Complex formation of ML324, the histone demethylase inhibitor, with essential metal ions: Relationship between solution chemistry and anticancer activity

N-(3-(dimethylamino)propyl-4-(8-hydroxyquinolin-6-yl)benzamide (ML324, HL) is a potent inhibitor of the iron-containing histone demethylase KDM4, a recognized potential target of cancer therapeutics. Herein, we report the proton dissociation and complex formation processes of ML324 with essential metal ions such as Fe(II), Fe(III), Cu(II) and Zn(II) using UV-visible, fluorescence, electron paramagnetic resonance and 1H NMR spectroscopic methods. The electrochemical behaviour of the copper and iron complexes was characterized by cyclic voltammetry and spectroelectrochemistry. The solid phase structure of ML324 analysed by X-ray crystallography is also provided. Based on the solution equilibrium data, ML324 is present in solution in H2L+ form with a protonated dimethylammonium moiety at pH 7.4, and this (N,O) donor bearing ligand forms mono and bis complexes with all the studied metal ions and the tris-ligand species is also observed with Fe(III). At pH 7.4 the metal binding ability of ML324 follows the order: Fe(II) < Zn(II) < Cu(II) < Fe(III). Complexation with iron resulted in a negative redox potential (E'1/2 = -145 mV vs. NHE), further suggesting that the ligand has a preference for Fe(III) over Fe(II). ML324 was tested for its anticancer activity in chemosensitive and resistant human cancer cells overexpressing the efflux pump P-glycoprotein. ML324 exerted similar activity in all tested cells (IC50 = 1.9-3.6 μM). Co-incubation and complexation of the compound with Cu(II) and Zn(II) had no impact on the cytotoxicity of ML324, whereas Fe(III) decreased the toxicity in a concentration-dependent manner, and this effect was more pronounced in the multidrug resistant cells.

Organometallic Ru(II), Rh(III) and Re(I) complexes of sterane-based bidentate ligands: synthesis, solution speciation, interaction with biomolecules and anticancer activity

In this study, we present the synthesis, characterization and in vitro cytotoxicity of six organometallic [Ru(II)(η6-p-cymene)(N,N)Cl]Cl, [Rh(III)(η5-C5Me5)(N,N)Cl]Cl and [Re(I)(CO)3(N,N)Cl] complexes, in which the (N,N) ligands are sterane-based 2,2'-bipyridine derivatives (4-Me-bpy-St-OH, 4-Ph-bpy-St-OH). The solution chemical behavior of the ligands and the complexes was explored by UV-visible spectrophotometry and 1H NMR spectroscopy. The ligands and their Re(I) complexes are neutral at pH = 7.40; this contributes to their highly lipophilic character (log D7.40 > +3). The Ru(II) and Rh(III) half-sandwich complexes are much more hydrophilic, and this property is greatly affected by the actual chloride ion content of the medium. The half-sandwich Ru and Rh complexes are highly stable in 30% (v/v) DMSO/water (<5% dissociation at pH = 7.40); this is further increased in water. The Rh(III)(η5-C5Me5) complexes were characterized by higher water/chloride exchange and pKa constants compared to their Ru(II)(η6-p-cymene) counterparts. The Re(I)(CO)3 complexes are also stable in solution over a wide pH range (2-12) without the release of the bidentate ligand; only the chlorido co-ligand can be replaced with OH- at higher pH values. A comprehensive discussion of the binding affinity of the half-sandwich Ru(II) and Rh(III) complexes toward human serum albumin and calf-thymus DNA is also provided. The Ru(II)(η6-p-cymene) complexes interact with human serum albumin via intermolecular forces, while for the Rh(III)(η5-C5Me5) complexes the coordinative binding mode is suggested as well. They are also able to interact with calf-thymus DNA, most likely via the coordination of the guanine nitrogen. The Ru(II)(η6-p-cymene) complexes were found to be the most promising among the tested compounds as they exhibited moderate-to-strong cytotoxic activity (IC50 = 3-11 μM) in LNCaP as well as in PC3 prostate cells in an androgen receptor-independent manner. They were also significantly cytotoxic in breast and colon adenocarcinoma cancer cell lines and showed good selectivity for cancer cells.

A Comparative Study on the Complexation of the Anticancer Iron Chelator VLX600 with Essential Metal Ions

As cancer cells exhibit an increased uptake of iron, targeting the interaction with iron has become a straightforward strategy in the fight against cancer. This work comprehensively characterizes the chemical properties of 6-methyl-3-{(2E)-2-[1-(2-pyridinyl)ethylidene]hydrazino}-5H-[1,2,4]triazino[5,6-b]indole (VLX600), a clinically investigated iron chelator, in solution. Its protonation processes, lipophilicity, and membrane permeability as well as its complexation with essential metal ions were investigated using UV-visible, electron paramagnetic resonance, and NMR spectroscopic and computational methods. Formation constants revealed the following order of metal binding affinity at pH 7.4: Cu(II) > Fe(II) > Zn(II). The structures of VLX600 (denoted as HL) and the coordination modes in its metal complexes [Cu(II)(LH)Cl2], [Cu(II)(L)(CH3OH)Cl], [Zn(II)(LH)Cl2], and [Fe(II)(LH)2](NO3)2 were elucidated by single-crystal X-ray diffraction. Redox properties of the iron complexes characterized by cyclic voltammetry showed strong preference of VLX600 toward Fe(II) over Fe(III). In vitro cytotoxicity of VLX600 was determined in six different human cancer cell lines, with IC50 values ranging from 0.039 to 0.51 μM. Premixing VLX600 with Fe(III), Zn(II), and Cu(II) salts in stoichiometric ratios had a rather little effect overall, thus neither potentiating nor abolishing cytotoxicity. Together, although clinically investigated as an iron chelator, this is the first comprehensive solution study of VLX600 and its interaction with physiologically essential metal ions.

Novel family of [RuCp(N,N)(P)]+ compounds with simultaneous anticancer and antibacterial activity: Biological evaluation and solution chemistry studies

A family of ten novel ruthenium(II)-cyclopentadienyl organometallics of general formula [Ru(η5-C5H5)(N,N)(PPh2(C6H4COOR)][CF3SO3] (1-10) in which (N,N) = 4,4'-R'-2,2'-bipyridyl (R = -H or -CH2CH2OH; R' = -H, -CH3, -OCH3, -CH2OH, and -CH2-biotin) was prepared from [Ru(η5-C5H5)(PPh2(C6H4COOH))2Cl]. All compounds were fully characterized by means of several spectroscopic and analytical techniques, and the molecular structures of [Ru(η5-C5H5)(PPh2(C6H4COOH))2Cl], 1, 3 and 4 have been additionally studied by single-crystal X-ray diffraction. The anticancer activity of all compounds was evaluated in sensitive and multidrug-resistant counterpart cell lines from human colorectal cancer (Colo 205 and Colo 320) and non-small cell lung cancer NSCLC (A549, NCI-H460 versus NCI-H460/R) as well. Notably, compounds 6 and 7 (R CH2CH2OH and (N,N) = bipy or Me2bipy, respectively) showed antiproliferative effect against both cell lines with high intrinsic selectivity towards cancer cells. The antibacterial activity of all compounds was also evaluated against both Gram negative and Gram positive strains, and some compounds in the series showed potent antibacterial activity against Staphylococcus aureus strains, including the methicillin-resistant MRSA strains. Solution speciation studies revealed that the complexes bearing the PPh2(C6H4COO-) ligand are neutral at physiological pH (7.4) in contrast with their ethylene glycol derivatives that have a permanent positive charge. While all compounds are lipophilic, the difference in the distribution coefficient for neutral and charged complexes is around one order of magnitude. Complexes 6 and 7 exhibited excellent biological activity and were selected for further studies. Spectrofluorometric methods were used to investigate their interaction with biomolecules such as human serum albumin (HSA) and calf thymus DNA (ct-DNA). For these complexes, binding site II of HSA is a possible binding pocket through non-covalent interactions. The release of ethidium from the DNA adduct by the charged complexes proves their interaction with DNA in contrast to the neutral ones. In conclusion, Ru(II)-cyclopentadienyl complexes with 2,2'-bipyridyl-derivatives and an ethylene glycol moiety tethered to the phenylphosphane co-ligand are very promising from a therapeutic perspective, in particular complexes 6 and 7 that display remarkable antibacterial activity with a high anti-proliferative effect against colon and non-small cell lung cancers, both clinically challenging neoplasias in need of effective solutions.

Structural and solution speciation studies on selected [Cu(NN)(OO)] complexes and an investigation of their biomimetic activity, ROS generation and their cytotoxicity in normoxic, hypoxic and anoxic environments in MCF-7 breast cancer-derived cells

Reactive oxygen species(ROS) generation with subsequent DNA damage is one of the principle mechanisms of action assigned to copper-based anticancer complexes. The efficacy of this type of chemotherapeutic may be reduced in the low oxygen environment of tumours. In this study the cytotoxicity of three complexes, [Cu(dips)(phen)] (1), [Cu(ph)(phen)]·2H2O (2) and [Cu(ph)(bpy)]·H2O (3) (disp: 3,5-diisopropylsalicylate, phen: 1,10- phenanthroline, ph: phthalate, bpy: 2,2'-bipyridyl) were assessed for anticancer activity in the breast-cancer derived MCF-7 line under normoxic, hypoxic and anoxic conditions. In an immortalised keratinocyte HaCaT cell line, the cytotoxicity of complexes 2 and 3 was significantly reduced under both normoxic and hypoxic conditions, whilst the cytotoxicity of complex 1 was increased under hypoxic conditions. The ability of the complexes to generate ROS in the MCF-7 cell line was evaluated as was their ability to act as superoxide dismutase(SOD) and catalase mimics using a yeast cell assay. ROS generation was significant for complexes 2 and 3, less so for complex 1 though all three complexes had SOD mimetic ability. Given the ternary nature of the complexes, solution speciation studies were undertaken but were only successful for complex 3, due to solubility issues with the other two complexes. The concentration distribution of various species, formed in aqueous solution, was evaluated as a function of pH and confirmed that complex 3 is the dominant species at physiological pH in the mM concentration range. However, as its concentration diminishes, it experiences a progressive dissociation, leading to the formation of binary complexes of bpy alongside unbound phthalate.

Iron(II)-cyclopentadienyl compounds are cytotoxic against colon adenocarcinoma cell lines: Ethylenebis(diphenylphosphane) vs. triphenylphosphane

Structure-activity studies aiming to understand the role of each coligand in the formulation of new metallodrugs is an important subject. In that frame, six new compounds with general formula [Fe(η5-C5H5)(dppe)(L)][CF3SO3] with L = benzonitriles (1-4) or carbon monoxide (5) and compound [Fe(η5-C5H5)(CO)(PPh3)2][CF3SO3] (6) were synthesized and compared with three other previously reported compounds [Fe(η5-C5H5)(CO)(L)(PPh3)][CF3SO3]. We were particularly interested in assessing the effect of dppe vs. (PPh3 + CO) for this set of compounds. For that, all compounds were tested against two human colon adenocarcinoma cell lines, Colo205 and the refractile Colo320 (expressing ABCB1, an efflux pump causing multidrug resistance), showing IC50 values in the micromolar range. The presence of dppe in the compound's coordination sphere over (PPh3 + CO) allows for more redox stable compounds showing higher cytotoxicity and selectivity, with improved cytotoxicity towards resistant cells that is not related to the inhibition of ABCB1. Further studies with GSH and H2O2 for selected compounds indicated that their antioxidant ability is not probably the main responsible for their cytotoxicity.

In Situ Bioconjugation of a Maleimide-Functionalized Ruthenium-Based Photosensitizer to Albumin for Photodynamic Therapy

Maleimide-containing prodrugs can quickly and selectively react with circulating serum albumin following their injection in the bloodstream. The drug-albumin complex then benefits from longer blood circulation times and better tumor accumulation. Herein, we have applied this strategy to a previously reported highly phototoxic Ru polypyridyl complex-based photosensitizer to increase its accumulation at the tumor, reduce off-target cytotoxicity, and therefore improve its pharmacological profile. Specifically, two complexes were synthesized bearing a maleimide group: one complex with the maleimide directly incorporated into the bipyridyl ligand, and the other has a hydrophilic linker between the ligand and the maleimide group. Their interaction with albumin was studied in-depth, revealing their ability to efficiently bind both covalently and noncovalently to the plasma protein. A crucial finding is that the maleimide-functionalized complexes exhibited significantly lower cytotoxicity in noncancerous cells under dark conditions compared to the nonfunctionalized complex, which is a highly desirable property for a photosensitizer. The binding to albumin also led to a decrease in the phototoxicity of the Ru bioconjugates in comparison to the nonfunctionalized complex, probably due to a decreased cellular uptake. Unfortunately, this decrease in phototoxicity was not compensated by a dramatic increase in tumor accumulation, as was demonstrated in a tumor-bearing mouse model using inductively coupled plasma mass spectrometry (ICP-MS) studies. Consequently, this study provides valuable insight into the future design of in situ albumin-binding complexes for photodynamic therapy in order to maximize their effectiveness and realize their full potential.

Cu(II) and Zn(II) Complexes of New 8-Hydroxyquinoline Schiff Bases: Investigating Their Structure, Solution Speciation, and Anticancer Potential

We report the synthesis and characterization of three novel Schiff bases (L1-L3) derived from the condensation of 2-carbaldehyde-8-hydroxyquinoline with amines containing morpholine or piperidine moieties. These were reacted with CuCl₂ and ZnCl₂ yielding six new coordination compounds, with the general formula ML₂, where M = Cu(II) or Zn(II) and L = L1-L3, which were all characterized by analytical, spectroscopic (Fourier transform infrared (FTIR), UV-visible absorption, nuclear magnetic resonance (NMR), or electron paramagnetic resonance (EPR)), and mass spectrometric techniques, as well as by single-crystal X-ray diffraction. In the solid state, two Cu(II) complexes, with L1 and L2, are obtained as dinuclear compounds, with relatively short Cu-Cu distances (3.146 and 3.171 Å for Cu₂(L1)₄ and Cu₂(L2)₄, respectively). The free ligands show moderate lipophilicity, while their complexes are more lipophilic. The pK_a values of L1-L3 and formation constants of the complex (for ML and ML₂) species were determined by spectrophotometric titrations, with the Cu(II) complexes showing higher stability than the Zn(II) complexes. EPR indicated the presence of several species in solution as pH varied and binding modes were proposed. The binding of the complexes to bovine serum albumin (BSA) was evaluated by fluorescence and circular dichroism (CD) spectroscopies. All complexes bind BSA, and as demonstrated by CD, the process takes several hours to reach equilibrium. The antiproliferative activity was evaluated in malignant melanoma cells (A375) and in noncancerous keratinocytes (HaCaT). All complexes display significant cytotoxicity (IC₅₀ < 10 μM) but modest selectivity. The complexes show higher activity than the free ligands, the Cu(II) complexes being more active than the Zn(II) complexes, and approximately twice more cytotoxic than cisplatin. A Guava ViaCount assay corroborated the antiproliferative activity.

First iron(II) organometallic compound acting as ABCB1 inhibitor

Five new iron (II) complexes bearing imidazole-based (Imi-R) ligands with the general formula [Fe(η⁵-C₅H₅)(CO)(PPh₃)(Imi-R)][CF₃SO₃] were synthesized and fully characterized by several spectroscopic and analytical techniques. All compounds crystallize in centrosymmetric space groups in a typical "piano stool" distribution. Given the growing importance of finding alternatives to overcome different forms of multidrug resistance, all compounds were tested against cancer cell lines with different ABCB1 efflux pump expression, namely, the doxorubicin-sensitive (Colo205) and doxorubicin-resistant (Colo320) human colon adenocarcinoma cell lines. Compound 3 bearing 1-benzylimidazole was the most active in both cell lines with IC₅₀ values of 1.26 ± 0.11 and 2.21 ± 0.26 μM, respectively, being also slightly selective against the cancer cells (vs. MRC5 normal human embryonic fibroblast cell lines). This compound, together with compound 2 bearing 1H-1,3-benzodiazole, were found to display very potent ABCB1 inhibitory effect. Compound 3 also showed the ability to induce cell apoptosis. Iron cellular accumulation studies by ICP-MS and ICP-OES methods revealed that the compounds' cytotoxicity is not related to the extent of iron accumulation. Yet, it is worth mentioning that, from the compounds tested, 3 was the only one where iron accumulation was greater in the resistant cell line than in the sensitive one, validating the possible role of ABCB1 inhibition in its mechanism of action.

A comparative study on the metal complexes of an anticancer estradiol-hydroxamate conjugate and salicylhydroxamic acid

Hydroxamic acids bearing an (O,O) donor set are well-known metal-chelating compounds with diverse biological activities including anticancer activity. Since steroid conjugation with a pharmacophoric moiety may have the potential to improve this effect, a salicylhydroxamic acid-estradiol hybrid molecule (E2HA) was synthesized. Only minimal effect of the conjugation on the proton dissociation constants was observed in comparison to salicylhydroxamic acid (SHA). The complexation with essential metal ions (iron, copper) was characterized, since E2HA may exert its cytotoxicity through the binding of these ions in cells. UV-visible spectrophotometric and pH-potentiometric titrations revealed the formation of high-stability complexes, while the Fe(III) preference over Fe(II) was proved by cyclic voltammetry and spectroelectrochemical measurements. Complex formation with half-sandwich Rh(III)(η⁵-Cp*) and Ru(II)(η⁶-p-cymene) organometallic cations was also studied as it may improve the anticancer effect and the pharmacokinetic profile of the ligand. At equimolar concentration the speciation is complicated because of the presence of mononuclear and binuclear complexes. The complexes readily react with small molecules e.g. glutathione, 1-methylimidazole and nucleosides, having major effect on solution speciation, namely mixed-ligand complex formation and ligand displacement occur. These processes serve as models for the interactions with biomolecules in the body. E2HA exerted moderate anticancer activity (IC₅₀ = 25-59 μM) in the tested three human cancer cell lines (Colo205, Colo320 and MCF-7), while being non-toxic on non-cancerous MRC-5 cells. Meanwhile, SHA was inactive in the same cells. Complexation with half-sandwich Rh(III) and Ru(II) cations had only a minor improvement on the cytotoxic effect of E2HA.

Half-Sandwich Rhodium Complexes with Releasable N-Donor Monodentate Ligands: Solution Chemical Properties and the Possibility for Acidosis Activation

Cancer chemotherapeutics usually have serious side effects. Targeting the special properties of cancer and activation of the anticancer drug in the tumor microenvironment in situ may decrease the intensity of the side effects and improve the efficacy of therapy. In this study, half-sandwich Rh complexes are introduced, which may be activated at the acidic, extracellular pH of the tumor tissue. The synthesis and aqueous stability of mixed-ligand complexes with a general formula of [Rh(η⁵-Cp*)(N,N/O)(N)]^2+/+ are reported, where (N,N/O) indicates bidentate 8-quinolate, ethylenediamine and 1,10-phenanthroline and (N) represents the releasable monodentate ligand with a nitrogen donor atom. UV-visible spectrophotometry, ¹H NMR, and pH-potentiometry were used to determine the protonation constants of the monodentate ligands, the proton dissociation constants of the coordinated water molecules in the aqua complexes, and the formation constants of the mixed-ligand complexes. The obtained data were compared to those of the analogous Ru(η⁶-p-cymene) complexes. The developed mixed-ligand complexes were tested in drug-sensitive and resistant colon cancer cell lines (Colo205 and Colo320, respectively) and in four bacterial strains (Gram-positive and Gram-negative, drug-sensitive, and resistant) at different pH values (5-8). The mixed-ligand complexes with 1-methylimidazole displayed sufficient stability at pH 7.4, and their activation was found in cancer cells with decreasing pH; moreover, the mixed-ligand complexes demonstrated antimicrobial activity in Gram-positive and Gram-negative bacteria, including the resistant MRSA strain. This study proved the viability of incorporating releasable monodentate ligands into mixed-ligand half-sandwich complexes, which is supported by the biological assays.

Ruthenium(II) polypyridyl complexes with benzothiophene and benzimidazole derivatives: Synthesis, antitumor activity, solution studies and biospeciation

With the aim to incorporate pharmacophore motifs into the Ru(II)-polypyridyl framework, compounds [Ru(II)(1,10-phenantroline)₂(2-(2-pyridyl)benzo[b]thiophene)](CF₃SO₃)₂ (1) and [Ru(II)(1,10-phenantroline)₂(2-(2-pyridyl)benzimidazole)](CF₃SO₃)₂ (2) were prepared, characterized and tested for their antitumor potential. The solid-state structure of the compounds was confirmed by single-crystal X-ray diffraction analysis. The solution behavior of both complexes was investigated, namely their solubility, stability, and lipophilicity in physiological mimetic conditions, as well as an eventual uptake by passive diffusion. In vitro anticancer activity of the complexes on ovarian and different colon cancer cells and apoptosis induction by the complexes were studied. A slow transformation process was observed for complex 1 in aqueous solution when exposed to sunlight, while complex 2 undergoes deprotonation (pK_a = 7.59). The lipophilicity of this latter complex depends strongly on the pH and ionic strength. In contrast, 1 is rather hydrophilic under various conditions. Complex 1 was highly cytotoxic on Colo-205 human colon (IC₅₀ = 7.87 μM) and A2780 ovarian (IC₅₀ = 2.2 μM) adenocarcinoma cell lines, while 2 displayed moderate anticancer activity (30.9 μM and 18.0 μM, respectively). The complexes induced late apoptosis and necrosis. Only a weak binding of the complexes to human serum albumin, the main transport protein in blood serum, was found. However, a more significant binding to calf thymus DNA was observed in UV-visible titrations and fluorometric dye displacement studies. Detailed analysis of fluorescence lifetime data collected for the latter systems reveals not only the partial intercalation of the complexes, but goes beyond the usual simplified interpretations.

Promising anticancer agents based on 8-hydroxyquinoline hydrazone copper(II) complexes

We report the synthesis and characterization of a group of benzoylhydrazones (Lⁿ) derived from 2-carbaldehyde-8-hydroxyquinoline and benzylhydrazides containing distinct para substituents (R = H, Cl, F, CH₃, OCH₃, OH and NH₂, for L^1-7, respectively; in L⁸ isonicotinohydrazide was used instead of benzylhydrazide). Cu(II) complexes were prepared by reaction of each benzoylhydrazone with Cu(II) acetate. All compounds were characterized by elemental analysis and mass spectrometry as well as by FTIR, UV-visible absorption, NMR or electron paramagnetic resonance spectroscopies. Complexes isolated in the solid state (1-8) are either formulated as [Cu(HL)acetate] (with L¹ and L⁴) or as [Cu(Lⁿ)]₃ (n = 2, 3, 5, 6, 7 and 8). Single crystal X-ray diffraction studies were done for L⁵ and [Cu(L⁵)]₃, confirming the trinuclear formulation of several complexes. Proton dissociation constants, lipophilicity and solubility were determined for all free ligands by UV-Vis spectrophotometry in 30% (v/v) DMSO/H₂O. Formation constants were determined for [Cu(LH)], [Cu(L)] and [Cu(LH_-1)] for L = L¹, L⁵ and L⁶, and also [Cu(LH_-2)] for L = L⁶, and binding modes are proposed, [Cu(L)] predominating at physiological pH. The redox properties of complexes formed with L¹, L⁵ and L⁶ are investigated by cyclic voltammetry; the formal redox potentials fall in the range of +377 to +395 mV vs. NHE. The binding of the Cu(II)-complexes to bovine serum albumin was evaluated by fluorescence spectroscopy, showing moderate-to-strong interaction and suggesting formation of a ground state complex. The interaction of L¹, L³, L⁵ and L⁷, and of the corresponding complexes with calf thymus DNA was evaluated by thermal denaturation. The antiproliferative activity of all compounds was evaluated in malignant melanoma (A-375) and lung (A-549) cancer cells. The complexes show higher activity than the corresponding free ligand, and most complexes are more active than cisplatin. Compounds 1, 3, 5, and 8 were selected for additional studies: while these complexes induce reactive oxygen species and double-strand breaks in both cancer cells, their ability to induce cell-death by apoptosis varies. Within the set of compounds tested, 8 emerges as the most promising one, presenting low IC₅₀ values, and high induction of oxidative stress and DNA damage, which eventually lead to high rates of apoptosis.

Metal Complexes of a 5-Nitro-8-Hydroxyquinoline-Proline Hybrid with Enhanced Water Solubility Targeting Multidrug Resistant Cancer Cells

Multidrug resistance (MDR) in cancer is one of the major obstacles of chemotherapy. We have recently identified a series of 8-hydroxyquinoline Mannich base derivatives with MDR-selective toxicity, however with limited solubility. In this work, a novel 5-nitro-8-hydroxyquinoline-proline hybrid and its Rh(η5-C5Me5) and Ru(η6-p-cymene) complexes with excellent aqueous solubility were developed, characterized, and tested against sensitive and MDR cells. Complex formation of the ligand with essential metal ions was also investigated using UV-visible, circular dichroism, 1H NMR (Zn(II)), and electron paramagnetic resonance (Cu(II)) spectroscopic methods. Formation of mono and bis complexes was found in all cases with versatile coordination modes, while tris complexes were also formed with Fe(II) and Fe(III) ions, revealing the metal binding affinity of the ligand at pH 7.4: Cu(II) > Zn(II) > Fe(II) > Fe(III). The ligand and its Rh(III) complex displayed enhanced cytotoxicity against the resistant MES-SA/Dx5 and Colo320 human cancer cell lines compared to their chemosensitive counterparts. Both organometallic complexes possess high stability in solution, however the Ru(II) complex has lower chloride ion affinity and slower ligand exchange processes, along with the readiness to lose the arene ring that is likely connected to its inactivity.

Metal Coordination and Biological Screening of a Schiff Base Derived from 8-Hydroxyquinoline and Benzothiazole

Designing new metallodrugs for anticancer therapy is a driving force in the scientific community. Aiming to contribute to this field, we hereby report the development of a Schiff base (H2L) derived from the condensation of 2-carbaldehyde-8-hydroxyquinoline with 2-hydrazinobenzothiazole and its complexation with transition metal ions. All compounds were characterised by analytical and spectroscopic techniques, which disclosed their structure: [Cu(HL)Cl], [Cu(HL)2], [Ni(HL)(acetate)], [Ni(HL)2], [Ru(HL)Cl(DMSO)], [VO(HL)2] and [Fe(HL)2Cl(H2O)]. Different binding modes were proposed, showing the ligand’s coordination versatility. The ligand proton dissociation constants were determined, and the tested compounds showed high lipophilicity and light sensitivity. The stability of all complexes in aqueous media and their ability to bind to albumin were screened. Based on an antiproliferative in vitro screening, [Ni(HL)(acetate)] and [Ru(HL)Cl(DMSO)] were selected for further studies aiming to investigate their mechanisms of action and therapeutic potential towards colon cancer. The complexes displayed IC50 < 21 μM towards murine (CT-26) and human (HCT-116) colon cancer cell lines. Importantly, both complexes exhibited superior antiproliferative properties compared to the clinically approved 5-fluorouracil. [Ni(HL)(acetate)] induced cell cycle arrest in S phase in CT-26 cells. For [Ru(HL)Cl(DMSO)] this effect was observed in both colon cancer cell lines. Additionally, both compounds significantly inhibited cell migration particularly in the human colon cancer cell line, HCT-116. Overall, the therapeutic potential of both metal complexes was demonstrated.

Structure-Activity Relationships of 8-Hydroxyquinoline-Derived Mannich Bases with Tertiary Amines Targeting Multidrug-Resistant Cancer

A recently proposed strategy to overcome multidrug resistance (MDR) in cancer is to target the collateral sensitivity of otherwise resistant cells. We designed a library of 120 compounds to explore the chemical space around previously identified 8-hydroxyquinoline-derived Mannich bases with robust MDR-selective toxicity. We included compounds to study the effect of halogen and alkoxymethyl substitutions in R5 in combination with different Mannich bases in R7, a shift of the Mannich base from R7 to R5, as well as the introduction of an aromatic moiety. Cytotoxicity tests performed on a panel of parental and MDR cells highlight a strong influence of experimentally determined pK_a values of the donor atom moieties, indicating that protonation and metal chelation are important factors modulating the MDR-selective anticancer activity of the studied compounds. Our results identify structural requirements increasing MDR-selective anticancer activity, providing guidelines for the development of more effective anticancer chelators targeting MDR cancer.

8-Hydroxyquinoline-Amino Acid Hybrids and Their Half-Sandwich Rh and Ru Complexes: Synthesis, Anticancer Activities, Solution Chemistry and Interaction with Biomolecules

Solution chemical properties of two novel 8-hydroxyquinoline-D-proline and homo-proline hybrids were investigated along with their complex formation with [Rh(η⁵-C₅Me₅)(H₂O)₃]²⁺ and [Ru(η⁶-p-cymene)(H₂O)₃]²⁺ ions by pH-potentiometry, UV-visible spectrophotometry and ¹H NMR spectroscopy. Due to the zwitterionic structure of the ligands, they possess excellent water solubility as well as their complexes. The complexes exhibit high solution stability in a wide pH range; no significant dissociation occurs at physiological pH. The hybrids and their Rh(η⁵-C₅Me₅) complexes displayed enhanced cytotoxicity in human colon adenocarcinoma cell lines and exhibited multidrug resistance selectivity. In addition, the Rh(η⁵-C₅Me₅) complexes showed increased selectivity to the chemosensitive cancer cells over the normal cells; meanwhile, the Ru(η⁶-p-cymene) complexes were inactive, most likely due to arene loss. Interaction of the complexes with human serum albumin (HSA) and calf-thymus DNA (ct-DNA) was investigated by capillary electrophoresis, fluorometry and circular dichroism. The complexes are able to bind strongly to HSA and ct-DNA, but DNA cleavage was not observed. Changing the five-membered proline ring to the six-membered homoproline resulted in increased lipophilicity and cytotoxicity of the Rh(η⁵-C₅Me₅) complexes while changing the configuration (L vs. D) rather has an impact on HSA or ct-DNA binding.

Estrone-salicylaldehyde N-methylated thiosemicarbazone hybrids and their copper complexes: solution structure, stability and anticancer activity in tumour spheroids

The terminal N-mono- and dimethylated derivatives of an estrone-salicylaldehyde thiosemicarbazone hybrid and their highly cytotoxic Cu(II) complexes were synthesized and characterized in addition to their structurally related simpler bicyclic analogues. Solution stability and structure of the complexes were determined by UV-visible spectrophotometry and electron paramagnetic resonance spectroscopy. The monomethylation has a minor influence on the pK_a values, while the dimethylation results in somewhat more acidic derivatives compared to the non-methylated derivatives, although all the compounds are neutral at physiological pH. Based on the speciation studies performed in a 30% (v/v) dimethyl sulfoxide/water mixture, the four novel ligands form fairly high-stability complexes with Cu(II) ions, in which they coordinate in mono-anionic (O^‒,N,S) or di-anionic (O^‒,N,S^‒) binding modes. [CuLH_‒1] species with (O^‒,N,S^‒)(H₂O) coordination mode are present in solution at neutral pH, and these complexes were isolated and further studied. The Cu(II) complexes formed with the estrone hybrids were more stable in comparison with the bicyclic analogues. The terminal N-dimethylation results in the most stable complexes in a given ligand series. In vitro cytotoxicity of all the Cu(II) complexes was measured in 3D spheroids of HCT-116, A-549 and CH-1 human cancer cells which showed fairly low IC₅₀ values (3.9‒17.1 μM). The Cu(II) complexes caused reduced tumour growth, and they activated the caspase-3 and caspase-7 endoproteases leading to apoptosis except the case of the complex formed with the monomethylated bicyclic derivative, where other type of mechanisms of action seems to induce the cell death. Anticancer Cu(II) complexes of mono- and dimethylated salicylaldehyde thiosemicarbazone-estrone hybrids possessing high solution stability and strong cytotoxic effect against 3D spheroids of a series of human cancer cells. 398x273 mm (150 x 150 DPI).

Complex formation and cytotoxicity of Triapine derivatives: a comparative solution study on the effect of the chalcogen atom and NH-methylation

α-N-Heterocyclic thiosemicarbazones are an important class of investigational anticancer drugs. The most prominent representative is 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine), which has shown promising results in clinical trials and is currently evaluated in phase III. In this study, we investigated the influence of a chalcogen atom exchange from S (Triapine) to O (O-Triapine) and Se (Se-Triapine) and the methylation of the hydrazonic NH moiety (Me-Triapine) on their complexation with Fe(ii), Fe(iii) and Cu(ii) ions and their cytotoxicity. The main aim of this study was to characterize and compare the most feasible chemical forms in solution, their stability and redox properties, as well as to reveal the relationships of the solution speciation and kinetic data with cytotoxic activity. The complex equilibria and redox properties of the complexes were characterized by the combined use of pH-potentiometry, UV-visible spectrophotometry, electron paramagnetic resonance spectroscopy, and cyclic voltammetry. These revealed that Se-Triapine forms Cu(ii) complexes with higher, and O-Triapine with lower stability as compared with Triapine. Me-Triapine, which is not able to coordinate via the typical (N,N,S-) donor set, nevertheless coordinates to Cu(ii) with unexpected high stability. The Cu(ii) complexes of Se-Triapine and Me-Triapine can be relatively slowly reduced by glutathione at pH 7.4 (but not by ascorbate), similarly to Cu(ii)-Triapine. In contrast, the Cu(ii)-O-Triapine complex can be reduced by both reducing agents in rapid redox reactions. Se-Triapine and Triapine form high stability complexes with both Fe(ii) and Fe(iii) ions, while O-Triapine has a much stronger preference towards Fe(iii) and Me-Triapine towards Fe(ii). This difference in the iron preference of the ligands seems to have a strong impact on their cytotoxic effects, which was measured in a human uterine sarcoma cell line (MES-SA) and its multidrug-resistant subline (MES-SA/Dx5). The Cu(ii) complexes of these calcogensemicarbazones are moderately toxic, and the highest level of ROS generation was found for the Cu(ii) complex of O-Triapine, which is the most reducible.

Critical factors affecting the albumin binding of half-sandwich Ru(ii) and Rh(iii) complexes of 8-hydroxyquinolines and oligopyridines

There is significant interest today in the interaction of half-sandwich anticancer organometallic complexes with proteins. It is considered as a crucial factor in the transport and mode of action of these compounds; thus it can affect their overall pharmacological and toxicological profiles. Albumin binding of high stability Ru(ii)(η⁶-p-cymene) and Rh(iii)(η⁵-C₅Me₅) complexes formed with 8-hydroxyquinoline, its 5-chloro-7-((proline-1-yl)methyl) substituted derivative, 2,2'-bipyridine and 1,10-phenanthroline is discussed herein. The interaction with human serum albumin in terms of kinetic aspects, binding strength and possible binding sites was studied in detail by means of various methods such as ¹H NMR spectroscopy, UV-visible spectrophotometry, steady-state and time-resolved fluorometry, ultrafiltration and capillary zone electrophoresis. Ru(ii)(η⁶-p-cymene)(2,2'-bipyridine) and Ru(ii)(η⁶-p-cymene)(1,10-phenanthroline) complexes do not bind to the protein measurably, most probably due to kinetic reasons. However, other complexes bind significantly to albumin with fairly different kinetics to albumin. The binding affinity towards hydrophobic binding pockets shows correlation with lipophilicity along with the actual charge of the respective complexes. The studied complexes preserve their original structure upon interaction with albumin. Formation constants computed for the binding of these metal complexes to histidine-containing model oligopeptides demonstrated significant ternary complex formation, pointing out the importance of histidine coordination in the binding of these types of complexes.

Deposition of pentamidine analogues in the human body - spectroscopic and computational approaches

Bis-benzamidines are a diverse group of compounds with high potential in pharmacotherapy, and among them, pentamidine is a drug of great therapeutic significance in Pneumocystis jiroveci pneumonia (PJP) prophylaxis and therapy. Pharmacokinetic properties of these cationic species such as transport, acid/base equilibria, and interactions with potential target molecules are still of interest, especially for recently designed compounds. To broaden our knowledge drug-likeness, human serum albumin binding, and acidity constants (K_a) were experimentally and theoretically examined for five pentamidine analogues 1 - 5 with -NH-CO-chain-CO-NH-bridges of increasing length and O, N, and S atoms in the chain. The studied analogues display very marked activity against Pneumocystis carinii without cytotoxicity that inspired us to perform an in silico analysis of their mode of action based on the hypothesis that the small DNA groove of rich in adenine-thymine pairs is their molecular target. These studies allowed us to classify them as very promising lead molecules.

Comparison of Solution Chemical Properties and Biological Activity of Ruthenium Complexes of Selected β-Diketone, 8-Hydroxyquinoline and Pyrithione Ligands

In this work, the various biological activities of eight organoruthenium(II) complexes were evaluated to reveal correlations with their stability and reactivity in aqueous media. Complexes with general formula [Ru(η6-p-cymene)(X,Y)(Z)] were prepared, where (X,Y) represents either an O,O-ligand (β-diketone), N,O-ligand (8-hydroxyquinoline) or O,S-pyrithione-type ligands (pyrithione = 1-hydroxypyridine-2(1H)-thione) with Cl- or 1,3,5-triaza-7-phosphaadamantane (PTA) as a co-ligand (Z). The tested complexes inhibit the chlamydial growth on HeLa cells, and one of the complexes inhibits the growth of the human herpes simplex virus-2. The chlorido complexes with N,O- and O,S-ligands displayed strong antibacterial activity on Gram-positive strains including the resistant S. aureus (MRSA) and were cytotoxic in adenocarcinoma cell lines. Effect of the structural variation on the biological properties and solution stability was clearly revealed. The decreased bioactivity of the β-diketone complexes can be related to their lower stability in solution. In contrast, the O,S-pyrithione-type complexes are highly stable in solution and the complexation prevents the oxidation of the O,S-ligands. Comparing the binding of PTA and the chlorido co-ligands, it can be concluded that PTA is generally more strongly coordinated to ruthenium, which at the same time decreased the reactivity of complexes with human serum albumin or 1-methylimidazole as well as diminished their bioactivity.

Multifunctional Pt(iv) prodrug candidates featuring the carboplatin core and deferoxamine

The synergistic combination of the anticancer drug carboplatin and the iron chelator deferoxamine (DFO) served as a foundation for the development of novel multifunctional prodrugs. Hence, five platinum(iv) complexes, featuring the equatorial coordination sphere of carboplatin, and one or two DFO units incorporated at axial positions, were synthesized and characterized using ESI-HRMS, multinuclear (¹H, ¹³C, ¹⁵N, ¹⁹⁵Pt) NMR spectroscopy and elemental analysis. Analytical studies demonstrated that the chelating properties of the DFO moiety were not compromised after coupling to the platinum(iv) core. The cytotoxic activity of the compounds was evaluated in monolayer (2D) and spheroid (3D) cancer cell models, derived from ovarian teratocarcinoma (CH1/PA-1), colon carcinoma (SW480) and non-small cell lung cancer (A549). The platinum(iv)-DFO prodrugs demonstrated moderate in vitro cytotoxicity (a consequence of their slow activation kinetics) but with less pronounced differences between intrinsically chemoresistant and chemosensitive cell lines as well as between 2D and 3D models than the clinically used platinum(ii) drug carboplatin.

Complex formation of an estrone-salicylaldehyde semicarbazone hybrid with copper(II) and gallium(III): Solution equilibria and biological activity

The solution chemical properties such as proton dissociation, complex formation with copper(II) and gallium(III) ions in addition to antibacterial and antitumor activity of a novel tridentate salicyaldehyde semicarbazone-estrone hybrid (estrone-SC) and a related bicyclic compound (thn-SC) were investigated. The crystal structure of complex [Cu(thn-SCH_-1)Cl] was studied by single crystal X-ray diffraction method. Estrone-SC and thn-SC form mono-ligand complexes with Cu(II) characterized by relatively high stability, however, they are much less stable than their thiosemicarbazone analogues. The neutral Cu(II) complexes with (O^-,N,O^-)(H₂O) coordination mode predominate at physiological pH. Estrone-SC and thn-SC are more efficient Ga(III) binders in comparison with thiosemicarbazones, although the complexes also suffer dissociation at pH 7.4. The Cu(II) complex of estrone-SC displayed significant cytotoxicity in A549, SW480 and CH1/PA cancer cells, and moderate apoptosis induction and ROS formation. The semicarbazone compounds did not exhibit antibacterial effect; unlike the related Cu(II)-thiosemicarbazone complexes represented by the fairly low MIC values (3-50 μM) obtained on the Gram-positive Staphylococcus aureus and Enterococcus faecalis bacteria.

Synthesis and conversion of primary and secondary 2-aminoestradiols into A-ring-integrated benzoxazolone hybrids and their in vitro anticancer activity

Hybrid systems are often endowed with completely different and improved properties compared to their parent compounds. In order to extend the chemical space toward sterane-based molecular hybrids, a number of estradiol-derived benzoxazol-2-ones with combined aromatic rings were synthesized via the corresponding 2-aminophenol intermediates. 2-Aminoestradiol was first prepared from estrone by a two-step nitration/reduction sequence under mild reaction conditions. Subsequent reductive aminations with different arylaldehydes furnished secondary 2-aminoestradiol derivatives in good yields. The proton dissociation processes of the aminoestradiols were investigated in aqueous solution by UV-visible spectrophotometric titrations to reveal their actual chemical forms at physiological pH. The determined pK₁ and pK₂ values are attributed to the ⁺NH₃ or ⁺NH₂R and OH moieties, and both varied by the different R substituents of the amino group. Primary and secondary 2-aminoestradiols were next reacted with carbonyldiimidazole as a phosgene equivalent to introduce a carbonyl group with simultaneous ring-closure to give A-ring-fused oxazolone derivatives in high yields. The novel aminoestradiols and benzoxazolones were subjected to in vitro cytotoxicity analysis and were found to exert cancer cell specific activity.

Estradiol–benzoxazolone hybrids with a common aromatic moiety were efficiently synthesized via primary and secondary aminophenol intermediates, and their anticancer activities were investigated.

Half-sandwich organometallic Ru and Rh complexes of (N,N) donor compounds: effect of ligand methylation on solution speciation and anticancer activity

A series of half-sandwich polypyridyl complexes was synthesized and compared focusing on structural, cytotoxic and aqueous solution behaviour. The formula of the synthesized complexes is [M(arene)(N,N)Cl]Cl, where M: Ru or Rh, arene: p-cymene, toluene or C5Me5-, (N,N): 2,2'-bipyridine (bpy), 4,4'-dimethyl-2,2'-bipyridine (dmb), 1,10-phenanthroline (phen) or 2,9-dimethyl-1,10-phenanthroline (neo). The structures of five half-sandwich complexes were determined by X-ray crystallography. It was found that introducing methyl groups next to the coordinating nitrogen atoms of the bidentate ligand causes steric congestion around the metal centre which changes the angle between ligand planes. The ligands and the Rh complexes showed significant cytotoxicity in A2780 and MES-SA cancer cell lines (IC50 = 0.1-56 μM) and in the cisplatin-resistant A2780cis cells. Paradoxically, phen and dmb as well as their half-sandwich Rh complexes showed increased toxicity against multidrug resistant MES-SA/Dx5 cells. In contrast, coordination to Ru caused loss of toxicity. Solution equilibrium constants showed that the studied metal complexes have high stability, and no dissociation was found for Ru and Rh complexes even at micromolar concentrations in a wide pH range. However, in the case of Ru complexes a slow and irreversible decomposition, namely arene loss, was also observed, which was more pronounced in light exposure in aqueous solution. In the case of neo, the methyl groups next to the nitrogen atoms significantly decrease the stability of complexes. For Rh complexes, the order of the stability constants corrected with ligand basicity (log K*): 9.78 (phen) > 9.01 (dmb) > 8.89 (bpy) > 3.93 (neo). The coordinated neo resulted in an enormous decrease in the chloride ion affinity of Ru compounds. Based on the results, a universal model was introduced for the prediction of chloride ion capability of half-sandwich Rh and Ru complexes. It combines the effects of the bidentate ligand and the M(arene) part using only two terms, performing multilinear regression procedure.

Improving the Stability of EGFR Inhibitor Cobalt(III) Prodrugs

Although tyrosine kinase inhibitors (TKIs) have revolutionized cancer therapy in the past two decades, severe drawbacks such as strong adverse effects and drug resistance limit their clinical application. Prodrugs represent a valuable approach to overcoming these disadvantages by administration of an inactive drug with tumor-specific activation. We have recently shown that hypoxic prodrug activation is a promising strategy for a cobalt(III) complex bearing a TKI of the epidermal growth factor receptor (EGFR). The aim of this study was the optimization of the physicochemical properties and enhancement of the stability of this compound class. Therefore, we synthesized a series of novel derivatives to investigate the influence of the electron-donating properties of methyl substituents at the metal-chelating moiety of the EGFR inhibitor and/or the ancillary acetylacetonate (acac) ligand. To understand the effect of the different methylations on the redox properties, the newly synthesized complexes were analyzed by cyclic voltammetry and their behavior was studied in the presence of natural low-molecular weight reducing agents. Furthermore, it was proven that reduction to cobalt(II) resulted in a lower stability of the complexes and subsequent release of the coordinated TKI ligand. Moreover, the stability of the cobalt(III) prodrugs was investigated in blood serum as well as in cell culture by diverse cell and molecular biological methods. These analyses revealed that the complexes bearing the methylated acac ligand are characterized by distinctly enhanced stability. Finally, the cytotoxic activity of all new compounds was tested in cell culture under normoxic and various hypoxic conditions, and their prodrug nature could be correlated convincingly with the stability data. In summary, the performed chemical modifications resulted in new cobalt(III) prodrugs with strongly improved stabilities together with retained hypoxia-activatable properties.

This study presents the synthesis of improved EGFR inhibitor cobalt(III) prodrugs activatable by hypoxia. By modification of the ancillary ligands, the redox potential could be lowered and the stability of the complexes could be distinctly increased in blood serum. Their physico-chemical properties were in detail characterized, the reductive behavior analyzed by different methods and the biological properties investigated in cancer cells.

Cancer Cell Resistance Against the Clinically Investigated Thiosemicarbazone COTI-2 Is Based on Formation of Intracellular Copper Complex Glutathione Adducts and ABCC1-Mediated Efflux

COTI-2 is a novel anticancer thiosemicarbazone in phase I clinical trial. However, the effects of metal complexation (a main characteristic of thiosemicarbazones) and acquired resistance mechanisms are widely unknown. Therefore, in this study, the copper and iron complexes of COTI-2 were synthesized and evaluated for their anticancer activity and impact on drug resistance in comparison to metal-free thiosemicarbazones. Investigations using Triapine-resistant SW480/Tria and newly established COTI-2-resistant SW480/Coti cells revealed distinct structure–activity relationships. SW480/Coti cells were found to overexpress ABCC1, and COTI-2 being a substrate for this efflux pump. This was unexpected, as ABCC1 has strong selectivity for glutathione adducts. The recognition by ABCC1 could be explained by the reduction kinetics of a ternary Cu-COTI-2 complex with glutathione. Thus, only thiosemicarbazones forming stable, nonreducible copper(II)-glutathione adducts are recognized and, in turn, effluxed by ABCC1. This reveals a crucial connection between copper complex chemistry, glutathione interaction, and the resistance profile of clinically relevant thiosemicarbazones.

High Copper Complex Stability and Slow Reduction Kinetics as Key Parameters for Improved Activity, Paraptosis Induction, and Impact on Drug-Resistant Cells of Anticancer Thiosemicarbazones

Aims: Due to their significant biological activity, thiosemicarbazones (TSCs) are promising candidates for anticancer therapy. In part, the efficacy of TSCs is linked to their ability to chelate essential metal ions such as copper and iron. Triapine, the best-studied anticancer TSC, has been tested clinically with promising results in hematological diseases. During the past few years, a novel subclass of TSCs with improved anticancer activity was found to induce paraptosis, a recently characterized form of cell death. The aim of this study was to identify structural and chemical properties associated with anticancer activity and paraptosis induction of TSCs. Results: When testing a panel of structurally related TSCs, compounds with nanomolar anticancer activity and paraptosis-inducing properties showed higher copper(II) complex solution stability and a slower reduction rate, which resulted in reduced redox activity. In contrast, TSCs with lower anticancer activity induced higher levels of superoxide that rapidly stimulated superoxide dismutase expression in treated cells, effectively protecting the cells from drug-induced redox stress. Innovation: Consequently, we hypothesize that in the case of close Triapine derivatives, intracellular reduction leads to rapid dissociation of intracellularly formed copper complexes. In contrast, TSCs characterized by highly stable, slowly reducible copper(II) complexes are able to reach new intracellular targets such as the endoplasmic reticulum-resident protein disulfide isomerase. Conclusion: The additional modes of actions observed with highly active TSC derivatives are based on intracellular formation of stable copper complexes, offering a new approach to combat (drug-resistant) cancer cells.

Abstract 1939: Improved activity and paraptosis-induction of anticancer thiosemicarbazones requires high copper(II) complex stability and slow reduction kinetics

Triapine, which is currently tested in a clinical phase III trial, is the best-studied thiosemicarbazone (TSC) for anticancer therapy. With regard to the mode of action, anticancer activity of TSCs is frequently linked to their ability to chelate essential metal ions such as copper and iron. As Triapine monotherapy showed promising results mainly against hematological diseases, novel TSC derivatives have been developed and clinically investigated for their activity against solid tumors. These novel TSCs belong to a subclass with enhanced anticancer activity in vitro and in vivo which were recently discovered to induce paraptosis, a form of programmed but caspase-independent cell death. As these TSCs are characterized by a up to 1000-fold higher activity in cell culture compared to Triapine, the aim of this study was to elucidate the mechanism of action as well as the underlying structural and chemical requirements of these effects.

For this purpose, a panel of structurally related Triapine derivatives was examined for anticancer activity (as metal-free ligand and copper(II) complex), paraptosis-inducing potential as well as solution stability and redox properties of their copper(II) complexes. Correlation studies between these chemical and biological properties revealed that the increased anticancer activity and paraptosis-inducing potential of the nanomolar active TSCs is related to a higher copper(II) complex solution stability and slower reduction rate. Unexpectedly, the TSCs with lower activity produced higher superoxide levels in a cell-free setting. This paradox could be explained by their lower copper(II) complex stability and increased readiness to be reduced, which resulted in a fast reduction of intracellular complexes and release of the metal-free ligand. Although this process resulted in the generation of superoxide, cell damage seemed to be prevented by rapid upregulation of the superoxide dismutase (in vitro and in vivo). In contrast, the copper complexes of the highly active TSCs are stable enough to reach intracellular targets such as the ER-resident protein disulfide isomerase, whose inhibition is crucial for paraptosis induction by TSCs.

In conclusion, copper complex stability is a crucial parameter of TSC activity, influencing the (intracellular) formation or dissociation of copper complexes. This intracellular stability of complexes, affects their mechanism of action as well as cell death induction. Overall, this study points out the importance of the redox parameters in order to understand and predict the TSC anticancer activity as well as their mechanism of action and, thereby, will pave the way for the development of improved anticancer agents.

Citation Format: Sonja Hager, Veronika F. Pape, Vivien Pósa, Bianca Montsch, Lukas Uhlik, Gergely Szakàcs, Szilárd Tóth, Nikolett Jabronka, Bernhard K. Keppler, Walter Berger, Christian R. Kowol, Éva A. Enyedy, Petra Heffeter. Improved activity and paraptosis-induction of anticancer thiosemicarbazones requires high copper(II) complex stability and slow reduction kinetics [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1939.

An 8-hydroxyquinoline-proline hybrid with multidrug resistance reversal activity and the solution chemistry of its half-sandwich organometallic Ru and Rh complexes

Herein the design and synthesis of a new 8-hydroxyquinoline derivative, (S)-5-chloro-7-((proline-1-yl)methyl)8-hydroxyquinoline (HQCl-Pro), with good water solubility and multidrug resistance reversal activity are reported. In this work the proton dissociation processes of HQCl-Pro and its complex formation with [Rh(η⁵-C₅Me₅)(H₂O)₃]²⁺, [Ru(η⁶-p-cymene)(H₂O)₃]²⁺ and [Ru(η⁶-toluene)(H₂O)₃]²⁺ were investigated by the combined use of pH-potentiometry, UV-visible spectrometry and ¹H NMR spectroscopy. Our results revealed the prominent solution stability of the complexes in all cases. The lipophilicity of the complexes increased with the chloride ion concentration, and the complexes showed moderate log D values (-0.8 to +0.4) at pH 7.4 at all tested Cl^- concentrations. The formation of mixed hydroxido complexes from the aqua complexes was characterized by relatively high pK_a values (8.45-9.62 in chloride-free medium). Complexation processes are much slower with the Ru(η⁶-arene) triaqua cations than with [Rh(η⁵-C₅Me₅)(H₂O)₃]²⁺. Both the pK_a values and H₂O/Cl^- exchange constants of the Ru-complexes are lower by 0.5-1.0 orders of magnitude than those of the Rh analogue. Arene loss (p-cymene and toluene) and oxidation were found in the case of Ru-complexes when an excess of HQCl-Pro and aromatic (N,N) bidentate ligands was added. The cytotoxicity and antiproliferative effect of HQCl-Pro and its complexes were assayed in vitro. In contrast to the structurally familiar 8-hydroxyquinoline, HQCl-Pro and its Rh(η⁵-C₅Me₅) complex were somewhat more effective against drug resistant Colo 320 adenocarcinoma human cells compared to the drug sensitive Colo 205 cells. The Ru- and Rh-complexes showed a similar metal uptake level after 4 h, while a longer incubation time resulted in higher cellular Rh concentration.

Synthesis and characterisation of Co(iii) complexes of N-formyl hydroxylamines and antibacterial activity of a Co(iii) peptide deformylase inhibitor complex

The X-ray crystal structure and pKa values of GSK322, a well-known and effective peptide deformylase inhibitor and antibacterial drug candidate, are reported. The first examples of Co(iii) complexes of N-formyl hydroxylamines are reported. Reaction of N-hydroxy-N-phenylformamide (HFA) with [Co(tren)Cl2]Cl and [Co(tpa)Cl2]Cl (where tren = tris(2-aminoethyl)amine, tpa = tris(2-pyridylmethyl)amine) with one equivalent of NaOH in H2O afforded [Co(tren)(HFA-1H)](PF6)1.5Cl0.5 (1) and [Co(tpa)(HFA-1H)]Cl2 (2), respectively. X-ray crystal structures of both complexes revealed that the N-formyl hydroxylamine group acts as a bidentate ligand, coordinating the Co(iii) centres via the carbonyl oxygen and deprotonated hydroxy group (O,O'), a coordination mode typically observed for closely related mono-deprotonated hydroxamic acids. Reaction of the N-formyl hydroxylamine-based GSK322 with [Co(tpa)Cl2]Cl afforded the corresponding Co(iii) chaperone complex of the peptide deformylase inhibitor, [Co(tpa)(GSK322-1H)](PF6)2. GSK322 and [Co(tpa)(GSK322-1H)](PF6)2 exhibited better Gram-positive activity than Gram-negative, where low MICs (1.56-6.25 μM) were determined for S. aureus strains, independent of their antibiotic susceptibility.

Salicylaldehyde thiosemicarbazone copper complexes: impact of hybridization with estrone on cytotoxicity, solution stability and redox activity

Copper complex of a novel estrone–thiosemicarbazone hybrid with significant cytotoxicity, lipophilicity and solution stability in addition to its structurally related bicyclic analogue.

Solution equilibrium, structural and cytotoxicity studies on Ru(η6-p-cymene) and copper complexes of pyrazolyl thiosemicarbazones

Solution chemical properties of two bidentate pyrazolyl thiosemicarbazones 2-((3-methyl-1-phenyl-1H-pyrazol-4-yl)methylene)hydrazinecarbothioamide (Me-pyrTSC), 2-((1,3-diphenyl-1H-pyrazol-4-yl)methylene)hydrazinecarbothioamide (Ph-pyrTSC), stability of their Cu(II) and Ru(η⁶-p-cymene) complexes were characterized in aqueous solution (with 30% DMSO) by the combined use of UV-visible spectrophotometry, ¹H NMR spectroscopy and electrospray ionization mass spectrometry in addition to their solid phase isolation. The solid phase structures of Me-pyrTSC∙H₂O, [Ru(η⁶-p-cymene)(Me-pyrTSC)Cl]Cl and [Cu(Ph-pyrTSCH_-1)₂] were determined by single crystal X-ray diffraction. High stability mononuclear Ru(η⁶-p-cymene) complexes with (N,S) coordination mode are formed in the acidic pH range, and increasing the pH the predominating dinuclear [(Ru(η⁶-p-cymene))₂(L)₂]²⁺ complex with μ₂-bridging sulphur donor atoms is formed (where L^- is the deprotonated thiosemicarbazone). [CuL]⁺ and [CuL₂] complexes show much higher stability compared to that of complexes of the reference compound benzaldehyde thiosemicarbazone. [CuL₂] complexes predominate at neutral pH. Me-pyrTSC and Ph-pyrTSC exhibited moderate cytotoxicity against human colonic adenocarcinoma cell lines (IC₅₀ = 33-76 μM), while their complexation with Ru(η⁶-p-cymene) (IC₅₀ = 11-24 μM) and especially Cu(II) (IC₅₀ = 3-6 μM) resulted in higher cytotoxicity. Cu(II) complexes of the tested thiosemicarbazones were also cytotoxic in three breast cancer and in a hepatocellular carcinoma cell line. No reactive oxygen species production was detected and the relatively high catalase activity of SUM159 breast cancer cells was decreased upon addition of the ligands and the complexes. In the latter cell line the tested compounds interfered with the glutathione synthesis as they decreased the concentration of this cellular reductant.

Binding mechanisms of half-sandwich Rh(III) and Ru(II) arene complexes on human serum albumin: a comparative study

Abstract

Various half-sandwich ruthenium(II) arene complexes and rhodium(III) arene complexes have been intensively investigated due to their prominent anticancer activity. The interaction of the organometallic complexes of Ru(η⁶-p-cymene) and Rh(η⁵-C₅Me₅) with human serum albumin (HSA) was studied in detail by a combination of various methods such as ultrafiltration, capillary electrophoresis, ¹H NMR spectroscopy, fluorometry and UV–visible spectrophotometry in the presence of 100 mM chloride ions. Binding characteristics of the organometallic ions and their complexes with deferiprone, 2-picolinic acid, maltol, 6-methyl-2-picolinic acid and 2-quinaldic acid were evaluated. Kinetic aspects and reversibility of the albumin binding are also discussed. The effect of low-molecular-mass blood components on the protein binding was studied in addition to the interaction of organorhodium complexes with cell culture medium components. The organometallic ions were found to bind to HSA to a high extent via a coordination bond. Release of the bound metal ions was kinetically hindered and could not be induced by the denaturation of the protein. Binding of the Ru(η⁶-p-cymene) triaqua cation was much slower (ca. 24 h) compared to the rhodium congener (few min), while their complexes interacted with the protein relatively fast (1–2 h). The studied complexes were bound to HSA coordinatively. The highly stable and kinetically inert 2-picolinate Ru(η⁶-p-cymene) complex bound in an associative manner preserving its original entity, while lower stability complexes decomposed partly or completely upon binding to HSA. Fast, non-specific and high-affinity binding of the complexes on HSA highlights their coordinative interaction with various types of proteins possibly decreasing effective drug concentration.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1007/s00775-019-01683-0) contains supplementary material, which is available to authorized users.

Impact of copper and iron binding properties on the anticancer activity of 8-hydroxyquinoline derived Mannich bases

The anticancer activity of 8-hydroxyquinolines relies on complex formation with redox active copper and iron ions. Here we employ UV-visible spectrophotometry and EPR spectroscopy to compare proton dissociation and complex formation processes of the reference compound 8-hydroxyquinoline (Q-1) and three related Mannich bases to reveal possible correlations with biological activity. The studied derivatives harbor a CH2-N moiety at position 7 linked to morpholine (Q-2), piperidine (Q-3), and chlorine and fluorobenzylamino (Q-4) substituents. Solid phase structures of Q-3, Q-4·HCl·H2O, [(Cu(HQ-2)2)2]·(CH3OH)2·Cl4·(H2O)2, [Cu(Q-3)2]·Cl2 and [Cu(HQ-4)2(CH3OH)]·ZnCl4·CH3OH were characterized by single-crystal X-ray diffraction analysis. In addition, the redox properties of the copper and iron complexes were studied by cyclic voltammetry, and the direct reaction with physiologically relevant reductants (glutathione and ascorbic acid) was monitored. In vitro cytotoxicity studies conducted with the human uterine sarcoma MES-SA/Dx5 cell line reveal the significant cytotoxicity of Q-2, Q-3, and Q-4 in the sub- to low micromolar range (IC50 values 0.2-3.3 μM). Correlation analysis of the anticancer activity and the metal binding properties of the compound series indicates that, at physiological pH, weaker copper(ii) and iron(iii) binding results in elevated toxicity (e.g.Q4: pCu = 13.0, pFe = 6.8, IC50 = 0.2 μM vs.Q1: pCu = 15.1, pFe = 13.0 IC50 = 2.5 μM). Although the studied 8-hydroxyquinolines preferentially bind copper(ii) over iron(iii), the cyclic voltammetry data revealed that the more cytotoxic ligands preferentially stabilize the lower oxidation state of the metal ions. A linear relationship between the pKa (OH) and IC50 values of the studied 8-hydroxyquinolines was found. In summary, we identify Q-4 as a potent and selective anticancer candidate with significant toxicity in drug resistant cells.

Anticancer Thiosemicarbazones: Chemical Properties, Interaction with Iron Metabolism, and Resistance Development

SIGNIFICANCE

During the past decades, thiosemicarbazones were clinically developed for a variety of diseases, including tuberculosis, viral infections, malaria, and cancer. With regard to malignant diseases, the class of α-N-heterocyclic thiosemicarbazones, and here especially 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine), was intensively developed in multiple clinical phase I/II trials. Recent Advances: Very recently, two new derivatives, namely COTI-2 and di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC) have entered phase I evaluation. Based on the strong metal-chelating/metal-interacting properties of thiosemicarbazones, interference with the cellular iron (and copper) homeostasis is assumed to play an important role in their biological activity.

CRITICAL ISSUES

In this review, we summarize and analyze the data on the interaction of (α-N-heterocyclic) thiosemicarbazones with iron, with the special aim of bridging the current knowledge on their mode of action from chemistry to (cell) biology. In addition, we highlight the difference to classical iron(III) chelators such as desferrioxamine (DFO), which are used for the treatment of iron overload.

FUTURE DIRECTIONS

We want to emphasize that thiosemicarbazones are not solely removing iron from the cells/organism. In contrast, they should be considered as iron-interacting drugs influencing diverse biological pathways in a complex and multi-faceted mode of action. Consequently, in addition to the discussion of physicochemical properties (e.g., complex stability, redox activity), this review contains an overview on the diversity of cellular thiosemicarbazone targets and drug resistance mechanisms.

Comparative solution and structural studies of half-sandwich rhodium and ruthenium complexes bearing curcumin and acetylacetone

Half-sandwich organometallic complexes of curcumin are extensively investigated as anticancer compounds. Speciation studies were performed to explore the solution stability of curcumin complexes formed with [Rh(η⁵-C₅Me₅)(H₂O)₃]²⁺. Acetylacetone (Hacac), as the simplest β-diketone ligand bearing (O,O) donor set, was involved for comparison and its Ru(η⁶‑p‑cymene), Ru(η⁶‑toluene) complexes were also studied. ¹H NMR, UV-visible and pH-potentiometric titrations revealed a clear trend of stability constants of the acac complexes: Ru(η⁶‑p‑cymene) > Ru(η⁶‑toluene) > Rh(η⁵-C₅Me₅). Despite this order, the highest extent of complex formation is seen for the Rh(η⁵-C₅Me₅) complexes at pH 7.4. Formation constant of [Rh(η⁵-C₅Me₅)(H₂curcumin)(H₂O)]⁺ reveals similar solution stability to that of the acac complex. Additionally, structures of two complexes were determined by X-ray crystallography. The in vitro cytotoxicity of curcumin was not improved by the complexation with these organometallic cations.

New Water-Soluble Copper(II) Complexes with Morpholine-Thiosemicarbazone Hybrids: Insights into the Anticancer and Antibacterial Mode of Action

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(L^1–6)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.

Evaluation of blood-brain barrier penetration and examination of binding to human serum albumin of 7-O-arylpiperazinylcoumarins as potential antipsychotic agents

The delivery of drugs to the brain is complicated by the multiple factors including low blood-brain barrier (BBB) passive permeability, active BBB efflux systems, and plasma protein binding. Thus, a detailed understanding of the transport of the new potent substances through the membranes is vitally important and their physico-chemical characteristics should be analyzed at first. This work presents an evaluation of drug likeness of eight 7-O-arylpiperazinylcoumarin derivatives with high affinity towards serotoninergic receptors 5-HT_1A and 5-HT_2A with particular analysis of the requirements for the CNS chemotherapeutics. The binding constants to human serum albumin (HSA) were determined at physiological pH using fluorescence spectroscopy, and then their mode of action was explained by analysis of theoretical HSA complexes. Dynamic simulation of systems allowed for reliable evaluation of the interaction strength. The analyzed coumarins were able to pass BBB, and they present good drug likeness properties. They showed high affinities to HSA (log K_Q = 5.3-6.0 which corresponds to -8.12 to -7.15 kcalmol^-1 of Gibbs free energy). The changes of the emission intensity upon binding to HSA were scrutinized showing the different mode of action for 4-phenylpiperazinylcoumarins. The values of computed Gibbs free energy and determined on the basis of experimentally obtained binding constants log K_Q coincide suggesting a good quality of the theoretical model. Overall the 8-acetyl-7-O-arylpiperazinyl-4-methylcoumarin derivatives represent valuable lead compounds to be further tested in various preclinical assays as a possible chemotherapeutics against CNS diseases. Studied coumarins can be metabolized by cytochrome P450 to aldehydes and hydroxy derivatives. The existence of other binding sites inside HSA than Sudlow's site 1 was postulated. The longer aliphatic linker between coumarin and piperazine moieties favored binding to HSA in other than Sudlow site 1 pocket.

Synthesis and biological evaluation of biotin-conjugated anticancer thiosemicarbazones and their iron(III) and copper(II) complexes

Triapine, the most prominent anticancer drug candidate from the substance class of thiosemicarbazones, was investigated in >30 clinical phase I and II studies. However, the results were rather disappointing against solid tumors, which can be explained (at least partially) due to inefficient delivery to the tumor site. Hence, we synthesized the first biotin-functionalized thiosemicarbazone derivatives in order to increase tumor specificity and accumulation. Additionally, for Triapine and one biotin conjugate the iron(III) and copper(II) complexes were prepared. Subsequently, the novel compounds were biologically evaluated on a cell line panel with different biotin uptake. The metal-free biotin-conjugated ligands showed comparable activity to the reference compound Triapine. However, astonishingly, the metal complexes of the biotinylated derivative showed strikingly decreased anticancer activity. To further analyze possible differences between the metal complexes, detailed physico- and electrochemical experiments were performed. However, neither lipophilicity or complex solution stability, nor the reduction potential or behavior in the presence of biologically relevant reducing agents showed strong variations between the biotinylated and non-biotinylated derivatives (only some differences in the reduction kinetics were observed). Nonetheless, the metal-free biotin-conjugate of Triapine revealed distinct activity in a colon cancer mouse model upon oral application comparable to Triapine. Therefore, this type of biotin-conjugated thiosemicarbazone is of interest for further synthetic strategies and biological studies.

Drug likeness prediction of 5-hydroxy-substituted coumarins with high affinity to 5-HT1A and 5-HT2A receptors

One of the latest trends is search for the new anti-psychotic drugs among coumarin derivatives with piperazine moiety. Their therapeutic potential can be hampered by poor physico-chemical parameters as low brain penetration or limited transport in the body fluid. Herein, we predicted the drug likeness of six coumarins with high affinity towards 5-HT_1A and 5-HT_2A receptors. Subsequent experimental determination of their binding constants to human serum albumin (HSA) revealed the binding with a moderate strength (logK=4.8-5.8) at the Sudlow's site 1, which represents a possibility of temporary storage of tested coumarins on HSA. Computational mapping of the binding of coumarins - HSA complexes showed that the coumarin rings of all tested compounds were similarly located within the hydrophobic binding pocket of HSA, while the rest of molecules (composed with alkyl chains, piperazine and benzene rings) decided about the difference in binding modes by the hydrogen bonding interactions. The proton dissociation constants (pK_a) of the compounds were also determined by UV-vis spectrophotometric titrations to obtain the distribution of the species in the different protonation states at physiological pH of 7.4. A good agreement of the computationally-determined free enthalpy values of the ligand - HSA complexes with the values determined by experimental fluorescence quenching data could be a promising prospect for proposed theoretical strategy.