The development of new metal anticancer compounds is a challenge for inorganic chemists. Four decades of research in this field have only produced a small number of clinically used compounds, most often developed through serendipity rather than through rational chemical design. Nevertheless, medicinal inorganic chemistry is probably mature for making significant steps forward. We suggest here a categorization of metal anticancer compounds into five classes based on their mode of action: (i) the metal has a functional role, i.e. it must bind to the biological target; (ii) the metal has a structural role, i.e. it is instrumental in determining the shape of the compound and binding to the biological target occurs through non-covalent interactions; (iii) the metal is a carrier for active ligands that are delivered in vivo; (iv) the metal compound is a catalyst; and (v) the metal compound is photoactive and behaves as a photo-sensitizer. Selected examples for each category are given.
