Purpose

Radioimmunotherapy uses tumor-specific antibodies to deliver therapeutic radionuclides, but hematological toxicity due to the long serum half-life of intact antibodies remains a challenge. We evaluated a smaller antibody fragment, the minibody, with faster kinetics and a potentially improved therapeutic index.

Procedures

The anti-prostate stem cell antigen (PSCA) minibody (A11 Mb) was radiolabeled with iodine-124 ([¹²⁴I]I-A11 Mb) or conjugated with deferoxamine (DFO) and labeled with zirconium-89 ([⁸⁹Zr]Zr-DFO-A11 Mb) for surrogate immunoPET to profile pharmacokinetics in a human prostate cancer xenograft model. Subsequently, minibodies labeled with two therapeutic beta emitters, directly iodinated [¹³¹I]I-A11 Mb (non-residualizing) and ¹⁷⁷Lu chelated using DTPA ([¹⁷⁷Lu]Lu-DTPA-A11 Mb) (residualizing), were compared for in vitro antigen-specific cytotoxicity. Full biodistribution studies (in 22Rv1-PSCA tumor bearing and hPSCA knock-in mice) were conducted for dosimetry calculations. Finally, the lead candidate [¹³¹I]I-A11 Mb was evaluated in a radioimmunotherapy experiment. Escalating single doses (3.7, 11, or 37 MBq) and saline control were administered to 22Rv1-PSCA tumor bearing mice and anti-tumor effects (tumor volume) and toxicity (body weight) were monitored.

Results

Minibodies radiolabeled with therapeutic beta emitters [¹³¹I]I-A11 Mb and [¹⁷⁷Lu]Lu-DTPA-A11 Mb exhibited comparable tumor cell growth inhibition in vitro. In vivo surrogate immunoPET imaging using [⁸⁹Zr]Zr-DFO-A11 Mb showed activity retention in liver and kidney up to 72 h, while [¹²⁴I]I-A11 Mb cleared from liver, kidney, and blood by 48 h. Based on full biodistribution and dosimetry calculations, administering 37 MBq [¹³¹I]I-A11 Mb was predicted to deliver a favorable dose to the tumor (35 Gy), with a therapeutic index of 22 (tumor:bone marrow). For [¹⁷⁷Lu]Lu-DTPA-A11 Mb, the kidneys would be dose-limiting, and the maximum tolerated activity (7.4 MBq) was not predicted to deliver an effective radiation dose to tumor. Radioimmunotherapy with a single dose of [¹³¹I]I-A11 Mb showed dose-dependent tumor inhibition with minimal off-target toxicity and improved median survival (19 and 24 days, P < 0.001) compared with untreated mice (12 days).

Conclusions

These findings show the potential of the anti-PSCA minibody for targeted radioimmunotherapy with minimal toxicity, and the application of immunoPET and dosimetry for personalized treatment.

中文翻译：

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[131I]I-和[177Lu]Lu-DTPA-A11微型抗体在表达PSCA的前列腺癌临床前模型中放射免疫治疗的评估。

目的

放射免疫疗法使用肿瘤特异性抗体来提供治疗性放射性核素，但由于完整抗体的血清半衰期长而导致的血液毒性仍然是一个挑战。我们评估了一种更小的抗体片段，即微型抗体，它具有更快的动力学和潜在的改善的治疗指数。

程序

抗前列腺干细胞抗原 (PSCA) 微型抗体 (A11 Mb) 用碘 124 ([ ¹²⁴ I]I-A11 Mb) 进行放射性标记或与去铁胺 (DFO) 缀合并用锆 89 ([ ⁸⁹ Zr]Zr -DFO-A11 Mb) 用于替代免疫PET，以分析人类前列腺癌异种移植模型中的药代动力学。随后，比较了用两种治疗性β发射体标记的微型抗体，即直接碘化的[ ¹³¹ I]I-A11 Mb（非残留化）和使用DTPA螯合的^{177 Lu（[ 177} Lu]Lu-DTPA-A11 Mb）（残留化）。体外抗原特异性细胞毒性。进行了完整的生物分布研究（在 22Rv1-PSCA 荷瘤小鼠和 hPSCA 敲入小鼠中）进行剂量测定计算。最后，在放射免疫治疗实验中评估了主要候选药物[ ^{131 I]I-A11 Mb。}对荷有 22Rv1-PSCA 肿瘤的小鼠进行逐步增加的单剂量（3.7、11 或 37 MBq）和盐水对照，并监测抗肿瘤作用（肿瘤体积）和毒性（体重）。

结果

用治疗性β发射体[ ^131I ]I-A11Mb和[ ^177Lu ]Lu-DTPA-A11Mb放射性标记的微型抗体在体外表现出相当的肿瘤细胞生长抑制作用。^{使用[ 89 Zr]Zr-DFO-A11 Mb 进行的}体内替代免疫PET 成像显示，肝脏和肾脏中的活性保留长达72 小时，而[ ¹²⁴ I]I-A11 Mb 在48 小时前从肝脏、肾脏和血液中清除。根据完整的生物分布和剂量测定计算，预计给予 37 MBq [ ¹³¹ I]I-A11 Mb 可为肿瘤提供有利的剂量 (35 Gy)，治疗指数为 22（肿瘤：骨髓）。对于[ ¹⁷⁷ Lu]Lu-DTPA-A11 Mb，肾脏将受到剂量限制，并且预计最大耐受活性（7.4 MBq）不会向肿瘤传递有效的辐射剂量。与未经治疗的小鼠（12 天）相比，单剂量 [ ¹³¹ I]I-A11 Mb 的放射免疫治疗显示出剂量依赖性肿瘤抑制作用，具有最小的脱靶毒性，并改善中位生存期（19 和 24 天，P  < 0.001）。

结论

这些发现显示了抗 PSCA 微型抗体在毒性最小的靶向放射免疫治疗中的潜力，以及免疫 PET 和剂量测定在个性化治疗中的应用。