Abstract 5038: Evaluation of anti-tumor immunity in response to [177Lu]Lu-PSMA in a mouse model of prostate cancer
Authors: Dias Gemma , Able Sarah , Skaripa-Koukelli Irini , Anderson Rachel , Wilson Gracie , Vallis Katherine A. 
Published: April 2023

Abstract
[177Lu]Lu-PSMA improves progression-free and overall survival in metastatic castration resistant prostate cancer. However, the role of immune stimulation by molecularly targeted radionuclide therapy (MRT) is poorly understood. The aim of this study was to evaluate the anti-tumor immune response induced by [177Lu]Lu-PSMA. The markers of immunogenic cell death (ICD), translocation of calreticulin (CRT) to the cell membrane, HMGB1 release and ATP release, were evaluated in the RM1 cell line (PSMA-negative murine prostate carcinoma) and its derivative, RM1-PGLS (stably transduced with human PSMA and SFG-EGFp/Luc), using flow cytometry, ELISA and a luminescence detection assay respectively. The in vivo biodistribution of [177Lu]Lu-PSMA in RM1-PGLS tumor-bearing C57BL/6 mice was investigated using microSPECT-CT imaging, and the in vivo efficacy of [177Lu]Lu-PSMA was tested in RM1-PGLS tumor growth inhibition studies. Vaccination/rechallenge experiments were performed to elucidate whether [177Lu]Lu-PSMA elicits ICD in vivo. The Nanostring Pan Cancer Immune Profiling Platform was used to assess changes in the tumor immune microenvironment (TIME) in response to [177Lu]Lu-PSMA. External radiation (EBRT; 137Cs γ-radiation; 6-10 Gy) was used as a comparator treatment in most experiments. We observed a dose-dependent increase in cell surface CRT following [177Lu]Lu-PSMA and EBRT at 24 and 72 h and of HMGB1 release in response to [177Lu]Lu-PSMA and EBRT at 96 and 144 h. At 24 h following [177Lu]Lu-PSMA (60 MBq) there was a significant increase in ATP release compared to untreated samples. Collectively these data indicate [177Lu]Lu-PSMA causes ICD. A high RM1-PGLS tumor:muscle uptake ratio (210 ± 59) was noted in biodistribution experiments confirming highly specific tumor accumulation of [177Lu]Lu-PSMA. In tumor growth inhibition studies the time for tumors to reach a volume of 400 mm3 was delayed in mice treated with [177Lu]Lu-PSMA (60 MBq) compared to no treatment (30 versus 21 days; P&lt;0.01). We showed that a subcutaneous inoculum of [177Lu]Lu-PSMA (60 MBq)-treated RM1-PGLS cells prevents tumor growth in approximately 50% of cases on rechallenge with viable RM1-PGLS cells one week later. This indicates that [177Lu]Lu-PSMA induces ICD in vivo. We observed a significant reduction in most tumor infiltrating lymphocyte (TIL) types at 2 days after in vivo exposure to [177Lu]Lu-PSMA (80 MBq), followed by an increase in TILs by Day 7. The observed pattern is similar to that after EBRT (6 Gy). There was overlap in the top 10 differentially expressed genes (treatment versus no treatment) for the [177Lu]Lu-PSMA and EBRT groups. The gene showing the greatest level of upregulation at day 7 in [177Lu]Lu-PSMA-treated tumors, IDO1, is known to be involved in immune tolerance and radioresistance. In conclusion, [177Lu]Lu-PSMA induces ICD in vitro and in vivo and modulates the TIME.
Citation Format: Gemma Dias, Sarah Able, Irini Skaripa-Koukelli, Rachel Anderson, Gracie Wilson, Katherine A. Vallis. Evaluation of anti-tumor immunity in response to [177Lu]Lu-PSMA in a mouse model of prostate cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5038.