由8個胺基酸組成的環型胜肽，octreotide，是一種體抑素類似物，對體抑素受體具有高度親和力。近年，利用系統性的受器標靶或代謝性引導的放射核種治療術用於治療神經內分泌腫瘤在核子醫學界引起廣泛的興趣。[90Y-DOTA0-Tyr3]octreotide和[177Lu-DOTA0,Tyr3]octreotate為目前臨床上用以治療此類腫瘤的藥物。然而，它們會造成在腎臟或骨髓的高輻射劑量，對人體產生一定的副作用。
本研究欲嘗試以錸188(I)三羰基離子[188Re(OH2)3(CO)3]+為前驅物直接標誌耦合有3個組胺酸（histidine）之octreotide，簡稱為188Re(I)-his3-octreotide，作為治療表現體抑素受器之胰臟腫瘤治療劑。
　　實驗方法：本研究以固相胜肽合成法製備his3-octreotide。以市售的sst2 receptor進行his3-octreotide受器競爭試驗。以錸188(I)三羰基離子為前驅物直接標誌his3-octreotide，得188Re(I)-his3-octreotide，經高效能液相層析(HPLC)進行放射化學純度分析，並以C18 Sep-Pak管柱進行純化，去除未反應的[188Re(OH2)3(CO)3]+或188ReO4-。測試188Re(I)-his3-octreotide在生理食鹽水及血清中之穩定度。以植有AR42J腫瘤的小鼠，進行生物分佈及造影等試驗。
結果：由體抑素受器競爭實驗，證實his3-octreotide對體抑素受器保持有高度親和力，其IC50接近octreotide之值。[188Re(OH2)3(CO)3]+合成產率可達85 %左右。經純化的188Re(I)-his3-octreotide的放化純度可達95 %以上。在穩定度試驗中，188Re(I)-his3-octreotide在生理食鹽水中的穩定度在48小時後降至60 %，可能受水中氧氧化的緣故，造成不穩定。在人類血清中之穩定度在48小時降至50 %，而在大鼠血清中則降至10 %。在生物分佈試驗中，發現在肝臟、腎臟及脾臟有大量放射活度累積。肝臟累積可能是由於188Re(I)-his3-octreotide 在血清中穩定度不佳，分解出之[188Re(OH2)3(CO)3]+與血清蛋白結合之產物大量累積於肝臟。而在腎臟累積則符合文獻報導胜肽生物分佈的結果。在腫瘤之累積在1小時及4小時分別為1.17 ± 0.41 %ID/g及2.83 ± 0.74 %ID/g，在8小時仍維持在2.41 ± 0.45 %ID/g，但在24小時則降至1.19 ± 0.3 %ID/g，在48小時降至0.88 ± 0.2 %ID/g。在活體動物造影與生物分佈趨勢相似，大量放射活度累積在肝臟、腎臟及脾臟，腫瘤位置在注射後1小時及4小時可見放射活度累積的影像，顯示188Re(I)-his3-octreotide對具體抑素受體之腫瘤的專一性，但可能因此188Re(I)標誌octreotide試劑穩定度不佳，經長時間（24小時後）不見有腫瘤累積。在另注入過量未標誌的his3-octreotide發現AR42J腫瘤從一開始即不見有放射活度累積的影像，印證188Re(I)-his3-octreotide對體抑素受器確實具有專一性。
結論：由實驗結果顯示，在體抑素受器競爭實驗，his3-octreotide與體抑素受器有很高的親和力。但在活體內試驗，可能因188Re(I)-his3-octreotide在血液中穩定度不佳，僅在4小時內有較些微的腫瘤累積。由本研究所得結論，若將his3-octreotide進一步改為his6-octreotide，再以[188Re(OH2)3(CO)3]+標誌，製備為188Re(I)-his6-octreotide，預期若此188Re標誌胜肽耦合物有良好穩定度，值得發展為新的神經內分泌腫瘤的造影及治療試劑。

Octreotide, a somatostatin analogue, is consisted of eight amino acids as a cyclic octapeptide and has a high binding affinity to somatostatin receptor 2 (sst2). Recently, systematic receptor-targeted or metabolically directed radiotherapy using various radioisotopes for therapy of gastroenteropancreatic neuroendocrine tumors (GEP-NETs) has aroused extensive interest in nuclear medicine. [90Y-DOTA0-Tyr3]octreotide and [177Lu-DOTA0,Tyr3]octreotate are currently clinically used for GEP-NETs therapy. However, they may cause high radiation dose in kidney or bone marrow. The aim of this study was to develop a novel radiolabeled octreotide analogue, 188Re(I)-his3-octreotide, as an imaging and therapeutic agent for neuroendocrine tumors.
Methods：First, octreotide was conjugated with three histidines, referred to his3-octreotide, by a solid phase peptide synthesizer. Binding affinity of his3-octreotide for sst2 was measured by competitive binding assay. Then, [188Re(OH2)3(CO)3]+ was prepared as a precursor and labeled directly to his3-octreotide. The resultant solution containing 188Re(I)-his3-octreotide was measured for its radiochemical purity by high performance liquid chromatography (HPLC) and purified by C18 Sep-Pak cartridge. In vitro stability and in vivo targeting in AR42J-bearing mice were assessed.
Results：In competitive binding assay, his3-octreotide showed high affinity for sst2 and IC50 value was similar to the value of octreotide. The radiochemical purity of [188Re(OH2)3(CO)3]+ reached about 85 %. The radiochemical purity of purified 188Re(I)-his3-octreotide achieved greater than 95 %. In stability test, the stability of 188Re(I)-his3-octreotide decreased to 60 % in saline at 48 h post-injection. However, in human serum and rat serum, the stability of 188Re(I)-his3-octreotide decreased to 50 % and 10 %, respectively at 48 h post-injection. In biodistribution study, it was indicated that 188Re(I)-his3-octreotide had high accumulations in liver, kidneys, and spleen. In microSPECT/CT imaging, the AR42J tumor in the SCID mouse could be slightly visualized at 1 and 4 h postinjection. The AR42J tumor uptake of 188Re(I)-his3-octreotide was blocked completely in the case of additional administration of a large excess amount (100 μg) of unlabeled his3-octreotide.
Conclusion：From the work, it is evident from the in vivo study that his3-octreotide exhibits a high affinity for sst2. However, the accumulation of the 188Re(I) labeled peptide in AR42J tumor in the animal was slightly displayed within 4 h post-injection but become dim later. The poor uptake in the tumor might be due to the unstability of the agent in blood. If 188Re(I)-his3-octreotide could be modified further to be 188Re(I)-his6-octreotide, the new designed 188Re(I) labeled octreotide would become stable and worthy to be developed as an imaging and therapeutic agent for GEP-NETs.

1.W. H. Bakker, R. Albert, C. Bruns, W. A. P. Breeman, L. J. Hofland, P. Marbach, J. Pless, D. Pralet, B. Stolz, J. W. Koper, S. W. J. Lamberts, T. J. Visser, and E. P. Krenning, [111In-DTPA-D-Phe1]-octreotide, a potential radiopharmaceutical for imaging of somatostatin receptor-positive tumors: synthesis, radiolabeling and in vitro validation. Life Sciences, 1991. 49(22): p. 1583-1591.
2.Gregory A. Kaltsas, G. Michael Besser, and Ashley B. Grossman, The Diag-nosis and Medical Management of Advanced Neuroendocrine Tumors. Endo-crine Reviews, 2004. 25(3): p. 458-511.
3.Gisbert Weckbecker, Ian Lewis, Rainer Albert, Herbert A. Schmid, Daniel Hoyer, and Christian Bruns, Opportunities in somatostatin research biological, chemical and therapeutic aspects. Nature Review Drug Discovery, 2003. 2(12): p. 999-1017.
4.Agnes Schonbrunn and Armen H. Tashjian, Jr., Characterization of functional receptors for somatostatin in rat pituitary cells in culture. The Journal of Bio-logical Chemistry, 1978. 253(18): p. 6473-6483.
5.Terry Reisine, and Graeme I. Bell, Molecular Biology of Somatostatin Recep-tors*. Endocrine Reviews, 1995. 16(4): p. 427-442.
6.Jeffrey P. Norenberg, Boudewijn J. Krenning, Inge R. H. M. Konings, Donna F. Kusewitt, Tapan K. Nayak, Tamara L. Anderson, Marion de Jong, Kayhan Garmestani, Martin W. Brechbiel, and Larry K. Kvols, 213Bi-[DOTA0,Tyr3]Octreotide Peptide Receptor Radionuclide Therapy of Pancreatic Tumors in a Preclinical Animal Model. Clinical Cancer Research, 2006. 12(3): p. 897-903.
7.Paul Brazeau, Wylie Vale, Roger Burgus, Nicholas Ling, Madalyn Butcher, Jean Rivier, and Roger Guillemin, Hypothalamic Polypeptide That Inhibits the Secretion of Imunoreactive Pituitary Growth Hormone. Science, 1973. 179(4068): p. 77-79.
8.Sylvie Froidevaux, and Alex N. Eberle, Somatostatin Analogs and Radiopep-tides in Cancer Therapy. Biopolymers (Peptide Science), 2002. 66: p. 161-183.
9.Steven W. J. Lamberts, Aart-Jan Van Der Lely, Wouter W. De Herder, and Leo J. Hofland, Octreotide. The New England Journal Of Medicine, 1996. 334(4): p. 246-254.
10.Kjell Oberg, Future Aspects of Somatostatin-Receptor-Mediated Therapy. Neuro-endocrinology, 2004. 80(suppl 1): p. 57-61.
11.Roger Schibli, Rolf Schwarzbach, Roger Alberto, Kirstin Ortner, Helmut Schmalle, Cecile Dumas, Andre Egli, and P. August Schubiger, Steps toward high specific activity labeling of biomolecules for therapeutic application: preparation of precursor [188Re(OH2)3(CO)3]+ and synthesis of tailor-made bifunctional ligand systems. Bioconjugate Chemistry, 2002. 13: p. 750-756.
12.E. Garcia-Garayoa, R. Schibli, and P. A. Schubiger, Peptides radiolabeled with Re-186/188 and Tc-99m as potential diagnostic and therapeutic agents. Nuclear Science and Techniques, 2007. 18(2): p. 88-100.
13.Roger Alberto, Roger Schibli, Andre Egli and August P. Schubiger, A novel organometallic aqua complex of technetium for the labeling of biomolecules: [99mTc(OH2)3(CO)3]+ from [99mTcO4]- in aqueous solution and its reaction with a bifunctional ligands. Journal of American Chemical Society, 1998. 120(31): p. 7987-7988.
14.Kuo-Ting Chen, Te-Wei Lee, and Jem-Mau Lo, In vivo examination of 188Re(I)-tricarbonyl-labeled trastuzumab to target HER2-overexpressing breast cancer. Nuclear Medicine and Biology, 2009. 36: p. 355-361.
15.H. J. Pietzsch, A. Gupta, M. Reisgys, A. Drews, S. Seifert, R. Syhre, H. Spies, R. Alberto, U. Abram, P. A. Schubiger, and B. Johannsen, Chemical and Bio-logical characterization of technetium(I) and rhenium(I) tricarbonyl complexes with dithioether ligands serving as linkers for coupling the Tc(CO)3 and Re(CO)3 moieties to biologically active molecules. Bioconjugate Chemistry, 2000. 11: p. 414-424.
16.André Egli, Roger Alberto, Lesley Tannahill, Roger Schibli, Ulrich Abram, Andreas Schaffland, Robert Waibel, Dirk Tourwé, Lauren Jeannin, Koen Iter-beke and P. August Schubiger, Organometallic 99mTc-Aquaion Labels Peptide to an Unprecedented High Specific Activity The Journal of Nuclear Medicine, 1999. 40(11): p. 1913-1917.
17.Jorg Willuda, Susanne Kubetzko, Robert Waibel, P. August Schubiger, Uwe Zangemeister-Wittke, and Andreas Pluckthun, Tumor Targeting of Mono-, Di-, and Tetravalent Anti-p185HER-2 Miniantibodies Multimerized by Self-associating Peptides*. The Journal of Biological Chemistry, 2001. 276(17): p. 14385–14392.
18.Wan-Jou Chen, Shu-Pei Chiu, Yu-En Cheng, Te-Wei Lee, Chih-Hao Kao, and Jem-Mau Lo, New Octreotide Derivatives for 99mTc-Tricarbonyl Labeling to Target Somatostatin sst2 Receptor. Journal of Labeled Compounds ＆ Radi-opharnaceuticals, 2009. 52(Supp. 1): p. S1-S540.
19.Jiyun Shi, Bing Jia, Zhaofei Liu, Zhi Yang, Zilin Yu, Kai Chen, Xiaoyuan Chen, Shuang Liu, and Fan Wang, 99mTc-Labeled Bombesin(7-14)NH2 with Favorable Properties for SPECT Imaging of Colon Cancer. Bioconjugate Chemistry, 2008. 19(6): p. 1170–1178.
20.Matthias Miederer, Gjermund Henriksen, Andrea Alke, Ilona Mossbrugger, Leticia Quintanilla-Martinez, Reingard Senekowitsch-Schmidtke, and Markus Essler, Preclinical Evaluation of the alpha-Particle Generator Nuclide 225Ac for Somatostatin Receptor Radiotherapy of Neuroendocrine Tumors. Clinical Cancer Research, 2008. 14(11): p. 3555-3561.
21.Roger Schibli, Roberto La Bella, Roger Alberto, Elisa Garcia-Garayoa, Kirstin Ortner, Ulrich Abram, and P. A. Schubiger, Influence of the Denticity of Ligand Systems on the in Vitro and in Vivo Behavior of 99mTc(I)-Tricarbonyl Complexes: A Hint for the Future Functionalization of Biomolecules. Bioconjugate Chemistry, 2000. 11: p. 345-351.