Matthias Tacke

Bioorganometallic fulvene-derived titanocene anti-cancer drugs

6-Substituted fulvenes are interesting and easily accessible starting materials for the synthesis of novel substituted titanocenes via reductive dimerisation, carbolithiation or hydridolithiation reactions, which are followed by a transmetallation reaction with titanium tetrachloride in the latter two cases. Depending on the substitution pattern, these titanocenes prove to be bioorganometallic anti-cancer drugs, which have significant potential against advanced or metastatic renal-cell cancer. Patients bearing these stages of kidney cancer have a poor prognosis so far and therefore real progress in the area of metal-based anti-cancer drugs may come from this simple and effective synthetic approach. This tutorial review provides an insight into the synthesis of fulvene-derived titanocenes and their activity in preclinical experiments.

In-vitro anti-tumor activity studies of bridged and unbridged benzyl-substituted titanocenes

The benzyl-substituted ansa-titanocenes [1,2-di(cyclopentadienyl)-1,2-di-(4-N,N-dimethylaminophenyl)ethanediyl] titanium dichloride (Titanocene X) and [1,2-di(cyclopentadienyl)-1,2-bis(m-dimethoxyphenyl)ethanediyl] titanium dichloride (Titanocene Z), and the benzyl-substituted unbridged titanocene bis-[(p-methoxybenzyl)cyclopentadienyl] titanium(IV) dichloride (Titanocene Y) were tested on the growth of a wide variety of tumor cells in vitro on a panel of 36 human tumor cell lines containing 14 different tumor types investigated in a cellular proliferation assay. Titanocene Y with a mean IC50 value of 65.8×10−6 mol/l over the full panel of 36 cancer cell lines reaches the activity of cisplatin with 14.7×10−6 mol/l within a factor of 4, whereas Titanocene X and Z show significantly less cytotoxic activity. Titanocene Y is most effective on pleura mesothelioma, and uterine and renal cell cancer, where the IC50 values are comparable or significantly better than for cisplatin. In particular, in the case of renal cell cancer and pleura mesothelioma there is an obvious lack of chemotherapeutic reagents, which might be filled by Titanocene Y, where a very promising cytotoxic effect in comparison with cisplatin could be shown.

Anti-tumor activity of Titanocene Y in xenografted Caki-1 tumors in mice.

The benzyl-substituted unbridged titanocene bis-[(p-methoxybenzyl)cyclopentadienyl] titanium(IV) dichloride (Titanocene Y) was tested in vitro against human renal cancer cells (Caki-1), in which it showed an IC50 value of 36×10–6 mol/l. Titanocene Y was then given in vivo in doses of 10, 20, 30, 40 and 50 mg/kg on 5 consecutive days to Caki-1-bearing mice, and it showed concentration-dependent and statistically significant tumor growth reduction with respect to a solvent-treated control cohort. The maximum tolerable dose of Titanocene Y was determined to be 40 mg/kg and it showed significantly better tumor volume growth reduction than cisplatin given at a dose of 2 mg/kg. This superior activity of Titanocene Y with respect to cisplatin will hopefully lead to clinical tests against metastatic renal cell cancer in the near future.

Novel titanocene anti-cancer drugs and their effect on apoptosis and the apoptotic pathway in prostate cancer cells

Advanced prostate cancer is not curable by current treatment strategies indicating a significant need for new chemotherapeutic options. Highly substituted ansa-titanocene compounds have shown promising cytotoxic activity in a range of cancers. The objectives of this study are to examine the effects of these titanocene compounds on prostate cancer cells.Prostate cell lines were treated with three novel titanocene compounds and compared to titanocene dichloride and cisplatin. Percent apoptosis, viability and cell cycle were assessed using propidium iodide DNA incorporation with flow cytometry. Cytochrome C was assessed by western blotting of mitochondrial and cytoplasmic fractions. Apoptosis Inducing Factor was assessed by confocal microscopy.These novel compounds induced more apoptosis compared to cisplatin in a dose dependent manner. Compound Y had the most significant effect on cell cycle and apoptosis. Despite the release of cytochrome C from the mitochondrial fraction there was no inhibition of apoptosis with the pan caspase inhibitor, ZVAD-FMK. AIF was shown to translocate from the cytosol to the nucleus mediating a caspase independent cell death. Bcl-2 over expressing PC-3 cells, which were resistant to cisplatin induced apoptosis, underwent apoptosis following treatment with all the titanocene compounds.This study demonstrates possible mechanisms by which these novel titanocene compounds can mediate their apoptotic effect in vitro. The fact that they can induce more apoptosis than cisplatin in advanced cancer cell lines would confer an advantage over cisplatin. They represent exciting new agents with future potential for the treatment of advanced prostate cancer.

Gold(I)-NHC complexes of antitumoral diarylimidazoles: structures, cellular uptake routes and anticancer activities.

Five new heterocyclic gold carbene complexes were prepared, four chlorido-[1,3-dimethyl-4,5-diarylimidazol-2-ylidene]gold complexes 6a-d and a chlorido-[1,3-dibenzylimidazol-2-ylidene]gold complex 11, and three of them were characterised by X-ray single crystal analyses. They were tested for cytotoxicity against a panel of four human cancer cell lines and non-malignant fibroblasts, for tubulin interaction, and for the pathways of their uptake into 518A2 melanoma cells. All complexes showed cytotoxic activity in the micromolar IC(50) range with distinct selectivities for certain cell lines. In stark contrast to related metal-free 1-methyl-4,5-diarylimidazoles, the complexes 6 and 11 did not noticeably inhibit the polymerisation of tubulin to give microtubules. The cellular uptake of complexes 6 occurred mainly via the copper transporter (Ctr1) and the organic cation transporters (OCT-1/2). Complex 11 was accumulated preferentially via the organic cation transporters and by Na(+)/K(+)-dependent endocytosis. The new gold carbene complexes seem to operate by a mechanism different from that of the parent 1-methylimidazolium ligands.

Substituted titanocenes induce caspase-dependent apoptosis in human epidermoid carcinoma cells in vitro and exhibit antitumour activity in vivo

Titanocene compounds are a novel series of agents that exhibit cytotoxic effects in a variety of human cancer cells in vitro and in vivo. In this study, the antiproliferative activity of two titanocenes (Titanocenes X and Y) was evaluated in human epidermoid cancer cells in vitro. Titanocenes X and Y induce apoptotic cell death in epidermoid cancer cells, with IC50 values that are comparable to cisplatin. Characterisation of the cell death pathway induced by titanocene compounds in A431 cells revealed that apoptosis is preceded by cell cycle arrest and the inhibition of cell proliferation. The induction of apoptosis is dependent on the activation of caspase-3 and -7 but not caspase-8. Furthermore, the antitumour activity of Titanocene Y was tested in an A431 xenograft model of epidermoid cancer. Results indicate that Titanocene Y significantly reduced the growth of A431 xenografts with an antitumour effect similar to cisplatin. These results suggest that titanocenes represent a novel series of promising antitumour agents.

Oxali‐Titanocene Y: A Potent Anticancer Drug

Outside of platinum and ruthenium anticancer drugs, there is significant unexplored space for further metal-based drugs that target cancer, such as titanocene dichloride. One of these promising drug candidates is bis-[(p-methoxybenzyl)cyclopentadienyl]titanium(IV) dichloride (titanocene Y, 1), which is accessible via the hydridolithiation reaction of 6-anisylfulvene and subsequent transmetallation with titanium tetrachloride. Compound 1 exhibits an IC50 value of 21 mm toward the LLC-PK cell line, which has proven to be a good mimic of a kidney carcinoma cell line and a reliable tool for the optimisation of titanocenes against this type of cancer. Additionally, the antiproliferative activity of 1 and other titanocenes has been studied in 36 human tumour cell lines and against explanted human tumours. 6] These in vitro and ex vivo experiments showed that renal cell cancer is the prime target for this novel class of titanocenes, but there is significant activity against ovarian, prostate, cervical, lung, colon, and breast cancers as well. Furthermore, titanocene derivatives give a positive immune response by up-regulating the number of natural killer (NK) cells in mice. These results were underscored by initial mechanistic studies of the effects of these titanocenes on apoptosis and the apoptotic pathway in prostate and cervical cancer cells. Recently, animal studies demonstrated the successful treatment of mice bearing xenografted A431, Caki-1, and MCF-7 tumours with 1. Herein the synthesis and initial cytotoxicity studies of titanocene oxalate and oxali-titanocene Y (2) are presented. The structures and syntheses of 1 and 2 are shown in Scheme 1. A simple anion-exchange reaction in THF employing silver oxalate eliminates insoluble silver chloride and produces 2 or titanocene oxalate. An X-ray crystallographic study established the molecular structure of 2 (Figure 1). Despite the addition of the oxalate bidentate ligand in replacement of the two chlorides on the titanium centre, there is almost no apparent variance in the molecular structures of 1 and 2. The length of the bond between the titanium centre and the carbon atoms of the cyclopentadienide rings are very similar for both 1 and 2. They vary from 2.34 to 2.41 C for 1 and from 2.33 to 2.40 C for 2. Very slight differences can be observed in comparing the titanium centroid distances in 1 (2.06 C) and 2 (2.04 C). The centroid–titanium–centroid angle is 130.78 for 1, whereas the corresponding angle in 2 is 134.88. This widening goes nicely with the more blade-like shape of the oxalate anion, compared with the spherical chloride anions. As expected, the average titanium–chloride bond in 1 is considerably longer (2.37 C) than the average titanium–oxygen bond in 2 (1.99 C). The average carbon–oxygen single and double bonds in 2 are, with 1.30 C and 1.21 C, quite close to those found in free carboxylic acids, showing that there is very little, if any, negative charge located on the oxygen atoms that are not bonded to titanium. In 1 the chloride–titanium–chloride angle is 95.98, which is in the range of other titanocene dichlorides. In 1, however, the oxygen atoms are part of a chelate ring which forces the oxygen–titanium–oxygen angle to 79.08. Despite the addition of the bidentate oxalate ligand. minimal structural change incurred. In tests against LLC-PK cells, which have proven to be a valuable in vitro model for kidney cancer, titanocene oxalate shows an IC50 value of 140 mm, which is a significant improvement (14-fold) relative to titanocene dichloride, with an IC50 value of 2000 mm. Similar behaviour is observed for the p-dimethoxybenzyl-substituted species; 2 is 13-fold more cytotoxic against LLC-PK than 1 and exhibits an IC50 value of 1.6 mm (see Figure 2), which is twice as cytotoxic as cisplatin (3.3 mm). The Scheme 1. Structures and syntheses of titanocene Y (1) and oxali-titanocene Y (2): a) 2LiBEt3H, Et2O, 2BEt3 ; b) TiCl4, THF, 2LiCl(s) ; c) Ag2C2O4, THF, 2AgCl(s).

Novel derivatives of ansa-titanocenes procured from 6-phenylfulvene: a combined experimental and theoretical study

Abstract The previously prepared trans-[(1,2-diphenyl-1,2-dicyclopentadienyl)ethanediyl] titanium(IV) dichloride, [1,2-(Ph)2C2H2{η5-C5H4}2]Ti(Cl)2, was synthesised using an alternative procedure, from which its crystal structure was determined. Using this compound, a variety of other ansa-titanocene derivatives were synthesised by replacement of the chloride ligands with selected substituents. Thus RTi(X)(Y) systems where R=1,2-(Ph)2C2H2η5-C5H42; X=Y=CH3; X=CH3, Y=Cl; X=Y=NCS; X=Y=NCO; X=Y=OPh and (X/Y)=O have been synthesised and characterised. DFT calculations were performed on the complexes trans-[(1,2-diphenyl-1,2-dicyclopentadienyl)-ethanediyl] titanium(IV) dichloride, bis-(6,6-diphenylfulvene)titanium and bis-(6,6-diphenylfulvene)iron. This demonstrated the role that the metal centre plays in their formation, generating either an ansa-metallocene, a ‘tucked in’ fulvene complex or a metallocene coordinating fulvene anions.

N-heterocyclic carbene metal complexes as bio-organometallic antimicrobial and anticancer drugs

Late transition metal complexes that bear N-heterocyclic carbene (NHC) ligands have seen a speedy growth in their use as both, metal-based drug candidates and potentially active homogeneous catalysts in a plethora of C-C and C-N bond forming reactions. This review article focuses on the recent developments and advances in preparation and characterization of NHC-metal complexes (metal: silver, gold, copper, palladium, nickel and ruthenium) and their biomedical applications. Their design, syntheses and characterization have been reviewed and correlated to their antimicrobial and anticancer efficacies. All these initial discoveries help validate the great potential of NHC-metal derivatives as a class of effective antimicrobial and anticancer agents.

Synthesis and Structural Characterisation of a Novel Chiral ansa-Cobaltocenium Hexafluorophosphate

trans-[(1,2-Diphenyl-1,2-dicyclopentadienyl)ethanediyl]cobalt(III) hexafluorophosphate (1) was synthesised and its structure derived by X-ray crystallography and spectroscopic methods in conjunction with theoretical calculations. To gain an insight into the effect of these bridging atoms and the exocyclic substituents on the sandwich structure, a study was undertaken in which comparisons were drawn between this ansa-cobaltocenium ion and other previously reported structurally characterised metallocenes and ansa-metallocenes. DFT calculations were also performed to provide further data for this analysis, including reaction energies for the formation of, and breaking of, the bridging C−C bond to ascertain the extent to which this moiety imposes strain on the system. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)