Receptor-target imaging of small cell lung cancer (SCLC) xenograft in nude mice with 99mTc-labeled neurotensin peptide 8-13 (NT)

1550 Objectives: To prepare neurotension peptides (NT) labeled with 99mTechnetium ( 99mTc) for radiopharmaceutical application and evaluate the feasibility of imaging oncogene NT receptors overexpressed in human small cell lung cancer (SCLC) cells. Methods: NT analogue, (Na-His)Ac-NT(8-13) was synthesized such that histidine was attached at the N-terminus. The analogue was labeled with organometallic aquaion [99mTc(H2O)3(CO)3] at pH 7. In vitro stability of 99mTc-(Na-His)Ac-NT(8-13) was determined by challenging the compound with 100 times excess of DTPA, HSA and Cysteine. The affinity, 99mTc-(Na-His)Ac-NT(8-13) binding to the cell of the human small cell lung cancer (SCLC) cells line NCI-H446, was studied in vitro. Biodistribution studies were performed at 4 and 12h respectively after 99mTc-(Na-His)Ac-NT(8-13) was injected, and tissue distribution after receptor blocking was also performed at 4h in nude mice bearing human SCLC NCI H446. Imaging with 99mTc-(Na-His)Ac-NT(8-13) was performed at different time post-injection in nude mice bearing the SCLC tumor, and imaging after receptor blocking was also performed at 4h post-injection. Blood clearance was tested in normal mice. Results: The affinity constant which 99mTc-(Na-His)Ac-NT(8-13) binds to the cells of SCLC was obtained (Kd=0.56nmol/L). When 99mTc-(Na-His)Ac-NT(8-13) challenged with 100 times molar excess of DTPA, HSA, or cysteine, more than 97% radioactivity remained as 99mTc-(Na-His)Ac-NT(8-13). Biodistribution showed that the tumor to muscle ratio was 3.35±1.01 at 4h and 4.20±1.35 at 12h postinjection respectively. The excretory route of 99mTc-(Na-His)Ac-NT(8-13) was chiefly through the renal pathway in vivo in the nude mice bearing SCLC cells. In receptor-blocking mice treated with unlabeled (Na-His)Ac-NT(8-13), the uptake of 99mTc-(Na-His)Ac-NT(8-13) decreased in the tumor. The ratio of tumor to muscle at 4h (1.25±0.55) was significantly lower than that (3.35±1.01) in receptor-unblocking mice treated without unlabeled (Na-His)Ac-NT(8-13). The ratio of tumor to contralateral limb (4.81±0.89) at 4h postinjection obtained by technique of region of interest (ROI) was significantly higher than that (1.58±0.86) at 4h postinjection in receptor-blocking mice. Conclusions: The results suggest that the 99mTc-(Na-His)Ac-NT(8-13) specifically binds to the SCLC cells, with the significantly higher tumor uptake of 99mTc-(Na-His)Ac-NT(8-13), made 99mTc-(Na-His)Ac-NT(8-13) (Na-His)Ac-NT(8-13) highly desirable for further studies in SPECT imaging of oncogene receptors overexpressed in SCLC cells.