Abstract 5353: PSMA-targeted thorium conjugate and darolutamide combination show synergistic anti-tumor activity and reduced expression of DNA damage repair genes in prostate cancer models
Authors: Baumgart Simon J. , Lesche Ralf , Schlicker Andreas , Baumgart Urs , Haendler Bernard , Lejeune Pascale , Scholz Arne , Mumberg Dominik , Hammer Stefanie , Schatz Christoph A. 
Published: September 2020

Abstract
Androgen receptor (AR) inhibitors are standard of care for the treatment of prostate cancer. Despite an initial response to treatment, patients eventually progress and novel therapeutic approaches are required. Prostate-specific membrane antigen (PSMA; FOLH1) is an integral membrane glycoprotein that is highly expressed in prostate cancer but has limited expression in normal tissues. PSMA-TTC consists of the alpha emitter thorium-227 complexed to a 3,2-HOPO chelator conjugated to a PSMA targeting antibody delivering radiation dose to PSMA expressing tumor cells. Here we evaluated the efficacy of PSMA-TTC in combination with darolutamide, a novel AR inhibitor and explored the mode of action of the combination treatment by transcriptome-wide analysis followed by gene set enrichment analysis (GSEA) using HALLMARK gene sets. In vitro, darolutamide induced the surface expression of PSMA in the androgen-sensitive cell lines VCaP and LNCaP more than 10-fold and 3-fold, respectively. Synergy of darolutamide plus PSMA-TTC treatment was seen in proliferation assays in VCaP and LNCaP cells with a combination index of 0.12 and 0.51, respectively. Expression of the apoptosis marker CDKN1A was significantly induced in the combination group compared to vehicle and single-agent groups in VCaP cells. In vivo, a single i.v. injection of PSMA-TTC at 250 kBq/kg resulted in a tumor/control (T/C) ratio of 0.37 on day 19 in the hormone-sensitive patient-derived prostate cancer model ST1273 while a daily oral treatment with 100 mg/kg darolutamide showed a T/C ratio of 0.21. The combination of darolutamide and PSMA-TTC resulted in a much higher efficacy with a T/C ratio of 0.03 and nine out of 10 mice remaining tumor-free 100 days after PSMA-TTC injection. We performed mRNA sequencing of tumors after treatment. Gene set enrichment analysis showed strongest activation for the hallmark TP53 and apoptosis pathways in both PSMA-TTC treatment groups. RNA expression of the DNA damage response (DDR) genes was significantly increased in the PSMA-TTC group compared to the control but remained unchanged in the PSMA-TTC plus darolutamide combination group. To investigate whether the lack of DDR gene expression in the combination group resulted in a higher amount of DNA damage, double strand breaks were investigated by immunohistochemistry. DNA damage indicated by γH2AX signal on histological slides was significantly higher in the combination group compared to PSMA-TTC alone. Synergistic in vivo effects were confirmed in a second subcutaneous in vivo model using VCaP cells. The combination of PSMA-TTC plus darolutamide resulted in significantly lower final tumor weights compared to all other treatment groups. These data indicate that darolutamide induces PSMA expression and impairs DDR genes expression, which may have contributed to increased DNA damage and stronger efficacy when combined with PSMA-TTC.
Citation Format: Simon J. Baumgart, Ralf Lesche, Andreas Schlicker, Urs Baumgart, Bernard Haendler, Pascale Lejeune, Arne Scholz, Dominik Mumberg, Stefanie Hammer, Christoph A. Schatz. PSMA-targeted thorium conjugate and darolutamide combination show synergistic anti-tumor activity and reduced expression of DNA damage repair genes in prostate cancer models [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5353.