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Activity and electrochemical properties: iron complexes of the anticancer drug triapine and its analogs

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Abstract

Triapine (3-AP), is an iron-binding ligand and anticancer drug that is an inhibitor of human ribonucleotide reductase (RNR). Inhibition of RNR by 3-AP results in the depletion of dNTP precursors of DNA, thereby selectively starving fast-replicating cancer cells of nucleotides for survival. The redox-active form of 3-AP directly responsible for inhibition of RNR is the Fe(II)(3-AP)2 complex. In this work, we synthesize 12 analogs of 3-AP, test their inhibition of RNR in vitro, and study the electronic properties of their iron complexes. The reduction and oxidation events of 3-AP iron complexes that are crucial for the inhibition of RNR are modeled with solution studies. We monitor the pH necessary to induce reduction in iron complexes of 3-AP analogs in a reducing environment, as well as the kinetics of oxidation in an oxidizing environment. The oxidation state of the complex is monitored using UV–Vis spectroscopy. Isoquinoline analogs of 3-AP favor the maintenance of the biologically active reduced complex and possess oxidation kinetics that allow redox cycling, consistent with their effective inhibition of RNR seen in our in vitro experiments. In contrast, methylation on the thiosemicarbazone secondary amine moiety of 3-AP produces analogs that form iron complexes with much higher redox potentials, that do not redox cycle, and are inactive against RNR in vitro.

Graphic abstract

The catalytic subunit of human Ribonucleotide Reductase (RNR), contains a tyrosyl radical in the enzyme active site. Fe(II) complexes of 3-AP and its analogs can quench the radical and, subsequently, inactivate RNR. The potency of RNR inhibitors is highly dependent on the redox properties of the iron complexes, which can be tuned by ligand modifications. Complexes are found to be active within a narrow redox window imposed by the cellular environment.

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Acknowledgements

Special thanks to Jon Brosmer for helpful discussions.

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  1. Daniel Sun and Juno Van Valkenburgh contributed equally.

Authors and Affiliations

  1. Department of Chemistry, University of California, Los Angeles, CA, 90095, USA

    Sheba Plamthottam, Daniel Sun, Juno Van Valkenburgh, Jeffrey Valenzuela, Bastian Ruehle, Dalton Steele, Maxim Marshalik, Selena Hernandez & Jeffrey I. Zink

  2. Department of Molecular and Medical Pharmacology, University of California, Los Angeles, CA, 90095, USA

    Sheba Plamthottam, Daniel Sun, Juno Van Valkenburgh, Dalton Steele, Soumya Poddar & Caius Gabriel Radu

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  1. Sheba Plamthottam
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775_2019_1675_MOESM1_ESM.pdf

Supplementary material 1 The following files are available free of charge. Complete Fe(II) titrations, pH titrations, oxidation spectra, and cyclic voltammograms of all 3-AP analog complexes (Figures S1–S5); Synthesis procedures for compounds; HPLC traces of 3-AP analogs; 1H-NMR and 13C-NMR spectra (PDF) (PDF 5714 kb)

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Plamthottam, S., Sun, D., Van Valkenburgh, J. et al. Activity and electrochemical properties: iron complexes of the anticancer drug triapine and its analogs. J Biol Inorg Chem 24, 621–632 (2019). https://doi.org/10.1007/s00775-019-01675-0

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  • DOI: https://doi.org/10.1007/s00775-019-01675-0

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