A Clear Increase In Tils And Modest Tumor Growth Inhibition By Pembrolizumab In Prostate Cancer Tumors Growing In Bone Of Cd34+Engrafted Nog Mice

Background The recent KEYNOTE-199 trial raises hope for new treatment options for prostate cancer patients with the encouraging results of checkpoint inhibitor activity in a subset of prostate cancer patients, also including patients with bone-predominant disease. However, the patient subset that benefited from the treatment was small, needing identification predictive biomarkers1. Proper preclinical models can help in the biomarker quest as well as in the search and selection of the best possible combination partners for further clinical trials. Methods In this study the bone-metastatic disease was modeled by intratibial inoculation of LNCaP human prostate cancer cells to male CIEA NOG® (NOG) mice and NOG mice engrafted with human CD34+ hematopoietic stem cells (huNOG, Taconic Biosciences). Tumor growth was followed by serum PSA measurements and tumor-induced bone changes by X-ray images. At study week 4, the PSA positive mice were stratified to two groups (n=10) treated with IgG4 isotype control or pembrolizumab (5 mg/kg, i.p., Q5D) until the end of the study. Tumor-induced bone changes were followed by X-ray 4, 8 and 10 weeks after inoculation. The study was terminated 10 weeks after inoculation and tumors were processed for histological and immunohistochemical (IHC) analysis of tumor infiltrating lymphocytes (TILs). Changes in blood cell counts were assessed by flow cytometry and hematology (n=5/group). Results At sacrifice, tumor-induced bone changes were observed in all mice, and there was no difference between the groups. Even though the PSA was not significantly lower in the pembrolizumab-treated group, the average histological tumorous surface was lower. In flow cytometry of peripheral blood, increases in the portions of CD3+ leukocytes and double positive CD4+CD8+ cells were observed, but no differences were found in CD4+ nor CD8+ T-cells. However, CD8+ T-cells were radically increased within the tumor as analyzed by IHC. Conclusions The model successfully mimicked the prevalent clinical situation, where clear responses in PSA or target lesions are not observed. However, a dramatic increase of cytotoxic T-cells in the tumor was observed, revealing the effects of pembrolizumab in a model of prostate cancer growth in bone of huNOG mice. The model presents a suitable platform for studying combination partners with pembrolizumab, that would boost or unlock the anti-tumor activity of the increased TILs. Ethics Approval This study was approved by the National Animal Experiment Board in Finland; license number ESAVI-2331-04 10 07-2017. Reference Antonarakis ES, Piulats JM, Gross-Goupil M, et al. Pembrolizumab for Treatment-Refractory Metastatic Castration-Resistant Prostate Cancer: Multicohort, Open-Label Phase II KEYNOTE-199 Study. J Clin Oncol 2020;38:395–405.