MO tripeptide diastereomers (M=99/99mTc, re):: Models to identify the structure of 99mTc peptide targeted radiopharmaceuticals
MO tripeptide diastereomers (M=99/99mTc, re):: Models to identify the structure of 99mTc peptide targeted radiopharmaceuticals
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DOI：
10.1021/ic070077p
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发表时间：
2007-09-03
影响因子：
4.6
通讯作者：
Francesconi, Lynn C.
中科院分区：
文献类型：
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作者：
Cantorias, Melchor V.;Howell, Robertha C.;Francesconi, Lynn C.
许多肽等生物活性分子可作为基于 Tc-99m 的放射性药物的靶向载体。三肽可以是合适的螯合物，可以轻松方便地合成并通过固相肽合成连接到肽靶向载体并形成稳定的(TcO)-O-V复合物。与 [TcO](3+) 络合后，形成两种产物：这些是顺式和反式非对映异构体，它们通常具有不同的生物学行为。已批准用于肺癌成像的放射性药物 [(TcO)-Tc-99m]depreotide（[(TcO)-Tc-99m]P829，NeoTect）就是这种情况。 [(TcO)-Tc-99m]德普肽确实在其 HPLC 曲线中显示出两个产物峰，但已证明将产物峰分配给非对映异构体很困难，因为金属肽复合物难以结晶以进行结构分析。在这项研究中，我们分离了几种三肽配体的 [(TcO)-Tc-99] 和 [ReO] 复合物的非对映体，这些三肽配体模拟了 [(TcO)-Tc-99m] depreotide 的金属螯合剂区域。使用X射线晶体学，我们观察到早期洗脱峰（A）对应于反非对映异构体，其中TcO基团位于配体主链相对于三肽配体的侧基形成的平面的相对侧，而后期洗脱峰（B）对应于顺式非对映异构体，其中TcO基团位于与三肽残基的平面的同一侧。 H-1 NMR 和圆二色性 (CD) 光谱报告了金属环境，并证明可以诊断顺式或反式非对映体，我们从这些技术中确定了特征特征，可用于指定 Tc-99m 肽放射性药物（如 [(TcO)-Tc-99m]depreotide 和 Re-188 肽放射治疗剂中的非对映体谱）。晶体学、电位滴定和核磁共振结果提供了对生理条件下发生的化学反应的见解。三肽复合物，其中赖氨酸是第二个氨基酸，以去质子化金属酰胺形式结晶，具有短 N-1-M 键。 N-1 胺的 pKa(a) 测量值（pKa 类似于 5.6）表明，金属络合和赖氨酸残基的存在使该胺的酸性更强。此外，掺入赖氨酸的肽螯合剂（如[TcO]depreotide 螯合剂）可能在体内以去质子化形式存在，包含中性金属中心。去质子化可能介导顺式和反式非对映异构体之间的相互转化过程。不含赖氨酸的金属肽上的 N-1 胺基不具有酸性（pK(a) 类似于 6.8），并且不会像金属酰胺物质那样去质子化和结晶。用 Tc-99m 对三种三肽配体（FGC、FSC 和 FKC）进行放射性标记，并分离出各个顺式和反式异构体，用于正常雌性裸鼠中的生物分布研究。主要摄取器官是肝脏、肠和肾脏，其中FGC化合物的肝脏摄取量最高。在比较非对映体时，顺式化合物比反式化合物具有显着更高的器官摄取和更慢的血液清除率。
Biologically active molecules, such as many peptides, serve as targeting vectors for radiopharmaceuticals based on Tc-99m. Tripeptides can be suitable chelates and are easily and conveniently synthesized and linked to peptide targeting vectors through solid-phase peptide synthesis and form stable (TcO)-O-V complexes. Upon complexation with [TcO](3+), two products form; these are syn and anti diastereomers, and they often have different biological behavior. This is the case with the approved radiopharmaceutical [(TcO)-Tc-99m]depreotide ([(TcO)-Tc-99m]P829, NeoTect) that is used to image lung cancer. [(TcO)-Tc-99m]depreotide indeed exhibits two product peaks in its HPLC profile, but assignment of the product peaks to the diastereomers has proven to be difficult because the metal peptide complex is difficult to crystallize for structural analysis. In this study, we isolated diastereomers of [(TcO)-Tc-99] and [ReO] complexes of several tripeptide ligands that model the metal chelator region of [(TcO)-Tc-99m]depreotide. Using X-ray crystallography, we observed that the early eluting peak (A) corresponds to the anti diastereomer, where the TcO group is on the opposite side of the plane formed by the ligand backbone relative to the pendant groups of the tripeptide ligand, and the later eluting peak (B) corresponds to the syn diastereomer, where the TcO group is on the same side of the plane as the residues of the tripeptide. H-1 NMR and circular dichroism (CD) spectroscopy report on the metal environment and prove to be diagnostic for syn or anti diastereomers, and we identified characteristic features from these techniques that can be used to assign the diastereomer profile in Tc-99m peptide radiopharmaceuticals like [(TcO)-Tc-99m]depreotide and in Re-188 peptide radiotherapeutic agents. Crystallography, potentiometric titration, and NMR results presented insights into the chemistry occurring under physiological conditions. The tripeptide complexes where lysine is the second amino acid crystallized in a deprotonated metallo-amide form, possessing a short N-1-M bond. The pK(a) measurements of the N-1 amine (pKa similar to 5.6) suggested that this amine is rendered more acidic by both metal complexation and the presence of the lysine residue. Furthermore, peptide chelators incorporating a lysine (like the chelator of [TcO]depreotide) likely exist in the deprotonated form in vivo, comprising a neutral metal center. Deprotonation possibly mediates the interconversion process between the syn and anti diastereomers. The N-1 amine group on non-lysine-containing metallopeptides is not as acidic (pK(a) similar to 6.8) and does not deprotonate and crystallize as do the metallo-amide species. Three of the tripeptide ligands (FGC, FSC, and FKC) were radiolabeled with Tc-99m, and the individual syn and anti isomers were isolated for biodistribution studies in normal female nude mice. The main organs of uptake were the liver, intestines, and kidneys, with the FGC compounds exhibiting the highest liver uptake. In comparing the diastereomers, the syn compounds had substantially higher organ uptake and slower blood clearance than the anti compounds.