Purpose Progression of prostate cancer (PC) to the lethal castrate-resistant (CR) stage coincides with loss of responsiveness to androgen deprivation and requires development of novel therapies. We previously provided proof-of-concept that Stat5a/b is a therapeutic target protein for PC. Here we demonstrate that pharmacological targeting of Jak2-dependent Stat5a/b signaling by the Jak2 inhibitor AZD1480 blocks CR growth of PC. Experimental Design Efficacy of AZD1480 in disrupting Jak2-Stat5a/b signaling and decreasing PC cell viability was evaluated in PC cells. A unique PC xenograft mouse model (CWR22Pc), which mimics PC clinical progression in patients, was used to assess in vivo responsiveness of primary and CR PC to AZD1480. Patient-derived clinical PCs, grown ex vivo in organ explant cultures, were tested for responsiveness to AZD1480. Results AZD1480 robustly inhibited Stat5a/b phosphorylation, dimerization, nuclear translocation, DNA binding and transcriptional activity in PC cells. AZD1480 reduced PC cell viability sustained by Jak2-Stat5a/b signaling through induction of apoptosis, which was rescued by constitutively active Stat5a/b. In mice, pharmacological targeting of Stat5a/b by AZD1480 potently blocked growth of primary androgen-dependent as well as recurrent CR CWR22Pc xenograft tumors, and prolonged survival of tumor-bearing mice vs. vehicle or docetaxel-treated mice. Finally, 9 of 13 clinical PCs responded to AZD1480 by extensive apoptotic epithelial cell loss, concurrent with reduced levels of nuclear Stat5a/b. Conclusions We report the first evidence for efficacy of pharmacological targeting of Stat5a/b as a strategy to inhibit CR growth of PC, supporting further clinical development of Stat5a/b inhibitors as therapy for advanced PC.

Active Stat5a/b predicts early recurrence and disease-specific death in prostate cancer (PC), which both typically are caused by development of metastatic disease. Herein, we demonstrate that Stat5a/b induces epithelial-to-mesenchymal transition (EMT) of PC cells, as shown by Stat5a/b regulation of EMT marker expression (Twist1, E-cadherin, N-cadherin, vimentin, and fibronectin) in PC cell lines, xenograft tumors in vivo, and patient-derived PCs ex vivo using organ explant cultures. Jak2-Stat5a/b signaling induced functional end points of EMT as well, indicated by disruption of epithelial cell monolayers and increased migration and adhesion of PC cells to fibronectin. Knockdown of Twist1 suppressed Jak2-Stat5a/beinduced EMT properties of PC cells, which were rescued by re-introduction of Twist1, indicating that Twist1 mediates Stat5a/b-induced EMT in PC cells. While promoting EMT, Jak2-Stat5a/b signaling induced stemlike properties in PC cells, such as sphere formation and expression of cancer stem cell markers, including BMI1. Mechanistically, both Twist1 and BMI1 were critical for Stat5a/b induction of stem-like features, because genetic knockdown of Twist1 suppressed Stat5a/b-induced BMI1 expression and sphere formation in stem cell culture conditions, which were rescued by re-introduction of BMI1. By using human prolactin knock-in mice, we demonstrate that prolactin-Stat5a/b signaling promoted metastases formation of PC cells in vivo. In conclusion, our data support the concept that Jak2-Stat5a/b signaling promotes metastatic progression of PC by inducing EMT and stem cell properties in PC cells. Most prostate cancer (PC)erelated deaths are because of development of metastatic disease. A central process in metastatic dissemination of PC is epithelial-to-mesenchymal transition (EMT), during which cancer cells attain more motile and invasive properties, invade through the basement membrane, and survive in systemic circulation. 1e3 Extravasation at distant organ sites is followed by adhesion of cancer cells to extracellular matrix proteins, such as fibronectin, 4e6 leading to formation of premetastatic niches and subsequent formation of macroscopic metastases. 7e9 Hallmarks of EMT in PC include disruption of adherens junctions through down-regulation of E-cadherin, 2 concomitant with a development of a migratory phenotype and up-regulation of mesenchymal markers, such as N-cadherin, vimentin, and fibronectin. 10,11 Loss of E-cadherin results from mutations, DNA methylation, or silencing of E-cadherin promoter

Bypassing tyrosine kinases responsible for Stat5a/b phosphorylation would be advantageous for therapy development for Stat5a/b-regulated cancers. Here, we sought to identify small-molecule inhibitors of Stat5a/b for lead optimization and therapy development for prostate cancer (PC) and Bcr-Abl-driven leukemias. In silico screening of chemical structure databases combined with medicinal chemistry was used for identification of a panel of small-molecule inhibitors to block SH2-domain-mediated docking of Stat5a/b to the receptor-kinase complex and subsequent phosphorylation and dimerization. We tested the efficacy of the lead-compound, IST5-002, in experimental models and patient samples of two known Stat5a/b-driven cancers, prostate cancer (PC) and chronic myeloid leukemia (CML). The lead compound Inhibitor of Stat5-002 (IST5-002) prevented both Jak2 and Bcr-Abl-mediated phosphorylation and dimerization of Stat5a/b, and selectively inhibited transcriptional activity of Stat5a (IC50 1.5 μM) and Stat5b (IC50 3.5 μM). IST5-002 suppressed nuclear translocation of Stat5a/b, binding to DNA and Stat5a/b target gene expression. IST5-002 induced extensive apoptosis of PC cells, impaired growth of PC xenograft tumors and induced cell death in patient-derived PCs when tested ex vivo in explant organ cultures. Importantly, IST5-002 induced robust apoptotic death not only of imatinib-sensitive but also imatinib-resistant chronic myeloid leukemia (CML) cell lines and primary CML cells from patients. IST5-002 provides a lead structure for further chemical modifications for clinical development for Stat5a/b-driven solid tumors and hematological malignancies.