Abstract 1391: Self-assembling and disassembling bispecific antibody platform for pretargeted radioimmunotherapy against GPA33 in a xenograft model of colorectal peritoneal carcinomatosis
Authors: Chung Sebastian , Wang Mao , Veach Darren , Cheal Sarah , Vargas Daniela Burnes , Seo Shin Hyuk , Punzalan Blesida , Santich Brian , Guo Hong-fen , Xu Hong , Nash Garrett , Cercek Andrea , Chandler Christopher , Larson Steven , Cheung Nai-Kong 
Published: July 2021

Abstract
Objectives: There is increasing interest in the potential role of radioimmunotherapy in the treatment of peritoneal carcinomatosis (PC), locoregional metastasis throughout the peritoneal cavity. We recently described a SADA platform when applied to GD2-specific PRIT using beta and alpha emitting radioisotopes achieved exceptional therapeutic indices (TI) achieving tumor cures without myelo-, renal-, neuro-, or hepatic toxicities (1). We applied SADA technology to a tumor associated antigen GPA33 expressed by colorectal cancer in a PC model, and compared that to a 3 step PRIT which requires a clearing agent (CA), previously reported to produce durable responses including histologic cures of GPA33-expressing SW1222 subcutaneous (2) and intraperitoneal (ip) colorectal xenograft models (3).
Methods: SADA BsAb built from a humanized anti-GPA33 single chain Fv (scFv) and a humanized anti-DOTA scFv were expressed using the Expi293 Expression System and purified by affinity chromatography (1). Biochemical purity of &gt;90% was confirmed via size exclusion HPLC and antigen binding confirmed by surface plasmon resonance and by flow cytometry on GPA33(+) cell lines. 8 wk old female athymic nude mice were inoculated ip with 5E6 cells of stably luciferase/GFP-transfected SW1222 in 200 µL media. PC tumor progression was monitored by in vivo BLI. GPA33-SADA and control GD2-SADA were injected intravenously (iv) on day 26, followed 48 hr later with 1 mCi (37 MBq,200 pmol) [177Lu]Lu-DOTA-Bn ip. In the 3 step PRIT group, mice were treated with 250µg (1.19nmol) GPA33-BsAb (n=4) followed 24 hr later by 20 µg (2.21 nmol) of CA iv, before the injection of 1 mCi (37 MBq,200 pmol) [177Lu]Lu-DOTA-Bn ip 4 hr later. In an additional control group, no SADA was given before 1 mCi (37 MBq,200 pmol) [177Lu]Lu-DOTA-Bn ip. Biodistribution was performed 24 hr after injection of ip 177Lu radiohapten.
Results: Radioactivity absorbed in harvested ip SW1222 xenograft tumors treated with SADA-GPA33 (4.92 + 1.59 [mean + SD] percentage injected dose per gram [%ID/g]) was greater than that of harvested tumors treated with the traditional 3 step PRIT to GPA33 (2.68 + 0.179 %ID/g, p=0.043) or with negative control SADA-GD2 (0.513 + 0.293 %ID/g, p&lt;0.001). In mice treated with SADA-GPA33, the ratio of tumor-to-organ radioactivity uptake was 19.6 + 5.08 for all organs, 4.12 + 0.734 for kidneys, 30.5 + 6.14 for liver, and 122.8 + 22.6 for blood.
Conclusions: SADA-PRIT targeting GPA33 has favorable TI for colorectal PC. When compared to 3 step PRIT, SADA showed more selectivity and TI.
{{References}}: 1. Santich BH, Cheal SM, Ahmed M, et al. Clin Cancer Res. 2020; 2. Cheal SM, Xu H, Guo HF, et al. Eur J Nucl Med Mol Imaging. 2016;43(5):925-937.3. Chandler C, Cheal SM, Nash G, et al. Journal of Nuclear Medicine. 2020;61.
Citation Format: Sebastian Chung, Mao Wang, Darren Veach, Sarah Cheal, Daniela Burnes Vargas, Shin Hyuk Seo, Blesida Punzalan, Brian Santich, Hong-fen Guo, Hong Xu, Garrett Nash, Andrea Cercek, Christopher Chandler, Steven Larson, Nai-Kong Cheung. Self-assembling and disassembling bispecific antibody platform for pretargeted radioimmunotherapy against GPA33 in a xenograft model of colorectal peritoneal carcinomatosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1391.