OBjective: Vascular endothelial growth factor receptor-2(KDR)is one of the potential ideal target for molecular imaging and molecular therapy of tumor.HTMYYHHYQHHL (K237) was selected from random peptides libraries using phage display technology and can bind selectively to the KDR expressed on cytomembrane of new vascular endothelial cell in tumor.The purpose of this study was to investgate the possibility of 99Tcm labelled K237 modified with MAG3 as a molecular imaging agent for detecting of KDR-expressing tumors.Methods: K237 was prepared by conventional Fmoc solid phase technology.During synthesis,three glycine residues were linked to the N-terminal of K237, acylation reactions of GGGHTMYYHHYQHHL at N-terminal were performed and the producte of S-acetyl-MAG3-K237 (MAG3-K237) was obtained.It was labeled with technetium-99m using stannous tartrate as reductant, optimized labelling conditions were determined by using a orthogonal design.Radiochemical analysis and stability studys of the radiolabeled peptide were performed by HPLC, Whatman 3MM ascending paper chromatography, Oil-Water partition test,cysteine challenge assay and serum protein binding test respectively.Tracerkinetics,biodistribution and scintigraphy studies were carried out in health rabbits, mice and hepatocellular carcinoma xenograft nude mice respectively.Results: The chromatographic purity of MAG3-K237 was 98.0%.The labelling yield and specific activity of 99Tcm-MAG3-K237 was (97.12 ± 0.49)% and (30.58 ± 0.14) TBq/mmol respectively;oil/water apparent partition coefficient was (2.46 ± 0.12)%; radiochemical purity was still greater 95% after placeing at room temperature for 8 h; no significant displacement of 99Tcm from 99Tcm-MAG3-K237 was observed by using 400 mmol/L cysteine; incorporation of the labeled peptide with serum proteins was only about 2.1%.Pharmacokinetics of the radiolabeled conjugate fitted by the three-compartment model with a weight of 1/c2 in health rabbits, the t1/2α, t1/2β and t1/2γ was (4.0 ± 3.5), (24.5 ± 9.8) and (852.2 ± 444.0) min respectively.Biodistribution studys showed that both the blood and liver radioactivity decreased 90.6% and 93.8% within first 10 min and 30 min postinjection in normal mice, the radioactivity was mainly eliminated and quickly excreted through kidneys and hepatobiliary system, and the uptake of radioactivity was quite low in the brain, heart and muscle.Scintigraphy studys in health rabbits demonstrated that radioactivity excretion of the liver and gallbladder was obviously faster than that of the biodistribution in mice,and then no visualization could be observed both in thyroid and stomach within 240 min.Serial planar imaging of tumor bearing HepG2 xenograft nude mice using SPECT showed that radioactivity accumulation was clearly visualized at the tumor site within 6.0 h.Comparing the uptake in the tumor vs that in the muscle on contralat distric, the T/NT ratio was 3.96, 4.78, 4.55,4.19 and 4.05 respectively at 0.5, 1.0, 2.0, 4.0 and 6.0 h after injection.The image of tumor was inhibited by K237 in dose-dependent.When injected dose of K237 was 0, 10, 50 and 500 μtg respectively, the T/NT ratio was 4.78, 3.76, 2.99 and 2.10 at 60 min after injection, and the inhibition ratio of K237 blocking uptake of 99mTc-MAG3-K237 in the tumor was 21.3%, 37.4%and 56.1% respectively.Conclusions: This study indicates that 99Tcm-MAG3-K237 is a potential molecular imaging agent for diagnosis of KDR-expressing tumors.