Purpose

In vivo imaging of programmed death ligand 1 (PD-L1) during immunotherapy could potentially monitor changing PD-L1 expression and PD-L1 expression heterogeneity within and across tumors. Some protein constructs can be used for same-day positron emission tomography (PET) imaging. Previously, we evaluated the PD-L1-targeting Affibody molecule [¹⁸F]AlF-NOTA-Z_{PD-L1_1} as a PET tracer in a mouse tumor model of human PD-L1 expression. In this study, we evaluated the affinity-matured Affibody molecule Z_{PD-L1_4}, to determine if improved affinity for PD-L1 resulted in increased in vivo targeting of PD-L1.

Procedures

Z_{PD-L1_4} was conjugated with NOTA and radiolabeled with either [¹⁸F]AlF or ⁶⁸Ga. [¹⁸F]AlF-NOTA-Z_{PD-L1_4} and [⁶⁸Ga]NOTA-Z_{PD-L1_4} were evaluated in immunocompromised mice with LOX (PD-L1+) and SUDHL6 (PD-L1-) tumors with PET and ex vivo biodistribution measurements. In addition, whole-body PET studies were performed in rhesus monkeys to predict human biodistribution in a model with tracer binding to endogenous PD-L1, and to calculate absorbed radiation doses.

Results

Ex vivo biodistribution measurements showed that both tracers had > 25 fold higher accumulation in LOX tumors than SUDHL6 ([¹⁸F]AlF-NOTA-Z_{PD-L1_4}: LOX: 8.7 ± 0.7 %ID/g (N = 4) SUDHL6: 0.2 ± 0.01 %ID/g (N = 6), [⁶⁸Ga]NOTA-Z_{PD-L1_4}: LOX: 15.8 ± 1.0 %ID/g (N = 6) SUDHL6: 0.6 ± 0.1 %ID/g (N = 6)), considerably higher than Z_{PD-L1_1}. In rhesus monkeys, both PET tracers showed fast clearance through kidneys and low background signal in the liver ([¹⁸F]AlF-NOTA-Z_{PD-L1_4}: 1.26 ± 0.13 SUV, [⁶⁸Ga]NOTA-Z_{PD-L1_4}: 1.11 ± 0.06 SUV). PD-L1-expressing lymph nodes were visible in PET images, indicating in vivo PD-L1 targeting. Dosimetry estimates suggest that both PET tracers can be used for repeated clinical studies, although high kidney accumulation may limit allowable radioactive doses.

Conclusions

[¹⁸F]AlF-NOTA-Z_{PD-L1_4} and [⁶⁸Ga]NOTA-Z_{PD-L1_4} are promising candidates for same-day clinical PD-L1 PET imaging, warranting clinical evaluation. The ability to use either [¹⁸F] or [⁶⁸Ga] may expand access to clinical sites.

中文翻译：

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靶向 PD-L1 的高亲和力 Affibody PET 示踪剂的体内评估和剂量测定估计

目的

免疫治疗期间程序性死亡配体 1 (PD-L1) 的体内成像可能会监测肿瘤内和肿瘤间 PD-L1 表达的变化和 PD-L1 表达异质性。一些蛋白质构建体可用于当日正电子发射断层扫描 (PET) 成像。之前，我们在人类 PD-L1 表达的小鼠肿瘤模型中评估了靶向 PD-L1 的 Affibody 分子 [ ¹⁸ F]AlF-NOTA-Z _{PD-L1_1}作为 PET 示踪剂。在本研究中，我们评估了亲和力成熟的 Affibody 分子 Z _{PD-L1_4}，以确定提高对 PD-L1 的亲和力是否会导致 PD-L1 的体内靶向增加。

程序

Z _{PD-L1_4}与 NOTA 结合并用 [ ¹⁸ F]AlF 或⁶⁸ Ga 进行放射性标记。 [ ¹⁸ F]AlF-NOTA-Z _{PD-L1_4}和 [ ⁶⁸ Ga]NOTA-Z _{PD-L1_4}在免疫功能低下的小鼠中进行了评估LOX (PD-L1+) 和 SUDHL6 (PD-L1-) 肿瘤与 PET 和离体生物分布测量。此外，在恒河猴身上进行了全身 PET 研究，以预测与内源性 PD-L1 结合的示踪剂模型中的人体生物分布，并计算吸收的辐射剂量。

结果

离体生物分布测量表明，两种示踪剂在 LOX 肿瘤中的积累比 SUDHL6 高 > 25 倍（[ ¹⁸ F]AlF-NOTA-Z _{PD-L1_4}：LOX：8.7 ± 0.7 %ID/g（N = 4）SUDHL6：0.2 ± 0.01 %ID/g ( N = 6), [ ⁶⁸ Ga]NOTA-Z _{PD-L1_4} : LOX: 15.8 ± 1.0 %ID/g ( N = 6) SUDHL6: 0.6 ± 0.1 %ID/g ( N = 6 ))，远高于 Z _{PD-L1_1}。在恒河猴中，两种 PET 示踪剂都显示出通过肾脏的快速清除和肝脏中的低背景信号（[ ¹⁸ F]AlF-NOTA-Z _{PD-L1_4}：1.26 ± 0.13 SUV，[ ⁶⁸ Ga]NOTA-Z _{PD-L1_4}: 1.11 ± 0.06 SUV)。在 PET 图像中可以看到表达 PD-L1 的淋巴结，表明体内PD-L1 靶向。剂量学估计表明，两种 PET 示踪剂均可用于重复的临床研究，尽管高肾脏积累可能会限制允许的放射性剂量。

结论

[ ¹⁸ F]AlF-NOTA-Z _{PD-L1_4}和[ ⁶⁸ Ga]NOTA-Z _{PD-L1_4}是当日临床 PD-L1 PET 成像的有希望的候选者，_值得进行临床评估。使用 [ ¹⁸ F] 或 [ ⁶⁸ Ga] 的能力可以扩大对临床站点的访问。