Abstract LB-A01: Establishment of a novel prostate cancer bone metastasis model in humanized mice and early efficacy results of pembrolizumab
Authors: Kähkönen Tiina E , Suominen Mari I , Mäki-Jouppila Jenni HE , Tanaka Azusa , Dube Philip , Halleen Jussi M , Bernoulli Jenni 
Published: March 2020

Abstract
Background: Programmed cell death 1 (PD-1) and programmed death ligand 1 (PD-L1) expression is typically low in primary prostate cancer but increases at advanced stages. However, targeting PD-1 or PD-L1 has not proven efficacy in prostate cancer patients, and recent clinical trials are directed to combination therapies. About 85% of advanced prostate cancer patients develop skeletal metastases, and the focus in this context should be addressed to the treatment of these metastases. These efforts have been hindered by the lack of relevant preclinical bone metastasis models in immunocompetent mice. We aimed to establish a prostate cancer bone metastasis model in humanized mice and to assess pembrolizumab efficacy in the established model. Materials and methods: Two million LNCaP human prostate cancer cells were inoculated into tibia bone marrow of male CIEA NOG® mice engrafted with human CD34+ hematopoietic stem cells to generate humanized mice. Serum prostate-specific antigen (PSA) levels were measured at 4 weeks, and the mice were allocated to receive either pembrolizumab (anti-PD-1, Keytruda®, 5 mg/kg, Q5D) or human IgG4 isotype control for 6 weeks. Tumor growth was monitored by measuring serum PSA levels. Tumor-induced bone changes were monitored by measuring serum levels of the bone formation marker N-terminal propeptide of type I procollagen (PINP), and by X-ray imaging of tibia. Changes in quantity of circulating T cells was monitored by flow cytometry. At study termination, tissue samples were collected for histological and immunohistochemical evaluation of tumor-infiltrating lymphocytes (TILs). Results: A tumor take of 90% was observed in the humanized mice evaluated by serum PSA levels at endpoint. No significant differences were observed in serum PSA levels, but in a few mice treated with pembrolizumab, only modest tumor growth was observed. Intratibial LNCaP tumors induced osteoblastic-mixed lesions as evaluated by X-ray imaging. No differences were observed in the bone lesion area or in serum PINP levels between the isotype control and pembrolizumab groups. No changes in circulating levels of CD4+, CD8+, or CD4+CD8+ cells were observed. Histology will be performed to confirm tumor morphology and overall tumor viability, and immunohistochemical analysis will reveal the quantity and location of TILs in the tumor. Conclusions: A novel preclinical model of bone metastasis in humanized mice was established. Intratibial prostate cancer tumors induced osteoblastic mixed bone lesions and increased serum PSA levels, mimicking the clinical situation in patients. Resembling recent clinical findings, no or modest responses with pembrolizumab as monotherapy were observed.
Citation Format: Tiina E Kähkönen, Mari I Suominen, Jenni HE Mäki-Jouppila, Azusa Tanaka, Philip Dube, Jussi M Halleen, Jenni Bernoulli. Establishment of a novel prostate cancer bone metastasis model in humanized mice and early efficacy results of pembrolizumab [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr LB-A01. doi:10.1158/1535-7163.TARG-19-LB-A01