A nanoparticle system for the treatment of metastatic cancer is provided. The nanoparticle system includes a plurality of nanoparticles. Each of the nanoparticles includes a biodegradable matrix, a polysulphonated naphthylurea, and a chemotherapeutic agent. The biodegradable matrix can be a natural polysaccharide, a protein, a peptide, or a derivative thereof. Further, the polysulphonated naphthylurea can include suramin or a pharmaceutically acceptable salt thereof, and the chemotherapeutic agent can include an anthracycline. In addition, each of the nanoparticles has a diameter ranging from about 20 nanometers to about 400 nanometers. A method of forming the nanoparticle system and a method of treating cancer in a mammal with the nanoparticle system are also provided.