Influence of the Linker Length on the Cytotoxicity of Homobinuclear Ruthenium(II) and Gold(I) Complexes

Viral peptide conjugates for metal-warhead delivery to chromatin

The presence of heavy metal groups can endow compounds with unique structural and chemical attributes beneficial for developing highly potent therapeutic agents and effective molecular labels. However, metallocompound binding site specificity is a major challenge that dictates the level of off-site targeting, which is a limiting factor in finding safer and more effective metal-based drugs. Here we designed and tested a family of metallopeptide conjugates based on two different chromatin-tethering viral proteins and a drug being repurposed for cancer, the Au(i) anti-arthritic auranofin. The viral peptides associate with the acidic patch of the nucleosome while the gold moiety can bind allosterically to the H3 H113 imidazole. To achieve synthesis of the conjugates, we also engineered a sulfur-free, nucleosome-binding Kaposi's sarcoma herpesvirus LANA peptide with a methionine-to-ornithine substitution and coupled the peptide to the metal group in a final step using click chemistry. The four conjugates tested are all selectively cytotoxic towards tumor cell lines, but the choice of viral peptide and mode of linkage to the Au(i) group influences metal binding site preference. Our findings suggest that viral peptide-metalloconjugates have potential for use in chromatin delivery of therapeutic warheads and as nucleosome-specific tags.

Ruthenium(II)-Cyclopentadienyl-Derived Complexes as New Emerging Anti-Colorectal Cancer Drugs

Colorectal cancer (CRC) is one of the most common malignancies and one of the leading causes of cancer-related death worldwide, urging the need for new and more efficient therapeutic approaches. Ruthenium complexes have emerged as attractive alternatives to traditional platinum-based compounds in the treatment of CRC. This work aims to evaluate anti-CRC properties, as well as to identify the mechanisms of action of ruthenium complexes with the general formula [Ru(η⁵-C₅H₄R)(PPh₃)(4,4′-R′-2,2′-bipyridine)][CF₃SO₃], where R = CH₃, CHO or CH₂OH and R′ = H, CH₃, CH₂OH, or dibiotin ester. The complexes (Ru 1–7) displayed high bioactivity, as shown by low IC₅₀ concentrations against CRC cells, namely, RKO and SW480. Four of the most promising ruthenium complexes (Ru 2, 5–7) were phenotypically characterized and were shown to inhibit cell viability by decreasing cell proliferation, inducing cell cycle arrest, and increasing apoptosis. These findings were in accordance with the inhibition of MEK/ERK and PI3K/AKT signaling pathways. Ruthenium complexes also led to a decrease in cellular clonogenic ability and cell migration, which was associated with the disruption of F-actin cytoskeleton integrity. Here, we demonstrated that ruthenium complexes, especially Ru7, have a high anticancer effect against CRC cells and are promising drugs to be used as a new therapeutical strategy for CRC treatment.

Recent development of gold(I) and gold(III) complexes as therapeutic agents for cancer diseases

Metal complexes have demonstrated significant antitumor activities and platinum complexes are well established in the clinical application of cancer chemotherapy. However, the platinum-based treatment of different types of cancers is massively hampered by severe side effects and resistance development. Consequently, the development of novel metal-based drugs with different mechanism of action and pharmaceutical profile attracts modern medicinal chemists to design and synthesize novel metal-based agents. Among non-platinum anticancer drugs, gold complexes have gained considerable attention due to their significant antiproliferative potency and efficacy. In most situations, the gold complexes exhibit anticancer activities by targeting thioredoxin reductase (TrxR) or other thiol-rich proteins and enzymes and trigger cell death via reactive oxygen species (ROS). Interestingly, gold complexes were recently reported to elicit biochemical hallmarks of immunogenic cell death (ICD) as an ICD inducer. In this review, the recent progress of gold(I) and gold(III) complexes is comprehensively summarized, and their activities and mechanism of action are documented.

In-vitro anticancer profile of recent ruthenium complexes against liver cancer

Ruthenium complexes are considered as the most favorable alternatives to traditional platinum-based cancer drugs owing to their acceptable toxicity level, selectivity, variant oxidation states and ability to treat platinum-resistant cancer cells. They have similar ligand exchange kinetics as platinum drugs but can be tailored according to our desire by ligands influence. In the current study, we illustrate the in-vitro anticancer profile of some ruthenium complexes (2016–2021) against human hepatocellular carcinoma (HepG2). The anticancer activity of ruthenium complexes is determined by comparing their IC50 values with one another and positive controls. Fortunately, some ruthenium complexes including 3, 4, 6, 14, 15, 20, 42, and 48 exhibit surpassed in-vitro anticancer profile than that of positive controls promising as potential candidates against liver cancer. We also explored the structure-activity relationship (SAR) which is a key factor in the rational designing and synthesis of new ruthenium drugs. It covers the factors affecting anticancer activity including lipophilicity, planarity, area and bulkiness, the steric influence of different ligands, and electronic effects induced by ligands, stability, aqueous solubility and bioavailability to the target sites. The data reported here will provide strong support in the plausible design and synthesis of ruthenium anticancer drugs in the upcoming days.

Investigation on Optical and Biological Properties of 2-(4-Dimethylaminophenyl)benzothiazole Based Cycloplatinated Complexes

The optical and biological properties of 2-(4-dimethylaminophenyl)benzothiazole cycloplatinated complexes featuring bioactive ligands ([{Pt(Me2 N-pbt)(C6 F5 )}L] [L=Me2 N-pbtH 1, p-dpbH (4-(diphenylphosphino)benzoic acid) 2, o-dpbH (2-(diphenylphosphino)benzoic acid) 3), [Pt(Me2 N-pbt)(o-dpb)] 4, [{Pt(Me2 N-pbt)(C6 F5 )}2 (μ-PRn P)] [PR4 P=O(CH2 CH2 OC(O)C6 H4 PPh2 )2 5, PR12 P=O{(CH2 CH2 O)3 C(O)C6 H4 PPh2 }2 6] are presented. Complexes 1-6 display 1 ILCT and metal-perturbed 3 ILCT dual emissions. The ratio between both bands is excitation dependent, accomplishing warm-white emissions for 2, 5 and 6. The phosphorescent emission is lost in aerated solutions owing to photoinduced electron transfer to 3 O2 and the formation of 1 O2 , as confirmed in complexes 2 and 4. They also exhibit photoinduced phosphorescence enhancement in non-degassed DMSO due to local oxidation of DMSO by sensitized 1 O2 , which causes a local degassing. Me2 N-pbtH and the complexes specifically accumulate in the Golgi apparatus, although only 2, 3 and 6 were active against A549 and HeLa cancer cell lines, 6 being highly selective in respect to nontumoral cells. The potential photodynamic property of these complexes was demonstrated with complex 4.

Antitumor Effect of Organometallic Half-Sandwich Ru(II)-Arene Complexes Bearing a Glutathione S-Transferase Inhibitor

The facile modification of the ligands in organometallic Ru(II)-arene complexes offers more opportunities to optimize their pharmacological profiles. Herein, three Ru(II)-arene complexes containing a glutathione S-transferase (GST) inhibitor (NBDHEX) in chelate ligand have been designed and synthesized in this study. In vitro results indicated that the ligation with NBDHEX significantly increased the activities and selectivities of the organometallic Ru(II)-arene complexes against tumor cells, especially complex 3, which was the most active compound among the tested compounds. DFT calculations and hydrolysis results demonstrated that complex 3 with more alkyl groups in the arene ligand has increased electron density at the Ru(II) center as compared with complexes 1 and 2, thus resulting in the improved hydrolysis rate, which may be responsible for its higher anticancer activity. Further studies showed that complexes 1-3 can cause the loss of the mitochondrial membrane potential and upregulate the expression of Bcl-2 and Bax in A549 cells, suggesting that complexes 1-3-induced cell death may be mediated via the mitochondrial apoptotic pathway. Thus, these findings suggested that simultaneous modification of the chelate ligands and arene rings in the organometallic Ru(II)-arene complexes is an effective way to improve their pharmacological properties.

Utilization of Guanidine-Based Ancillary Ligands in Arene-Ruthenium Complexes for Selective Cytotoxicity

A family of three water-soluble half-sandwich arene-ruthenium complexes, depicted as C ₁ -C ₃ , having the general formula [Ru(p-cymene)(L)Cl]Cl has been synthesized, where L represents (1H-benzo[d]imidazol-2-yl)guanidine (L ₁ ) or (benzo[d]oxazol-2-yl)guanidine (L ₂ ) or (benzo[d]thiazol-2-yl)guanidine (L ₃ ). The crystal structure of complex C ₃ has been determined. The complexes show several absorption bands in the visible and ultraviolet regions, and they also show prominent emission in the visible region while excited near 400 nm. Studies on the interaction of ligands L ₁ -L ₃ and complexes C ₁ -C ₃ with calf thymus DNA reveal that the complexes are better DNA binders than the ligands, which is attributable to the imposed planarity of the ruthenium-bound guanidine-based ligand, enabling it to serve as a better intercalator. Molecular docking studies show that the complexes effectively bind with DNA through electrostatic and H-bonding interactions and partial intercalation of the guanidine-based ligands. Cytotoxicity studies carried out on two carcinoma cell lines (PC3 and A549) and on two non-cancer cell lines (BPH1 and WI-38) show a marked improvement in antitumor activity owing to complex formation, which is attributed to improvement in cellular uptake on complex formation. The C ₁ complex is found to exhibit the most prominent activity against the PC3 cell line. Inclusion of the guanidine-based ligands in the half-sandwich ruthenium-arene complexes is found to be effective for displaying selective cytotoxicity to cancer cells and also for convenient tracing of the complexes in cells due to their prominent emissive nature.

Conjugates Containing Two and Three Trithiolato-Bridged Dinuclear Ruthenium(II)-Arene Units as In Vitro Antiparasitic and Anticancer Agents

The synthesis, characterization, and in vitro antiparasitic and anticancer activity evaluation of new conjugates containing two and three dinuclear trithiolato-bridged ruthenium(II)-arene units are presented. Antiparasitic activity was evaluated using transgenic Toxoplasmagondii tachyzoites constitutively expressing β-galactosidase grown in human foreskin fibroblasts (HFF). The compounds inhibited T.gondii proliferation with IC50 values ranging from 90 to 539 nM, and seven derivatives displayed IC50 values lower than the reference compound pyrimethamine, which is currently used for treatment of toxoplasmosis. Overall, compound flexibility and size impacted on the anti-Toxoplasma activity. The anticancer activity of 14 compounds was assessed against cancer cell lines A2780, A2780cisR (human ovarian cisplatin sensitive and resistant), A24, (D-)A24cisPt8.0 (human lung adenocarcinoma cells wild type and cisPt resistant subline). The compounds displayed IC50 values ranging from 23 to 650 nM. In A2780cisR, A24 and (D-)A24cisPt8.0 cells, all compounds were considerably more cytotoxic than cisplatin, with IC50 values lower by two orders of magnitude. Irrespective of the nature of the connectors (alkyl/aryl) or the numbers of the di-ruthenium units (two/three), ester conjugates 6-10 and 20 exhibited similar antiproliferative profiles, and were more cytotoxic than amide analogues 11-14, 23, and 24. Polynuclear conjugates with multiple trithiolato-bridged di-ruthenium(II)-arene moieties deserve further investigation.

Luminescent Bimetallic IrIII /AuI Peptide Bioconjugates as Potential Theranostic Agents

Diverse iridium peptide bioconjugates and the corresponding iridium/gold bimetallic complexes have been synthesized starting from a cyclometallated carboxylic acid substituted IrIII complex [Ir(ppy)2 (Phen-5-COO)] by solid phase peptide synthesis (SPPS). The selected peptide sequences were an enkephalin derivative Tyr-Gly-Gly-Phe-Leu together with the propargyl-substituted species Tyr-Gly-Pgl-Phe-Leu to allow gold coordination (Pgl: propyrgyl-glycine, HC≡C-Gly), and a specific short peptide, Ala-Cys-Ala-Phen, containing a cysteine residue. Introduction of the gold center has been achieved via a click reaction with the alkynyl group leading to an organometallic Au-C(triazole) species, or by direct coordination to the sulfur atom of the cysteine. The photophysical properties of these species revealed predominantly an emission originating from the Ir complex, using mixed metal-to-ligand and ligand-to-ligand charge transfer excited states of triplet multiplicity. The formation of the peptide bioconjugates caused a systematic redshift of the emission profiles. Lysosomal accumulation was observed for all the complexes, in contrast to the expected mitochondrial accumulation triggered by the gold complexes. Only the cysteine-containing Ir/Au bioconjugate displayed cytotoxic activity. The absence of activity may be related to the lack of endosomal/lysosomal escape for the cationic peptide conjugates. Interestingly, the different coordination sphere of the gold atom may play a crucial role, as the Au-S(cysteine) bond may be more readily cleaved in a biological environment than the Au-C(triazole) bond, and thus the Au fragment could be released from or trapped in the lysosomes, respectively. This work represents a starting point in the development of bimetallic peptide bioconjugates as theranostics and in the knowledge of factors that contribute to anti-proliferative activity.

Structure-activity relationships for osmium(II) arene phenylazopyridine anticancer complexes functionalised with alkoxy and glycolic substituents

Twenty-four novel organometallic osmium(II) phenylazopyridine (AZPY) complexes have been synthesised and characterised; [Os(η⁶-arene)(5-RO-AZPY)X]Y, where arene = p-cym or bip, AZPY is functionalized with an alkoxyl (O-R, R = Me, Et, ⁿPr, ⁱPr, ⁿBu) or glycolic (O-{CH₂CH₂O}_nR*, n = 1-4, R* = H, Me, or Et) substituent on the pyridyl ring para to the azo-bond, X is a monodentate halido ligand (Cl, Br or I), and Y is a counter-anion (PF₆^-, CF₃SO₃^- or IO₃^-). X-ray crystal structures of two complexes confirmed their 'half-sandwich' structures. Aqueous solubility depended on X, the AZPY substituents, arene, and Y. Iodido complexes are highly stable in water (X = I ⋙ Br > Cl), and exhibit the highest antiproliferative activity against A2780 (ovarian), MCF-7 (breast), SUNE1 (nasopharyngeal), and OE19 (oesophageal) cancer cells, some attaining nanomolar potency and good cancer-cell selectivity. Their activity and distinctive mechanism of action is discussed in relation to hydrophobicity (RP-HPLC capacity factor and Log P_o/w), cellular accumulation, electrochemical reduction (activation of azo bond), cell cycle analysis, apoptosis and induction of reactive oxygen species (ROS). Two complexes show ca. 4× higher activity than cisplatin in the National Cancer Institute (NCI) 60-cell line five-dose screen. The COMPARE algorithm of their datasets reveals a strong correlation with one another, as well as anticancer agents olivomycin, phyllanthoside, bouvardin and gamitrinib, but only a weak correlation with cisplatin, indicative of a different mechanism of action.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.

Dinuclear platinum(II) complexes of imidazophenanthroline-based bridging ligands as potential anticancer agents: synthesis, characterization, and in vitro cytotoxicity studies

The synthesis and characterization of the dinucleating ligands 1,2-bis(2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenoxy)ethane (L1) and 1,2-bis(2-(1H-imidazo[4,5-f][1, 10]phenanthrolin-2-yl)phenoxy)hexane (L2) and their dinuclear complexes [Pt₂(L1)Cl₄] (1) and [Pt₂(L2)Cl₄] (2) and the in vitro cytotoxicity of the complexes against HeLa, HepG2, and MCF-7 cell lines are reported. Ligand L1 crystallizes in the orthorhombic system with the space group Pbca. The complexes 1 and 2 undergo aquation following first-order kinetics. The MTT and trypan blue assays indicate higher cytotoxicity of the complexes towards the HepG2 and MCF-7 cell lines compared to cisplatin. The AO/EB assay and flow cytometry by Annexin V alexa fluor^®488/PI double staining assay demonstrate distinct morphological changes of apoptosis in a dose dependent manner. The cell cycle analysis shows a marked decrease in the DNA content in the G0/G1 phase with an increase in the G2/M phase on increasing the concentration of the complexes. The potential of the complexes as anticancer agents is demonstrated by their antiproliferative activity on the cell lines. The complexes interact with the major groove of DNA through H-bonding between the imidazole N-H protons and the nucleotide residues DC`21/N4 (cytosine) for complex 1 and DT`7/O2 (thymine) and DT`19/O2 (thymine) for complex 2, with the binding energy of - 1.98 and - 4.45 kcal/mol, respectively. Dinuclear Pt(II) complexes of imidazophenanthroline-based dinucleating ligands exhibit antiproliferative activity against HeLa, HepG2, and MCF-7 cell lines.

Anticancer organorhodium and -iridium complexes with low toxicity in vivo but high potency in vitro: DNA damage, reactive oxygen species formation, and haemolytic activity

Dinuclear Rh^III(Cp*) and Ir^III(Cp*) complexes demonstrated potent in vitro anticancer activity while exhibiting low toxicity in haemolysis studies and in vivo zebrafish models.

Synthesis and preliminary biological evaluation for the anticancer activity of organochalcogen (S/se) tethered chrysin-based organometallic RuII(η6-p-cymene) complexes

Organochalcogen (S/Se) functionalized chrysin derivatives were synthesized and coordinated with Ru^II(η⁶-p-cymene) to efficiently form ruthenium-based chemotherapeutic drug entities [C₃₁H₃₅O₄SRuCl]; [C₃₁H₃₅O₄SeRuCl]; [C₃₃H₃₁O₄SRuCl]; and [C₃₃H₃₁O₄SeRuCl]. The complexes were thoroughly characterized by analytical and various spectroscopic techniques which include elemental analysis, UV-vis, IR, NMR (¹H, ¹³C, and ⁷⁷Se NMR), and HR-MS. The interaction studies of these Ru(II) complexes were carried out with CT DNA/HSA by employing UV-vis, fluorescence and circular dichroic techniques in view to examine their chemotherapeutic potential. The complexes demonstrated predominant binding toward CTDNA via electrostatic interaction while, the extent of binding was quantified by calculating intrinsic binding constant (K_b) and binding constant (K) values which revealed higher binding affinity of selenium-based chrysin complexes as compared to their thio-analogs, following the order [C₃₁H₃₅O₄SeRuCl] > [C₃₃H₃₁O₄SeRuCl] > [C₃₁H₃₅O₄SRuCl] > [C₃₃H₃₁O₄SRuCl]. Moreover, interaction of these complexes with human serum albumin (HSA) was also investigated which suggested spontaneous interactions of complexes with the protein by hydrogen bonding and van der Waals forces. To visualize the preferential binding sites and affinity of complexes with DNA and HSA molecular docking studies were performed. Additionally, in vitro anticancer activity of the complexes were evaluated by SRB assay on selected cancer cell lines viz., HeLa (cervical), MIA-PA-CA-2 (pancreatic), MCF-7 (breast), Hep-G2 (Hepatoma), and SK-OV-3 (ovarian) which exhibited the superior cytotoxicity of complex [C₃₁H₃₅O₄SeRuCl] as compared to other analogs on selective cancer phenotypes. Communicated by Ramaswamy H. Sarma.

Ruthenium arene complexes with triphenylphosphane ligands: cytotoxicity towards pancreatic cancer cells, interaction with model proteins, and effect of ethacrynic acid substitution

The anticancer behaviour of Ru arene complexes can be tuned by an appropriate choice of the site and linkage of the bioactive group to the phosphane ligand.