therapeutic particles contain metal ions and are characterized by the use of unique ligand sets capable of making the metal ion complex soluble in biological media to induce selective toxicity in diseased cells . the particles may comprise a polymeric base particle , at least one pharmaceutically active metal ion , including metal ions from more than one metal element , a ligand that is covalently attached to the polymeric base particle and attached to the metal ion via a stimuli - responsive bond , and a cell targeting component . when the metal ion - containing particle enters a pre - defined environment , the ligands binding the metal to the particle are broken , triggering release of the free metal ion while the original ligands remain covalently bound to the particle .