[en] Abstract Background: B2-S22-AFA is a kind of small molecule mimetic polypeptide that can conjugate to epidermal growth factor receptor 2 (HER-2) specifically and has obvious depressant effect on breast cancer cells with overexpressing HER-2. Purpose: This study aims to investigate the specific killing effect of ¹³¹I-labeled B2-S22-AFA on breast cancer cell with overexpressing HER-2. Methods: ¹³¹I-B2-S22-AFA was prepared by using N-bromosuccinimide method to measure the labeling efficiency, radiochemical purity, stability, and immunocompetence. The expression levels of HER-2 in SKBR3 and MDA-MB-231 cells were detected by Immunohistochemistry and Western Blot. The inhibitory rate of B2-S22-AFA on cell growth was observed with epidermal growth factors (EGF) at different concentrations. Five groups i.e, B2-S22-AFA group (B2-S22-AFA final concentration: 1 μg·mL^-1), ¹³¹I-B2-S22-AFA group (¹³¹I-B2-S22-AFA final concentration: 144 kBq/1 μg·mL^-1), ¹³¹I group (¹³¹I final concentration: 144 kBq·mL^-1), negative control group and reagent blank group, were selected for contrast tests with the final concentration of EGF in each group at 5.0 pg·mL^-1. The proliferation and activity of cells were measured by Methyl thiazolyl tetrazolium assay. The inhibitory rates were compared among ¹³¹I-B2-S22-AFA group, B2-S22-AFA group and ¹³¹I group in the growth of the two kinds of cells at different time. Results: 1) HER-2 receptors were strongly expressed in SKBR3 cells while weakly (low) or no expressed in MDA-MB-231 cells. The radiochemical purity, labeling rate and specific activity of ¹³¹I-B2-S22-AFA were 95.0%-97.6%, 64.8%-79.6% and 151.7 MBq·mg^-1, respectively. Radiochemical purity of ¹³¹I-B2-S22-AFA was 95.4%, 93.5%, 91.4%, 88.2% and 77.4% when it was placed at 37 ℃ in serum for 4 h, 12 h, 24 h, 48 h and 72 h, respectively. The maximum binding rates of SKBR3 and MDA-MB-231 cells were (66.47 ± 3.24)% and (3.89 ± 0.81)% (tested for 3 times) respectively. 2) The inhibitory effect of B2-S22-AFA on the growth of SKBR3 cells only in the presence of EGF. 3) The killing effect of ¹³¹I-B2-S22-AFA on SKBR3 cells was significantly stronger than that of B2-S22-AFA and ¹³¹I (p < 0.05), and the inhibitory effect of B2-S22-AFA on the growth of SKBR3 cells was significantly higher than that of ¹³¹I (p < 0.05). ¹³¹I-B2-S22-AFA and B2-S22-AFA had no obvious killing effect on MDA-MB-231 cells. Conclusion: ¹³¹I-B2-S22-AFA can significantly enhance and accelerate the specific killing effect of B2-S22-AFA on breast cancer cells with high HER-2 expression. (authors)