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Background: The intractability of castration-resistant prostate cancer (CRPC) is exacerbated by tumour heterogeneity, including diverse alterations to the androgen receptor (AR) axis and AR-independent phenotypes. The availability of additional models encompassing this heterogeneity would facilitate the identification of more effective therapies for CRPC., Objective: To discover therapeutic strategies by exploiting patient-derived models that exemplify the heterogeneity of CRPC., Design, Setting, and Participants: Four new patient-derived xenografts (PDXs) were established from independent metastases of two patients and characterised using integrative genomics. A panel of rationally selected drugs was tested using an innovative ex vivo PDX culture system., Intervention: The following drugs were evaluated: AR signalling inhibitors (enzalutamide and galeterone), a PARP inhibitor (talazoparib), a chemotherapeutic (cisplatin), a CDK4/6 inhibitor (ribociclib), bromodomain and extraterminal (BET) protein inhibitors (iBET151 and JQ1), and inhibitors of ribosome biogenesis/function (RNA polymerase I inhibitor CX-5461 and pan-PIM kinase inhibitor CX-6258)., Outcome Measurements and Statistical Analysis: Drug efficacy in ex vivo cultures of PDX tissues was evaluated using immunohistochemistry for Ki67 and cleaved caspase-3 levels. Candidate drugs were also tested for antitumour efficacy in vivo, with tumour volume being the primary endpoint. Two-tailed t tests were used to compare drug and control treatments., Results and Limitations: Integrative genomics revealed that the new PDXs exhibited heterogeneous mechanisms of resistance, including known and novel AR mutations, genomic structural rearrangements of the AR gene, and a neuroendocrine-like AR-null phenotype. Despite their heterogeneity, all models were sensitive to the combination of ribosome-targeting agents CX-5461 and CX-6258., Conclusions: This study demonstrates that ribosome-targeting drugs may be effective against diverse CRPC subtypes including AR-null disease, and highlights the potential of contemporary patient-derived models to prioritise treatment strategies for clinical translation., Patient Summary: Diverse types of therapy-resistant prostate cancers are sensitive to a new combination of drugs that inhibit protein synthesis pathways in cancer cells., (Copyright © 2018 European Association of Urology. Published by Elsevier B.V. All rights reserved.)

Current treatments for castration-resistant prostate cancer (CRPC) that target androgen receptor (AR) signaling improve patient survival, yet ultimately fail. Here, we provide novel insights into treatment response for the antiandrogen abiraterone by analyses of a genetically engineered mouse (GEM) model with combined inactivation of Trp53 and Pten , which are frequently comutated in human CRPC. These NPp53 mice fail to respond to abiraterone and display accelerated progression to tumors resembling treatment-related CRPC with neuroendocrine differentiation (CRPC-NE) in humans. Cross-species computational analyses identify master regulators of adverse response that are conserved with human CRPC-NE, including the neural differentiation factor SOX11 , which promotes neuroendocrine differentiation in cells derived from NPp53 tumors. Furthermore, abiraterone-treated NPp53 prostate tumors contain regions of focal and/or overt neuroendocrine differentiation, distinguished by their proliferative potential. Notably, lineage tracing in vivo provides definitive and quantitative evidence that focal and overt neuroendocrine regions arise by transdifferentiation of luminal adenocarcinoma cells. These findings underscore principal roles for TP53 and PTEN inactivation in abiraterone resistance and progression from adenocarcinoma to CRPC-NE by transdifferentiation. Significance: Understanding adverse treatment response and identifying patients likely to fail treatment represent fundamental clinical challenges. By integrating analyses of GEM models and human clinical data, we provide direct genetic evidence for transdifferentiation as a mechanism of drug resistance as well as for stratifying patients for treatment with antiandrogens. Cancer Discov; 7(7); 736-49. ©2017 AACR. See related commentary by Sinha and Nelson, p. 673 This article is highlighted in the In This Issue feature, p. 653 ., (©2017 American Association for Cancer Research.)