Voltage-gated sodium channel activity enhances the motilityand oncogene expression of metastasic cancer cells thatexpress a neonatal alternatively spliced form of the Na V 1.5isoform. We reported previously that FS50, a salivary proteinfrom Xenopsylla cheopis, showed inhibitory activity againstthe Na V 1.5 channel when assayed in HEK 293T cells andantiarrhythmia effects on rats and monkeys after inductionof arrhythmia by BaCl 2 . This study aims to identify the effectof FS50 on voltage-gated sodium channel activity and themotility of MDA-MB-231 human breast cancer cells in vitro.Na V 1.5 was abnormally expressed in the highly metastaticbreast cancer cell line MDA-MB-231, but not in the MCF-7cell line. FS50 significantly inhibited sodium current,migration, and invasion in a dose-dependent manner, buthad no effect on the proliferation of MDA-MB-231 cells atthe working concentrations (1.5–12μmol/l) after a long-term treatment for 48h. Meanwhile, FS50 decreased Na V 1.5mRNA expression without altering the total protein level inMDA-MB-231 cells. Correspondingly, the results alsoshowed that MMP-9 activity and the ratio of MMP-9 mRNAto TIMP-1 mRNA were markedly decreased by FS50. Takentogether, our findings highlighted for the first time aninhibitory effect of a salivary protein from a blood-feedingarthropod on breast cancer cells through the Na V 1.5channel. Furthermore, this study provided a new candidateleading molecule against antitumor cells expressing