Abstract 5343: Depletion of gut microbiota with broad spectrum antibiotics drives Pten-null prostate cancer growth in mice

Abstract Gut microbes have the potential to be either promoters or inhibitors of cancer. Use of antibiotics during cancer therapy can lead to dysbiosis, which may affect treatment outcomes. Here, we examine the effects of broad-spectrum antibiotics on cancer progression in mice with Pten-null castration resistant prostate cancer (CRPC). After five or eight weeks, orchiectomized conditional Pten-null knockout (KO) mice aged 50 and 90 weeks were given two biweekly cycles of a broad-spectrum antibody cocktail (BSAbx), respectively. The effects of BSAbx on tumor burden were compared, and molecular and immunological responses were analyzed by quantitative IHC (qIHC) and qRt-PCR at week four after antibiotic treatment. BSAbx treatment resulted in tumor burdens of 1.2 and 2.4-fold higher than in control mice in mice aged 50 and 90 weeks, respectively. qIHC analysis of KI67 showed increased proliferation in stromal and cancer cells. Additionally, androgen receptor (AR) protein and gene expression levels of Ar and Ar target genes (Fkbp5, Tmprss2, and Timp4) were elevated in tumors of BSAbx-treated mice. Restoring gut microbiota by fecal microbiota transplantation (FMT) from KO mice CRPC after BSAbx restored AR transcriptional activity to basal levels and lessened tumor growth. FMT from non-castrated KO with castration-naïve prostate cancer or heathy wildtype mice further reduced AR Transcriptional activity. Moreover, altering the gut microbiota of CRPC mice with FMT from heathy mice resulted in a tendency for lower tumor burden, 21% reduction versus control mice. Immunologically, BSAbx treatment resulted in decreased CD11b+/GR1+ cells in tumors and were associated with decreased interferon-gamma (IFN-gamma), dendritic cell functions and T cell-mediated cytotoxicity gene signatures. Mice that received FMT form health mice showed enhanced T cell mediated immunity as demonstrated by higher IFN-gamma signatures and tumor abundances of CD8+ T cells expression IFN-gamma, TNF-alpha and the cytotoxic granule granzyme B. Our study shows that broad spectrum antibiotics caused dysbiosis and promoted prostate cancer growth in mice with Pten-null prostate cancer. The effects were partly mediated through the upregulation of AR signaling and the impairment of neutrophil and T cell-mediated immune responses in tumors. Citation Format: Marco A. De Velasco, Yurie Kura, Kazuko Sakai, Mitsuhisa Nishimoto, Yasunori Mori, Kazuhiro Yoshimura, Kazutoshi Fujita, Kazuto Nishio, Hirotsugu Uemura. Depletion of gut microbiota with broad spectrum antibiotics drives Pten-null prostate cancer growth in mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5343.