Six morpholine-(iso)thiosemicarbazone hybrids HL1–HL6 and their Cu(II) complexes with good-to-moderate solubility and stability in water were synthesized and characterized. Cu(II) complexes [Cu(L1–6)Cl] (1–6) formed weak dimeric associates in the solid state, which did not remain intact in solution as evidenced by ESI-MS. The lead proligands and Cu(II) complexes displayed higher antiproliferative activity in cancer cells than triapine. In addition, complexes 2–5 were found to specifically inhibit the growth of Gram-positive bacteria Staphylococcus aureus with MIC50 values at 2–5 μg/mL. Insights into the processes controlling intracellular accumulation and mechanism of action were investigated for 2 and 5, including the role of ribonucleotide reductase (RNR) inhibition, endoplasmic reticulum stress induction, and regulation of other cancer signaling pathways. Their ability to moderately inhibit R2 RNR protein in the presence of dithiothreitol is likely related to Fe chelating properties of the proligands liberated upon reduction.

A series of water-soluble sodium salts of 3-formyl-4-hydroxybenzenesulfonic acid thiosemicarbazones (or sodium 5-sulfonate-salicylaldehyde thiosemicarbazones) containing different substituents at the terminal nitrogen atom (H, Me, Et, Ph) and their copper(ii) complexes have been prepared and characterised by elemental analysis, spectroscopic techniques (IR, UV-vis, H NMR), ESI mass spectrometry, X-ray crystallography and cyclic voltammetry. The proligands and their copper(ii) complexes exhibit moderate water solubility and good stability in aqueous environment, determined by investigating their proton dissociation and complex formation equilibria. The copper(ii) complexes showed moderate anticancer activity in established human cancer cell lines, while the proligands were devoid of cytotoxicity. The anticancer activity of the copper(ii) complexes correlates with their ability to induce ROS accumulation in cells, consistent with their redox potentials within the biological window, triggering the activation of antioxidation defense mechanisms in response to the ROS insult. These studies pave the way for the investigation of ROS-inducing copper(ii) complexes as prospective antiproliferative agents in cancer chemotherapy.

Thiosemicarbazones continue to attract the interest of researchers as potential anticancer drugs. For example, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone, or triapine, is the most well-known representative of this class of compounds that has entered multiple phase I and II clinical trials. Two new triapine derivatives HL1 and HL2 were prepared by condensation reactions of 2-pyridinamidrazone and S-methylisothiosemicarbazidium chloride with 3-N-(tert-butyloxycarbonyl) amino-pyridine-2-carboxaldehyde, followed by a Boc-deprotection procedure. Subsequent reaction of HL1 and HL2 with CuCl2·2H2O in 1:1 molar ratio in methanol produced the complexes [CuII(HL1)Cl2]·H2O (1·H2O) and [CuII(HL2)Cl2] (2). The reaction of HL2 with Fe(NO3)3∙9H2O in 2:1 molar ratio in the presence of triethylamine afforded the complex [FeIII(L2)2]NO3∙0.75H2O (3∙0.75H2O), in which the isothiosemicarbazone acts as a tridentate monoanionic ligand. The crystal structures of HL1, HL2 and metal complexes 1 and 2 were determined by single crystal X-ray diffraction. The UV-Vis and EPR spectroelectrochemical measurements revealed that complexes 1 and 2 underwent irreversible reduction of Cu(II) with subsequent ligand release, while 3 showed an almost reversible electrochemical reduction in dimethyl sulfoxide (DMSO). Aqueous solution behaviour of HL1 and 1, as well as of HL2 and its complex 2, was monitored as well. Complexes 1−3 were tested against ovarian carcinoma cells, as well as noncancerous embryonic kidney cells, in comparison to respective free ligands, triapine and cisplatin. While the free ligands HL1 and HL2 were devoid of antiproliferative activity, their respective metal complexes showed remarkable antiproliferative activity in a micromolar concentration range. The activity was not related to the inhibition of ribonucleotide reductase (RNR) R2 protein, but rather to cancer cell homeostasis disturbance—leading to the disruption of cancer cell signalling.

A novel proligand HLH has been prepared from 2-pyridinamidrazone and 6-(morpholinomethyl)­pyridine-2-carboxaldehyde. Subsequently, starting from HLH and CuCl2·2H2O in the presence of triethylamine, a series of homo- and heterometallic complexes with [CuICl2]− as counteranion, namely mononuclear complex [CuIICl­(HLOMe)]­[CuICl2] (1), dinuclear Ru­(II)–Cu­(II) complex [CuIIRuII(η6-p-cymene)­Cl2(LH)]­[CuICl2]·1.5H2O (2·1.5H2O), Os­(II)–Cu­(II) complex [CuIIOsII(η6-p-cymene)­Cl2(LH)]­[CuICl2]·H2O (3·H2O), and tetranuclear complex [CuII 4(L1)2(L2)2Cl2]·5MeOH (4·5MeOH), where HLOMe, L1, and L2 are Schiff base ligands derived from HLH, were synthesized. The structures of metal complexes were established by X-ray diffraction. Complexes 1–3 demonstrated quasireversible one-electron reduction of Cu­(II) in complex cations and one-electron oxidation of [CuICl2]− counterion, confirmed by UV–vis and EPR spectroelectrochemical measurements. They exhibited moderate antiproliferative activity against ovarian carcinoma and cervical adenocarcinoma cell lines in a micromolar concentration range, which was superior to the activity of HLH, CuCl2, and [CuICl2]−. The observed anticancer effects might be related to the ability of novel complexes to generate ROS, which was established by spin trapping experiments in cell-free media as well as by confocal microscopic imaging in ovarian carcinoma cells.

In C6H6N2O2·H2O, the N-hy­droxy­picolinamide mol­ecule adopts a strongly flattened conformation. O—H⋯O inter­actions and π–π stacking inter­actions between the pyridine rings organize the crystal components into columns extending along the b axis while N—H⋯N hydrogen bonds link these columns into a two-dimensional framework parallel to (100).