Abstract 5540: Tumor phenotypic plasticity and resistance to immune-mediated cytotoxicity in models of castration resistant prostate cancer
Authors: Dahut Madeline , Fousek Kristen , Horn Lucas , Qin Haiyan , Schlom Jeffrey , Palena Claudia 
Published: June 2022

Abstract
Androgen receptor (AR)-targeted therapies are routinely used to treat prostate cancer, including the use of next-generation hormonal therapies such as abiraterone or enzalutamide. Although these therapies are initially effective, a significant proportion of patients exhibit resistance to treatment, therefore developing castration-resistant prostate cancer (CRPC). While immune checkpoint blockade therapies have led to remarkable clinical responses in patients with several tumor types, CRPC remains resistant to immunotherapy. The characterization of tumor resistance mechanisms has led to the identification of various tumor variants that could emerge along the progression of prostate cancer, including tumors undergoing phenotypic plasticity. Our laboratory has previously shown that phenotypic plasticity is a driver of resistance to immunotherapy. Here we investigated whether changes in tumor phenotype could affect the response of CRPC to immune-based therapies, and ways these mechanisms can be mitigated. Models of resistance to enzalutamide (LNCAP-EnzaR) or abiraterone (LNCAP-AbiR) were derived from the androgen sensitive LNCAP prostate cancer cell line. Resistant and parental LNCAP cells were comparatively evaluated for phenotypic features via RT-PCR, ELISA, western blot, immunofluorescence, and RNAseq analysis. Changes in the susceptibility to NK-cell mediated cytotoxicity were evaluated with NK cells isolated from peripheral blood from healthy donors. LNCAP-EnzaR and LNCAP parental cells were also grown in vivo in NSG MHCI/II-deficient mice, and tumors were characterized for phenotypic markers and potential therapeutic targets. Our results demonstrated that acquisition of resistance to both enzalutamide and abiraterone was associated with a significant increase in mesenchymal tumor features, including increased levels of vimentin and fibronectin, and the loss of epithelial E-cadherin. The susceptibility of LNCAP-Enza-R and LNCAP-AbiR cells to NK-cell mediated cytotoxicity and antibody-dependent cell cytotoxicity (ADCC) were significantly reduced (up to 90%), compared with parental cells. RNAseq analysis identified several upregulated gene and gene pathways, including estrogen receptor 1 (ESR1), which was particularly overexpressed in LNCAP-EnzaR cells. In a xenograft model of LNCAP-EnzaR cells, we corroborated the maintenance of tumor phenotypic plasticity and the expression of actionable targets. Our data indicates that acquisition of resistance to AR inhibition is associated with significant reduction of susceptibility to immune attack, and the acquisition of tumor phenotypic plasticity features. Future studies plan to investigate approaches that revert tumor plasticity, including blockade of increased markers such as ESR1, TGF-beta or IL-8, for potential improvement of tumor susceptibility to immune attack in CRPC.
Citation Format: Madeline Dahut, Kristen Fousek, Lucas Horn, Haiyan Qin, Jeffrey Schlom, Claudia Palena. Tumor phenotypic plasticity and resistance to immune-mediated cytotoxicity in models of castration resistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5540.