Tc-99m-labeled vasoactive intestinal peptide receptor agonist: Functional studies

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 Tc-99m-a generator-produced, inexpensive radionuclide that possesses ideal characteristics for scintigraphic imaging-and to evaluate T c-99m-VIP for bioactivity and its ability to detect experimental tumors. Me thods: VIP28 was modified at the carboxy terminus by the addition of four a mino acids that provided an N-4 configuration for a strong chelation of Tc- 99m. To eliminate steric hindrance, 4-aminobutyric acid (Aba) was used as a spacer. VIP28 was labeled with I-125, which served as a control. Biologica l activity of the modified VIP28 agonist (TP3654) was examined in vitro usi ng a cell-binding assay and an opossum internal anal sphincter (IAS) smooth muscle relaxivity assay. Tissue distribution studies were performed at 4 a nd 24 h after injection, and receptor-blocking assays were also performed i n nude mice bearing human colorectal cancer LS174T. Blood clearance was exa mined in normal Sprague-Dawley rats. Results: The yield of Tc-99m-TP3654 wa s quantitative, and the yields of I-125-VIP and I-125-TP3654 were >90%. All in vitro data strongly suggested that the biological activity of 99mTc-TP3 654 agonist was equivalent to that of VIP28. As the time after injection in creased, radioactivity in all tissues decreased, except in the receptor-enr iched tumor(P = 0.84) and in the lungs (P = 0.78). The tumor uptake (0.23 p ercentage injected dose per gram of tissue [%ID/g]) was several-fold higher than I-125-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 d ecreased in all VIP receptor-rich tissues except the kidneys. The blood cle arance was biphasic; the alpha half-time was 5 min and the beta half-time w as approximately 120 min. Conclusion: VIP28 was modified and successfully l abeled with 99mTc. The results of all in vitro examinations indicated that the biological activity of TP3654 was equivalent to that of native VIP28 an d tumor binding was receptor specific.