An Efficient Method for Labeling Single Domain Antibody Fragments with <sup>18</sup>F Using Tetrazine-<i>Trans</i>-Cyclooctene Ligation and a Renal Brush Border Enzyme-Cleavable Linker
作者:Zhengyuan Zhou、Nick Devoogdt、Michael R. Zalutsky、Ganesan Vaidyanathan
DOI:10.1021/acs.bioconjchem.8b00699
日期:2018.12.19
Single domain antibody fragments (sdAbs) labeled with 18F have shown promise for assessing the status of oncological targets such as the human epidermal growth factor receptor 2 (HER2) by positron emission tomography (PET). Earlier, we evaluated two residualizing prosthetic agents for 18F-labeling of anti-HER2 sdAbs; however, these methods resulted in poor labeling yields and high uptake of 18F activity in the kidneys. To potentially mitigate these limitations, we have now developed an 18F labeling method that utilizes the trans-cyclooctene (TCO)-tetrazine (Tz)-based inverse-electron demand Diels–Alder reaction (IEDDAR) in tandem with a renal brush border enzyme-cleavable glycine-lysine (GK) linker in the prosthetic moiety. The HER2-targeted sdAb 2Rs15d was derivatized with TCO-GK-PEG4-NHS or TCO-PEG4-NHS, which lacks the cleavable linker. As an additional control, the non HER2-specific sdAb R3B23 was derivatized with TCO-GK-PEG4-NHS. The resultant sdAb conjugates were labeled with 18F by IEDDAR using [18F]AlF-NOTA-PEG4-methyltetrazine. As a positive control, the 2Rs15d sdAb was radioiodinated using the well-characterized residualizing prosthetic agent, N-succinimidyl 4-guanidinomethyl-3-[125I]iodobenzoate ([125I]SGMIB). Synthesis of [18F]AlF-NOTA-Tz-TCO-GK-2Rs15d was achieved with an overall radiochemical yield (RCY) of 17.8 ± 1.5% (n = 5) in 90 min, a significant improvement over prior methods (3–4% in 2–3 h). In vitro assays indicated that [18F]AlF-NOTA-Tz-TCO-GK-2Rs15d bound with high affinity and immunoreactivity to HER2. In normal mice, when normalized to coinjected [125I]SGMIB-2Rs15d, the kidney uptake of [18F]AlF-NOTA-Tz-TCO-GK-2Rs15d was 15- and 28-fold lower (P < 0.001) than that seen for the noncleavable control ([18F]AlF-NOTA-Tz-TCO-2Rs15d) at 1 and 3 h, respectively. Uptake of [18F]AlF-NOTA-Tz-TCO-GK-2Rs15d in HER2-expressing SKOV-3 ovarian carcinoma xenografts implanted in athymic mice was about 80% of that seen for coinjected [125I]SGMIB-2Rs15d. On the other hand, kidney uptake was 5–6-fold lower, and as a result, tumor-to-kidney ratios were 4-fold higher for [18F]AlF-NOTA-Tz-TCO-GK-2Rs15d than those for [125I]SGMIB-2Rs15d. SKOV-3 xenografts were clearly delineated even at 1 h after administration of [18F]AlF-NOTA-Tz-TCO-GK-2Rs15d by Micro-PET/CT imaging with even higher contrast observed thereafter. In conclusion, this strategy warrants further evaluation for labeling small proteins such as sdAbs because it offers the benefits of good radiochemical yields and enhanced tumor-to-normal tissue ratios, particularly in the kidney.
带有 18F 标记的单域抗体片段 (sdAbs) 已显示出通过正电子发射断层扫描 (PET) 评估人表皮生长因子受体 2 (HER2) 等肿瘤学靶点状态的潜力。早期,我们评估了两种残余化辅基用于 18F 标记抗 HER2 sdAbs;然而,这些方法导致标记产率较低且肾脏中 18F 活性摄取较高。为了潜在地缓解这些局限性,我们现在开发了一种 18F 标记方法,该方法利用反式-环辛烯 (TCO)-四嗪 (Tz) 基反电子需求的 Diels-Alder 反应 (IEDDAR) 与肾刷状缘酶可切割的甘氨酸-赖氨酸 (GK) 连接子相结合的策略。HER2 靶向的 sdAb 2Rs15d 被衍生化为 TCO-GK-PEG4-NHS 或不含可切割连接子的 TCO-PEG4-NHS。作为额外对照,非 HER2 特异性的 sdAb R3B23 被衍生化为 TCO-GK-PEG4-NHS。所得的 sdAb 偶联物通过使用 [18F]AlF-NOTA-PEG4-甲基四嗪的 IEDDAR 反应进行 18F 标记。作为阳性对照,2Rs15d sdAb 通过使用广为人知的残余化辅基 N-琥珀酰亚胺基 4-胍基甲基-3-[125I]碘苯甲酸 ([125I]SGMIB) 进行放射碘标记。[18F]AlF-NOTA-Tz-TCO-GK-2Rs15d 的合成在 90 分钟内实现了 17.8 ± 1.5%(n = 5)的总放射化学产率 (RCY),这相对于先前方法(2-3 小时内的 3-4%)有了显著改进。体外实验表明,[18F]AlF-NOTA-Tz-TCO-GK-2Rs15d 对 HER2 具有高亲和力和免疫反应性。在正常小鼠中,当相对于共注射的 [125I]SGMIB-2Rs15d 进行归一化时,[18F]AlF-NOTA-Tz-TCO-GK-2Rs15d 在 1 和 3 小时时的肾脏摄取分别比不可切割对照组([18F]AlF-NOTA-Tz-TCO-2Rs15d)低 15 倍和 28 倍(P < 0.001)。在无胸腺小鼠中植入的 HER2 表达的 SKOV-3 卵巢癌异种移植模型中,[18F]AlF-NOTA-Tz-TCO-GK-2Rs15d 的摄取约为共注射的 [125I]SGMIB-2Rs15d 的 80%。另一方面,肾脏摄取低 5-6 倍,因此肿瘤与肾脏比率比 [125I]SGMIB-2Rs15d 高 4 倍。即使在注射 [18F]AlF-NOTA-Tz-TCO-GK-2Rs15d 1 小时后,SKOV-3 异种移植体也能通过 Micro-PET/CT 成像清晰地描绘出来,随后观察到更高的对比度。总之,这种方法值得进一步评估用于标记小蛋白如 sdAbs,因为它提供了良好的放射化学产率和增强的肿瘤与正常组织比率,特别是在肾脏方面。