been repeatedly linked with maintenance of a stem cell phenotype across cancers. Additionally, drug resistance has been hypothesized to occur via enrichment of cancer stem-like cells (CSC), a phenotype associated with poor survival in patients. Therefore, we propose a shift of focus to target CSC phenotype using small molecule inhibitor of STAT3 called GPA500, instead of the AR axis to deal with ENZ resistance.

Methods and Results: We developed a unique model of ENZ-resistance and found that cell lines derived from serially passaged ENZR tumors displayed broad genetic diversity and differential AR activity. Notably, the cell lines 42DENZR and 42FENZR are PSAlow, harbor an expanded CSC population and have STAT3 hyper-activation compared to CRPC controls measured by STAT3-luc reporter. Accordingly, Crystal Violet and MTT proliferation assays showed that 42DENZR and 42FENZR cell lines were more sensitive to the STAT3 inhibitor GPA500 compared to CRPC control. Targeting the ENZ-R cells with GPA500 reduced mRNA levels of CD133, CD44, SOX2, OCT4 and Nanog. The reduction of these CSC markers was accompanied by reduced self-renewal capacity measured by spheroid assay. Cytometry analysis revealed that treatment with GPA500 reduced α2β1+, CD44+ and CD133+ (CSC) population in the 42D and 42F cells.

Conclusion: In this study, we provide pre-clinical proof that targeting the STAT3 pathway using GPA500 in ENZ resistance as well as CRPC reduces cell proliferation as well as expression of stem cell markers.

Impact: Exploring mechanisms of ENZR resistance serves a critical unmet need in PCa oncology, as the number of men with ENZ resistant CRPC continues to rise. Targeting STAT3 with the small molecule inhibitor GPA500 may provide an effective method to treat ENZR patients or delay the emergence of ENZ resistance in CRPC by reducing the emergence of stem cells.

Citation Format: Sepideh Vahid, Daksh Thaper, Alistair Davies, Micaela Janse Van Rensburg, Kate Gibson, Krisi Ketola, Martin Johansson, Jennifer L. Bishop, Amina Zoubeidi. Galiellalactone derivative targets stem cell population in ENZ-resistant prostate cancer through inhibition of STAT3. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3342.