Graduate School of Pharmaceutical Sciences, Chiba University, Chiba 260-8675, Japan.
Showa Pharmaceutical University, Machida 194-8543, Japan.
Mol Pharm. 2020 May 4;17(5):1621-1628. doi: 10.1021/acs.molpharmaceut.0c00070. Epub 2020 Apr 22.
The accumulation of Tc-labeled probes targeting saturable systems of the body is hindered by the presence of a large excess of unlabeled ligands needed to ensure high radiochemical yields in a short reaction time. To address the issue, we recently reported a novel concept of a metal-coordination-mediated synthesis of a bivalent Tc-labeled probe from a monovalent ligand using d-penicillamine (Pen) as a chelating molecule and c(RGDfK) as a model targeting device. The Pen-conjugated c(RGDfK) via a hexanoate linkage (Pen-Hx-c(RGDfK)) provided a bivalent [Tc]Tc-[(Pen-Hx-c(RGDfK)) that possessed much higher integrin αβ binding affinity than Pen-Hx-c(RGDfK) and visualized a murine tumor without purification. However, high radioactivity levels were observed in the abdominal regions, which necessitated improved pharmacokinetics of the probes for practical applications. In this study, a pharmacokinetic (PK) modifier was introduced to manipulate the pharmacokinetics of the Tc-Pen-based bivalent probe. The Hx linkage in Pen-Hx-c(RGDfK) was replaced with acetyl-d-serine-d-serine-glycine (Ac-ssG) or hexanoyl-d-serine-d-serine-d-serine (Hx-sss) to prepare Pen-Ac-ssG-c(RGDfK) or Pen-Hx-sss-c(RGDfK). Pen-Ac-ssG-c(RGDfK) impaired the complexation ability of Pen-Hx-c(RGDfK), and a monovalent Tc-labeled compound was generated at low ligand concentration. However, Pen-Hx-sss-c(RGDfK) provided the objective bivalent Tc-labeled probe in high radiochemical yields at a concentration similar to that of Pen-Hx-c(RGDfK). [Tc]Tc-[Pen-Hx-sss-c(RGDfK)] also possessed stability and integrin αβ binding affinity similar to those of [Tc]Tc-[Pen-Hx-c(RGDfK)]. As a result, [Tc]Tc-[Pen-Hx-sss-c(RGDfK)] exhibited tumor and abdominal radioactivity levels similar to and significantly lower than those of [Tc]Tc-[Pen-Hx-c(RGDfK)]. These findings indicate the incorporation of a tripeptide PK modifier to Pen-Hx-c(RGDfK) preserved the complexation ability and improved the pharmacokinetics of the resulting Tc-labeled bivalent probe without impairing the targeting ability. Thus, the [Pen-Hx-(PK modifier)-(targeting device)] would constitute a basic formulation for preparing the Tc-Pen-based bivalent probes for imaging saturable targets of the body.
针对体内可饱和系统的 Tc 标记探针的积累受到大量未标记配体的阻碍，这些配体需要确保在短反应时间内获得高放射化学产率。为了解决这个问题，我们最近报道了一种使用 D-青霉胺（Pen）作为螯合分子和 c（RGDfK）作为模型靶向装置，从单价配体通过金属配位介导合成双价 Tc 标记探针的新方法。通过己酸酯键连接的 Pen 共轭 c（RGDfK）（Pen-Hx-c（RGDfK））提供了双价 [Tc]Tc-[(Pen-Hx-c（RGDfK）），其与 Pen-Hx-c（RGDfK）相比具有更高的整联蛋白 αβ 结合亲和力，并在未经纯化的情况下可视化了鼠肿瘤。然而，在腹部区域观察到高放射性水平，这需要改进探针的药代动力学以进行实际应用。在这项研究中，引入了一种药代动力学（PK）修饰剂来操纵基于 Tc-Pen 的双价探针的药代动力学。用乙酰-D-丝氨酸-D-丝氨酸-甘氨酸（Ac-ssG）或己酰基-D-丝氨酸-D-丝氨酸-D-丝氨酸（Hx-sss）取代 Pen-Hx-c（RGDfK）中的 Hx 键，制备 Pen-Ac-ssG-c（RGDfK）或 Pen-Hx-sss-c（RGDfK）。Pen-Ac-ssG-c（RGDfK）削弱了 Pen-Hx-c（RGDfK）的络合能力，并且在低配体浓度下生成单价 Tc 标记化合物。然而，Pen-Hx-sss-c（RGDfK）以类似于 Pen-Hx-c（RGDfK）的浓度提供了目标双价 Tc 标记探针。[Tc]Tc-[Pen-Hx-sss-c（RGDfK）]也具有与 [Tc]Tc-[Pen-Hx-c（RGDfK）]相似的稳定性和整联蛋白 αβ 结合亲和力。结果，[Tc]Tc-[Pen-Hx-sss-c（RGDfK）]的肿瘤和腹部放射性水平与 [Tc]Tc-[Pen-Hx-c（RGDfK）]相似，但明显低于后者。这些发现表明，将三肽 PK 修饰剂掺入到 Pen-Hx-c（RGDfK）中可以保留络合能力，并改善所得 Tc 标记双价探针的药代动力学，而不会损害靶向能力。因此，[Pen-Hx-（PK 修饰剂）-（靶向装置）]将构成用于成像体内可饱和靶标的 Tc-Pen 基双价探针的基本制剂。
