CHEMICAL DESIGN OF RADIOIODINATED LABELING AGENTS WITH A CLEAVABLE LINKAGE FOR RADIOIMMUNODETECTION AND RADIOIMMUNOTHERAPY OF TUMOR

已经作出努力，利用单抗实现靶向选择性的放射性传递。最近对放射性标记多肽和抗体的代谢研究表明，溶酶体蛋白分解后产生的放射性代谢物对组织的放射性水平起着至关重要的作用。基于这些发现，我们通过多肽连接将偏碘马尿酸连接到抗体上，这种连接提供了放射性碘化合物与血浆中抗体的稳定结合，同时促进了放射性标记化合物在溶酶体中的快速和选择性释放。在不损害靶区放射性水平的情况下实现了肝脏放射性水平的降低。另一方面，由于放射性碘标记氨基酸(IAA)在肿瘤…中的滞留时间较长，已被发现可用作肿瘤成像的放射性药物比非靶细胞中的细胞多。这些特性使得IAA作为从溶酶体中的抗体中释放出来的放射性代谢物具有吸引力，用于靶向成像和治疗。在这个设计中，抗体和IAA之间的连接在血浆中应该足够稳定，以保持抗体传递的靶体内的放射性水平，而溶酶体隔间需要快速和选择性地从抗体中释放所设计的IAA。由于溶酶体的低pH环境是众所周知的，因此对溶酶体选择性释放IAA的适用性进行了评估。我们以前的研究中选择了体内代谢稳定性高、尿量大的IAA。与艾氏腹水癌细胞孵育时，放射性碘α-甲基-L酪氨酸(I-L-AMT)和D-酪氨酸(I-D-MIT)在肿瘤细胞中有较高的蓄积和滞留。放射性碘标记的酪氨酸衍生物I-L-AMT和I-D-MIT具有显著的肿瘤蓄积活性。为了评价其对酸不稳定键的pH依赖性断裂作用，以对羟基去甲麻黄碱为模型化合物，分别与顺丁烯二酸酐偶联，制得甲基丙烯酸甲酯或柠檬酸酐。纯化后，用氯胺T法对MNE和CNE进行放射性碘化。两种放射性碘化合物均在37℃、不同pH的缓冲溶液中孵育。虽然I-MNE和I-CNE在pH 7.0和7.4下孵育48小时后都保持稳定，但在较低的pH值下，两种结合物都选择性地释放了I-PHNE。此外，I-CNE释放I-PHNE的速度比I-MNE快得多，这可能是由于CNE中存在较大的侧链所致。这项研究的发现将为未来设计释放溶酶体内放射性碘化氨基酸的放射性标记抗体提供良好的基础。较少

Efforts have been made to achieve target-selective radioactivity delivery using monoclonal antibodies. Recent metabolic studies of radiolabeled polypeptides and antibodies indicated that radiometabolites generated after lysosomal proteolysis play a critical role in the radioactivity levels of the tissues. Based on the findings, we conjugated meta-iodohippuric acid to antibodies via a peptide linkage that provides a stable attachment of the radioiodinated compound with antibodies in plasma while facilitating rapid and selective release of the radiolabeled compound in lysosomes. Reduction of hepatic radioactivity levels was achieved without impairing the radioactivity levels in the target. However, this approach may also reduce target radioactivity levels when applied to antibodies that are internalized into target cells.On the other hand, radioiodinated amino acids (IAAs) have been found to be useful as radiopharmaceuticals for tumor imaging due to their longer residence times in tumor … More cells than those in non-target cells. Such characteristics rendered IAAs attractive as radiometabolites liberated from antibodies in lysosomes for targeted imaging and therapy. In this design, the linkage between antibodies and IAAs should be stable enough in plasma to preserve the radioactivity levels in the target delivered by antibodies, whereas rapid and selective release of the designed IAAs from antibodies are required in the lysosomal compartment.In the present study, tumor accumulation and retention of some IAAs were evaluated. Since lower pH environment of the lysosomes is well known, applicability of acid-labile linkages was estimated for lysosome-selective release of the IAAs. The IAAs that possess high in vivo metabolic stability and high urinary excretion characteristics were selected in our previous studies. When incubated with Ehrlich acite tumor cells, radioiodinated α-methyl-L-tyrosine (I-L-AMT) and D-tyrosine (I-D-MIT) showed high accumulation and retention in the tumor cells. Radioiodinated tyrosine derivatives such as I-L-AMT and I-D-MIT were found to be prominent in tumor accumulation.In order to evaluate the pH-dependent cleavage of acid-labile linkages, p-hydroxynorephedrine (PHNE), used as a model compound, was conjugated with maleic anhydride to prepare MNE or citraconic anhydride to prepare CNE, respectively. After purification, radioiodination of MNE and CNE was performed by chloramine T method. Both radioiodinated compounds were incubated at 37℃ in buffered-solutions at various pH. Although both I-MNE and I-CNE remained stable after 48 hr incubation at pH 7.0 and 7.4, selective release of I-PHNE was observed from both conjugates at lower pH values. In addition, I-CNE liberated I-PHNE at a rate much faster than that of I-MNE, presumably due to a presence of bulky side chain in CNE. The findings in this study would provide a good basis for future design of radiolabeled antibodies that liberate radioiodinated amino acids in the lysosomal compartments. Less