Cellular Selectivity and Biological Impact of Cytotoxic Rhodium(III) and Iridium(III) Complexes Containing Methyl‐Substituted Phenanthroline Ligands

Mitochondria-targeted cyclometalated iridium (III) complexes: Dual induction of A549 cells apoptosis and autophagy

In this study, we synthesized 4 cyclometalated iridium complexes using N-(1,10-phenanthrolin-5-yl)picolinamide (PPA) as the main ligand, denoted as [Ir(ppy)2PPA]PF6 (ppy = 2-phenylpyridine, Ir1), [Ir(bzq)2PPA]PF6 (bzq = benzo[h]quinoline, Ir2), [Ir(dfppy)2PPA]PF6 (dfppy = 2-(3,5-difluorophenyl)pyridine, Ir3), and [Ir(thpy)2PPA]PF6 (thpy = 2-(thiophene-2-yl)pyridine, Ir4). Compared to cisplatin and oxaliplatin, all four complexes exhibited significant anti-tumor activity. Among them, Ir2 demonstrated higher cytotoxicity against A549 cells, with an IC50 value of 1.6 ± 0.2 μM. The experimental results indicated that Ir2 primarily localized in the mitochondria, inducing a large amount of reactive oxygen species (ROS) generation, that decreased in mitochondrial membrane potential (MMP), reduced ATP production, and further impaired mitochondrial function, leading to cytochrome c release. Additionally, Ir2 caused cell cycle arrest at the S phase and induced apoptosis through the AKT-mediated signaling pathway. Further investigations revealed that Ir2 could simultaneously induce both apoptosis and autophagy in A549 cells, with the latter acting as a non-protective mechanism that promoted cell death. More importantly, Ir2 exhibited low toxicity to both normal LO2 cells in vitro and zebrafish embryos in vivo. Consequently, these newly developed Ir(III) complexes show great potential in the development of novel and low-toxicity anticancer agents.

Cationic N,S-chelate half-sandwich iridium complexes: synthesis, characterization, anticancer and antiplasmodial activity

A series of pyrazole-based ligands and their corresponding cationic N,S-chelate half-sandwich iridium complexes were successfully synthesized. All iridium complexes exhibited good anticancer activity against the MCF-7 and MDA-MB-231 human breast cancer cells. The cytotoxic activity of unsubstituted iridium complex 1 is greater than that of cisplatin against MCF-7 cells. In addition, the cationic half-sandwich iridium complexes are also efficient in antiplasmodial study and complex 1 displayed the best activity as its IC50 was observed to be approximately 0.11 μM against the CQS-NF54 strain. These iridium complexes generally exhibited enhanced activity against the CQS-NF54 strain in comparison with that against the CQR-K1 strain. An "IC50 speed assay" investigation against the CQS-NF54 strain indicated complexes 1-3 to be fast-acting complexes that reach their lowest IC50 values within 16 hours. All complexes were fully characterized by IR spectroscopy, NMR spectroscopy, and elemental analysis, and the structure of the iridium complex was confirmed by single-crystal X-ray diffraction.

Cell nucleus localization and high anticancer activity of quinoline-benzopyran rhodium(III) metal complexes as therapeutic and fluorescence imaging agents

Four novel rhodium(III) complexes, [Rh^III(QB1)Cl₃(DMSO)] (RhN1), [Rh^III(QB2)Cl₃(CH₃OH)]·CH₃OH (RhN2), [Rh^III(QB3)Cl₃(CH₃OH)]·CH₃OH (RhS), and [Rh^III(QB4)Cl₃(DMSO)] (RhQ), bearing quinoline-benzopyran ligands (QB1-QB4) were synthesized and used to develop highly anticancer therapeutic and fluorescence imaging agents. Compared with the QB1-QB4 ligands (IC₅₀ > 89.2 ± 1.7 μM for A549/DDP), RhN1, RhN2, RhS and RhQ exhibit selective cytotoxicity against lung carcinoma cisplatin-resistant A549/DDP (A549CDDP) cancer cells, with IC₅₀ values in the range of 0.08-2.7 μM. The fluorescent imaging agent RhQ with the more extended planar QB4 ligand exhibited high anticancer activity in A549CDDP cells and was found in the cell nucleus fraction, whereas RhS had no fluorescence properties. RhQ and RhS may trigger cell apoptosis by causing DNA damage and initiating the mitochondrial dysfunction pathway. Furthermore, RhQ has a higher antitumor efficacy (ca. 55.3%) than RhS (46.4%) and cisplatin (CDDP, 33.1%), and RhQ demonstrated significantly lower toxicity in vivo than CDDP, making it a promising Rh(III)-based anticancer therapeutic and fluorescence imaging agent.

Highly potent half-sandwich iridium and ruthenium complexes as lysosome-targeted imaging and anticancer agents

A new class of half-sandwich Ir and Ru compounds containing P^P-chelating ligands can be developed as potential multifunctional theranostic platforms that combine bioimaging and anticancer capabilities.

Rhodium(I) N-Heterocyclic Carbene Bioorganometallics as in Vitro Antiproliferative Agents with Distinct Effects on Cellular Signaling

Organometallics with N-heterocyclic carbene (NHC) ligands have triggered major interest in inorganic medicinal chemistry. Complexes of the type Rh(I)(NHC)(COD)X (where X is Cl or I, COD is cyclooctadiene, and NHC is a dimethylbenzimidazolylidene) represent a promising type of new metallodrugs that have been explored by advanced biomedical methods only recently. In this work, we have synthesized and characterized several complexes of this type. As observed by mass spectrometry, these complexes remained stable over at least 3 h in aqueous solution, after which hydrolysis of the halido ligands occurred and release of the NHC ligand was evident. Effects against mitochondria and general cell tumor metabolism were noted at higher concentrations, whereas phosphorylation of HSP27, p38, ERK1/2, FAK, and p70S6K was induced substantially already at lower exposure levels. Regarding the antiproliferative activity in tumor cells, a clear preference for iodido over chlorido secondary ligands was noted, as well as effects of the substituents of the NHC ligand.

Metallomics insights into the programmed cell death induced by metal-based anticancer compounds

Since the discovery of cisplatin more than 40 years ago, enormous research efforts have been dedicated to developing metal-based anticancer agents and to elucidating the mechanisms involved in the action of these compounds. Abnormal metabolism and the evasion of apoptosis are important hallmarks of malignant transformation, and the induction of apoptotic cell death has been considered to be a main pathway by which cytotoxic metal complexes combat cancer. However, many cancers have cellular defects involving the apoptotic machinery, which results in an acquired resistance to apoptotic cell death and therefore reduced chemotherapeutic effectiveness. Over the past decade, it has been revealed that a growing number of cell death pathways induced by metal complexes are not dependent on apoptosis. Metal complexes specifically triggering these alternative cell death pathways have been identified and explored as novel cancer treatment options. In this review, we discuss recent examples of metallomics studies on the different types of cell death induced by metal-based anticancer drugs, especially on the three major forms of programmed cell death (PCD) in mammalian cells: apoptosis, autophagy and regulated necrosis, also called necroptosis.

A TrxR inhibiting gold(I) NHC complex induces apoptosis through ASK1-p38-MAPK signaling in pancreatic cancer cells

Background

Cancer cells in the advanced stage show aberrant antioxidant capacity to detoxify excessive ROS resulting in the compensation for intrinsic oxidative stress and therapeutic resistance. PDAC is one of the most lethal cancers and often associated with a high accumulation of ROS. Recent studies identified gold(I) NHC complexes as potent TrxR inhibitors suppressing cell growth in a wide spectrum of human malignant cell lines at the low micromolar concentration. However, the mechanism of action is not completely elucidated yet.

Methods

To understand the biological function of gold(I) NHC complexes in PDAC, we used a recently published gold(I) NHC complex, MC3, and evaluated its anti-proliferative effect in four PDAC cell lines, determined by MTT and SRB assays. In further detailed analysis, we analyzed cellular ROS levels using the ROS indicator DHE and mitochondrial membrane potential indicated by the dye JC-1 in Panc1. We also analyzed cell cycle arrest and apoptosis by FACS. To elucidate the role of specific cell signaling pathways in MC3-induced cell death, co-incubation with ROS scavengers, a p38-MAPK inhibitor and siRNA mediated depletion of ASK1 were performed, and results were analyzed by immunoblotting, ELISA-microarrays, qRT-PCR and immunoprecipitation.

Results

Our data demonstrate that MC3 efficiently suppressed cell growth, and induced cell cycle arrest and apoptosis in pancreatic cancer cells, in particular in the gemcitabine-resistant cancer cells Panc1 and ASPC1. Treatment with MC3 resulted in a substantial alteration of the cellular redox homeostasis leading to increased ROS levels and a decrease in the mitochondrial membrane potential. ROS scavengers suppressed ROS formation and rescued cells from damage. On the molecular level, MC3 blocked the interaction of Trx with ASK1 and subsequently activated p38-associated signaling. Furthermore, inhibition of this pathway by using ASK1 siRNA or a p38 inhibitor clearly attenuated the effect of MC3 on cell proliferation in Panc1 and ASPC1.

Conclusions

Our results confirm that MC3 is a TrxR inhibitor and show MC3 induced apoptosis in gemcitabine-resistant PDACs. MC3 mediated cell death could be blocked by using anti-oxidants, ASK1 siRNA or p38 inhibitor suggesting that the Trx-ASK1-p38 signal cascade played an important role in gold(I) NHC complexes-mediated cellular damage.

Electronic supplementary material

The online version of this article (doi:10.1186/1476-4598-13-221) contains supplementary material, which is available to authorized users.

Anticancer potential of (pentamethylcyclopentadienyl)chloridoiridium(III) complexes bearing κP and κP,κS-coordinated Ph2 PCH2 CH2 CH2 S(O)x Ph (x=0-2) ligands

Iridium(III) complexes of the type [Ir(η(5) -C5 Me5 )Cl2 {Ph2 PCH2 CH2 CH2 S(O)x Ph-κP}] (x=0-2; 1-3) and [Ir(η(5) -C5 Me5 )Cl{Ph2 PCH2 CH2 CH2 S(O)x Ph-κP,κS}][PF6 ] (x=0-1; 4 and 5) with 3-(diphenylphosphino)propyl phenyl sulfide, sulfoxide, and sulfone ligands Ph2 PCH2 CH2 CH2 S(O)x Ph were designed, synthesized, and characterized fully, including X-ray diffraction analyses for complexes 3 and 4. In vitro studies against human thyroid carcinoma (8505C), submandibular carcinoma (A253), breast adenocarcinoma (MCF-7), colon adenocarcinoma (SW480), and melanoma (518A2) cell lines provided evidence for the high biological potential of the neutral and cationic iridium(III) complexes. Neutral iridium(III) complex 5 proved to be the most active, with IC50 values up to about 0.1 μM, representing activities of up to one order of magnitude higher than cisplatin. Using 8505C cells, apoptosis was shown to be the main mechanism through which complex 5 exerts its tumoricidal action. The described iridium(III) complexes represent potential leads in the search for novel metal-based anticancer agents.

A chemical-biological evaluation of rhodium(I) N-heterocyclic carbene complexes as prospective anticancer drugs

Rhodium(I) complexes bearing N-heterocyclic carbene (NHC) ligands have been widely used in catalytic chemistry, but there are very few reports of biological properties of these organometallics. A series of Rh(I)-NHC derivatives with 1,5-cyclooctadiene and CO as secondary ligands were synthesized, characterized, and biologically investigated as prospective antitumor drug candidates. Pronounced antiproliferative effects were noted for all complexes, along with moderate inhibitory activity of thioredoxin reductase (TrxR) and efficient binding to biomolecules (DNA, albumin). Biodistribution studies showed that the presence of albumin lowered the cellular uptake and confirmed the transport of rhodium into the nuclei. Changes in the mitochondrial membrane potential (MMP) were observed as well as DNA fragmentation in wild-type and daunorubicin- or vincristine-resistant Nalm-6 leukemia cells. Overall, these studies indicated that Rh(I)-NHC fragments could be used as partial structures of new antitumor agents, in particular in those drugs designed to address resistant malignant tissues.

Water-soluble Co(III) complexes of substituted phenanthrolines with cell selective anticancer activity

Transition metal complexes with substituted phenanthrolines as ligands represent potential anticancer products without the drawbacks of platinum complexes that are currently marketed. Here, we report the synthesis and cell selective anticancer activity of five new water-soluble Co(III) complexes with methyl substituted phenanthroline ligands. The complexes were characterized by elemental analysis, NMR, FAB-mass spectrometry, FTIR, electronic spectroscopy, and single crystal X-ray diffraction. Possible interaction of these complexes with DNA was assessed by a combination of circular dichroism, UV-vis spectroscopy titration, and ethidium bromide displacement assay, and the results indicated that DNA interaction is weak for these complexes. Cellular uptake and cytotoxicity of complexes at low concentrations were assessed by flow cytometry on PC-3 cells, while their effect on intracellular mitochondrial function was measured by MTS assay on HeLa and PC-3 cell lines. These complexes showed selective cytotoxicity with a significantly higher effect on intracellular mitochondrial function in PC-3 cells than in HeLa cells. At low concentrations, complex 2 had the highest cytotoxic effect on PC-3 cells, inducing around 38% cell death, and the correlation of cytotoxicity of these complexes to their hydrophobicity indicates that an appropriate value of the hydrophobicity is essential for high antitumor activity.

Biological activity of neutral and cationic iridium(III) complexes with κP and κP,κS coordinated Ph₂PCH₂S(O)xPh (x = 0-2) ligands

Neutral iridium(III) complexes of the type [Ir(η(5)-C₅Me₅)Cl₂{Ph₂PCH₂S(O)xPh-κP}] (1-3) with diphenylphosphino-functionalized methyl phenyl sulfides, sulfoxides, and sulfones Ph₂PCH₂S(O)xPh (x = 0, L1; 1, L2; 2, L3) and the cationic complex [Ir(η(5)-C₅Me₅)Cl{Ph₂PCH₂SPh-κP,κS}][PF6] (4) were synthesized and fully characterized analytically and spectroscopically. Furthermore, the structure of 2 was determined by X-ray diffraction analysis. The biological potential of the neutral and cationic iridium(III) complexes was tested in vitro against the cell lines 8505C, A253, MCF-7, SW480 and 518A2. Complex [Ir(η(5)-C₅Me₅)Cl₂{Ph₂PCH₂S(O)Ph-κP}] (2), with ligand L2 κP coordinated containing a pendent sulfinyl group, is the most active one (IC₅₀ values of about 3 μM), thus, with activities comparable to cisplatin. Complex 2 proved to have an even a higher antiproliferative activity than cisplatin against 8505C and SW480 cell lines, used as a model system of highly anaplastic cancers with low sensitivity to conventional chemotherapeutics such as cisplatin. Additional experiments demonstrated that apoptosis and autophagic cell death contribute to the drug's tumoricidal action.

Organometallic Palladium Complexes with a Water-Soluble Iminophosphorane Ligand as Potential Anticancer Agents

The synthesis and characterization of a new water-soluble iminophosphorane ligand TPA=N-C(O)-2BrC(6)H(4) (C,N-IM; TPA = 1,3,5-triaza-7-phosphaadamantane) 1 is reported. Oxidative addition of 1 to Pd(2)(dba)(3) affords the orthopalladated dimer [Pd(μ-Br){C(6)H(4)(C(O)N=TPA-kC,N)-2}](2) (2) as a mixture of cis and trans isomers (1:1 molar ratio) where the iminophosphorane moeity behaves as a C,N-pincer ligand. By addition of different neutral or monoanionic ligands to 2, the bridging bromide can be cleaved and a variety of hydrophilic or water-soluble mononuclear organometallic palladium(II) complexes of the type [Pd{C(6)H(4)(C(O)N=TPA-kC,N)-2}(L-L)] (L-L = acac (3); S(2)CNMe(2) (4); 4,7-Diphenyl-1,10-phenanthrolinedisulfonic acid disodium salt C(12)H(6)N(2)(C(6)H(4)SO(3)Na)(2) (5)); [Pd{C(6)H(4)(C(O)N=TPA-kC,N)-2}(L)Br] (L = P(mC(6)H(4)SO(3)Na)(3) (6); P(3-Pyridyl)(3) (7)) and, [Pd(C(6)H(4)(C(O)N=TPA)-2}(TPA)(2)Br] (8) are obtained as single isomers. All new complexes were tested as potential anticancer agents and their cytotoxicity properties were evaluated in vitro against human Jurkat-T acute lymphoblastic leukemia cells, normal T-lymphocytes (PBMC) and DU-145 human prostate cancer cells. Compounds [Pd(μ-Br){C(6)H(4)(C(O)N=TPA-kC,N)-2}](2) (2) and [Pd{C(6)H(4)(C(O)N=TPA-kC,N)-2}(acac)] 3 (which has been crystallographically characterized) display the higher cytotoxicity against the above mentioned cancer cell lines while being less toxic to normal T-lymphocytes (peripheral blood mononuclear cells: PBMC). In addition, 3 is very toxic to cisplatin resistant Jurkat shBak indicating a cell death pathway that may be different to that of cisplatin. The interaction of 2 and 3 with plasmid (pBR322) DNA is much weaker than that of cisplatin pointing to an alternative biomolecular target for these cytotoxic compounds. All the compounds show an interaction with human serum albumin (HSA) faster than that of cisplatin.

Synthesis and cellular impact of diene-ruthenium(II) complexes: a new class of organoruthenium anticancer agents

The cytostatic properties and cellular effects of novel diene-ruthenium(II) complexes of the types OC-6-13-[RuCl(2)(pp)(cod)] 1-5 (pp=2,2'-bipyridyl (bpy), phen=1,10-phenanthroline (phen), 5,6-dimethylphenanthroline (5,6-Me2phen), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq), ethylenediamine (en)) and OC-6-24-[RuCl{(Me(2)N)(2)CS}(pp)(cod)](CF(3)SO(3)) 6-8 (pp=phen, 5,6-Me(2)phen, dpq) have been studied for the human cancer cell lines MCF-7 and HT-29 and for Jurkat leukemia cells. CD spectra indicate that 7 causes a massive distortion of the CT DNA B double helix toward the A form. Whereas the neutral complexes 1, 2 and 5 exhibit only modest antiproliferative activity toward MCF-7 and HT-29 cells, the monocationic complexes are significantly more active, in particular the DNA-distorting complex 7 with its IC(50) values of 0.73 and 0.42 μM, respectively. As established by online monitoring with a cell-based sensor chip, this potent 5,6-Me(2)phen complex invokes dose-dependent decreases in MCF-7 cellular respiration and extracellular acidification rates and causes a time-delayed decrease in the impedance of the cell layers, that can be ascribed to cell death. Treatment of Jurkat cells with 7 leads to high concentrations of reactive oxygen species and the induction of apoptosis. The pronounced dose-dependent inhibition of oxygen consumption by isolated mice mitochondria indicates the involvement of an intrinsic mitochondrial pathway in the programmed cell death process.

Highly cytotoxic substitutionally inert rhodium(III) tris(chelate) complexes: DNA binding modes and biological impact on human cancer cells

The antiproliferative properties and cellular impact of novel substitutionally inert rhodium(III) complexes of the types [Rh{(CH₃)₂ NCS₂}₂(pp)]Cl 3-5 (pp=5,6-Me₂phen, dpq, dppz) and OC-6-23-[Rh(2-S-py)₂(pp)]Cl 6 and 7 (2-S-py=pyridine-2-thiolate; pp=dpq, dppz) have been investigated for the adherent human cancer cell lines MCF-7 and HT-29 and for non-adherent Jurkat cells. Whereas CD and viscosity measurements indicate that the polypyridyl ligands of 4 and 5 intercalate into CT DNA, this is not the case for the analogous pyridine-2-thiolate complexes 6 and 7. Complexes 3-7 all exhibit a high antiproliferative activity towards MCF-7 and HT-29 cells, with IC(50) values in the range 0.055-0.285 μM. As established by online monitoring with a cell-based sensor chip, the highly cytostatic complex 6 (IC(50)=0.059 and 0.078 μM) invokes an immediate concentration-dependent reduction of MCF-7 cell respiration and a time-delayed decrease in cellular impedance, which can be ascribed to the induction of cell death. Annexin V/PI assays demonstrated that 6 also has a pronounced antiproliferative activity towards Jurkat cells and that it invokes extensive apoptosis and high concentrations of reactive oxygen species in these leukemia cells. The observation of a dose-dependent inhibition of the oxygen consumption of isolated mice mitochondria indicates the involvement of an intrinsic mitochondrial pathway in this process.