Triapine Derivatives Act as Copper Delivery Vehicles to Induce Deadly Metal Overload in Cancer Cells

Abstract

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.

Coumarin-Based Triapine Derivatives and Their Copper(II) Complexes: Synthesis, Cytotoxicity and mR2 RNR Inhibition Activity

“…Endogenous thiols are likely able to reduce copper(II) complexes of TSCs [68] to less stable copper(I) species that may dissociate. The released proligand can act as an Fe-chelator as was already unambiguously confirmed for other TSCs, including morpholine-thiosemicarbazone hybrids [26] and 3-amino-2pyridinecarboxaldehyde-S-methylisothiosemicarbazone [64], and consequently may reduce the tyrosyl radical in R2 RNR. Additionally, thiols may react with Cu(II) ions to form mixtures of Cu(I) and Cu(II) complexes, which could behave either as antioxidants or pro-oxidants, depending on the molar ratio of reactants [69].…”

“…Absorption spectra measured upon cathodic reduction of 2 in the region of the first reduction peak revealed an increase of the initial optical bands at 338 nm and simultaneous decrease of the of the S→Cu(II) CT band (~420 nm) (Figure 3a). There is only partial recovery of the initial optical band after scan reversal (Figure 3b), which provides further evidence that Cu(I) state is not stable, and the complex decomposes with the ligand (or new ligand product) release [64]. Characteristic for Cu(II), the intensity of the room temperature X-band EPR spectrum of 2 decreased upon stepwise application of a negative potential at the first reduction peak, indicating the formation of a d 10 EPRsilent Cu(I) species (see inset in Figure 3b).…”

“…It is worth noting that the presence of DTT did not have an effect on radical destruction by this complex, indicating that, although the diferric center of mR2 becomes reduced by DTT and therefore causes the tyrosyl radical to be more susceptible for reduction [13,72], complex 3 may be too large to enter the mR2 hydrophobic pocket that harbours the tyrosyl radical, due to the bulky dimethyl-protected phenolic group of the proligand HL 3 . Furthermore, when compared to triapine, which reduces 100% tyrosyl radical in 3 min, [64] neither HL 1 nor 1 are shown to be efficient mR2 inhibitors. Although HL 1 may chelate iron from mR2 and form the redox active [Fe II (L 1 ) 2 ] complex, it is evident that this species is devoid of the potent inhibitory activity observed previously for [Fe II (3-AP) 2 ] [12,13].…”

“…Cyclic voltammograms (CVs) of 1-4 show one irreversible or quasi-reversible oneelectron reduction wave between −0.87 and −0.91 V vs. Fc + /Fc (Figure 2a and Figure S4a in ESI †) that corresponds to the reduction of the Cu(II) to Cu(I) [26,64]. The irreversible oxidation wave at E pa = 0.1 V vs. Fc + /Fc (Figure 2b) can be only seen for complex 3 with potentially redox-active 2,6-dimethyl-4-aminophenol unit.…”

Modulation of Intracellular Copper Levels as the Mechanism of Action of Anticancer Copper Complexes: Clinical Relevance

“…Several Cu(II) complexes of 3-AP and its analogues did not demonstrate any improvement in cytotoxicity [ 188 , 292 , 293 ]. However, a large number of other pre-formed Cu(II)-TSC complexes were significantly more cytotoxic than their metal-free counterparts [ 139 , 290 , 294 , 295 ]. Moreover, the most cytotoxic Cu(II)-TSC complexes (nanomolar concentration range) demonstrated quick cancer killing activity (≈3 h), which was related to the rapid generation of the deadly ROS insult [ 290 ].…”