Schiff bases and their metal complexes to target and overcome (multidrug) resistance in cancer

Overcoming multidrug resistance (MDR) is one of the major challenges in cancer therapy. In this respect, Schiff base-related compounds (bearing a R1R2CNR3 bond) gained high interest during the past decades. Schiff bases are considered privileged ligands for various reasons, including the easiness of their preparation and the possibility to form complexes with almost all transition metal ions. Schiff bases and their metal complexes exhibit many types of biological activities and are used for the treatment and diagnosis of various diseases. Until now, 13 Schiff bases have been investigated in clinical trials for cancer treatment and hypoxia imaging. This review represents the first collection of Schiff bases and their complexes which demonstrated MDR-reversal activity. The areas of drug resistance covered in this article involve: 1) Modulation of ABC transporter function, 2) Targeting lysosomal ABCB1 overexpression, 3) Circumvention of ABC transporter-mediated drug efflux by alternative routes of drug uptake, 4) Selective activity against MDR cancer models (collateral sensitivity), 5) Targeting GSH-detoxifying systems, 6) Overcoming apoptosis resistance by inducing necrosis and paraptosis, 7) Reactivation of mutated p53, 8) Restoration of sensitivity to DNA-damaging anticancer therapy, and 9) Overcoming drug resistance through modulation of the immune system. Through this approach, we would like to draw attention to Schiff bases and their metal complexes representing highly interesting anticancer drug candidates with the ability to overcome MDR.

Strategies for the Nuclear Delivery of Metal Complexes to Cancer Cells

The nucleus is an essential organelle for the function of cells. It holds most of the genetic material and plays a crucial role in the regulation of cell growth and proliferation. Since many antitumoral therapies target nucleic acids to induce cell death, tumor-specific nuclear drug delivery could potentiate therapeutic effects and prevent potential off-target side effects on healthy tissue. Due to their great structural variety, good biocompatibility, and unique physico-chemical properties, organometallic complexes and other metal-based compounds have sparked great interest as promising anticancer agents. In this review, strategies for specific nuclear delivery of metal complexes are summarized and discussed to highlight crucial parameters to consider for the design of new metal complexes as anticancer drug candidates. Moreover, the existing opportunities and challenges of tumor-specific, nucleus-targeting metal complexes are emphasized to outline some new perspectives and help in the design of new cancer treatments.

8-hydroxyquinoline-N-oxide copper(II)- and zinc(II)-phenanthroline and bipyridine coordination compounds: Design, synthesis, structures, and antitumor evaluation

Fourteen novel tumor-targeting copper(II) and zinc(II) complexes, [Cu(ONQ)(QD1)(NO3)]·CH3OH (NQ3), [Cu(ONQ)(QD2)(NO3)] (NQ2), [Cu(NQ)(QD2)Cl] (NQ3), [Cu(ONQ)(QD1)Cl] (NQ4), [Cu(ONQ)(QD3)](NO3) (NQ5), [Cu(ONQ)(QD3)Cl] (NQ6), [Zn(ONQ)(QD4)Cl] (NQ7), [Zn(ONQ)(QD1)Cl] (NQ8), [Zn(ONQ)(QD5)Cl] (NQ9), [Zn(ONQ)(QD2)Cl] (NQ10), [Zn(ONQ)(QD6)Cl] (NQ11), [Zn(ONQ)(QD7)Cl] (NQ12), and [Zn(ONQ)(QD3)Cl] (NQ13) supported on 8-hydroxyquinoline-N-oxide (H-ONQ), 2,2'-dipyridyl (QD1), 5,5'-dimethyl-2,2'-bipyridyl (QD2), 1,10-phenanthroline (QD3), 4,4'-dimethoxy-2,2'-bipyridyl (QD4), 4,4'-dimethyl-2,2'-bipyridyl (QD5), 5-chloro-1,10-phenanthroline (QD6), and bathophenanthroline (QD7), were first synthesized and characterized using various spectroscopic techniques. Furthermore, NQ1-NQ13 exhibited higher antiproliferative activity and selectivity for cisplatin-resistant SK-OV-3/DDP tumor cells (CiSK3) compared to normal HL-7702 cells based on results obtained from the cell counting Kit-8 (CCK-8) assay. The complexation of copper(II) ion with QD2 and ONQ ligands resulted in an evident increase in the antiproliferation of NQ1-NQ6, with NQ6 exhibiting the highest antitumor potency against CiSK3 cells compared to NQ1-NQ5, H-ONQ, QD1-QD7, and NQ7-NQ13 as well as the reference cisplatin drug with an IC50 value of 0.17 ± 0.05 μM. Mechanistic studies revealed that NQ4 and NQ6 induced apoptosis of CiSK3 cells via mitophagy pathway regulation and adenosine triphosphate (ATP) depletion. Further, the differential induction of mitophagy decreased in the order of NQ6 > NQ4, which can be attributed to the major impact of the QD3 ligand with a large planar geometry and the Cl leaving group within the NQ6 complex. In summary, these results confirmed that the newly synthesized H-ONQ copper(II) and zinc(II) coordination metal compounds NQ1-NQ13 exhibit potential as anticancer drugs for cisplatin-resistant ovarian CiSK3 cancer treatment.

Characterization of a Zeolite-Y-Encapsulated Zn(II)Salmphen Complex with Targeted Anticancer Property

Resistance and severe side effects of classical chemotherapeutic drugs are major challenges to cancer therapy. New therapeutic agents and combination therapy are considered potential solutions that enhance the efficacy of the drug as well as reduce drug resistance. The success of a platinum-based anticancer drug, cisplatin, has paved the way to explore metal-centered anticancer therapeutic agents. Herein, the zeolite-Y-encapsulated Zn(II)Salmphen complex is synthesized using a flexible ligand approach. The Zn(II)Salmphen complex and its encapsulation within the supercage of zeolite-Y were characterized by elemental analysis, Fourier transform infrared (FTIR) spectroscopy, UV-vis, fluorescence, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), NMR, and high-resolution mass spectrometry (HRMS) techniques. Elemental analysis, PXRD, and SEM, all together confirm the integrity of the zeolite framework after the encapsulation of Zn(II)Salmphen complex in it, and elemental analysis provides the Si/Al ratio and Zn content present. FTIR and XPS studies indicate the successful encapsulation of the complex. NMR and HRMS studies confirm that the Zn(II)Salmphen complex is dimer; however, within the supercage of zeolite-Y, it is expected to exist as a monomer. The extent of structural modification of the encapsulated Zn(II)Salmphen complex is intimated by electronic spectroscopic studies. The free-state Zn(II)Salmphen is a fluorescent complex, and even the encapsulated Zn(II)Salmphen complex, when taken in dimethyl sulfoxide (DMSO), shows fluorescence. In comparison to cisplatin, encapsulated Zn(II)Salmphen complex displays comparable cytotoxicity (IC50 = 2.0 ± 0.5 μg/mL at 48 h) toward breast cancer cell line, whereas free Zn(II)Salmphen has better cytotoxicity (IC50 = 1.5 ± 0.5 μg/mL at 48 h). Importantly, elemental analysis has revealed that the IC50 value, if calculated only in terms of Zn(II)Salmphen within Zn(II)Salmphen-Y, is as low as 54.59 ng/mL, indicating a very high efficacy of the drug. Interestingly, a 48 h treatment with the encapsulated Zn(II)Salmphen complex shows no toxicity toward immortal noncancerous keratinocyte cells (HaCaT), whereas cisplatin has an IC50 value of 1.75 ± 0.5 μg/mL. Internalization studies indicate that zeolite-Y targets cancer cells better than it does noncancerous ones. Hence, cellular uptake of the zeolite-encapsulated Zn(II)Salmphen complex in cancer cells is more than that in HaCaT cells, resulting in the generation of more reactive oxygen species and cell death. Significant upregulation of DNA damage response protein indicates that DNA-damage-induced cellular apoptosis could be the mechanism of drug action. Overall, the zeolite-encapsulated Zn(II)Salmphen complex could be a better alternative to the traditional drug cisplatin with minimal effect on noncancerous HaCaT cells and can also be utilized as a fluorescent probe in exploring the mechanistic pathway of its activity against cancer cells.

Anticancer activity of Ni(II) and Zn(II) complexes based on new unsymmetrical salophen-type ligands: synthesis, characterization and single-crystal X-ray diffraction

The discovery of new coordination compounds with anticancer properties is an active field of research due to the severe side effects of platinum-based compounds currently used in chemotherapy. In the search for new agents for the treatment of cancer, unsymmetrical N2O2-tetradentate ligand (H2L1 and H2L2) and their Ni(II) and Zn(II) asymmetric complexes (NiII-L1-2 and ZnII-L1-2) have been synthesized and fully characterized. 1H NMR studies revealed that the ligands and complexes were stable in mixtures of DMSO : D2O (9 : 1). Complementary UV-Vis studies confirmed that ZnII derivatives also exhibit high stability in mixtures DMSO : buffer (6 : 4) after 24 h. Single-crystal X-ray diffraction studies confirmed the molecular structures of H2L1, H2L2, NiII-L1, and NiII-L2. At the molecular level, complexes were completely planar without significant distortions of the square-planar geometry according to τ4 parameter. Furthermore, the crystalline structures revealed non-classical intermolecular interactions of the C-H⋯O and the Ni⋯Ni type. The ligands and complexes were screened against the human osteosarcoma (MG-63), human colon cancer (HCT-116), breast cancer (MDA-MB-231) cell lines, and non-cancerous cells (L929). H2L1 and H2L2 ligands not caused cytotoxic effects at a concentration of 100 μM, while NiII-L2, ZnII-L1, and ZnII-L2 complexes induce cytotoxic effects in all cell lines. NiII-L2 was a more active complex against MG-63 (3.9 ± 1.5) and HCT-116 (3.4 ± 1.7) cell lines with IC50 values in the low micromolar range. In addition, this compound was 10-, 5-, and 11-fold more potent than cisplatin in MG-63 (39 ± 1.8), HCT-116 (17.2), and MDA-MB-231 (131 ± 18), respectively. Three complexes exhibited great selectivity for tumoral cells with SI values ranging from 1.6 to 7.4.

Synthesis and anticancer mechanisms of zinc(II)-8-hydroxyquinoline complexes with 1,10-phenanthroline ancillary ligands

Twenty new zinc(II) complexes with 8-hydroxyquinoline (H-Q1-H-Q6) in the presence of 1,10-phenanthroline derivatives (D1-D10) were synthesized and formulated as [Zn(Q1)₂(D1)] (DQ1), [Zn(Q2)₂(D2)]·CH₃OH (DQ2), [Zn(Q1)₂(D3)] (DQ3), [Zn(Q1)₂(D4)] (DQ4), [Zn(Q3)₂(D5)] (DQ5), [Zn(Q3)₂(D4)] (DQ6), [Zn(Q4)₂(D5)]·CH₃OH (DQ7), [Zn(Q4)₂(D6)] (DQ8), [Zn(Q4)₂(D3)]·CH₃OH (DQ9), [Zn(Q4)₂(D1)]·H₂O (DQ10), [Zn(Q5)₂(D4)] (DQ11), [Zn(Q6)₂(D6)]·CH₃OH (DQ12), [Zn(Q5)₂(D2)]·5CH₃OH·H₂O (DQ13), [Zn(Q5)₂(D7)]·CH₃OH (DQ14), [Zn(Q5)₂(D8)]·CH₂Cl₂ (DQ15), [Zn(Q5)₂(D9)] (DQ16), [Zn(Q5)₂(D1)] (DQ17), [Zn(Q5)₂(D5)] (DQ18), [Zn(Q5)₂(D10)]·CH₂Cl₂ (DQ19) and [Zn(Q5)₂(D3)] (DQ20). They were characterized using multiple techniques. The cytotoxicity of DQ1-DQ20 was screened using human cisplatin-resistant SK-OV-3/DDP ovarian cancer (SK-OV-3CR) cells and normal hepatocyte (HL-7702) cells. Complex DQ6 showed low IC₅₀ values (2.25 ± 0.13 μM) on SK-OV-3CR cells, more than 3.0-8.0 times more cytotoxic than DQ1-DQ5 and DQ7-DQ20 (≥6.78 μM), and even 22.2 times more cytotoxic than the standard cisplatin, the corresponding free H-Q1-H-Q6 and D1-D10 alone (>50 μM). As a comparison, DQ1-DQ20 displayed nontoxic rates against healthy HL-7702 cells. Furthermore, DQ6 and DQ11 induced significant apoptosis via mitophagy pathways. DQ6 also significantly inhibited tumor growth in an in vivo SK-OV-3-xenograft model (ca. 49.7%). Thus, DQ6 may serve as a lead complex for the discovery of new antitumor agents.

Dual stimuli-responsive nanocarriers based on polyethylene glycol-mediated schiff base interactions for overcoming tumour chemoresistance

Multifunctional and stimulus-sensitive intelligent nanodrug delivery systems (NDDSs) can significantly optimize the effectiveness of theranostic agents for cancer treatment. In this study, redox and pH dual-responsive nanocarriers (CPNPs) were prepared through molecular assembly by utilizing the Schiff base interactions of cystamine (Cys), PEG-NH₂ and formaldehyde (FA) under aqueous conditions with a one-pot, one-step technique. First, the degradation products of CPNPs exhibited good biocompatibility, and the high concentration of intact CPNPs (200 µg/mL) could inhibit the growth of cells. In addition, doxorubicin (DOX) was encapsulated in CPNPs simply by changing the pH (DOX@CPNPs), and pH/GSH-responsive release behaviour was confirmed. In vitro, CPNPs significantly increased the uptake of DOX and enhanced the cytotoxicity of DOX to tumour cells. More importantly, DOX@CPNPs strongly reversed drug resistance in three different types of cancer cells, exhibiting significant anticancer effects. Collectively, this study presents the easy preparation of nanomedicines that respond to multiple stimuli, which highlights the advantages of Schiff base-based nanomedicines for cancer therapy and reversing chemoresistance.

Complexes of Zn(II) with a mixed tryptanthrin derivative and curcumin chelating ligands as new promising anticancer agents

In this study, two novel curcumin (H-Cur)-tryptanthrin metal compounds-[Zn(TA)Cl₂], i.e., Zn(TA), and [Zn(TA)(Cur)]Cl, i.e., Zn(TAC)-were synthesized and investigated using 5-(bis-pyridin-2-ylmethyl-amino)-pentanoic acid (6,12-dioxo-6,12-dihydro-indolo[2,1-b]quinazolin-8-yl)-amide (TA) and H-Cur as the targeting and high-activity anticancer chemotherapeutic moieties, respectively. They were then compared with the di-(2-picolyl)amine (PA) Zn(II) complex [Zn(PA)Cl₂], i.e., Zn(PA). When compared with Zn(PA) and cisplatin, the IC₅₀ values of Zn(TA) and Zn(TAC) indicated that the compounds had high cytotoxicity against A549/DDP cancer cells, implying that the H-Cur-tryptanthrin Zn(II) compounds have the potential for use as anticancer drugs. We propose the use of synthesized theragnostic H-Cur-tryptanthrin Zn(II) complexes with nuclear-targeting and DNA-damaging capabilities as a simple therapeutic strategy against tumors. The Zn(TA) and Zn(TAC) complexes could be traced via red fluorescence and were found to accumulate in the cell nuclei and induce DNA damage, cell cycle arrest, mitochondrial dysfunction, and cell apoptosis both in vitro and in vivo. In addition, Zn(TAC) exhibited a higher antiproliferative effect on A549/DDP than Zn(TA) and Zn(PA), which was undoubtedly associated with the key roles of the novel tryptanthrin derivative TA and H-Cur in the Zn(TAC) complex.

Influence of the introduction of a triphenylphosphine group on the anticancer activity of a copper complex

Aiming at obtaining new copper complexes with good cytotoxicity against cancer cells, triphenylphosphine (TPP) was introduced to obtain insight into the influence of the co-ligands. In this paper, two copper complexes, Cu(2-pbmq)(CH₃OH)Br₂ (1) and [Cu(2-pbmq)(TPP)Br]₂ (2) were designed, synthesized, and characterized by X-ray crystallography, 2-((2-(pyrazin-2-yl)-1H-benzo[d]imidazol-1-yl)methyl))quinolone (2-pbmq), to investigate the influence of the TPP group on the anticancer activity of the metal complex. Although the presence of the TPP group diminished the intensity of the interaction properties of the complex with DNA, the in vitro anticancer activity and cellular uptake of the TPP-containing complex were markedly superior to those of its TPP-lacking counterpart. Detailed studies on the more potently cytotoxic complex 2 revealed that it accumulated in nucleus, arrested the cell cycle at the G0-G1 phase, causing mitochondrial dysfunction, involving the potential simultaneous mitochondrial membrane collapse, cellular ATP level depletion, and Ca²⁺ leakage, eventually inducing cell apoptosis. In summary, the introduction of a TPP group enhances the biological activity and cytotoxicity of the complex.

Sodium, Potassium, and Lithium Complexes of Phenanthroline and Diclofenac: First Report on Anticancer Studies

Diclofenac or 2-[(2',6'-dichlorophenyl)amino]phenyl}acetic acid (dcf) is a nonsteroidal anti-inflammatory drug, and 1,10-phenanthroline (phen) is a well-known enzyme inhibitor. In this study, three new alkali metal complexes (1-3) containing both phen and dcf were prepared, and their structures were characterized by a variety of analytical techniques including infrared and UV-vis spectroscopy, 1H NMR and 13C NMR elemental analysis, mass spectrometry, and single-crystal X-ray diffraction analysis. In these complexes, phen binds via a N,N'-chelate pocket, while the monoanionic dcf-ligand remains either uncoordinated (in the case of 1 and 3) or coordinated in a bidentate fashion (in the case of 2). All three complexes crystallize in the triclinic space group P-1. [Na2(phen)2 (H2O)4][dcf]2 (1) is a dinuclear sodium complex, where two crystallographically identical Na+ cations adopt a distorted five-coordinate spherical square-pyramidal geometry, with a [N2O3] donor set. [K2(phen)2(dcf)2(H2O)4] (2) is also a dinuclear complex where the crystallographically unique K+ cation adopts a distorted seven-coordinate geometry comprising a [N2O5] donor set. [Li(phen)(H2O)2][dcf] (3) is a mononuclear lithium complex where the Li+ cation adopts a four-coordinate distorted tetrahedral geometry comprising a [N2O2] donor set. The complexes were evaluated for their anticancer activity against lung and oral cancer cell lines as well as for their antibacterial potential. The prepared complexes displayed very good antibacterial and anticancer activities with an excellent bioavailability.