Radioimmunotherapy of solid tumors using antibody-targeted radionuclides has been limited by low therapeutic indices (TIs). We recently reported a novel 3-step pretargeted radioimmunotherapy (PRIT) strategy based on a glycoprotein A33 (GPA33)–targeting bispecific antibody and a small-molecule radioactive hapten, a complex of ¹⁷⁷Lu and S-2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (¹⁷⁷Lu-DOTA-Bn), that leads to high TIs for radiosensitive tissues such as blood (TI = 73) and kidney (TI = 12). We tested our hypothesis that a fractionated anti-GPA33 DOTA-PRIT regimen calibrated to deliver a radiation absorbed dose to tumor of more than 100 Gy would lead to a high probability of tumor cure while being well tolerated by nude mice bearing subcutaneous GPA33-positive SW1222 xenografts. Methods: We treated groups of nude mice bearing 7-d-old SW1222 xenografts with a fractionated 3-cycle anti-GPA33 DOTA-PRIT regimen (total administered ¹⁷⁷Lu-DOTA-Bn activity, 167 MBq/mouse; estimated radiation absorbed dose to tumor, 110 Gy). In randomly selected mice undergoing treatment, serial SPECT/CT imaging was used to monitor treatment response and calculate radiation absorbed doses to tumor. Necropsy was done on surviving animals 100–200 d after treatment to determine frequency of cure and assess select normal tissues for treatment-related histopathologies. Results: Rapid exponential tumor progression was observed in control treatment groups (i.e., no treatment or ¹⁷⁷Lu-DOTA-Bn only), leading to euthanasia due to excessive tumor burden, whereas 10 of 10 complete responses were observed for the DOTA-PRIT–treated animals within 30 d. Treatment was well tolerated, and 100% histologic cure was achieved in 9 of 9 assessable animals without detectable radiation damage to critical organs, including bone marrow and kidney. Radiation absorbed doses to tumor derived from SPECT/CT (102 Gy) and from biodistribution (110 Gy) agreed to within 6.9%. Of the total dose of approximately 100 Gy, the first dose contributes 30%, the second dose 60%, and the third dose 10%. Conclusion: In a GPA33-positive human colorectal cancer xenograft mouse model, we validated a SPECT/CT-based theranostic PRIT regimen that led to 100% complete responses and 100% cures without any treatment-related toxicities, based on high TIs for radiosensitive tissues. These studies support the view that anti-GPA33 DOTA-PRIT will be a potent radioimmunotherapy regimen for GPA33-positive colorectal cancer tumors in humans.

使用靶向抗体的放射性核素对实体瘤进行放射免疫治疗受到低治疗指数（TI）的限制。我们最近报道了一种新颖的3步预靶向放射免疫疗法（PRIT）策略，该策略基于靶向糖蛋白A33（GPA33）的双特异性抗体和小分子放射性半抗原，即¹⁷⁷ Lu和S -2-（4-氨基苄基）- 1,4,7,10-四氮杂环十二烷四乙酸（¹⁷⁷Lu-DOTA-Bn），导致放射敏感性组织（如血液（TI = 73）和肾脏（TI = 12））的TI较高。我们检验了这样的假设：经过校准的分级抗GPA33 DOTA-PRIT方案可为肿瘤提供超过100 Gy的放射吸收剂量，这将导致肿瘤治愈的可能性很高，而带有皮下GPA33阳性SW1222的裸鼠则可以很好地耐受异种移植。方法：我们采用分阶段的3周期抗GPA33 DOTA-PRIT方案治疗了携带7日龄SW1222异种移植物的裸鼠组（共给药^177种）Lu-DOTA-Bn活性，167 MBq /小鼠；估计对肿瘤的辐射吸收剂量（110 Gy）。在随机选择的接受治疗的小鼠中，连续SPECT / CT成像用于监测治疗反应并计算对肿瘤的辐射吸收剂量。在治疗后100-200 d对存活的动物进行尸检，以确定治愈的频率并评估与治疗相关的组织病理学的选定正常组织。结果：在对照组中观察到了快速的指数性肿瘤进展（即，没有治疗或¹⁷⁷仅Lu-DOTA-Bn），由于过多的肿瘤负担而导致安乐死，而在30天内，用DOTA-PRIT治疗的动物共观察到10个完整反应中的10个。治疗耐受性良好，在9只可评估动物中有9只获得了100％的组织学治愈，而对包括骨髓和肾脏在内的关键器官没有可检测到的辐射损伤。来自SPECT / CT（102 Gy）和生物分布（110 Gy）的肿瘤的放射吸收剂量一致在6.9％以内。在大约100 Gy的总剂量中，第一剂量占30％，第二剂量占60％，第三剂量占10％。结论：在GPA33阳性的人类结直肠癌异种移植小鼠模型中，我们基于放射敏感性组织的高TIs，验证了基于SPECT / CT的治疗性PRIT方案可导致100％完全缓解和100％治愈，而没有任何与治疗相关的毒性。这些研究支持这样的观点，即抗GPA33 DOTA-PRIT将是针对人类GPA33阳性结直肠癌肿瘤的有效放射免疫疗法。