AIM: To evaluate the multi-step pretargeting radioimmunoimaging (RII) and radioimmunotherapy (RIT) in nude mice bearing human colon carcinoma with avidin-biotin system labeled with 153Sm.METHODS: Two- and three-step strategies for avidinbiotin system pretargeting techniques were established.In a three-step procedure, human colon carcinoma bearing nude mice were first injected with biotinylated monoclonal antibody (McAb-Bt) followed by cold avidin (Av) 48 h later and then 153Sm-DB2 24 h thereafter;whereas the twostep procedure consisted of injection of 153Sm-SA 48 h after pretargeting with biotinylated anti-CEA monoclonal antibody (CEA McAb-Bt). SPECT imaging and biodistribution were performed at 4, 24, 48, or 72 h after injection of 153Sm-labeled compounds. Five groups of nude mice subcutaneously grafted with human colon carcinoma were treated 3 d after grafting. One group received the injection with 100 μg CEA McAb-Bt followed by cold avidin (80 μg)after 2 d and 11.1 MBq 153Sm-DB2 after 1d. Four control groups were treated respectively with 11.1 MBq 153SmCEA McAb, 11.1 MBq 153Sm-nmIgG, 11.1 MBq 153Sm-DB2,100 Μl normal saline. Toxicity was evaluated by changes of leukocyte count, and the efficacy by variation in tumor volume. Histological analyses of tumors were performed.RESULTS: The three-step procedure allowed faster blood clearance and yielded higher tumor blood ratios (5.76 at4 h and 12.94 at 24 h) of the 153Sm-DB2. The tumor was clearly visualized at 4 h in y-imaging after the injection of 153Sm-DB2, while a significant accumulation of 153Sm-SA in the tumor was observed only 24 h after the injection and tumor blood ratios at 4 and 24 h were 1.00 and 2.03,respectively, in the two-step procedure. Pretargeting RIT and 153Sm-CEA McAb had a strong tumor-inhibiting effect.The tumor inhibitory rate was 80.67% and 78.44%,respectively, five weeks after therapy. Histopathological evidence also indicated radioactive damage in tumor tissues as necrosis of tumor cells, while in the other organs such as liver and kidney no radioactive damage was observed. Leukocyte counts showed significant decrease after treatment in groups of 153Sm-CEA Mc Ab and 153SmnmIgG.CONCLUSION: The two kinds of pretargeting strategies can elevate the target-to-nontarget ratio, decrease the blood background and shorten the imaging time compared to 153Sm-CEA McAb. Three-step pretargeting RIT is as efficient as 153Sm-CEA Mc Ab, but markedly less toxic. This study provides experimental evidence for the clinical application of pretargeting RII and RIT.