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.

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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 x 10 mol/l over the full panel of 36 cancer cell lines reaches the activity of cisplatin with 14.7 x 10 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.

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.