<p>An agent in the form of nanoparticles in the range of 5-100 nanometer carrying radionuclides or metal poly-chelators within their coating material. The agent is produced by coating cores in the size of 2-50 nanometers having a hydrophobic layer from a fatty acid by a) mixing and reacting a poly maleic anhydride having a hydrophobic tale (compound I) with b) an amino terminal polymer (compound II) in an amount of at least one amino group from the amino terminal polymer( compound II) to at least one malemide group from (compound I) to form c) a multi-amid structure with amides and carboxylic groups side by side d) whereafter said structure is added to the hydrophobic layer coated surface of the cores, whereby the hydrophobic layer is made to get in contact with the hydrophobic tail of compound I and whereby the core is coated and a poly-chelator network is formed around the core during the coating process to form the final product which is a nanoparticle with the polymer (compound II) as the outer component of the formed nanoparticle as well as the use of the agent in radiotherapy and/or medical imaging.</p>
