Abstract: Objective To establish a labeling method for a specific lung cancer-targeting small molecule peptide cNGQGEQc
with 131I and observe the radioactivity distribution of the labeled peptide in rabbits using single-photon emission computed
tomography (SPECT). Methods Chloramine-T method was used for 131I labeling of the tyrosine amino group on cNGQGEQc,
and the labeling efficiency and radiochemical purity of 131I-cNGQGEQc were determined with paper chromatography. The
stability of 131I-cNGQGEQc in saline and human serum was assessed after incubation in water bath at 37 ℃ for 24 h. The
octanol-water partition coefficient lg P (the radioactivity counting ratio of 131I-cNGQGEQc dissolved in 100 μl octanol or in 100
μl saline) was calculated. SPECT was performed in 3 male New Zealand white rabbits after intravenous injection of
131I-cNGQGEQc to observe the dynamic distribution of the peptide with the time-radioactivity curve (T-A curve) of the region
of interest (ROI). Results With a labeling efficiency of 90%, 131I-cNGQGEQc showed a radiochemical purity of was 95% after
purification with HPLC. The radiochemical purity of 131I-cNGQGEQc was (93.12±1.18)% and (88.34±5.43)% after intubation in
saline and human serum for 24 h, respectively. The octanol-water partition coefficient lg P of 131I-cNGQGEQc was -1.75,
suggesting its hydrosolubility. In rabbits with intravenous injection of 131I-cNGQGEQc, SPECT visualized the kidneys at 1 min
after the injection; the imaging of the heart and liver became attenuated at 5 min when the bladder was visualized with an
increasing radioactivity. The radioactivity of the soft tissues began to fade at 30 min. No gallbladder visualization was
detected, and the radioactivity of the abdomen remained low. No obvious radioactivity concentration was observed in the
thyroid and stomach. The T-A curves of the ROI of all the tissues and organs descended over time. Conclusion Radiolabeling
of cNGQGEQc with 131I is simple and highly efficient. 131I-cNGQGEQc has good stability in vitro and good distribution
characteristics for in vivo imaging, and is cleared mainly by renal excretion due to its hydrosolubility. These results provide
experimental basis for further studies of diagnosis and therapy of lung cancer with targeting polypeptide.