Facile Preparation of a Thiol-Reactive (18)F-Labeling Agent and Synthesis of (18)F-DEG-VS-NT for PET Imaging of a Neurotensin Receptor-Positive Tumor.
Accumulating evidence suggests that neurotensin receptors (NTRs) play key roles in cancer growth and survival. In this study, we developed a simple and efficient method to radiolabel neurotensin peptide with (18)F for NTR-targeted imaging.


METHODS
The thiol-reactive reagent (18)F-(2-(2-(2-fluoroethoxy)ethoxy)ethylsulfonyl)ethane ((18)F-DEG-VS) was facilely prepared through 1-step radiofluorination. After high-pressure liquid chromatography purification, (18)F-DEG-VS was incubated with the c(RGDyC) and c(RGDyK) peptide mixture to evaluate its specificity toward the reactive thiol. Thiolated neurotensin peptide was then labeled with (18)F using this novel synthon, and the resulting imaging probe was subjected to receptor-binding assay and small-animal PET studies in a murine xenograft model. The imaging results and metabolic stability of (18)F-DEG-VS-NT were compared with the thiol-specific maleimide derivative N-[2-(4-(18)F-fluorobenzamido)ethyl]maleimide-neurotensin ((18)F-FBEM-NT).


RESULTS
(18)F-DEG-VS was obtained in high labeling yield. The reaction of (19)F-DEG-VS was highly specific for thiols at neutral pH, whereas the lysine of c(RGDyK) reacted at a pH greater than 8.5. (18)F-DEG-VS-c(RGDyC) was the preferred product when both c(RGDyK) and c(RGDyC) were incubated together with (18)F-DEG-VS. Thiolated neurotensin peptide (Cys-NT) efficiently reacted with (18)F-DEG-VS, with a 95% labeling yield (decay-corrected). The radiochemical purity of the (18)F-DEG-VS-NT was greater than 98%, and the specific activity was about 19.2 ± 4.3 TBq/mmol. Noninvasive small-animal PET demonstrated that (18)F-DEG-VS-NT had an NTR-specific tumor uptake in subcutaneous HT-29 xenografts. The tumor-to-muscle, tumor-to-liver, and tumor-to-kidney ratios reached 30.65 ± 22.31, 11.86 ± 1.98, and 1.91 ± 0.43 at 2 h after injection, respectively, based on the biodistribution study. Receptor specificity was demonstrated by blocking experiment. Compared with (18)F-FBEM-NT, (18)F-DEG-VS-NT was synthesized with fewer steps and provided significantly improved imaging quality in vivo.


CONCLUSIONS
We have established a facile (18)F-labeling method for site-specific labeling of the Cys-NT. Using this method, we synthesized an NTR-targeted PET agent, which demonstrated high tumor-to-background contrast.
Department of Diabetes and Metabolic Diseases Research, Beckman Research Institute of the City of Hope, Duarte, California.
Zhanhong
Z
Wu
Department of Immunology, Beckman Research Institute of the City of Hope, Duarte, California.
Lin
L
Li
Department of Radiology, Molecular Imaging Center, University of Southern California, Los Angeles, California; and.
Shuanglong
S
Liu
Department of Molecular Medicine, Beckman Research Institute of the City of Hope, Duarte, California.
Fumiko
F
Yakushijin
Department of Molecular Medicine, Beckman Research Institute of the City of Hope, Duarte, California.
Kenichi
K
Yakushijin
Department of Molecular Medicine, Beckman Research Institute of the City of Hope, Duarte, California.
David
D
Horne
Department of Radiology, Molecular Imaging Center, University of Southern California, Los Angeles, California; and.
Peter S
PS
Conti
Department of Radiology, Molecular Imaging Center, University of Southern California, Los Angeles, California; and ziboli@usc.edu JShively@coh.org.
Zibo
Z
Li
Department of Diabetes and Metabolic Diseases Research, Beckman Research Institute of the City of Hope, Duarte, California.
Fouad
F
Kandeel
Department of Immunology, Beckman Research Institute of the City of Hope, Duarte, California ziboli@usc.edu JShively@coh.org.
John E
JE
Shively
D000230:Adenocarcinoma / Q000000981:diagnostic imaging* / Q000378:metabolism / Q000473:pathology; D060326:Chemistry Techniques, Synthetic; D003110:Colonic Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism / Q000473:pathology; D004355:Drug Stability; D019073:HT29 Cells; D006801:Humans; D007553:Isotope Labeling; D009496:Neurotensin / Q000031:analogs & derivatives* / Q000138:chemical synthesis* / Q000378:metabolism / Q000493:pharmacokinetics; D049268:Positron-Emission Tomography / Q000379:methods*; D011854:Radiochemistry; D018028:Receptors, Neurotensin / Q000378:metabolism*; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D014018:Tissue Distribution; D014753:Vinyl Compounds / Q000138:chemical synthesis* / Q000378:metabolism / Q000493:pharmacokinetics
D018028:Receptors, Neurotensin; D013438:Sulfhydryl Compounds; D014753:Vinyl Compounds; D009496:Neurotensin
18F; PET; neurotensin; vinyl sulfone