Organometallic chemistry, biology and medicine: ruthenium arene anticancer complexes

The Midas touch in cancer chemotherapy: from platinum- to gold-dithiocarbamato complexes

The unquestionable therapeutic success of the anticancer drug cisplatin and its second- and third-generation analogues has triggered, in the past forty years, the development of several metal-based potential chemotherapeutic agents, most of which have failed to enter clinical trials. In this context, during the last decade, our research group has been making quite an effort to design a number of metal-dithiocarbamato derivatives that were expected, at least in principle, to resemble the main features of cisplatin together with higher activity, improved selectivity and bioavailability, and lower side-effects. Among all, gold(III) complexes have shown outstanding in vitro and in vivo antitumour properties and reduced or no systemic and renal toxicity, compared to the reference drug. Here, we summarize the results achieved to date, focusing on the mechanistic studies and the potential future developments opened up by our research work.

Photocontrolled nucleobase binding to an organometallic Ru(II) arene complex

The complex [(p-cym)Ru(bpm)(py)][PF(6)](2) (where p-cym = para-cymene, bpm = 2,2'-bipyrimidine and py = pyridine) is the first ruthenium(ii) arene complex that can selectively photodissociate a monodentate ligand (py) when excited with visible light and form a reactive aqua derivative able to bind to a DNA base.

Synthetic strategies towards ruthenium-porphyrin conjugates for anticancer activity

The conjugation of porphyrins to metal fragments is a strategy for making new compounds that are expected to combine the phototoxicity and the tumour-localization properties of the porphyrin chromophore with the cytotoxicity of the metal fragment for additive antitumour effect. We report here the preparation of new classes of porphyrin-ruthenium conjugates with potential bio-medical applications. Ruthenium was chosen because several Ru compounds have shown promising anticancer activity. The conjugation with the porphyrin moiety was accomplished either through peripheral pyridyl rings (e.g.meso-4'-tetrapyridylporphyrin, 4'TPyP) or through bpy units (e.g.meso-(p-bpy-phenyl)porphyrins, bpy(n)-PPs, n = 1-4). The number of Ru fragments attached to the porphyrins ranges from 1 to 4 and the total charge of the conjugates from -4 to +8. Different types of peripheral fragments, both Ru(III) and Ru(II), have been used: in some cases they are structurally similar to established anticancer compounds. Examples are [Na](4)[4'TPyP{trans-RuCl(4)(dmso-S)}(4)] (2), that bears four NAMI-type Ru(III) fragments, or [4'TPyP{Ru([9]aneS3)(en)}(4)][CF(3)SO(3)](8) (3) and [bpy(4)-PP{Ru([9]aneS3)(dmso-S)}(4)][CF(3)SO(3)](8) (9) (en = ethane-1,2-diamine, [9]aneS3 = 1,4,7-trithiacyclononane) that have four half-sandwich Ru(II) compounds. The Ru fragments may either contain one or more labile ligands, such as in 2 or in 9, or be coordinatively saturated and substitutionally inert, such as in 3 or in [bpy(4)-PP{Ru([12]aneS4)}(4)][CF(3)SO(3)](8) (11) ([12]aneS4 = 1,4,7,10-tetrathiacyclododecane). Most of the ruthenium-porphyrin conjugates described in this work are soluble--at least moderately--in aqueous solution and are thus suitable for biological investigations, in particular for cytotoxicity and photo-cytotoxicity tests.

Recent developments in ruthenium anticancer drugs

Interest in Ru anticancer drugs has been growing rapidly since NAMI-A ((ImH(+))[Ru(III)Cl(4)(Im)(S-dmso)], where Im = imidazole and S-dmso = S-bound dimethylsulfoxide) or KP1019 ((IndH(+))[Ru(III)Cl(4)(Ind)(2)], where Ind = indazole) have successfully completed phase I clinical trials and an array of other Ru complexes have shown promise for future development. Herein, the recent literature is reviewed critically to ascertain likely mechanisms of action of Ru-based anticancer drugs, with the emphasis on their reactions with biological media. The most likely interactions of Ru complexes are with: (i) albumin and transferrin in blood plasma, the former serving as a Ru depot, and the latter possibly providing active transport of Ru into cells; (ii) collagens of the extracellular matrix and actins on the cell surface, which are likely to be involved in the specific anti-metastatic action of Ru complexes; (iii) regulatory enzymes within the cell membrane and/or in the cytoplasm; and (iv) DNA in the cell nucleus. Some types of Ru complexes can also promote the intracellular formation of free radical species, either through irradiation (photodynamic therapy), or through reactions with cellular reductants. The metabolic pathways involve competition among reduction, aquation, and hydrolysis in the extracellular medium; binding to transport proteins, the extracellular matrix, and cell-surface biomolecules; and diffusion into cells; with the extent to which individual drugs participate in various steps along these pathways being crucial factors in determining whether they are mainly anti-metastatic or cytotoxic. This diversity of modes of action of Ru anticancer drugs is also likely to enhance their anticancer activities and to reduce the potential for them to develop tumour resistance. New approaches to metabolic studies, such as X-ray absorption spectroscopy and X-ray fluorescence microscopy, are required to provide further mechanistic insights, which could lead to the rational design of improved Ru anticancer drugs.

Transferring the concept of multinuclearity to ruthenium complexes for improvement of anticancer activity

Multinuclear platinum anticancer complexes are a proven option to overcome resistance of established anticancer compounds. Transferring this concept to ruthenium complexes led to the synthesis of dinuclear Ru(II)-arene compounds containing a bis(pyridinone)alkane ligand linker. A pronounced influence of the spacer length on the in vitro anticancer activity was found, which is correlated to the lipophilicity of the complexes. IC(50) values in the same dimension as for established platinum drugs were found in human tumor cell lines. No cross-resistance to oxoplatin, a cisplatin prodrug, was observed for the most active complex in three resistant cell lines; in fact, a 10-fold reversal of sensitivity in two of the oxoplatin-resistant lines was found. (Bio)analytical characterization of the representative examples showed that the ruthenium complexes hydrolyze rapidly, forming predominantly diaqua species that exhibit affinity toward transferrin and DNA, indicating that both proteins and nucleobases are potential targets.

Bio- and air-tolerant carbon–carbon bond formations via organometallic ruthenium catalysis

Selected [2+2+2] cycloadditions, alkene–alkyne coupling and fusion of enyne with diazo compound, all triggered by an artificial organometallic ruthenium catalyst are demonstrated to proceed under ambient aerobic aqueous conditions in presence of bodily fluids or cell lysate. To the best of our knowledge, these are the first examples of bio- and air-tolerant C–C bond formation catalyzed by an artificial organometallic compound.

Synthesis, characterization, and reaction pathways for the formation of a GMP adduct of a cytotoxic thiocyanato ruthenium arene complex

The organoruthenium complex [(eta(6)-hmb)Ru(en)(Cl)][PF6] (hmb is hexamethylbenzene, en is ethylenediamine) undergoes facile aquation and then reacts with KSCN in unbuffered solution to give the S-coordinated thiocyanato product [(eta(6)-hmb)Ru(en)(S-SCN)]+ which slowly converts to the thermodynamically favored N-bound complex [(eta(6)-hmb)Ru(en)(N-NCS)]+ (1+). Complex 1 was synthesized and characterized by X-ray crystallography and mass spectrometry. Despite its lack of hydrolysis over 24 h, complex 1 exhibits moderate cytotoxicity (IC(50) 24 microM) towards the human ovarian cancer cell line A2780, comparable with that of the chlorido analogue which is thought to be activated (towards potential target DNA) via a rapid aquation (Wang et. al. in Proc Natl Acad Sci USA 102:18269-18274, 2005). Detailed kinetic studies suggest that complex 1 binds to guanosine 5'-monophosphate (GMP) through direct N7 substitution of the N-bound SCN ligand. In the presence of a high concentration of chloride (104 mM), however, complex 1 may bind partly to GMP via Cl substitution.

Synthesis and characterization of dicobalthexacarbonyl-alkyne derivatives of amino acids, peptides, and peptide nucleic acid (PNA) monomers

The reaction of Co(2)(CO)(8) with alkyne-containing amino acids [1a: phenylalanine (Phe) and 1b: methionine (Met)], two suitably alkyne-functionalized derivatives of the neuropeptide enkephalin (Enk) [3: Ac-Enk-Prop and 5: Ac-Enk(Pgl)-NH(2) (Ac--Acetyl; Pgl--propargylglycine; Prop--propargylamine)], a thymine Peptide Nucleic Acid (T-PNA) monomer (7), and a PNA-like monomer (9) derivative gave the respective dicobalthexacarbonyl bioconjugates in very good yields. Two different sites for labeling of the biomolecules were successfully used: The organometallic moiety was reacted with the C-terminus of alkyne-containing amino acids, peptide or PNA thymine monomers, and alternatively the organometallic compound was complexed to an internal site in the peptide or PNA. To this end, a simple glycine was replaced by propargylglycine in peptides, and a new alkyne-containing PNA-like monomer, in which an alkyne chain replaces the nucleobase, was used for PNA chemistry. For the synthesis of the two alkyne-containing enkephalin derivatives 3 and 5, two different resins, namely sulfamylbutyryl and Rink amid, were used as they allow to selectively insert, on the solid phase, an alkyne moiety at the C-terminus and on a side-chain of a peptide sequence, respectively. The identity and constitution of all cobalt complexes were confirmed by different analytical methods (IR, FAB, ESI-MS, and NMR). Most notably, IR spectroscopy shows intensive bands in the 2100-2000 cm(-1) region because of the Co(2)(CO)(6) moiety. In both (1)H NMR spectra of the dicobalthexacarbonyl PNA monomer derivatives 8 and 10, all signals are doubled because of the cis-trans isomerism about the central amide bond. The X-ray structure of a dicobalthexacarbonyl phenylalanine derivative (2a) confirms the proposed composition of the bioconjugates and shows that, as anticipated, the alkyne group of 2a is no longer linear upon complexation in comparison to the alkyne group of the bioconjugate precursor 1a, as indicated by a C-C[triple bond]C angle of about 143 degrees in 2a. Moreover, the C[triple bond]C bond of 1a was elongated by about 0.15 A upon Co(2) coordination.

Novel organometallic cationic ruthenium(II) pentamethylcyclopentadienyl benzenesulfonamide complexes targeted to inhibit carbonic anhydrase

Cationic ruthenium(II) pentamethylcyclopentadienyl benzenesulfonamide sandwich complexes have been synthesized and screened for enzymatic inhibition of the physiologically dominant carbonic anhydrase (CA) isozymes: human CA I and II, mitochondrial isozymes VA and VB, and the cancer-associated isozyme IX. The complexes demonstrated weaker binding to CAs compared with typical aromatic sulfonamides, inhibiting the enzyme at high nanomolar concentrations. An in vitro cytotoxic evaluation of the complexes was also undertaken against a range of tumorigenic cell lines and a healthy human cell line. Complexes inhibited the growth of cancerous cells at low micromolar concentrations while expressing lower levels of toxicity towards the normal human cell line. Factors influencing the synthesis, cytotoxicity, and enzyme affinity for this series of organometallic complexes are discussed.

Electronic structural investigations of ruthenium compounds and anticancer prodrugs

Several Ru(III) compounds are propitious anticancer agents although the precise mechanisms of action remain unknown. With this paper we start to establish an experimental library of X-ray absorption spectroscopy (XAS) data for ten Ru compounds wherein the ligands [Cl(-), dimethyl sulfoxide, imidazole, and indazole] were varied systematically to provide electronic structural information for future use in correlating spectroscopic signatures with chemical properties. Despite the considerable difference in the coordination environments of the complexes studied, the overall differences in spectral features and electronic structures calculated using density functional theory are unexpectedly small. However, the differences in the electronic structure of the Ru(III) prodrugs KP1019 ([IndH][trans-RuCl(4)(Ind)(2)], Ind is indazole) and ICR ([ImH][trans-RuCl(4)(Im)(2)], Im is imidazole) observed in the XAS data show correlation with known chemical and biological activities in addition to the donor abilities of imidazole compared with indazole and reduction potentials of the complexes. These semiquantitative results lay the groundwork for future biochemical studies into the structure-function relationships of Ru-based anticancer drugs.

Synthesis of a 1,4-benzodiazepine containing palladacycle with in vitro anticancer and cathepsin B activity

The reaction of the five-membered C,N-palladacycle [(L)PdCl](2), where LH = 1-methyl-5-phenyl-1H-1,4-benzodiazepin-2(3H)-one, with 1,2-ethanebis(diphenylphosphine), dppe, leads to the formation of the bridged palladacycle. [Pd(2)L(2)(mu-dppe)Cl(2)] 3, which was characterised in solution by (1)H and (31)P NMR spectroscopy and in the solid state by X-ray crystallography. Complex 3 was tested in vitro against a number of cell lines. For example, it inhibited K562 leukaemia cells with an IC(50) value of 4.3 microM (1 h exposure) and displayed cathepsin B inhibitory action with an IC(50) value of 3 microM.

Medicinal organometallic chemistry: designing metal arene complexes as anticancer agents

The field of medicinal inorganic chemistry is rapidly advancing. In particular organometallic complexes have much potential as therapeutic and diagnostic agents. The carbon-bound and other ligands allow the thermodynamic and kinetic reactivity of the metal ion to be controlled and also provide a scaffold for functionalization. The establishment of structure-activity relationships and elucidation of the speciation of complexes under conditions relevant to drug testing and formulation are crucial for the further development of promising medicinal applications of organometallic complexes. Specific examples involving the design of ruthenium and osmium arene complexes as anticancer agents are discussed.

In vitro anticancer activity and biologically relevant metabolization of organometallic ruthenium complexes with carbohydrate-based ligands

The synthesis and in vitro anticancer activity of dihalogenido(eta6-p-cymene)(3,5,6-bicyclophosphite-alpha-D-glucofuranoside)ruthenium(II) complexes are described. The compounds were characterized by NMR spectroscopy and ESI mass spectrometry, and the molecular structures of dichlorido-, dibromido- and diiodido(eta6-p-cymene)(3,5,6-bicyclophosphite-1,2-O-isopropylidene-alpha-D-glucofuranoside)ruthenium(II) were determined by X-ray diffraction analysis. The complexes were shown to undergo aquation of the first halido ligand in aqueous solution, followed by hydrolysis of a P--O bond of the phosphite ligand, and finally formation of dinuclear species. The hydrolysis mechanism was confirmed by DFT calculations. The aquation of the complexes was markedly suppressed in 100 mM NaCl solution, and notably only very slow hydrolysis of the P--O bond was observed. The complexes showed affinity towards albumin and transferrin and monoadduct formation with 9-ethylguanine. In vitro studies revealed that the 3,5,6-bicyclophosphite-1,2-O-cyclohexylidene-alpha-D-glucofuranoside complex is the most cytotoxic compound in human cancer cell lines (IC50 values from 30 to 300 microM depending on the cell line).

Applications of heteronuclear NMR spectroscopy in biological and medicinal inorganic chemistry

There is a wide range of potential applications of inorganic compounds, and metal coordination complexes in particular, in medicine but progress is hampered by a lack of methods to study their speciation. The biological activity of metal complexes is determined by the metal itself, its oxidation state, the types and number of coordinated ligands and their strength of binding, the geometry of the complex, redox potential and ligand exchange rates. For organic drugs a variety of readily observed spin I = 1/2 nuclei can be used (1H, 13C, 15N, 19F, 31P), but only a few metals fall into this category. Most are quadrupolar nuclei giving rise to broad lines with low detection sensitivity (for biological systems). However we show that, in some cases, heteronuclear NMR studies can provide new insights into the biological and medicinal chemistry of a range of elements and these data will stimulate further advances in this area.

DNA interactions of dinuclear RuII arene antitumor complexes in cell-free media

We recently synthesized and characterized water-soluble dinuclear Ru(II) arene complexes, in which two {(eta(6)-p-isopropyltoluene)RuCl[3-(oxo-kappaO)-2-methyl-4-pyridinonato-kappaO(4)]} units were linked by flexible chains of different length [(CH(2))(n) (n=4, 6, 8, 12)]. These new dinuclear ruthenium drugs were found to exert promising cytotoxic effects in human cancer cells. In the present work DNA modifications by these new dinuclear Ru(II) arene compounds, which differed in the length of the linker between the two Ru(II) centers, were examined by biochemical and biophysical methods. The complexes bind DNA forming intrastrand and interstrand cross-links in one DNA molecule in the absence of proteins. An intriguing aspect of the DNA-binding mode of these dinuclear Ru(II) compounds is that they can cross-link two DNA duplexes and also proteins to DNA--a feature not observed for other antitumor ruthenium complexes. Thus, the concept for the design of interhelical and DNA-protein cross-linking agents based on dinuclear Ru(II) arene complexes with sufficiently long linkers between two Ru centers may result in new compounds which exhibit a variety of biological effects and can be also useful in nucleic acids research.