Thiosemicarbazones : coordination properties in relation to biological activity

Thiosemicarbazones are considered to be potential therapeutics, because they possess a broad range of biological properties including antitumor, antimalarial and antimicrobial activity. Generally, the tiosemicarbazones coordinate to the metal centre by means of an (N,S) bidentate mode, and when an additional coordinating group is present, more diversified binding modes can occur such as a tridentate (X,N,S) coordination fashion. The stability of the metal complexes formed with the tiosemicarbazoness strongly depends on the character of the metal ion, the X-donor atom of the additional functional group and the position and type of the substituents at the tiosemicarbazones. The most prominent representative of this family is the α(N)-heterocyclic Triapine (3-aminopyridine- 2-carbaldehyde thiosemicarbazone; 3-AP), which is currently undergoing different phase-I and -II clinical trials as an antitumor agent, and demonstrates promising activity. Triapine is a very strong inhibitor of ribonucleotide reductase, the rate determining enzyme in the supply of deoxyribonucleotides for DNA synthesis required for cell proliferation. The mechanism of action involves most probably the formation of an iron(II)–Triapine complex, which reacts with molecular oxygen to result in the generation of reactive oxygen species. Subsequently, these reactive oxygen species are responsible for the quenching of the active-site tyrosyl radical of ribonucleotide reductase required for the enzymatic activity. As a result, the coordination chemistry of iron complexes of tiosemicarbazones has been receiving considerable attention. This review describes the coordination chemistry of tiosemicarbazones, in particular analogs of Triapine. The coordination compounds of d-block elements are discussed with respect to their bonding and structures. Several of complexes are mononuclear, with distorted tetrahedral, square planar, square pyramid or octahedral as their common geometries. The metal-binding ability of STSC at physiological pH was compared and shown. Further, various biological applications with emphasis an anticancer activity of the ligands/complexes are discussed in brief so as to indicate the importance of ligands under consideration