Abstract 570: Preclinical insights into 161Tb-trastuzumab and immune checkpoint combination therapy for cancer in a HER2-expressing murine cancer model

Abstract Introduction: Targeted radionuclide therapy (TRT) is a unique treatment option allowing for in vivo imaging and delivery of lethal radiation directly and specifically to solid cancers. As TRT is generally well-tolerated with limited side effects, it holds a significant potential for use in combination with other therapies such as immunotherapy. However, preclinical assessment of this combinational pair has been limited by the lack of relevant animal models. To address this, we established a fully immunocompetent animal model bearing murine tumors expressing a human target making the model eligible for both TRT and immune checkpoint inhibitor (ICI) therapy. Subsequently, the model was used for evaluating the in vivo biodistribution, tolerability and efficacy of Tb-161 trastuzumab in combination with anti-PD-1 (aPD-1) therapy. In addition, whole-body immune status following therapy was evaluated with CD8 ImmunoPET with 89Zr-Df-IAB42M1-14 (ImaginAb) and tumor microenvironment was profiled by flow cytometry. Methods: The murine EMT-6 breast cancer cell line was modified to express HER2 by lentiviral transduction with expression confirmed by flow cytometry. The engineered EMT-6-HER2 (1+) were implanted on Balb/c mice, which subsequently were included in a theranostic study receiving either vehicle, 161Tb-Trastuzumab, aPD-1 or a combination of 161Tb-Trastuzumab and aPD-1. SPECT/CT imaging was performed 72 and 168 hours post dosing of TRT single therapy to evaluate biodistribution. Eight days after treatment initiation, PET/CT imaging of CD8 T cells was performed 24 hours after injection 89Zr-Df-IAB42M1-14 (4 MBq, 0.2 MBq/µg) in all treatment groups. Autoradiography and IHC were performed on tumors to evaluate co-localization of target expression and compound accumulation. Results: Animals were included with a tumor volume of 101.5 mm3 (n=12-16/ group). 161Tb-Trastuzumab SPECT/CT imaging showed high tumor uptake (>15 %ID/g) and limited uptake in non-target expressing organs. Animals receiving 161Tb-Trastuzumab as a single therapy or in combination with aPD-1 had significantly smaller tumor volumes when compared to the vehicle control (Dunnett’s, p=0.003) with the smallest tumor volume observed in the combinatory treatment group. PET/CT imaging of 89Zr-Df-IAB42M1-14 targeting CD8 T cells revealed tumor uptake of 5-10 %ID/g. Autoradiography and IHC confirmed tumor border T cell infiltration and subcellular overlay between HER2 target expression and Tb-161. Conclusion: This study demonstrated an evaluation of the efficacy of TRT and ICI, both as single therapies and in combination in a relevant animal model. We furthermore confirmed targeting of 161Tb-Trastuzumab in vivo and showed that the immune response and CD8 T cell infiltration could be monitored with PET/CT imaging in animals previously dosed with Tb-161 radiolabeled trastuzumab. Citation Format: Sigrid Cold, Pragalath Sadasivam, Myrto Ischyropoulou, Sakeerthi K. Vinethkanth, Emma Papin, Jacob Hald, Sebastian Gnosa, Trine B. Engel, Lene Jessen, Andreas Kjaer, Carsten H. Nielsen, Lotte K. Kristensen. Preclinical insights into 161Tb-trastuzumab and immune checkpoint combination therapy for cancer in a HER2-expressing murine cancer model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 570.