The invention provides a low-molecular-weight (15 kDa) over-sulfated exopolysaccharide (GYS15) prepared from a marine native exopolysaccharide excreted by a mesophilic marine bacterium from a deep-sea hydrothermal environment, and relates to the use of this low-molecular-weight over-sulfated exopolysaccharide for the prevention or inhibition of metastases formation.