Titanocene Dichloride

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7.5.1 Chemistry and anti-cancer activity

In 1979, Kopf and Kopf-Maier reported42 on the first metallocene with antitumor activity, the complex [Cp2TiIVCl2] (bis(^-cyclopentadienyl)titanium(IV) dichloride, titanocene dichloride). The red crystalline titanium(IV) complex was first prepared by Wilkinson and Birmingham in 1954.43 The two planar C5H5— cyclopentadienyl rings each have a delocalized negative charge and are coordinated to the metal center via pentahapto metal carbon coordination in a 'bent sandwich' configuration. X-ray data show that the coordination about the titanium atom formed by two chlorine atoms and the two centroids of the cyclopentadienyl rings is that of a distorted tetrahedron. The average Ti—Cl bond distance is 2.364(3) A, and the average Ti-Cp-(centroid) distance is 2.06 A.44

Again, a number of structure-activity relation studies of the complexes Cp2MIVX2 have been carried out15 using, for example, the Ehrlich ascites tumor in mice. The main results of these tests may be summarized as follows.22 Titanocene derivatives with alkyl-substituted Cp rings, bridged Cp rings or only one Cp ring showed significantly reduced anti-tumor activity compared to the parent compound Cp2TiIVCl2. Modification of the cyclopentadienyl ligand, for instance by replacement of H by R (CH3, C2H5, Si(CH3)3 and N(CH3)2) groups ranging from mono-substitution to deca-substitution showed a dramatic reduction in the anti-tumor activity, the activity decreasing as the substitution increases.15,23 The main influence of alkylation of the Cp rings on the anti-tumor activity is believed to be mainly due to electronic effects rather than steric effects. Alkyl substitution, in addition, reduces the aqueous solubility and therefore may influence transport processes.

Again, the nature of the ion X does not appear to affect significantly the antitumor activity of the parent compounds. Reduced activity of complexes with X, for example p-nitrophenoxy or 2,4,6-trichlorophenoxy, was attributed to the lack of lability of the Ti—X bond, which is unable to dissociate in solution and generate the active species which was assumed to coordinate to DNA.22 However, a recent study involving mice inoculated with Ehrlich ascites tumor has shown that the compound Cp2TiIV(NCS)2 also exhibits a reduction in anti-tumor efficacy when compared to previous results with the original titanocene dichloride.45 In contrast to budotitane, variation of the central metal M is tolerated in the series of Cp2MX2 complexes. Optimum cure rates against Ehrlich ascites tumor in mice were obtained for Ti(IV), V(IV), Nb(IV) and Mo(IV). Sporadic cure rates were obtained for Ta(IV) and W(IV), whereas Zr(IV) and Hf(IV) analogues showed no activity.22

7.5.2 Reaction with biomolecules

By inductively coupled plasma (ICP) spectroscopy, it was shown that Cp2TiIVCl2 forms adducts of the type CpTiIV-DNA at pH = 5.3 and CpTiIV-DNA at pH = 7. The titanium-DNA adducts once formed are stable for up to 2 days.46 While the exact mechanism of the anti-cancer action of titanocene dichloride seems still unclear, there is some evidence that a Ti(IV) species is formed and stabilized in vivo that is transported into cells via blood transport peptides as, for example, glutathione. NMR studies of the metallocenes Cp2MIVCl2 (M = titanium, niobium or molybdenum), however, show that with the niobium or molybdenum complex adducts of glutathione are formed, whereas no binding of the titanium complex to glutathione was detected at pH = 2.4 or pH = 6.0.47 Examples of further studies of the interaction of Cp2TiIVCl2 and derivatives with biomolecules are summarized in Table 7.1.

Table 7.1 Selected examples of the interaction of titanocene dichloride and derivatives with biomolecules

Compound

Biomolecule(s)

Interaction

Ref.

Cp2TiIVCl2

amino acids

carboxyl binding

48, 49

Cp2TiIVCl2

human topoisomerase II

inhibition

50

Cp2TiIVCl2

,0-cyclodextrin

inclusion complex

51

Cp2TiIVCl2

glutathione

no interaction

47

Cp2TiIVCl2

nucleotides

phosphate binding

52

Cp2TiIVCl2

6-thiopurines

S(6)/N(7) chelation

53

Cp2TiIVCl2

2-thiocytosine

NH2/N(3) chelation

53

TiIVcitrate

human apo-transferrin

binding at Fe3+ site

29, 54

Cp2TiIVCl2

ATP

phosphate/N(7) binding

27

Cp2TiIVCl2

5'-dAMP

phosphate/N(7) binding

55

Cp2TiIVCl2

DNA

DNA adducts

46

7.5.3 Animal studies

The tumor inhibition influence upon human lung carcinomas xenografted into athymic mice by titanocene dichloride has been studied by Kopf-Maier.56 In the case of lung adenocarcinoma, titanocene dichloride inhibited tumor growth by more than 50% resulting in treated/control values of 20-50%.

It has been reported that titanocene dichloride suppresses angiogenesis, the formation of blood vessels from preexisting ones, and inhibits biosynthesis of collagenous proteins in the in vivo system of the chorioallantoic membrane of the chick embryo.57 At non-toxic dose regimens, titanocene dichloride retards the growth of Walker 256 carcinosarcoma transplants in rats and reduces the number of seeded implants. These data suggest that the anti-tumor activity of titanocene dichloride may be attributed, at least in part, to its ability to suppress angiogenesis.

In xenografted human renal cell carcinoma in athymic mice, titanocene dichloride showed better activity than the established drugs cyclophosphamide and vinblastine.58

Recently, the effect of titanocene dichloride on the activity of natural killer cells in Ehrlich ascites tumor-bearing immunodeficient mice was studied.59 After inoculation of the tumor, the natural killer cell function declined to subnormal levels. The treatment consisted of intraperitoneal administration of 15mg/kg/day of titanocene dichloride for 2 days. The treatment significantly enhanced natural killer cell function, and this function was restored to normal values by titanocene dichloride in all stages studied, but not by the established anti-tumor drug cyclophosphamide.

7.5.4 Clinical investigations

As a consequence of the hydrolytic instability of titanocene dichloride at pH >5, a formulation preventing hydrolysis and precipitation reactions had to be devel-oped.60 Formulations used for clinical studies include titanocene dichloride in a lyophilized powder dissolved in malate buffer at pH = 3.2 and titanocene dichloride in a lyophilized powder dissolved in malic acid at pH = 3.5.22

A first clinical phase I protocol, including 40 patients with refractory solid malignancies, evaluated a single-dose administration of titanocene dichloride, repeated every three weeks.61 The dose-limiting toxicity was renal toxicity, and the recommended dose for phase II studies was 240 mg/m2. Two minor responses (bladder carcinoma and non-small-cell lung cancer) were observed.

A second phase I study evaluated a weekly administration schedule involving 20 patients.62 The dose-limiting toxicity again was renal toxicity. The maximum tolerated dose was 140mg/m2/week. The dose recommended for phase II studies was also 140mg/m2. One patient with adenocarcinoma of unknown primary had a minor response.

In a further phase I clinical trial, a total of 10 patients with progressive advanced cancer were treated with 80 mg/m2 titanocene dichloride, administered as lyophilized powder, at days 1, 3 and 5 (repeated at day 22).63 No objective tumor remission was observed. Drug-related side effects observed were renal and hepatic, emesis and metallic taste. The split dose regimen had no advantage with respect to the toxicity profile.

In a phase II trial, 14 patients with metastatic renal-cell carcinoma received 270mg/m2 titanocene dichloride every three weeks for six weeks. Although the toxicities and side effects encountered were mild to moderate, no partial or complete response was detectable.64

A multicenter phase II clinical trial of titanocene dichloride in 12 patients with metastatic breast cancer has recently been reported by Kroger etal.65 All 12 patients had prior surgery and metastatic disease at study entry. Titanocene dichloride was intravenously administered at a dose of 270 mg/m2 every three weeks. The conclusion is that titanocene dichloride was not effective in patients with metastatic breast cancer.

7.5.5 Perspectives of titanocene dichloride

One possibility to overcome the limited solubility and low stability in water of neutral titanocene dihalides could be the application of ionic titanocene complexes. Therefore, complexes of the type [Cp2TiIVXL]+Y~ or [Cp2TiIVL2]2+(Y^)2, where X~ and Y~ are anionic ligands, for example Cl~, CF3SO3~, I", [FeCl4]~ or [AsF6]~, and L is a neutral ligand, for example phenantroline, CH3CN, 6-thioguanine, glycine or L-alanine, have been tested. All complexes showed good to moderate anti-tumor activity against fluid Ehrlich ascite tumor but not as active as the parent neutral compounds.23 Similar results recently have been observed with tin compounds.66 In order to overcome insolubility and hydrolysis problems, Mokdsi and Harding have prepared the titanocene derivatives (MeCp)2TiCl2, bis(methylcyclopentadienyl)titano-cene dichloride, and the bis-glycine analogue (MeCp)2Ti(O2CCH2NH3Cl)2 which both exhibit reasonable solubility in water and are relatively stable at pH = 7.0.55

Complex formation between ^-cyclodextrin and Cp2TiIVCl2 has been proven in solution.67 The encapsulation of Cp2TiIVCl2 in ^-cyclodextrin yields inclusion complexes interesting for pharmaceutical use, owing to the increased aqueous solubility of the drug, better oral absorption, and enhanced chemical and physical stability with respect to oxidation by air, sensitivity to light, rate of disproportionation or polymerization and acidic conditions.51

An interesting new approach involving heterobimetallic complexes containing platinum linked to titanium by an organic ligand with hard and soft donors has recently been published.68 For example, a relatively high in vitro cytotoxicity has been found for Cp2Ti[m-OSO2(CH2)2Ph2P]2PtCl2-4H2O, suggesting that treatment of cancer cells with a drug containing two disparate metal centers could be more effective than one containing either metal singly.

At present, transferrin is believed to be a potential carrier of Ti(IV) into the target place. The use of these species as chemotherapeutic agents remains relatively unexplored and awaits future investigation.23

Another new development is the combination of two pharmacologically active components in one molecular compound. For example, titanocene complexes with thionucleobases of the type [Cp2TiIV(L)]Cl2 (for example L = 6-thioguanine, an established anti-cancer drug) have been synthesized, which could render enhanced activity, since they contain two active agents within the same compound.53 In fact, synergism between titanocene dichloride and the anti-cancer drug 5-fluorouracil has already been reported.69 A series of platinum complexes of the anti-cancer drug 6-mercaptopurine, reflecting an analogous strategy to improve platinum drugs, has recently been characterized.70 This strategy is well known in gold drug development.71

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  • Negisti
    Why is titanocene dichloride used as an air sensitivity indicator?
    2 months ago

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