Margaret M. Harding

Antitumour metallocenes : Structure-activity studies and interactions with biomolecules

The metallocene dihalides are a relatively new class of small, hydrophobic organometallic anticancer agents that exhibit antitumour properties against numerous cell lines including leukemias P388 and L1210, colon 38 and Lewis lung carcinomas, B16 melanoma, solid and fluid Ehrlich ascites tumours and several human colon and lung carcinomas transplanted into athymic mice. Titanocene dichloride 1 has been the most widely studied metallocene and the drug is currently in phase II clinical trials. Formation of metallocene-DNA complexes has been implicated in the mechanism of antitumour properties of the metallocenes, as both titanocene dichloride 1 and vanadocene dichloride 2 inhibit DNA and RNA synthesis, and titanium and vanadium accumulate in nucleic acid-rich regions of tumour cells. However, in contrast to the well characterized platinum-based anticancer drugs, the active species responsible for antitumour activity in vivo has not been identified and the mechanism whereby irreparable DNA damage and/or structural modification of DNA or other cellular targets occurs is poorly understood. This review will focus on recent studies that have been carried out in order to identify the biologically active species and more fully understand the molecular level mechanism of action of the metallocene dihalides. Studies with nucleotides, oligonucleotides, DNA and proteins including topoisomerases, protein kinase C and transferrin have provided important insight into potential cellular transport mechanisms and the interaction of metallocenes with biomolecular targets. New structure activity studies including the design of hydrolytically stable metallocenes and the preparation of highly water soluble amino acid analogues have not led to improved anticancer activity of titanocene dichloride 1. The vastly different chemical and hydrolytic stability of each of the metallocenes points to a unique mechanism of action of each metallocene in vivo.

Inhibition of human topoisomerase II by the antitumor metallocenes

The ability of antitumor active metallocenes Cp2MCl2, (M=Ti, V, Mo, Nb) and the biologically inactive derivative (MeCp)2TiCl2, to inhibit the relaxation of supercoiled plasmid DNA pBR322 by human topoisomerase II has been studied by gel electrophoresis. All metallocenes inhibit the enzyme with maximum inhibition observed at 2.0 mM (Cp2TiCl2), 3.0 mM (Cp2MoCl2), 0.2 mM (Cp2NbCl2), 0.25 mM (Cp2VCl2) and 2.0 mM (MeCpTiCl2). The implications for the mechanism of antitumor activity of the metallocene dihalides are discussed.

Intracellular mapping of the distribution of metals derived from the antitumor metallocenes

The intracellular distribution of transition metals in V79 Chinese hamster lung cells treated with subtoxic doses of the organometallic anticancer complexes Cp2MCl2, where Cp is η5 -cyclopentadienyl and M is Mo, Nb, Ti, or V, has been studied by synchrotron-based X-ray fluorescence (XRF). While significantly higher concentrations of Mo and Nb were found in treated cells compared with control cells, distinct differences in the cellular distribution of each metal were observed. Analysis of thin sections of cells was consistent with some localization of Mo in the nucleus. Studies with a noncytotoxic thiol derivative of molybdocene dichloride showed an uneven distribution of Mo in the cells. For comparison, the low levels of Ti and V in cells treated with the more toxic titanocene and vanadocene complexes, respectively, resulted in metal concentrations at the detection limit of XRF. The results agree with independent chemical studies that have concluded that the biological chemistry of each of the metallocene dihalides is unique.

Control of reactivity at the porphyrin periphery by metal ion co-ordination: a general method for specific nitration at the β-pyrrolic position of 5,10,15,20-tetra-arylporphyrins

The site of nitration by nitrogen dioxide on a range of metalloporphyrins has been found to be dependent on the co-ordinated metal; copper(II), nickel(II), and palladium(II) complexes are nitrated specifically on the porphyrin β-pyrrolic position while magnesium(II), zinc(II), chloroiron(III), and cobalt(II) complexes also give products which result from reaction at the meso-position.

Inclusion complexes of the antitumour metallocenes Cp2MCl2 (M = Mo, Ti) with cucurbit[n]urils

The encapsulation of the aquated forms of molybdocene dichloride and titanocene dichloride by cucurbit[n]uril (Q[n], where n = 7 and 8) at different pD values has been studied by (1)H NMR spectroscopy and molecular modelling. (1)H NMR titration experiments indicate that both metallocenes form 1 : 1 host-guest complexes with both Q[7] and Q[8]. In these complexes, both the cyclopentadienyl ligands and metal centre are positioned deep within the cucurbituril cavity. In vitro cell proliferation studies using the cancer cell lines MCF-7 and 2008 showed that the encapsulated molybdocene complex was more active than the corresponding free metallocene, with GI(50) values of 210 and 400 muM respectively. However, unexpectedly the encapsulation of Cp(2)MoCl(2(aq))at pD 7 catalysed significant degradation of the cucurbituril framework in the presence of oxygen. Encapsulation of Cp(2)TiCl(2(aq)) by Q[7] greatly slowed the protonolysis of the cyclopentadienyl ligands in aqueous phosphate buffer (pD 7), while encapsulation in Q[8] only slightly retarded the hydrolytic degradation of the metallocene.

Synthesis and interaction with human serum albumin of the first 3,18-disubstituted derivative of bilirubin

Addition of excess (>500 equiv.) of p-fluorothiophenol to bilirubin in the presence of toluene-p-sulfonic acid catalyst afforded 3,18-didevinyl-3,18-bis[2-(p-fluorothiophenyl)ethyl] bilirubin 2 from anti-Markovnikov addition to both the exo and endo vinyl groups of bilirubin. Toluene-p-sulfonic acid is not essential as p-fluorothiophenol acts as the acid and nucleophile in the reaction. In contrast, in the presence of toluene-p-sulfonic acid, regioselective Markovnikov addition of thioacetic S-acid to the exo vinyl group occurs, in agreement with previous studies of the addition of a range of oxygen and sulfur nucleophiles to bilirubin (P. Manitto and D. Monti, Experientia, 1973, 29, 137). Binding of 2 to human serum albumin was measured by circular dichroism. The two bulky p-fluorothiophenyl groups do not appear to impede interaction with the protein. This result supports a model in which the reactive methylene bridge of bilirubin, that connects rings B and C, points into the binding pocket of human serum albumin.

Design and Synthesis of Antifreeze Glycoproteins and Mimics

Antifreeze glycoproteins are an important class of biological antifreezes that have potential applications in many areas of medicine, agriculture and industry in which ice crystal growth is damaging. While the synthesis of antifreeze glycoproteins as pure glycoforms has recently been achieved by using ligation and polymerisation strategies, the routine production of large quantities of pure glycoforms remains challenging. A range of C‐linked analogues that are readily produced by solid‐phase synthesis have delivered novel compounds that are not biological antifreezes, but are potent, non‐cytotoxic, ice‐recrystallisation inhibitors. Structure–activity studies, the identification of cyclic antifreeze glycoproteins and conformational studies have provided further insight into the requirements for antifreeze activity. These results, coupled with significant advances in approaches to the routine synthesis of different glycoproteins and mimics, present opportunities for the design and synthesis of novel ice‐growth‐inhibiting and antifreeze compounds.

Antitumour Metallocenes: Effect of DMSO on the Stability of Cp2TiX2and Implications for Anticancer Activity

The rate of hydrolysis of the aromatic rings of Cp2TiX2 [X = CI 1, O2CCCl3 8 and O2CCH2NH3Cl 13], in aqueous solutions, 10%DMSO and 100% DMSO have been studied by 1H NMR spectroscopy. Rapid hydrolysis of both the carboxylate and cyclopentadienyl ligands in Cp2TiX2[X = O2CCCl3,O2CCH2NH3Cl] occurs in DMSO to give biologically inactive species. The rate of these reactions are concentration dependent as dilution of these samples with saline or water to give the therapeutic conditions of 10%DMSO/90%H2O slows the hydrolysis chemistry. In contrast, samples of Cp2TiX2 [X = CI 1, O2CCH2NH3Cl13 ], dissolved in water give solutions containing the presumed antitumour active species in which the halide or glycine ligands have been hydrolysed but the Cp rings remain metal bound.

Applications of type I antifreeze proteins: studies with model membranes & cryoprotectant properties.

Antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs), found in the body fluids of many species of polar fish allow them to survive in waters colder than the equilibrium freezing point of their blood and other internal fluids. Despite their structural diversity, all AF(G)Ps kinetically depress the temperature at which ice grows in a non-colligative manner and hence exhibit thermal hysteresis. AF(G)Ps also share the ability to interact with and protect mammalian cells and tissues from hypothermic damage (e.g., improved storage of human blood platelets at low temperatures), and are able to stabilize or disrupt membrane composition during low temperature and freezing stress (e.g., cryoprotectant properties in stabilization of sperm and oocytes). This review will summarize studies of AFPs with phospholipids and plant lipids, proposed mechanisms for inhibition of leakage from membranes, and cryoprotectant studies with biological samples. The major focus will be on the alpha-helical type I antifreeze proteins, and synthetic mutants, that have been most widely studied. For completeness, data on glycoproteins will also be presented. While a number of models to explain stabilization and destabilization of different lipid systems have been proposed, it is currently not possible to predict whether a particular AFP will stabilize or destabilize a given lipid system. Furthermore the relationship between the antifreeze property of thermal hysteresis and membrane stabilization is unknown. This lack of detailed knowledge about how AFPs function in the presence of different types of materials has hampered progress toward the development of antifreezes for cold storage of cells, tissues, and organs.

A 31P NMR study of the interaction of the antitumor active metallocene Cp2MoCl2 with calf thymus DNA

Treatment of sonicated calf thymus DNA with the antitumor active metallocene Cp2MoCl2 afforded a metallocene‐DNA complex which was characterized by 31P NMR spectroscopy. In addition to the resonance for the phosphate backbone (δ −1.6), the spectrum contained 2 signals assigned to a phosphate bound Mo—DNA complex(es) (δ 37.2, 36.5) and a broad signal at δ 6.2 ppm. This result suggests that covalent attachment of the metallocene Cp2MoCl2 occurs via phosphate(O) coordination and is accompanied by local distortion of the DNA backbone. This result supports recent ICP studies with Cp2TiCl2 that have DNA detected DNA—metallocene adducts.