Abstract P168: Pretargeted radioimmunotherapy using 225Ac for intraperitoneal Her2-expressing epithelial ovarian carcinoma xenografts

Objectives Epithelial ovarian carcinoma (EOC) is a common and lethal gynecologic malignancy that frequently presents as advanced staged disease, such as peritoneal carcinomatosis (PC). We previously reported cures in BT-474 murine xenografts with Pretargeted Radioimmunotherapy (PRIT) using 177Lu radiohaptens targeting Her2 via a bispecific antibody (BsAb). Here-in we report the use of PRIT with alpha emitter 225Ac labelled PrDOTA to treat a PC model of SKOV3, a Her2+ cell line of EOC. Methods 6 wk old female athymic nude mice inoculated IP with 1E5 luciferase/GFP-transfected SKOV3 cells were separated into 5 groups (n=10). Treatment mice received 1 or 2 cycles of Anti-Her2-C825 BsAb + [225Ac]PrDOTA (Her2-Targeted), at 14 and 21 days after inoculation, respectively. Control groups received Anti-Her2 BsAb only, Anti-GPA33 BsAb + [225Ac]PrDOTA (Off-Targeted) or no treatment. On cycle day 1, the mice were injected IP with 0.25mg (1.19nmol) BsAb. On cycle day 2, 25µg (2.76nmol) CCA16-DOTAY clearing agent (CA) was given IV 22h from BsAb. Mice in therapy groups were injected IP with 1µCi (0.74-0.79nmol) [225Ac]PrDOTA-Bn 4h after CA. Weekly weights and BLIs with IP cavity ROIs were obtained and normalized to the respective values for each mouse at week 0 pre-treatment. End points: weight loss >20% baseline, moribund, or severe abdominal distension. At 154 days, 15 surviving treatment and 1 untreated control mice were submitted for hematology and histopathology. Results Histologic cures and prolonged survival were demonstrated in treatment mice (17/20 at 133 days) as compared to control mice (12/27 at 133 d, Logrank p0.05; all weeks). BLI of mice treated with 1 and 2 cycle of Her2 PRIT decreased 47% when compared to baseline within 1 week (T test p=0.04), suggesting treatment effects as early as 1 week. There were no differences in weights when compared to baseline (2-way ANOVA p>0.05). While the untreated mouse had high peritoneal adenocarcinoma tumor burden, there was no histologic evidence of viable neoplasia in 15/15 submitted treatment mice. Treatment mice had moderate renal tubular degenerative lesions on histology, but this did not affect renal function based on serum BUN or Cr. All hematologic parameters were within normal limits for treated mice. Conclusions 1 and 2 cycles of [225Ac]PrDOTA-PRIT against Her2 resulted in histologic cures and prolonged survival in IP SKOV3 xenografts with minimal toxicity. The anti-Her2 PrDOTA-PRIT system is a promising theranostic approach for otherwise incurable PC. Citation Format: Sebastian K. Chung, Christopher S. Chandler, Daniela Burnes Vargas, Shin H. Seo, Michael R. McDevitt, Darren Veach, Blesida Punzalan, Xu Hong, Hong-fen Guo, Garrett M. Nash, Andrea Cercek, Nai-Kong V. Cheung, Steven M. Larson, Sarah M. Cheal. Pretargeted radioimmunotherapy using 225Ac for intraperitoneal Her2-expressing epithelial ovarian carcinoma xenografts [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P168.