背景:由于基于抗体的配体通过血脑屏障(BBB)的进入受限,因此大脑是基于抗体的正电子发射断层扫描(免疫PET)成像的挑战性靶标。为了克服这个生理障碍,我们之前已经开发了双特异性抗体配体,其通过受体介导的胞吞作用通过BBB.虽然这些放射性标记的配体具有高亲和力和特异性,它们在血液和大脑中的停留时间很长,典型的大分子,对PET成像提出了另一个挑战。可行的解决方案可以是两步预靶向方法,其中包括施用标记的抗体,该抗体积聚在大脑的靶位点,然后从血液中清除,随后施用具有快速动力学的小放射性标记的分子。该放射性标记的分子可以与标记的抗体偶联,从而通过PET成像使抗体定位可见。体内连接可以通过使用反电子需求Diels-Alder反应(IEDDA)来实现,反式环辛烯(TCO)和四嗪基团作为反应物参与。在这项研究中,合成了两种新的18F标记的四嗪,并评估了它们作为预靶向成像剂的潜在用途,即,因为它们快速进入大脑的能力,如果未绑定,以最小的背景保留有效地清除。
结果:两种化合物,甲基四嗪[18F]MeTz和H-四嗪[18F]HTz使用两步程序通过在固体载体上合成的[18F]F-Py-TFP进行放射性标记,然后用含胺的四嗪酰胺化,产生24%和22%的放射化学产率,分别,放射化学纯度>96%.进行体内PET成像以评估它们对体内预靶向的适用性。PET扫描的时间-活性曲线显示[18F]MeTz是更适合药代动力学的药物,鉴于其在大脑中快速均匀的分布和快速清除。然而,就共振动力学而言,H-四嗪是有利的,在IEDDA与反式环辛烯等亲二烯体反应中表现出更快的反应速率,使[18F]HTz可能更有利于预定位应用。
结论:这项研究表明,[18F]MeTz和[18F]HTz由于其有效的脑摄取而具有作为预靶向PET脑成像剂的重要潜力,快速清除和适当的化学稳定性。
BACKGROUND: The brain is a challenging target for antibody-based positron emission tomography (immunoPET) imaging due to the restricted access of antibody-based ligands through the blood-brain barrier (BBB). To overcome this physiological obstacle, we have previously developed bispecific antibody ligands that pass through the BBB via receptor-mediated transcytosis. While these radiolabelled ligands have high affinity and specificity, their long residence time in the blood and brain, typical for large molecules, poses another challenge for PET imaging. A viable solution could be a two-step pre-targeting approach which involves the administration of a tagged antibody that accumulates at the target site in the brain and then clears from the blood, followed by administration of a small radiolabelled molecule with fast kinetics. This radiolabelled molecule can couple to the tagged antibody and thereby make the antibody localisation visible by PET imaging. The in vivo linkage can be achieved by using the inverse electron demand Diels-Alder reaction (IEDDA), with trans-cyclooctene (TCO) and
tetrazine groups participating as reactants. In this study, two novel 18F-labelled tetrazines were synthesized and evaluated for their potential use as pre-targeting imaging agents, i.e., for their ability to rapidly enter the brain and, if unbound, to be efficiently cleared with minimal background retention.
RESULTS: The two compounds, a methyl
tetrazine [18F]MeTz and an H-
tetrazine [18F]HTz were radiolabelled using a two-step procedure via [18F]F-Py-TFP synthesized on solid support followed by amidation with amine-bearing tetrazines, resulting in radiochemical yields of 24% and 22%, respectively, and a radiochemical purity of > 96%. In vivo PET imaging was performed to assess their suitability for in vivo pre-targeting. Time-activity curves from PET-scans showed [18F]MeTz to be the more pharmacokinetically suitable agent, given its fast and homogenous distribution in the brain and rapid clearance. However, in terms of rection kinetics, H-tetrazines are advantageous, exhibiting faster reaction rates in IEDDA reactions with dienophiles like trans-cyclooctenes, making [18F]HTz potentially more beneficial for pre-targeting applications.
CONCLUSIONS: This study demonstrates a significant potential of [18F]MeTz and [18F]HTz as agents for pre-targeted PET brain imaging due to their efficient brain uptake, swift clearance and appropriate chemical stability.