Triapine Analogues and Their Copper(II) Complexes: Synthesis, Characterization, Solution Speciation, Redox Activity, Cytotoxicity, and mR2 RNR Inhibition.

Iuliana Besleaga,Anatoly D Shutalev,Iryna Stepanenko,Ana Popović-Bijelić,Małgorzata A Marć,Gabriella Spengler,Peter Rapta,Marta Hammerstad,Eva A Enyedy,Tatsiana V Petrasheuskaya,Denisa Darvasiova,Alexander Prado-Roller,Vladimir B Arion,Martin Breza

doi:10.1021/acs.inorgchem.1c01275

Abstract

Three new thiosemicarbazones (TSCs) HL1–HL3 as triapine analogues bearing a redox-active phenolic moiety at the terminal nitrogen atom were prepared. Reactions of HL1–HL3 with CuCl2·2H2O in anoxic methanol afforded three copper(II) complexes, namely, Cu(HL1)Cl2 (1), [Cu(L2)Cl] (2′), and Cu(HL3)Cl2 (3), in good yields. Solution speciation studies revealed that the metal-free ligands are stable as HL1–HL3 at pH 7.4, while being air-sensitive in the basic pH range. In dimethyl sulfoxide they exist as a mixture of E and Z isomers. A mechanism of the E/Z isomerization with an inversion at the nitrogen atom of the Schiff base imine bond is proposed. The monocationic complexes [Cu(L1–3)]+ are the most abundant species in aqueous solutions at pH 7.4. Electrochemical and spectroelectrochemical studies of 1, 2′, and 3 confirmed their redox activity in both the cathodic and the anodic region of potentials. The one-electron reduction was identified as metal-centered by electron paramagnetic resonance spectroelectrochemistry. An electrochemical oxidation pointed out the ligand-centered oxidation, while chemical oxidations of HL1 and HL2 as well as 1 and 2′ afforded several two-electron and four-electron oxidation products, which were isolated and comprehensively characterized. Complexes 1 and 2′ showed an antiproliferative activity in Colo205 and Colo320 cancer cell lines with half-maximal inhibitory concentration values in the low micromolar concentration range, while 3 with the most closely related ligand to triapine displayed the best selectivity for cancer cells versus normal fibroblast cells (MRC-5). HL1 and 1 in the presence of 1,4-dithiothreitol are as potent inhibitors of mR2 ribonucleotide reductase as triapine.

Highlights

The X-ray diffraction study of 4 (Figure 6a) revealed that the ligand underwent an oxidative dehydrogenation accompanied by the intramolecular cyclization via a C−S coupling reaction between a phenolic carbon and a thione group into a five-membered thiazole ring instead of the expected oxidative dehydrogenation of the 3,5-dimethyl-1,4-aminophenol unit with formation of a 3,5-dimethyl-1,4-benzoquinone imine moiety
Indicate that the Z/E isomerization involves an inversion at the aldimine nitrogen atom, rather than a tautomeric shift of the thioamide N2H proton to the pyridine nitrogen, followed by a rotation around the C−N1 bond as suggested previously.[44]
The relatively high Gibbs free energy barrier (∼35.3 kcal/mol) for the Z/E conversion rules out the possibility of an isomerization at room temperature, in agreement with timedependent NMR data

Summary

■ INTRODUCTION

Thiosemicarbazones (TSCs) are known as biologically active compounds with a broad spectrum of pharmacological properties, including anticancer activity.[1−4] These properties can be modulated by coordination to physiologically relevant metal ions.[5,6] In addition, as versatile ligands, TSCs have tunable electronic and steric properties, which may have a favorable effect on their pharmacological profile.[7−10] α-N-. To a solution of 3-(tert-butoxycarbonyl)amino-2-formylpyridine (210 mg, 0.95 mmol) and 4-(4-hydroxy-3,5-dimethylphenyl)thiosemicarbazide (200 mg, 0.95 mmol) in a mixture of ethanol/water 3:1 (8 mL) was added dropwise 12 M HCl (0.19 mL, 2.28 mmol) This solution was stirred at room temperature for 1 h to give Boc-HL3·HCl Details about the synthesis and characterization of oxidized thiosemicarbazones and their copper(II) complexes, X-ray data collection and refinement (Tables S1−S3), elemental analysis, UV−vis titrations, kinetic measurements, lipophilicity determination, spectroelectrochemical studies, in vitro cell studies, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assays, and tyrosyl radical reduction in mouse R2 RNR protein as well as computational details are given in the Supporting Information (Sections 1 and 2)

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES