［Abstract］ Background and purpose: Researches had indicated that about over 85% of malignant tumors highly express telomerase activity. So t

elomerase has become one of the important methods in the research field of tumor diagnosis and treatment. Nowadays

several reports about malignant tumor which over expresses hTERT targeting imaging with radionuclide labeled hTERT ASON had been published. In these reports

high quality of pictures can hardly be acquired because of poor anatomical and spacial resolution in nuclear imaging itself. Accordingly

in this study

we developed a method of detecting human telomerase in vivo with magnetic resonance imaging (MRI) and evaluate its feasibility. Methods: Firstly

Uniformly phosphorothioate-modified human telomerase reverse transcriptase antisense oligonucleotide (hTERT ASON) was labeled with Gd

3+

through the bifunctional chelator 1

4

7

10-tetraazacyclododecane-N

N’

N’’

N’’’-tetraacetic acid (DOTA) and iv vitro experiments were performed to characterize the antisense probes (for biodistribution and cellular uptake

99m

Tc-DOTA-ASON was used in stead of Gd- DOTA-ASON). Then Gd-DOTA-ASON was injected intraperitoneally in pulmonary adenocarcinoma A375 nude mice tumor-bearing BALB/c for in vivo imaging using 7.0 T Micro MRI periodically

tumors and their surrounding tissues were defined as region of interest (ROI) to calculate the signal to noise ratio (SNR) of tumor to muscle using Gd-DTPA as control. Finally

immunohistochemical analysis of telomerase activity of each xenograft was operated 2 days after imaging. Results: The binding efficiency of Gd-DOTA-ASON reached was as high as 65% (63.2±2.4

n=6). And it can maintain 61% in fresh human serum and normal saline at 37 ℃ over 24 h; A375 cells showed an uptake of 8.5% when incubated with 99mTc -DOTA-ASON; In comparing with DOTA-ASON and Gd-DTPA

cells transected with Gd-DOTA-ASON had higher SI when performed MRI with T1WI. The hTERT-expressing xenografts were obviously enhanced by Gd-DOTA-ASON at 0.5–6 h after injection and the SNR can reach 2.37

whereas obvious enhancement only could be found within 2 h after injection of Gd-DTPA. Both

labeled and non-labeled antisense probes can suppress the activity of telomerase of A375 cells either in vitro or in vitro. Conclusion: Our research offers proof that Gd-DOTAASON can be used as tumor specific targeting MR probe for diagnosing malignant tumors with high expression of telomerase.