Pallela V R, Thakur M L, Chakder S, Rattan S
Department of Radiology, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania 19107, USA.
J Nucl Med. 1999 Feb;40(2):352-60.
Vasoactive intestinal peptide (VIP) is a naturally occurring 28-amino acid peptide with a wide range of biological activities. Recent reports suggest that VIP receptors are expressed on a variety of malignant tumor cells and that the receptor density is higher than for somatostatin. Our aims were to label VIP with 99mTc--a generator-produced, inexpensive radionuclide that possesses ideal characteristics for scintigraphic imaging--and to evaluate 99mTc-VIP for bioactivity and its ability to detect experimental tumors.
VIP28 was modified at the carboxy terminus by the addition of four amino acids that provided an N4 configuration for a strong chelation of 99mTc. To eliminate steric hindrance, 4-aminobutyric acid (Aba) was used as a spacer. VIP28 was labeled with 1251, which served as a control. Biological activity of the modified VIP28 agonist (TP3654) was examined in vitro using a cell-binding assay and an opossum internal anal sphincter (IAS) smooth muscle relaxivity assay. Tissue distribution studies were performed at 4 and 24 h after injection, and receptor-blocking assays were also performed in nude mice bearing human colorectal cancer LS174T. Blood clearance was examined in normal Sprague-Dawley rats.
The yield of 99mTc-TP3654 was quantitative, and the yields of 125I-VIP and 1251-TP3654 were >90%. All in vitro data strongly suggested that the biological activity of 99mTc-TP3654 agonist was equivalent to that of VIP28. As the time after injection increased, radioactivity in all tissues decreased, except in the receptor-enriched tumor (P = 0.84) and in the lungs (P = 0.78). The tumor uptake (0.23 percentage injected dose per gram of tissue [%ID/g]) was several-fold higher than 125I-VIP (0.06 %ID/g) at 24 h after injection in the similar system. In mice treated with unlabeled VIP or TP3654, the uptake of 99mTc-TP3654 decreased in all VIP receptor-rich tissues except the kidneys. The blood clearance was biphasic; the alpha half-time was 5 min and the beta half-time was approximately 120 min.
VIP28 was modified and successfully labeled with 99mTc. The results of all in vitro examinations indicated that the biological activity of TP3654 was equivalent to that of native VIP28 and tumor binding was receptor specific.
血管活性肠肽（VIP）是一种天然存在的28个氨基酸的肽，具有广泛的生物活性。最近的报道表明，VIP受体在多种恶性肿瘤细胞上表达，且受体密度高于生长抑素。我们的目的是用99mTc标记VIP——一种由发生器产生的、价格低廉的放射性核素，具有闪烁显像的理想特性——并评估99mTc-VIP的生物活性及其检测实验性肿瘤的能力。
通过添加四个氨基酸对VIP28的羧基末端进行修饰，为99mTc的强螯合提供N4构型。为消除空间位阻，使用4-氨基丁酸（Aba）作为间隔物。用125I标记VIP28作为对照。使用细胞结合试验和负鼠内括约肌（IAS）平滑肌松弛试验在体外检测修饰后的VIP28激动剂（TP3654）的生物活性。在注射后4小时和24小时进行组织分布研究，并且在荷人结肠直肠癌LS174T的裸鼠中也进行受体阻断试验。在正常的Sprague-Dawley大鼠中检测血液清除情况。
99mTc-TP3654的产率是定量的，125I-VIP和125I-TP3654的产率>90%。所有体外数据强烈表明99mTc-TP3654激动剂的生物活性与VIP28相当。随着注射后时间的增加，除富含受体的肿瘤（P = 0.84）和肺（P = 0.78）外，所有组织中的放射性均降低。在相似系统中，注射后24小时肿瘤摄取（每克组织0.23%注射剂量[％ID/g]）比125I-VIP（0.06％ID/g）高几倍。在用未标记的VIP或TP3654处理的小鼠中，除肾脏外，99mTc-TP3654在所有富含VIP受体的组织中的摄取均降低。血液清除呈双相性；α半衰期为5分钟，β半衰期约为120分钟。
VIP28经修饰后成功用99mTc标记。所有体外检查结果表明，TP3654的生物活性与天然VIP28相当，且肿瘤结合具有受体特异性。
