Timothy W. Failes

Cellular uptake and distribution of cobalt complexes of fluorescent ligands

The development of complexes that allow the monitoring of the release and distribution of fluorescent models of anticancer drugs initially bound to cobalt(III) moieties is reported. Strong quenching of fluorescence upon ligation to cobalt(III) was observed for both the carboxylate- and the hydroximate-bound fluorophores as was the partial return of fluorescence following addition of ascorbate and cysteine. The extent of the increase in the fluorescence intensity observed following addition of these potential reductants is indicative of the fluorophore being displaced from the complex by the action of ascorbate or cysteine, by ligand exchange. The cellular distribution of the fluorescence revealed that coordination to cobalt can dramatically alter the subcellular distribution of a bound fluorophore. This work shows that fluorescence can be an effective means of monitoring these agents in cells, and of determining their sites of activation. The results also reveal that the cytotoxicity of such agents correlates with their uptake and distribution patterns and that these are influenced by the types of ligands attached to the complex.

Bioreductive activation and drug chaperoning in cobalt pharmaceuticals

The potential for cobalt(III) complexes in medicine, as chaperones of bioactive ligands, and to target tumours through bioreductive activation, has been examined over the past 20 years. Despite this, chemical properties such as reduction potential and carrier ligands required for optimal tumour targeting and drug delivery have not been optimised. Here we review the chemistry of cobalt(III) drug design, and recent developments in the understanding of the cellular fate of these drugs.

Using XANES to Monitor the Oxidation State of Cobalt Complexes

X-Ray absorption near-edge structure (XANES) spectroscopy was used to monitor the oxidation state of cobalt following treatment of CoIII complexes with reducing agents such as ascorbate and cysteine. It was established that the XANES spectra of mixtures of CoII and CoIII complexes can be used to calculate proportions of the two oxidation states by monitoring the height of the Co K-edge. The relationships developed were used to estimate proportions of each complex in solutions of CoIII complexes treated with reducing agents.

Crystal Structures of Tris(hydroxamato) Complexes of Iron(III)

The crystal structures of [Fe(aha)3]·1.5H2O and [Fe(bha)3]·3H2O (ahaH = acetohydroxamic acid, bhaH = benzohydroxamic acid) have been determined. [Fe(aha)3]·1.5H2O crystallizes in the monoclinic space group P21/ c (a 19.169(4), b 8.5738(3), c 12.257(3) A, α = γ = 90°, β 92.255(4)°). The two isomeric forms (fac and mer) cocrystallize. The structure of [Fe(bha)3]·3H2O is as reported previously (monoclinic, P21/n, a 12.9998(16), b 13.2196(16), c 14.1154(17) A, α = γ = 90°, β 90.809(2)°) but with improved refinement giving an R value of 0.03 compared to 0.12. In each complex the geometry of the Fe centre is octahedral but with significant distortion.

Models of hypoxia activated prodrugs : Co(III) complexes of hydroxamic acids

Co(III) complexes of simple hydroxamic acids have been evaluated as models of hypoxia activated prodrugs containing MMP inhibitors. The complexes are based upon a proposed carrier system comprising the tripodal tetradentate ligand tris(2-methylpyridyl)amine (tpa) with the hydroxamate functionality occupying the remaining coordination sites of the Co centre. Acetohydroxamato (aha), propionhydroxamato (pha), and benzohydroxamato (bha) complexes were synthesised and characterised by single crystal X-ray diffraction. For aha and pha both the hydroxamato and hydroximato (deprotonated) forms were obtained and were readily interconverted by pH manipulation; for bha only the hydroximato complex was obtained as a stable species. Electrochemical analysis was used to probe the redox chemistry of the complexes and assess their ease of reduction. All of the complexes displayed irreversible reduction and had low cathodic peak potentials. This suggests that the Co-tpa carrier system would provide a suitably inert framework to deliver the drugs to target sites intact yet would release the ligands upon reduction to the more labile Co(II) oxidation state.

Crystal structures of organomercury(II) derivatives of cyclohexanone and benzaldehyde thiosemicarbazones

The crystal and molecular structures of the organomercury(II) complexes [Hg(C6H5)(chtsc)], 1, and [Hg(C6H5C5H4N)(btsc)], 2, obtained from the reaction of phenylmercury(II) acetate with cyclohexanone thiosemicarbazone (Hchtsc) and that of [2-(pyridin-2′-yl)]phenyl]mercury(II) acetate with benzaldehyde thiosemicarbazone (Hbtsc), respectively, are described. Both 1 and 2 are monoclinic, space group C2/c. Complex 1 has a distorted T-shaped geometry {C-Hg-S, 161.91(10)°} and 2 can be considered to have a distorted seesaw geometry {C-Hg-S, 171.2(10)°}. In both complexes the ligands act as bidentate chelating anions bonding through azomethine N1 and thiolato S atoms.

Evolution of resistance to Aurora kinase B inhibitors in leukaemia cells.

Aurora kinase inhibitors are new mitosis-targeting drugs currently in clinical trials for the treatment of haematological and solid malignancies. However, knowledge of the molecular factors that influence sensitivity and resistance remains limited. Herein, we developed and characterised an in vitro leukaemia model of resistance to the Aurora B inhibitor ZM447439. Human T-cell acute lymphoblastic leukaemia cells, CCRF-CEM, were selected for resistance in 4 µM ZM447439. CEM/AKB4 cells showed no cross-resistance to tubulin-targeted and DNA-damaging agents, but were hypersensitive to an Aurora kinase A inhibitor. Sequencing revealed a mutation in the Aurora B kinase domain corresponding to a G160E amino acid substitution. Molecular modelling of drug binding in Aurora B containing this mutation suggested that resistance is mediated by the glutamate substitution preventing formation of an active drug-binding motif. Progression of resistance in the more highly selected CEM/AKB8 and CEM/AKB16 cells, derived sequentially from CEM/AKB4 in 8 and 16 µM ZM447439 respectively, was mediated by additional defects. These defects were independent of Aurora B and multi-drug resistance pathways and are associated with reduced apoptosis mostly likely due to reduced inhibition of the catalytic activity of aurora kinase B in the presence of drug. Our findings are important in the context of the use of these new targeted agents in treatment regimes against leukaemia and suggest resistance to therapy may arise through multiple independent mechanisms.

Stabilization of Triam(m)inechloridoplatinum Complexes by Oxidation to PtIV

PtIV analogues of the active end groups {PtClN3} of multinuclear Pt anticancer drugs have been investigated. The crystal structure of trans,mer-[PtCl(OH)2(dien)]Cl shows that the bond lengths are similar to those in the dihydroxidoplatinum(iv) analogue of cisplatin. The axial ligands are shown to be the predominant influence on reduction potentials with the dihydroxido complex trans,mer-[PtCl(OH)2(NH3)3]Cl being the most resistant to reduction. X-ray absorption near-edge spectroscopy is shown to be suitable for monitoring the oxidation state of these complexes and reveals that trans,mer-[PtCl(OH)2(NH3)3]+ survives for more than 2 h in cancer cells.

Synthesis and anticancer evaluation of 3-substituted quinolin-4-ones and 2,3-dihydroquinolin-4-ones

A series of 3-aryl-5,7-dimethoxyquinolin-4-ones 8 and 3-aryl-5,7-dimethoxy-2,3-dihydroquinolin-4-ones 13 were synthesized in good yields. Demethylation under a range of conditions afforded the corresponding 5-hydroxy and 5,7-dihydroxy derivatives. Biological evaluation against a range of cancer cells lines showed that the quinolin-4-one scaffold was more cytotoxic than the reduced 2,3-dihydroquinolin-4-one scaffold. The most active monohydroxy compound 15f demonstrated 85.9-99% reduction in cell viability against the cell lines tested.

The first examples of platinum amine hydroxamate complexes: structures and biological activity

Two novel benzohydroxamate complexes of anticancer active Pt(II)–diamine moieties have been synthesised in which Pt–C bonds are present in the dinuclear structures. The complexes, [{Pt(en)}2(µ-bha)]ClO4·H2O (en = ethane-1,2-diamine) and [{Pt(R,R-chxn)}2(µ-bha)]NO3·2H2O (chxn = cyclohexane-1,2-diamine), have two platinum centres that are bridged through the bha ligand via (O,O) and (C,N) coordination modes, the latter mode occurring through deprotonation of the ortho carbon of the phenyl ring. The cytotoxicities of the complexes were tested against a panel of cell lines based on the A2780 ovarian cancer cell line and revealed that both dinuclear complexes were less active than their corresponding dichloro parent complexes. It is likely that they act in a similar manner to the parent complexes, but with the cytotoxicity mediated by factors influencing cellular uptake, such as the charge and lipophilicity of the compounds.