Aggressive Variant Prostate Cancers (AVPC) are lethal variants of the disease and usually occur in the setting of castration-resistant prostate cancer (CRPC). These neuroendocrine tumors are currently incurable, with most patients dying within 12-24 months of diagnosis. AVPCs are androgen-indifferent, and therefore do not respond to Androgen-Deprivation-Therapy (ADT), the mainstay treatment for prostate cancers. We recently discovered that these tumors are enriched with cancer cells with stem-cell properties (CSCs) that lack expression of androgen-receptor (AR) (PMID 26804168). Importantly, we observed that CSCs are generated when prostate cancer cells undergo a cell biological process called epithelial-to-mesenchymal-transition (or EMT). We also demonstrated that it is CSCs that facilitate neuroendocrine trans-differentiation, and promote drug-resistance via p38MAPK signaling. In this project, we aimed to inhibit EMT in patient-derived AVPC tumors (using SB203580, a p38MAPK inhibitor and anti-EMT drug), and identify clinically-relevant biological pathways upon EMT inhibition, using RNA-Seq analyses. We also aimed to develop a diagnostic CSC gene-expression score to identify/predict AVPC tumors. We tested the effect of SB203580 in two well-validated AR-negative PDX models of human AVPC (144-4 and 177-B). These models effectively recapitulate the heterogeneity and complex biology of AVPC. We first determined the appropriate dosing for SB203580 in these models that resulted in statistically significant target (p38MAPK) inhibition. Our data indicated that 10mg/kg SB significantly inhibits p38 activity (as judged by loss in MSK1 phosphorylation - MSK1 is a direct target of p38, and its phosphorylation is dependent on p38 activity). EMT inhibition also resulted in a significant reduction in the expression of SOX2, a known CSC marker for aggressive prostate cancers. In parallel, we observed a simultaneous up-regulation in expression of FOXA1, suggesting a shift to luminal epithelial phenotype upon EMT inhibition. We also developed a novel CSC gene expression score based on p38MAPK signaling components critical for EMT and neuroendocrine trans-differentiation in prostate cancer. We validated it in the Beltran dataset (PMID 26855148), wherein patients are categorized into CRPC-Adenocarcinoma and CRPC-Neuroendocrine type (aggressive tumors displaying extensive RB loss and increased expression of clinical neuroendocrine markers). We observed that our CSC score is highly represented in the CRPC-Neuro group, thus validating our rationale for anti-EMT therapy. This study thus highlights a potentially targetable signaling pathway for treatment of AVPC.

Acknowledgements: We thank Dr. Mahajan (Washington University School of Medicine), Dr. Wistuba, Ms. Mino, Dr. Lin (UTMDACC) and Dr. Tang (Rosewell Park Cancer Institute) for their contributions, & UTMDACC Prostate Cancer SPORE for financial support (RS, SAM).

Citation Format: Rama Soundararajan, Paul Allegakoen, Petra den Hollander, Anurag Paranjape, Robiya Joseph, Sandhya Sundaram, Devarajan Karunagaran, Peter Shepherd, Nora Navone, Ana Aparicio, Sankar Maity, Bradley Broom, Christopher Logothetis, Sendurai Mani. Targeting cancer stem-cells in aggressive variant prostate cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4674.