School of Biological Sciences, University of Wollongong, Wollongong, New South Wales 2522, Australia.
Nucl Med Biol. 2012 Aug;39(6):833-9. doi: 10.1016/j.nucmedbio.2012.01.006.
Tumour-associated urokinase plasminogen activator (uPA) is a critical marker of invasion and metastasis, and it is recognised as having strong prognostic relevance as well as being a therapeutic target. The specific uPA inhibitor plasminogen activator inhibitor type-2 (PAI-2, SerpinB2) specifically targets cell bound uPA and is internalised. Furthermore, preclinical studies have established the "proof-of-principle" of uPA-targeting by PAI-2-cytotoxin conjugates in human carcinoma models. However, these studies also suggest that PAI-2 is rapidly cleared via the renal system with low total dose reaching the tumour. In this study, a comparative single photon emission computed tomography (SPECT) and biodistribution (BD) analysis of different forms of PAI-2 labelled with the radioisotopes iodine-123 ((123)I) and technetium-99m ((99m)Tc) was undertaken.
The pharmacokinetic (PK) properties and BD of wild-type, ΔCD-loop and PEGylated ΔCD-loop PAI-2 labelled with the commonly used diagnostic SPECT radioisotopes (99m)Tc or (123)I were compared in mouse models of human prostate carcinoma. Whole body SPECT imaging was also performed.
Both wild-type and the shorter but active ΔCD-loop form of PAI-2 (123)I-labelled indirectly via conjugation to free amine groups (termed (123)I-Bn-PAI-2) exhibited low tumour uptake, rapid excretion and similar PK profiles. Preliminary studies with a short branched-chain PEGylated (123)I-Bn-PAI-2 ΔCD-loop indicated an increase in blood retention time and tumour uptake. All (123)I-Bn-labelled radiotracers were largely excreted through the kidneys. By comparison, both wild-type (123)I-PAI-2 (labelled directly via tyrosine residues) and (99m)Tc-PAI-2 displayed different PK/BD patterns compared to (123)I-Bn-PAI-2, suggesting greater liver based catabolism and thus slower elimination. SPECT imaging mimicked the BD results of all radiotracers.
The different labelling methods gave distinct PAI-2 BD and tumour uptake profiles, with radioiodination resulting in the best non-tumour organ clearance profiles. Preliminary analyses with short branched-chain PEGylated (123)I-Bn-PAI-2 ΔCD-loop suggest that further investigations with other PEGylation reagents are required to optimise this approach for tumour imaging. These findings impact on the use of PAI-2 for drug delivery and/or diagnostic development.
肿瘤相关尿激酶型纤溶酶原激活物（uPA）是侵袭和转移的关键标志物，它被认为具有很强的预后相关性，也是治疗靶点。特异性 uPA 抑制剂纤溶酶原激活物抑制剂-2（PAI-2，SerpinB2）特异性靶向细胞结合的 uPA 并被内化。此外，临床前研究已经在人类癌模型中建立了 uPA 靶向的 PAI-2-细胞毒素缀合物的“原理验证”。然而，这些研究还表明，PAI-2 主要通过肾脏系统快速清除，只有少量总剂量到达肿瘤。在这项研究中，用放射性同位素碘-123 ((123)I)和锝-99m ((99m)Tc)对不同形式的 PAI-2 进行了单光子发射计算机断层扫描 (SPECT) 和生物分布 (BD) 的比较分析。
在人类前列腺癌模型中比较了野生型、ΔCD 环和聚乙二醇化 ΔCD 环 PAI-2 与常用诊断 SPECT 放射性同位素 ((99m)Tc 或 (123)I) 标记的 PK 特性和 BD。还进行了全身 SPECT 成像。
野生型和较短但活性的 ΔCD 环形式的 PAI-2 (123)I 通过与游离胺基间接标记（称为 (123)I-Bn-PAI-2），显示出低肿瘤摄取、快速排泄和相似的 PK 特征。初步研究表明，短支链聚乙二醇化 (123)I-Bn-PAI-2 ΔCD 环可增加血液保留时间和肿瘤摄取。所有 (123)I-Bn 标记的放射性示踪剂主要通过肾脏排泄。相比之下，与 (123)I-Bn-PAI-2 相比，野生型 (123)I-PAI-2（直接通过酪氨酸残基标记）和 (99m)Tc-PAI-2 显示出不同的 PK/BD 模式，表明肝脏代谢更快，因此消除速度更慢。SPECT 成像模拟了所有放射性示踪剂的 BD 结果。
不同的标记方法赋予了 PAI-2 不同的 BD 和肿瘤摄取特征，放射性碘标记产生了最佳的非肿瘤器官清除特征。初步分析表明，需要使用其他聚乙二醇化试剂进一步研究短支链聚乙二醇化 (123)I-Bn-PAI-2 ΔCD 环，以优化该方法用于肿瘤成像。这些发现影响 PAI-2 在药物输送和/或诊断开发中的应用。
