Development of Half-Sandwich Ru, Os, Rh, and Ir Complexes Bearing the Pyridine-2-ylmethanimine Bidentate Ligand Derived from 7-Chloroquinazolin-4(3H)-one with Enhanced Antiproliferative Activity

Kinesin spindle protein (KSP) inhibitors are one of the most promising anticancer agents developed in recent years. Herein, we report the synthesis of ispinesib-core pyridine derivative conjugates, which are potent KSP inhibitors, with half-sandwich complexes of ruthenium, osmium, rhodium, and iridium. Conjugation of 7-chloroquinazolin-4(3H)-one with the pyridine-2-ylmethylimine group and the organometallic moiety resulted in up to a 36-fold increased cytotoxicity with IC50 values in the micromolar and nanomolar range also toward drug-resistant cells. All studied conjugates increased the percentage of cells in the G2/M phase, simultaneously decreasing the number of cells in the G1/G0 phase, suggesting mitotic arrest. Additionally, ruthenium derivatives were able to generate reactive oxygen species (ROS); however, no significant influence of the organometallic moiety on KSP inhibition was observed, which suggests that conjugation of a KSP inhibitor with the organometallic moiety modulates its mechanism of action.

Podophyllotoxin: Recent Advances in the Development of Hybridization Strategies to Enhance Its Antitumoral Profile

Podophyllotoxin is a naturally occurring cyclolignan isolated from rhizomes of Podophyllum sp. In the clinic, it is used mainly as an antiviral; however, its antitumor activity is even more interesting. While podophyllotoxin possesses severe side effects that limit its development as an anticancer agent, nevertheless, it has become a good lead compound for the synthesis of derivatives with fewer side effects and better selectivity. Several examples, such as etoposide, highlight the potential of this natural product for chemomodulation in the search for new antitumor agents. This review focuses on the recent chemical modifications (2017-mid-2023) of the podophyllotoxin skeleton performed mainly at the C-ring (but also at the lactone D-ring and at the trimethoxyphenyl E-ring) together with their biological properties. Special emphasis is placed on hybrids or conjugates with other natural products (either primary or secondary metabolites) and other molecules (heterocycles, benzoheterocycles, synthetic drugs, and other moieties) that contribute to improved podophyllotoxin bioactivity. In fact, hybridization has been a good strategy to design podophyllotoxin derivatives with enhanced bioactivity. The way in which the two components are joined (directly or through spacers) was also considered for the organization of this review. This comprehensive perspective is presented with the aim of guiding the medicinal chemistry community in the design of new podophyllotoxin-based drugs with improved anticancer properties.

Polypyridyl-based Co(III) complexes of vitamin B6 Schiff base for photoactivated antibacterial therapy

Herein, five novel polypyridyl-based Co(III) complexes of Schiff bases, viz., [Co(dpa)(L1)]Cl (1), [Co(dpa)(L2)]Cl (2), [Co(L3)(L2)]Cl (3), [Co(L3)(L1)]Cl (4), and [Co(L4)(L1)]Cl (5), where dpa (dipicolylamine) = bis(2-pyridylmethyl)amine; H2L1 = (E)-2-((2-hydroxybenzylidene)amino)phenol; H2L2 = (E)-5-(hydroxymethyl)-4-(((2-hydroxyphenyl)imino)methyl)-2-methylpyridin-3-ol; L3 = 4'-phenyl-2,2':6',2''-terpyridine (ph-tpy); and L4 = 4'-ferrocenyl-2,2':6',2''-terpyridine (Fc-tpy), were synthesized and characterized. Complexes 1, 3, and 4 were structurally characterized by single-crystal XRD, indicating an octahedral CoIIIN4O2 coordination core. The absorption bands of these complexes were observed in the visible range with a λmax at ∼430-485 nm. Complex 5 displayed an extra absorption band near 545 nm because of a ferrocene moiety. These absorptions in the visible region reflect the potential of the complexes to act as visible-light antimicrobial photodynamic therapy (aPDT) agents. All of these complexes showed reactive oxygen species (ROS)-mediated antibacterial effects against S. aureus (Gram-positive) and E. coli (Gram-negative bacteria) upon low-energy visible light (0.5 J cm-2, 400-700 nm) exposure. Additionally, 1-5 did not show any toxicity toward A549 (Human Lung adenocarcinoma) cells, reflecting their selective bacteria-killing abilities.

Has Ferrocene Really Delivered Its Role in Accentuating the Bioactivity of Organic Scaffolds?

Ferrocene is an important structural core in bioorganometallic chemistry because of its inherent stability, excellent redox properties, and low toxicity. Ferroquine and ferrocifen are two of the most notable contributions of ferrocene to medicinal chemistry with remarkable antimalarial and anticancer properties. The improved medicinal properties of these drug candidates highlight the impact that ferrocene can have on the molecular and biological properties of the bioactive compounds. In this Perspective, we investigate the scope and limitations of ferrocene incorporation into organic compounds/natural products on their mode of action and biological activities. We have also discussed the detailed role of ferrocene modifications in influencing the anticancer, antimalarial, and antimicrobial properties of various bioactive moieties to design safer and promising ferrocene-based drugs.

Impact of the ferrocenyl group on cytotoxicity and KSP inhibitory activity of ferrocenyl monastrol conjugates

The incorporation of the ferrocenyl moiety into a bioactive molecule may significantly alter the activity of the resulting conjugate. By applying this strategy, we designed ferrocenyl analogs of monastrol - the first low molecular weight kinesin spindle protein (KSP) inhibitor. The obtained compounds showed low micromolar antiproliferative activity towards a panel of sensitive and ABC-overexpressing cancer cells. Most cytotoxic compounds exhibited also higher KSP modulatory activity and ability for ROS generation compared to monastrol. The increased bioactivity of the studied compounds can be attributed to the presence of the ferrocenyl group.

Synthesis and Biological Activity of Ferrocenyl and Ruthenocenyl Analogues of Etoposide: Discovery of a Novel Dual Inhibitor of Topoisomerase II Activity and Tubulin Polymerization

Two series of the ferrocenyl and ruthenocenyl analogues of etoposide bearing 1,2,3-triazolyl or aminoalkyl linker were synthesized and evaluated for their cytotoxic properties, influence on the cell cycle, ability to induce tubulin polymerization, and inhibition of topoisomerase II activity. We found that the replacement of the etoposide carbohydrate moiety with a metallocenyl group led to organometallic conjugates exhibiting differentiated antiproliferative activity. Biological studies demonstrated that two ferrocenylalkylamino conjugates were notably more active than etoposide, with submicromolar or low-micromolar IC₅₀ values towards SW620, etoposide-resistant SW620E, and methotrexate-resistant SW620M cancer cell lines. Moreover, the simplest ferrocenylmethylamino conjugate exerted dual inhibitory action against tubulin polymerization and topoisomerase II activity while other studied compounds affected only topoisomerase II activity.

Metal-Dependent Cytotoxic and Kinesin Spindle Protein Inhibitory Activity of Ru, Os, Rh, and Ir Half-Sandwich Complexes of Ispinesib-Derived Ligands

Ispinesib is a potent inhibitor of kinesin spindle protein (KSP), which has been identified as a promising target for antimitotic anticancer drugs. Herein, we report the synthesis of half-sandwich complexes of Ru, Os, Rh, and Ir bearing the ispinesib-derived N,N-bidentate ligands (R)- and (S)-2-(1-amino-2-methylpropyl)-3-benzyl-7-chloroquinazolin-4(3H)-one and studies on their chemical and biological properties. Using the enantiomerically pure (R)- and (S)-forms of the ligand, depending on the organometallic moiety, either the S_M,R or R_M,S diastereomers, respectively, were observed in the molecular structures of the Ru- and Os(cym) (cym = η⁶-p-cymene) compounds, whereas the R_M,R or S_M,S diastereomers were found for the Rh- and Ir(Cp*) (Cp* = η⁵-pentamethylcyclopentadienyl) derivatives. However, density functional theory (DFT) calculations suggest that the energy difference between the diastereomers is very small, and therefore a mixture of both will be present in solution. The organometallics exhibited varying antiproliferative activity in a series of human cancer cell lines, with the complexes featuring the (R)-enantiomer of the ligand being more potent than the (S)-configured counterparts. Notably, the Rh and Ir complexes demonstrated high KSP inhibitory activity, even at 1 nM concentration, which was independent of the chirality of the ligand, whereas the Ru and especially the Os derivatives were much less active.

Ferrocene-containing hybrids as potential anticancer agents: Current developments, mechanisms of action and structure-activity relationships

Cancer is one of the most fatal threatens to human health throughout the world. The major challenges in the control and eradication of cancers are the continuous emergency of drug-resistant cancer and the low specificity of anticancer agents, creating an urgent need to develop novel anticancer agents. Organometallic compounds especially ferrocene derivatives possess remarkable structural and mechanistic diversity, inherent stability towards air, heat and light, low toxicity, low cost, reversible redox, ligand exchange, and catalytic properties, making them promising drug candidates for cancer therapy. Ferrocifen, a ferrocene-phenol hybrid, has demonstrated promising anticancer properties on drug-resistant cancers. Currently, Ferrocifen is in pre-clinical trial against cancers. Obviously, ferrocene moiety is a useful template for the development of novel anticancer agents. This review will provide an overview of ferrocene-containing hybrids with potential application in the treatment of cancers covering articles published between 2010 and 2020. The mechanisms of action, the critical aspects of design and structure-activity relationships are also discussed.

Iron Compounds as Anticancer Agents

Many ferrocene complexes have been prepared for their oncological potential. Some derive from molecules with known biological effects (taxanes, podophyllotoxine, artemisine, SAHA, etc.) while others are synthetic molecules selected for their cytotoxic effects (N-alkylaminoferrocenes and ferrocenyl alkylpyridinium). Although these complexes have received a great deal of attention, the field of iron metallodrugs is not limited to them. A number of inorganic complexes of iron(ii) and iron(iii) with possible anticancer effects have also been published, although research into their biological effects is often only at an early stage. This chapter also includes iron chelators, molecules that are administered in non-metallic form but whose cytotoxic species are their coordination complexes of iron generated in vivo. The most emblematic molecule of this family is bleomycin, used as an anticancer agent in many chemotherapies. To these can be added the iron chelates originally synthesized to treat iron overload, some of which have been shown to possess interesting anticancer properties. They have been, and continue to be, the subject of many clinical trials, whether alone or in combination. Thus, the area of iron metallodrugs includes molecules with very different structures and reactivity, studied from a number of different perspectives, but focused on increasing the number of molecules at our disposal for combatting cancer.

Biotin conjugated organic molecules and proteins for cancer therapy: A review

The main transporter for biotin is sodium dependent multivitamin transporter (SMVT), which is overexpressed in various aggressive cancer cell lines such as ovarian (OV 2008, ID8), leukemia (L1210FR), mastocytoma (P815), colon (Colo-26), breast (4T1, JC, MMT06056), renal (RENCA, RD0995), and lung (M109) cancer cell lines. Furthermore, its overexpression was found higher to that of folate receptor. Therefore, biotin demand in the rapidly growing tumors is higher than normal tissues. Several biotin conjugated organic molecules has been reported here for selective delivery of the drug in cancer cell. Biotin conjugated molecules are showing higher fold of cytotoxicity in biotin positive cancer cell lines than the normal cell. Nanoparticles and polymer surface modified drugs and biotin mediated cancer theranostic strategy was highlighted in this review. The cytotoxicity and selectivity of the drug in cancer cells has enhanced after biotin conjugation.

Synthesis and in vitro Biological Evaluation of Ferrocenyl Side-Chain-Functionalized Paclitaxel Derivatives

Taxanes, including paclitaxel, are widely used in cancer therapy. In an attempt to overcome some of the disadvantages entailed with taxane chemotherapy, we devised the synthesis of ferrocenyl-functionalized paclitaxel derivatives and studied their biological properties. The cytotoxic activity was measured with a panel of human cancer cell lines of various tissue origin, including multidrug-resistant lines. A structure-activity study of paclitaxel ferrocenylation revealed the N-benzoyl-ferrocenyl-substituted derivative to be the most cytotoxic. In contrast, substitution of the 3'-phenyl group of paclitaxel with a ferrocenyl moiety led to less potent antiproliferative compounds. However, these agents were able to overcome multidrug resistance, as they were virtually unrecognized by ABCB1, a major cellular exporter of taxanes. Interestingly, the redox properties of these ferrocenyl derivatives appear to play a less important role in their mode of action, as there was no correlation between intracellular redox activity and cytotoxicity/cell-cycle distribution. The antiproliferative activity of ferrocenyl taxanes strongly depends on the substitution position, and good tubulin polymerization inducers, as confirmed by molecular docking, were usually more cytotoxic, whereas compounds with stronger pro-oxidative properties exhibited lower antiproliferative activity.

Synthesis and evaluation of biological properties of ferrocenyl–podophyllotoxin conjugates

Ferrocenyl–podophyllotoxin conjugates have been synthesised and their antiproliferative activity, influence on cell cycle, and interactions with tubulin were evaluated.

Colchicine metallocenyl bioconjugates showing high antiproliferative activities against cancer cell lines

Ferrocenyl and ruthenocenyl conjugates with colchicine have been synthesised and their cytotoxic activity, influence on the cell cycle, and interactions with tubulin were evaluated.

The length of the bridging chain in ansa-metallocenes influences their antiproliferative activity against triple negative breast cancer cells (TNBC)

[n]Ferrocenophane and [n]ruthenocenophane derivatives have been synthesized and their antiproliferative activity evaluated against MDA-MB-231 cells.

Ferrocifen type anti cancer drugs

The mechanisms of action of ferrocifens depend on several features: chemical structures, used concentrations, nature of cancer cells.

Photoactivated cytotoxicity of ferrocenyl-terpyridine oxovanadium(IV) complexes of curcuminoids

Oxovanadium(IV) complexes, viz. [VO(Fc-tpy)(Curc)](ClO4) (1), [VO(Fc-tpy)(bDHC)](ClO4) (2), [VO(Fc-tpy)(bDMC)](ClO4) (3) and [VO(Ph-tpy)(Curc)](ClO4) (4), of 4'-ferrocenyl-2,2':6',2″-terpyridine (Fc-tpy) and 4'-phenyl-2,2':6',2″-terpyridine (Ph-tpy) and monoanionic curcumin (Curc), bis-dehydroxycurcmin (bDHC) and bis-demethoxycurcumin (bDMC) were prepared, characterized and their photo-induced DNA cleavage activity and photocytotoxicity in visible light studied. The ferrocenyl complexes 1-3 showed an intense metal-to-ligand charge transfer band near 585 nm in DMF and displayed Fc(+)/Fc and V(IV)/V(III) redox couples near 0.65 V and -1.05 V vs. SCE in DMF-0.1 M TBAP. The complexes as avid binders to calf thymus DNA showed significant photocleavage of plasmid DNA in red light of 647 nm forming OH radicals. The complexes showed photocytotoxicity in HeLa and Hep G2 cancer cells in visible light of 400-700 nm with low dark toxicity. ICP-MS and fluorescence microscopic studies exhibited significant cellular uptake of the complexes within 4 h of treatment with complexes. The treatment with complex 1 resulted in the formation of reactive oxygen species inside the HeLa cells which was evidenced from the DCFDA assay.

Auranofin and related heterometallic gold(I)-thiolates as potent inhibitors of methicillin-resistant Staphylococcus aureus bacterial strains

A series of new heterometallic gold(I) thiolates containing ferrocenyl-phoshines were synthesized. Their antimicrobial properties were studied and compared to that of FDA-approved drug, auranofin (Ridaura), prescribed for the treatment of rheumatoid arthritis. MIC in the order of one digit micromolar were found for most of the compounds against Gram-positive bacteria Staphylococcus aureus and CA MRSA strains US300 and US400. Remarkably, auranofin inhibited S. aureus, US300 and US400 in the order of 150-300 nM. This is the first time that the potent inhibitory effect of auranofin on MRSA strains has been described. The effects of a selected heterometallic compound and auranofin were also studied in a non-tumorigenic human embryonic kidney cell line (HEK-293).

Anti-cancer palladium complexes: a focus on PdX2L2, palladacycles and related complexes

Much success has been achieved with platinum-based chemotherapeutic agents, i.e. through interactions with DNA. The long-term application of Pt complexes is thwarted by issues, leading scientists to examine other metals such as palladium which could exhibit complementary modes of action (given emphasis wherever known). Over the last 10 years several research groups have focused on the application of an eclectic array of palladium complexes (of the type PdX2L2, palladacycles and related structures) as potential anti-cancer agents. This review therefore provides readers with an up to date account of the advances that have taken place over the past several decades.

Potential anticancer heterometallic Fe-Au and Fe-Pd agents: initial mechanistic insights

A series of gold(III) and palladium(II) heterometallic complexes with new iminophosphorane ligands derived from ferrocenylphosphanes [{Cp-P(Ph2)═N-Ph}2Fe] (1), [{Cp-P(Ph2)═N-CH2-2-NC5H4}2Fe] (2), and [{Cp-P(Ph2)═N-CH2-2-NC5H4}Fe(Cp)] (3) have been synthesized and structurally characterized. Ligands 2 and 3 afford stable coordination complexes [AuCl2(3)]ClO4, [{AuCl2}2(2)](ClO4)2, [PdCl2(3)], and [{PdCl2}2(2)]. The complexes have been evaluated for their antiproliferative properties in human ovarian cancer cells sensitive and resistant to cisplatin (A2780S/R), in human breast cancer cells (MCF7) and in a nontumorigenic human embryonic kidney cell line (HEK-293T). The highly cytotoxic trimetallic derivatives M2Fe (M = Au, Pd) are more cytotoxic to cancer cells than their corresponding monometallic fragments. Moreover, these complexes were significantly more cytotoxic than cisplatin in the resistant A2780R and the MCF7 cell lines. Studies of the interactions of the trimetallic compounds with DNA and the zinc-finger protein PARP-1 indicate that they exert anticancer effects in vitro based on different mechanisms of actions with respect to cisplatin.