Ribonucleotide reductase inhibition by metal complexes of Triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone): A combined experimental and theoretical study

Triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone, 3-AP) is currently the most promising chemotherapeutic compound among the class of α-N-heterocyclic thiosemicarbazones. Here we report further insights into the mechanism(s) of anticancer drug activity and inhibition of mouse ribonucleotide reductase (RNR) by Triapine. In addition to the metal-free ligand, its iron(III), gallium(III), zinc(II) and copper(II) complexes were studied, aiming to correlate their cytotoxic activities with their effects on the diferric/tyrosyl radical center of the RNR enzyme in vitro. In this study we propose for the first time a potential specific binding pocket for Triapine on the surface of the mouse R2 RNR protein. In our mechanistic model, interaction with Triapine results in the labilization of the diferric center in the R2 protein. Subsequently the Triapine molecules act as iron chelators. In the absence of external reductants, and in presence of the mouse R2 RNR protein, catalytic amounts of the iron(III)–Triapine are reduced to the iron(II)–Triapine complex. In the presence of an external reductant (dithiothreitol), stoichiometric amounts of the potently reactive iron(II)–Triapine complex are formed. Formation of the iron(II)–Triapine complex, as the essential part of the reaction outcome, promotes further reactions with molecular oxygen, which give rise to reactive oxygen species (ROS) and thereby damage the RNR enzyme. Triapine affects the diferric center of the mouse R2 protein and, unlike hydroxyurea, is not a potent reductant, not likely to act directly on the tyrosyl radical.

Graphical abstractAntiproliferative activity and inhibition of mouse ribonucleotide reductase by Triapine and its Fe(III), Ga(III), Zn(II) and Cu(II) complexes are reported. Molecular docking provides evidence for a specific binding pocket for Triapine on the surface of the R2 RNR protein. A mechanism of enzyme inhibition by Triapine is proposed.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Triapine binds specifically to mouse R2 RNR protein. ► Binding of Triapine results in labilization of diferric center in R2 RNR protein. ► Triapine does not directly reduce tyrosyl radical in R2 RNR protein. ► Metal ion that is part of Triapine complex affects the extent of R2 RNR inhibition.