The importance of science communication in cancer research: an interview with Martin Christlieb.

I trained as a chemist, studying for a PhD at the University of Cambridge. After I left Cambridge, I undertook postdoctoral appointments at Stanford and Oxford. These appointments led me into metal chemistry, initially using the metals as reagents, and finally using metal atoms as the key atom in a series of compounds designed to be radiolabeled markers of tumor hypoxia for positron emission tomography (PET) imaging. Finally I made the move to oncology to give my compounds a chance to be tested in a more biological setting. I am now the public engagement manager in the Department of Oncology at the University of Oxford, where I look for opportunities to discuss our science with school students, adult audiences and people who have been touched by cancer. During my spare time, I am the Adventure Training Officer for Thames Valley RAF Air Cadets, where I work to provide opportunities to allow young people to experience hill walking, climbing, paddlesport, and trail cycling. I am a keen hill walker myself and recently, made it to 6000 m in the Himalayas.

A dual radiolabelling approach for tracking metal complexes: investigating the speciation of copper bis(thiosemicarbazonates) in vitro and in vivo.

Copper(II)bis(thiosemicarbazonato) complexes such as [(64)Cu]Cu-ATSM continue to be investigated for positron emission tomography (PET) imaging of tumour hypoxia. However, the currently proposed mechanisms for the mode of action of these complexes are unable to account fully for their observed biological behaviour. In order to examine the roles of the copper metal and the ligand, we designed a pair of (123)I/(64)Cu-copper bis(thiosemicarbazonates), radiolabelled at either the metal or at the ligand. In vitro cellular retention studies of the orthogonal pair demonstrate for the first time that retention under hypoxia involves dissociation of the copper bis(thiosemicarbazone) complex, consistent with the previously suggested mechanism of reductive trapping of copper. In contrast, in vivo biodistribution and dynamic PET/SPECT imaging of the orthogonally labelled complexes underline our previous findings for [(64)Cu]Cu-ATSM and [(64)Cu]Cu-acetate, providing further support for the important contribution of copper metabolism in the in vivo hypoxia selectivity of Cu-ATSM. This dual radiolabelling approach may find applications for determining the speciation of other metal complexes in vitro and in vivo.

Synthesis of sulfonamide conjugates of Cu(II), Ga(III), In(III), Re(V) and Zn(II) complexes: carbonic anhydrase inhibition studies and cellular imaging investigations.

Carbonic anhydrase IX (CA IX) is currently generating great interest as a marker of tumour hypoxia and a potential chemotherapeutic target. In order to test the principle that a CA IX inhibitor could be used for targeting PET or SPECT metallic radioisotopes to tumours we have prepared a number of conjugates involving aryl-sulfonamides or an acetazolamide derivative linked to a range of copper, indium, rhenium, 99m-technetium and zinc complexes. Radiolabelled (64)Cu and (99m)Tc analogues of the 'cold' Cu and some of the Re complexes were prepared in good radiochemical incorporation. Inhibition of various human carbonic anhydrase isoforms (I, II, IX and XII) was tested with the 'cold', non-radiolabelled complexes, and compared with an acetazolamide standard (AZA). The molecular structure of a new, tri-sulfonated porphyrin-labeled sulfonamide was determined using synchrotron X-ray crystallography.

A comparison of the behavior of (64)Cu-acetate and (64)Cu-ATSM in vitro and in vivo.

UNLABELLED: (64)Cu-diacetyl-bis(N(4)-methylthiosemicarbazonate), (64)Cu-ATSM, continues to be investigated clinically as a PET agent both for delineation of tumor hypoxia and as an effective indicator of patient prognosis, but there are still aspects of the mechanism of action that are not fully understood. METHODS: The retention of radioactivity in tumors after administration of (64)Cu-ATSM in vivo is substantially higher for tumors with a significant hypoxic fraction. This hypoxia-dependent retention is believed to involve the reduction of Cu-ATSM, followed by the loss of copper to cellular copper processing. To shed light on a possible role of copper metabolism in hypoxia targeting, we have compared (64)Cu retention in vitro and in vivo in CaNT and EMT6 cells or cancers after the administration of (64)Cu-ATSM or (64)Cu-acetate. RESULTS: In vivo in mice bearing CaNT or EMT6 tumors, biodistributions and dynamic PET data are broadly similar for (64)Cu-ATSM and (64)Cu-acetate. Copper retention in tumors at 15 min is higher after injection of (64)Cu-acetate than (64)Cu-ATSM, but similar values result at 2 and 16 h for both. Colocalization with hypoxia as measured by EF5 immunohistochemistry is evident for both at 16 h after administration but not at 15 min or 2 h. Interestingly, at 2 h tumor retention for (64)Cu-acetate and (64)Cu-ATSM, although not colocalizing with hypoxia, is reduced by similar amounts by increased tumor oxygenation due to inhalation of increased O2. In vitro, substantially less uptake is observed for (64)Cu-acetate, although this uptake had some hypoxia selectivity. Although (64)Cu-ATSM is stable in mouse serum alone, there is rapid disappearance of intact complex from the blood in vivo and comparable amounts of serum bound activity for both (64)Cu-ATSM and (64)Cu-acetate. CONCLUSION: That in vivo, in the EMT6 and CaNT tumors studied, the distribution of radiocopper from (64)Cu-ATSM in tumors essentially mirrors that of (64)Cu-acetate suggests that copper metabolism may also play a role in the mechanism of selectivity of Cu-ATSM.

CH-01 is a hypoxia-activated prodrug that sensitizes cells to hypoxia/reoxygenation through inhibition of Chk1 and Aurora A.

The increased resistance of hypoxic cells to all forms of cancer therapy presents a major barrier to the successful treatment of most solid tumors. Inhibition of the essential kinase Checkpoint kinase 1 (Chk1) has been described as a promising cancer therapy for tumors with high levels of hypoxia-induced replication stress. However, as inhibition of Chk1 affects normal replication and induces DNA damage, these agents also have the potential to induce genomic instability and contribute to tumorigenesis. To overcome this problem, we have developed a bioreductive prodrug, which functions as a Chk1/Aurora A inhibitor specifically in hypoxic conditions. To achieve this activity, a key functionality on the Chk1 inhibitor (CH-01) is masked by a bioreductive group, rendering the compound inactive as a Chk1/Aurora A inhibitor. Reduction of the bioreductive group nitro moiety, under hypoxic conditions, reveals an electron-donating substituent that leads to fragmentation of the molecule, affording the active inhibitor. Most importantly, we show a significant loss of viability in cancer cell lines exposed to hypoxia in the presence of CH-01. This novel approach targets the most aggressive and therapy-resistant tumor fraction while protecting normal tissue from therapy-induced genomic instability.

Orthogonal 18F and 64Cu labelling of functionalised bis(thiosemicarbazonato) complexes.

The synthesis of three pairs of orthogonally labelled fluorinated Cu bis(thiosemicarbazonato) complexes is presented. These are the first examples of (18)F-labelled Cu(II)-complexes designed to serve as new hypoxia selective PET tracers and as mechanistic probes to study the mode of action of this class of markers. In vitro evaluation revealed that the fluorinated Cu-complex derived from amide coupling is suitable for in vivo work.

Bis(thiosemicarbazones) as bifunctional chelators for the room temperature 64-copper labeling of peptides.

A range of new carboxylate functionalised bis(thiosemicarbazone) ligands and their Cu(II) complexes have been prepared, fully characterised and radiolabeled in high yield with both (64)Cu and (99m)Tc. Conjugation to a bombesin derivative was achieved using standard solid phase synthetic methodologies and the (64)Cu-labeled conjugate was shown to have good tumour uptake in mice with xenografted PC-3 tumours.

Fluorescent copper(II) bis(thiosemicarbazonates): synthesis, structures, electron paramagnetic resonance, radiolabeling, in vitro cytotoxicity and confocal fluorescence microscopy studies.

Copper bis(4-ethyl-3-thiosemicarbazonato) acenaphthenequinone (1) and copper bis(4-methyl-3-thiosemicarbazonato) acenaphthenequinone (2) are synthesized and characterized in solution, in the solid state, and radiolabeled. Serum-protein binding radioassays show good stability in solution and about 25 % binding to protein over 1 h, which is comparable with the hypoxia selective tracer [(64)Cu(ATSM)]. Cyclic voltammetry shows fast and reversible reduction at redox potentials similar to the values known for hypoxia-selective copper compounds. However, despite this, complex 1 does not show any hypoxic-selective uptake in HeLa cells over 1-h standard assays. Possible reasons for this are studied by using the intrinsic fluorescence of the Cu(II) complexes to determine the cellular distributions and uptake mechanism by confocal microscopy. The complexes are found to bind to the external cell membrane and disperse evenly in the cytoplasm only after a very slow cell internalization (>1 h). No significant changes in distribution are observed by fluorescence imaging under hypoxic conditions. The rate of localization in the cytoplasm contrasts with their Zn(II) analogues, which are known to have fast cell uptake (up to 20 min) and a clear localization in lysosomes and mitochondria. The cytotoxicity mechanism of 1 over 24 h against a number of adherent cell lines is seen to be by membrane disruption and is of a comparable magnitude to that of [Cu(ATSM)], as demonstrated by methyl tetrazolium (MTT) and lactate dehydrogenase (LDH) assays.

In vitro and in vivo evaluations of a hydrophilic 64Cu-bis(thiosemicarbazonato)-glucose conjugate for hypoxia imaging.

UNLABELLED: A water-soluble glucose conjugate of the hypoxia tracer 64Cu-diacetyl-bis(N4-methylthiosemicarbazone) (64Cu-ATSM) was synthesized and radiolabeled (64Cu-ATSE/A-G). Here we report our initial biological experiments with 64Cu-ATSE/A-G and compare the results with those obtained for 64Cu-ATSM and 18F-FDG. METHODS: The uptake of 64Cu-ATSE/A-G and 64Cu-ATSM into HeLa cells in vitro was investigated at a range of dissolved oxygen concentrations representing normoxia, hypoxia, and anoxia. Small-animal PET with 64Cu-ATSE/A-G was performed in male BDIX rats implanted with P22 syngeneic carcinosarcomas. Images of 64Cu-ATSM and 18F-FDG were obtained in the same model for comparison. RESULTS: 64CuATSE/A-G showed oxygen concentration-dependent uptake in vitro and, under anoxic conditions, showed slightly lower levels of cellular uptake than 64Cu-ATSM; uptake levels under hypoxic conditions were also lower. Whereas the normoxic uptake of 64Cu-ATSM increased linearly over time, 64Cu-ATSE/A-G uptake remained at low levels over the entire time course. In the PET study, 64CuATSE/A-G showed good tumor uptake and a biodistribution pattern substantially different from that of each of the controls. In marked contrast to the findings for 64Cu-ATSM, renal clearance and accumulation in the bladder were observed. 64Cu-ATSE/A-G did not display the characteristic brain and heart uptake of 18F-FDG. CONCLUSION: The in vitro cell uptake studies demonstrated that 64Cu-ATSE/A-G retained hypoxia selectivity and had improved characteristics when compared with 64Cu-ATSM. The in vivo PET results indicated a difference in the excretion pathways, with a shift from primarily hepatointestinal for 64Cu-ATSM to partially renal with 64Cu-ATSE/A-G. This finding is consistent with the hydrophilic nature of the glucose conjugate. A comparison with 18F-FDG PET results revealed that 64Cu-ATSE/A-G was not a surrogate for glucose metabolism. We have demonstrated that our method for the modification of Cu-bis(thiosemicarbazonato) complexes allows their biodistribution to be modified without negating their hypoxia selectivity or tumor uptake properties.

Designing Zn(II) and Cu(II) derivatives as probes for in vitro fluorescence imaging.

New M(II) bis(thiosemicarbazonato) complexes (M = Ni(II), Cu(II) and Zn(II)) featuring allyl groups at the exocyclic nitrogens have been synthesised. The complexes were characterised in solution by spectroscopic methods and their solid state structures determined by single crystal X-ray diffraction using synchrotron radiation. The Zn(II) complex was found to be intrinsically fluorescent and soluble in biocompatible media. The uptake of this Zn(II) complex in HeLa, MCF-7 and IGROV cancer cells was monitored by fluorescence microscopies (epi- and confocal fluorescence imaging). The radiolabelling to (64)Cu(II) bis(thiosemicarbazonato) complex was performed cleanly by transmetallation from the corresponding Zn(II) species using (64)Cu(OAc)(2).

The exocyclic functionalisation of bis(thiosemicarbazonate) complexes of zinc and copper: the synthesis of monomeric and dimeric species.

This paper reports the synthesis of bimetallic zinc thiosemicarbazone complexes with rigid aromatic linkers, using either 1,3- or 1,4- benzenediamines or 1,3- or 1,4- benzenedialdehydes as the basis of the linking groups. Non-rigid aliphatic diamines and dialdehydes were also used to link the zinc chelating units. Reaction of a bis(thiosemicarbazone) with a pendant NHNH(2) group with monoaldehydes or ketones gives a range of monomeric complexes with exocylic imine groups bearing a range of substituents. The zinc complexes can be quantitatively and rapidly transmetallated to the corresponding copper complexes and this route or direct reaction with the free ligand can be used to radiolabel the monomeric species with (64)Cu. In vivo and in vitro studies of one of the (64)Cu imine complexes shows substantial hypoxic selectivity and high tumour uptake in a murine model.

Oxidative metabolism of combretastatin A-1 produces quinone intermediates with the potential to bind to nucleophiles and to enhance oxidative stress via free radicals.

Combretastatins are stilbene-based, tubulin depolymerization agents with selective activity against the tumor vasculature; two variants (A-1 and A-4) are currently undergoing clinical trials. Combretastatin A-1 (CA1) has a greater antitumor effect than combretastatin A-4 (CA4). We hypothesized that this reflects the enhanced reactivity conferred by the second (ortho) phenolic moiety in CA1. Oxidation of CA1 by peroxidase, tyrosinase, or Fe(III) generates a species with mass characteristics of the corresponding ortho-quinone Q1. After administration of CA1-bis(phosphate) to mice, the hydroquinone-thioether conjugate Q1H2-SG, formed from the nucleophilic addition of GSH to Q1, was detected in liver. In competition, electrocyclic ring closure of Q1, over a few minutes at pH 7.4, leads to a second ortho-quinone product Q2, characterized by exact mass and NMR. This product was also generated by human promyelocytic leukemia (HL-60) cells in vitro, provided that superoxide dismutase was added. Q2 is highly reactive toward glutathione (GSH) and ascorbate, stimulating oxygen consumption in a catalytic manner. Free radical intermediates formed during autoxidation of CA1 were characterized by EPR, and the effects of GSH and ascorbate on the signals were studied. Pulse radiolysis was used to initiate selective one-electron oxidation or reduction and provided further evidence, from the differing absorption spectra of the radicals formed on oxidation of CA1 or reduction of Q2, that two different quinones were formed on oxidation of CA1. The results demonstrate fundamental differences between the pharmacological properties of CA1 and CA4 that provide two possible explanations for their differential activities in vivo: oxidative activation to a quinone intermediate likely to bind to protein thiols and possibly to nucleic acids and stimulation of oxidative stress by enhancing superoxide/hydrogen peroxide production. The observation of the GSH conjugate Q1H2-SG in vivo provides a new marker for oxidative metabolism of relevance to current clinical trials of CA1-bis(phosphate) (OXi4503).

Functionalized bis(thiosemicarbazonato) complexes of zinc and copper: synthetic platforms toward site-specific radiopharmaceuticals.

Two new types of unsymmetrical bis(thiosemicarbazone) proligands and their neutral zinc(II) and copper(II) complexes have been synthesized. These bifunctional ligands both chelate the metal ions and provide pendent amino groups that can be readily functionalized with biologically active molecules. Functionalization has been demonstrated by the synthesis of three water-soluble glucose conjugates of the new zinc(II) bis(thiosemicarbazonato) complexes, and their copper(II) analogues have been prepared in aqueous solution via transmetalation. A range of techniques including NMR, electron paramagnetic resonance, cyclic voltammetry, high-performance liquid chromatography (HPLC), UV/vis, and fluorescence emission spectroscopy have been used to characterize the complexes. Four compounds, including two zinc(II) complexes, have been characterized by X-ray crystallography. The connectivity and conformation of the glucose conjugates have been assigned by NMR spectroscopy. Time-dependent density functional theory calculations have been used to assign the electronic transitions of the copper(II) bis(thiosemicarbazonato) chromophore. Two copper-64-radiolabeled complexes, including one glucose conjugate, have been prepared and characterized using radio-HPLC, and transmetalation is shown to be a viable method for radiolabeling compounds with copper radionuclides. Preliminary cell washout studies have been performed under normoxic conditions, and the uptake and intracellular distribution have been studied using confocal fluorescence microscopy.

New bimetallic compounds based on the bis(thiosemicarbazonato) motif.

Zinc and copper bis(thiosemicarbazonato) complexes containing more than one metal centre have been prepared with a view to examining their application for molecular imaging. The zinc complexes are fluorescent with excitation and emission at relatively long wavelengths. The dinuclear copper complex undergoes two sequential, quasi-reversible reductions.

Ligands for molecular imaging: the synthesis of bis(thiosemicarbazone) ligands.

Bis(thiosemicarbazones) have been of interest to chemists for over fifty years; they display antitumour, antibiotic and antiviral properties. Recently it has become apparent that they may also provide a convenient way of labelling biologically active molecules by using metallic radionuclides and/or fluorescence. Although apparently simple, the synthesis of bis(thiosemicarbazone) ligands can be problematic. This article provides a summary of the published literature, based on the synthetic strategies used and indicates some of the difficulties that may arise.

Enantioselective total synthesis of (-)-xialenon A.

The first total synthesis of (-)-xialenon A (1) via conjugate allylation of a 1,5-cyclooctadiene-derived bicyclo[3.3.0]octenone 3 and an alpha'-hydroxylation on the more hinderd face of enone 9 using hypervalent iodine chemistry, is described.