Abstract 3951: Apalutamide reworks the immune composition of prostate tumors
Authors: Velasco Marco A. De , Kura Yurie , Ando Naomi , Sato Noriko , Nozawa Masahiro , Yoshimura Kazuhiro , Sakai Kazuko , Yoshikawa Kazuhiro , Nishio Kazuto , Uemura Hirotsugu 
Published: August 2019

Prostate cancers depend on androgen receptor (AR) signaling for survival making it a major therapeutic target. Apalutamide is an oral nonsteroidal AR antagonist that is currently indicated for the treatment of patients with non-metastatic castration-resistant prostate cancer. Apalutamide hinders AR-mediated transcriptional activity by impeding AR nuclear translocation and binding to DNA in cancer cells. AR is also expressed by stromal and immune cells and can modulate innate and adaptive immune responses, moreover, androgen ablation can both hinder and enhance antitumor immunity. Thus far, success from immune checkpoint blockade monotherapy has been lacking in prostate cancer patients and novel strategies using of other modalities are being investigated. Here, we characterize the effect of apalutamide therapy on antitumor immunity in a preclinical mouse model of prostate cancer to assess its value as potential partner for combination with immunotherapy. Conditional prostate-specific Pten-knockout mice were treated with apalutamide (30 mg/kg/d, 5 days on/2 days off) or vehicle for a period of 4 or 8 weeks to coincide with androgen sensitivity and the shift towards castration resistance. Tumor reductions after 4 or 8 weeks of treatment with apalutamide were 38.2% (P&lt;0.01) and 46.2% (P&lt;0.001), respectively. Flow cytometric analysis of tumor tissues revealed 1.3 (P=0.033) and 1.9 (P&lt;0.001) -fold increases of leukocyte (CD45+) infiltration in mice treated with apalutamide for 4 or 8 weeks, respectively. CD8+ T cell infiltration was 2.7-fold higher at 8 weeks, however, tumor reactive PD1+/CD8 T cells were 2-3-fold greater for apalutamide-treated mice at weeks 4 and 8. Quantitative immunohistochemical analysis confirmed increased intraepithelial CD8+ T and granzyme B-positive cells. Apalutamide treatment was also associated with increased peritumoral T regulatory cells and intraepithelial neutrophils. Immuno-profiling using a qRT-PCR-based panel of immune-relevant genes associated apalutamide therapy with an increased IFN-γ inflammatory signature, antigen presentation/dendritic cell, natural killer cell, T regulatory cell, tumor associated macrophage, and myeloid-derived suppressor cells (MDSC). These data also revealed a trend towards decreased CD8 T cell activation at week 8 accompanied by increased expression of the immunological checkpoints Ctla4, Tim3, and 2b4. In conclusion, our findings suggests that apalutamide-induced tumor cell death attracted phagocytes (macrophages and dendritic cells) that led to a innate and adaptive immune cell responses. While, cytotoxic T cell activity was improved, it was limited by the development of an immunosuppressive tumor microenvironment replete with MDSCs and T regulatory cells. Nevertheless, treatment with apalutamide turned immunologically “cold” tumors into more immunologically reactive tumors that may become more susceptible to targeted immunotherapy.

Citation Format: Marco A. De Velasco, Yurie Kura, Naomi Ando, Noriko Sato, Masahiro Nozawa, Kazuhiro Yoshimura, Kazuko Sakai, Kazuhiro Yoshikawa, Kazuto Nishio, Hirotsugu Uemura. Apalutamide reworks the immune composition of prostate tumors [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 3951.