Anionic carbonato and oxalato cobalt(iii) nitrogen mustard complexesElectronic supplementary information (ESI) available: Table of hydrogen bonds for 6b and 8. See http://www.rsc.org/suppdata/dt/b3/b311091e/

Synthesis, characterization, and cancer cell-selective cytotoxicity of mixed-ligand cobalt(III) complexes of 8-hydroxyquinolines and phenanthroline bases

Transition metal complexes exhibiting selective toxicity towards a broad range of cancer types are highly desirable as potential anticancer agents. Herein, we report the synthesis, characterization, and cytotoxicity studies of six new mixed-ligand cobalt(III) complexes of general formula [Co(B)2(L)](ClO4)2 (1-6), where B is a N,N-donor phenanthroline base, namely, 1,10-phenanthroline (phen in 1, 2), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq in 3, 4), and dipyrido[3,2-a:2',3'-c]phenazine (dppz in 5, 6), and L is the monoanion of 8-hydroxyquinoline (HQ in 1, 3, 5) and 5-chloro-7-iodo-8-hydroxyquinoline (CQ in 2, 4, 6). The X-ray single crystal structures of complexes 1 and 2 as PF6- salts revealed a distorted octahedral CoN5O coordination environment. Complexes demonstrated good stability in an aqueous buffer medium and in the presence of ascorbic acid as a reductant. Cytotoxicity studies using a panel of nine cancer cell lines showed that complex 6, with the dppz and CQ ligands, was significantly toxic against most cancer cell types, yielding IC50 values in the range of 2 to 14 μM. Complexes 1, 3, and 5, containing the HQ ligand, displayed lower toxicity compared to their CQ counterparts. The phenanthroline complexes demonstrated marginal toxicity towards the tested cell lines, while the dpq complexes exhibited moderate toxicity. Interestingly, all complexes demonstrated negligible toxicity towards normal HEK-293 kidney cells (IC50 > 100 μM). The observed cytotoxicity of the complexes correlated well with their lipophilicities (dppz > dpq > phen). The cytotoxicity of complex 6 was comparable to that of the clinical drug cisplatin under similar conditions. Notably, neither the HQ nor the CQ ligands alone demonstrated noticeable toxicity against any of the tested cell lines. The Annexin-V-FITC and DCFDA assays revealed that the cell death mechanism induced by the complexes involved apoptosis, which could be attributed to the metal-assisted generation of reactive oxygen species. Overall, the dppz complex 6, with its remarkable cytotoxicity against a broad range of cancer cells and negligible toxicity toward normal cells, holds significant potential for cancer chemotherapeutic applications.

An Insight into the Effect of Schiff Base and their d and f Block Metal Complexes on Various Cancer Cell Lines as Anticancer Agents: A Review

Over the last few decades, an alarming rise in the percentage of individuals with cancer and those with multi-resistant illnesses has forced researchers to explore possibilities for novel therapeutic approaches. Numerous medications currently exist to treat various disorders, and the development of small molecules as anticancer agents has considerable potential. However, the widespread prevalence of resistance to multiple drugs in cancer indicates that it is necessary to discover novel and promising compounds with ideal characteristics that could overcome the multidrug resistance issue. The utilisation of metallo-drugs has served as a productive anticancer chemotherapeutic method, and this approach may be implemented for combating multi-resistant tumours more successfully. Schiff bases have been receiving a lot of attention as a group of compounds due to their adaptable metal chelating abilities, innate biologic properties, and versatility to tweak the structure to optimise it for a specific biological purpose. The biological relevance of Schiff base and related complexes, notably their anticancer effects, has increased in their popularity as bio-inorganic chemistry has progressed. As a result of learning about Schiff bases antitumor efficacy against multiple cancer cell lines and their complexes, researchers are motivated to develop novel, side-effect-free anticancer treatments. According to study reports from the past ten years, we are still seeking a powerful anticancer contender. This study highlights the potential of Schiff bases, a broad class of chemical molecules, as potent anticancer agents. In combination with other anticancer strategies, they enhance the efficacy of treatment by elevating the cytotoxicity of chemotherapy, surmounting drug resistance, and promoting targeted therapy. Schiff bases also cause cancer cell DNA repair, improve immunotherapy, prevent angiogenesis, cause apoptosis, and lessen the side effects of chemotherapy. The present review explores the development of potential Schiff base and their d and f block metal complexes as anticancer agents against various cancer cell lines.

Investigation of cobalt(III)-phenylalanine complexes for hypoxia-activated drug delivery

Four cobalt(iii)-phenylalanine complexes, [Co(Phe)(py2en)](ClO4)2·H2O (1), [Co(Phe)(TPA)](ClO4)2·H2O (2), [Co(Phe)(py2enMe2)](ClO4)2·H2O (3) and [Co(bipy)2(Phe)](ClO4)2·H2O (4), were investigated as prototype models for hypoxia-activated delivery of melphalan - a phenylalanine derivative anticancer drug of the class of nitrogen mustards. Single crystal X-ray diffraction analysis provided the molecular structures of 1-4, as a single isomer/conformer. According with NMR and theoretical calculations, the solid-state structures of 2 and 4 are maintained in solutions. For complexes 1 and 3, though, a mixture of isomers was found in DMSO solutions: Λ-cisα(exo,exo) and Δ-cisβ1(exo,exo) for 1 (3 : 2 ratio), and Λ-cisα(exo,exo) and Δ-cisα(exo,exo) for 3 (5 : 1 ratio). Theoretical calculations point to a re-equilibration reaction of the solid-state Λ-cisβ1 isomer of 1 in solution. Electrochemical analysis revealed a correlation between the electron-donor capacity of the ancillary ligands and the redox potentials of the complexes. The potentials varied from +0.01 for 1 to +0.31 V vs. SHE for 4 in aqueous media and indicate that reduction should be achieved in biological media. The integrity of the complexes in pH 5.5 and 7.4 buffered solutions was confirmed by UV-Vis monitoring up to 24 h at 25 °C. Reduction by ascorbic acid (AA) shows an O2-dependent dissociation of the l-Phe for complexes 1-3, with higher conversion rates at pH 7.4. For complex 4, a fast dissociation of l-Phe was observed, with conversion rates unaffected by the pH and presence of O2.

Development of a cobalt(iii)-based ponatinib prodrug system

Receptor tyrosine kinase inhibitors have become a central part of modern targeted cancer therapy. However, their curative potential is distinctly limited by both rapid resistance development and severe adverse effects. Consequently, tumor-specific drug activation based on prodrug designs, exploiting tumor-specific properties such as hypoxic oxygen conditions, is a feasible strategy to widen the therapeutic window. After proof-of-principal molecular docking studies, we have synthesized two cobalt(iii) complexes using a derivative of the clinically approved Abelson (ABL) kinase and fibroblast growth factor receptor (FGFR) inhibitor ponatinib. Acetylacetone (acac) or methylacetylacetone (Meacac) have been used as ancillary ligands to modulate the reduction potential. The ponatinib derivative, characterized by an ethylenediamine moiety instead of the piperazine ring, exhibited comparable cell-free target kinase inhibition potency. Hypoxia-dependent release of the ligand from the cobalt(iii) complexes was proven by changed fluorescence properties, enhanced downstream signaling inhibition and increased in vitro anticancer activity in BCR-ABL- and FGFR-driven cancer models. Respective tumor-inhibiting in vivo effects in the BCR-ABL-driven K-562 leukemia model were restricted to the cobalt(iii) complex with the higher reduction potential and confirmed in a FGFR-driven urothelial carcinoma xenograft model. Summarizing, we here present for the first time hypoxia-activatable prodrugs of the clinically approved tyrosine kinase inhibitor ponatinib and a correlation of the in vivo activity with their reduction potential.

Improving the Stability of EGFR Inhibitor Cobalt(III) Prodrugs

Although tyrosine kinase inhibitors (TKIs) have revolutionized cancer therapy in the past two decades, severe drawbacks such as strong adverse effects and drug resistance limit their clinical application. Prodrugs represent a valuable approach to overcoming these disadvantages by administration of an inactive drug with tumor-specific activation. We have recently shown that hypoxic prodrug activation is a promising strategy for a cobalt(III) complex bearing a TKI of the epidermal growth factor receptor (EGFR). The aim of this study was the optimization of the physicochemical properties and enhancement of the stability of this compound class. Therefore, we synthesized a series of novel derivatives to investigate the influence of the electron-donating properties of methyl substituents at the metal-chelating moiety of the EGFR inhibitor and/or the ancillary acetylacetonate (acac) ligand. To understand the effect of the different methylations on the redox properties, the newly synthesized complexes were analyzed by cyclic voltammetry and their behavior was studied in the presence of natural low-molecular weight reducing agents. Furthermore, it was proven that reduction to cobalt(II) resulted in a lower stability of the complexes and subsequent release of the coordinated TKI ligand. Moreover, the stability of the cobalt(III) prodrugs was investigated in blood serum as well as in cell culture by diverse cell and molecular biological methods. These analyses revealed that the complexes bearing the methylated acac ligand are characterized by distinctly enhanced stability. Finally, the cytotoxic activity of all new compounds was tested in cell culture under normoxic and various hypoxic conditions, and their prodrug nature could be correlated convincingly with the stability data. In summary, the performed chemical modifications resulted in new cobalt(III) prodrugs with strongly improved stabilities together with retained hypoxia-activatable properties.

This study presents the synthesis of improved EGFR inhibitor cobalt(III) prodrugs activatable by hypoxia. By modification of the ancillary ligands, the redox potential could be lowered and the stability of the complexes could be distinctly increased in blood serum. Their physico-chemical properties were in detail characterized, the reductive behavior analyzed by different methods and the biological properties investigated in cancer cells.

Bioconjugates of Co(III) complexes with Schiff base ligands and cell penetrating peptides: Solid phase synthesis, characterization and antiproliferative activity

In this work we synthesized a chelating Schiff base by a single condensation of salicylaldehyde with 3,4-diamino benzoic acid (1). This ligand was used further for complexation to CoCl₂·6H₂O under nitrogen. In the next step, three six-coordinate Co(III) complexes were synthesized by coordinating this complex with imidazole (2), 2-methyimidazole (3) and N-Boc-l-histidine methyl ester (4) (Boc: tert.-butoxycarbonyl) in axial positions with simultaneous oxidation of Co(II) to Co(III) under ambient environment. All Co(III) complexes were characterized by multinuclear NMR spectroscopy (¹H, ¹³C and ⁵⁹Co NMR), FT-IR, mass spectrometry and HPLC. The Co(III) complexes were conjugated to three different cell penetrating peptides: FFFF (P1), RRRRRRRRRGAL (P2) and FFFFRRRRRRRRRGAL (P3). Standard solid-phase peptide chemistry was used for the synthesis of cell penetrating peptides. Coupling of N-terminal peptides with the cobalt complexes, possessing a carboxylic group on the tetradentate Schiff base ligand, afforded Co(III)-peptide bioconjugates, which were purified by semi-preparative HPLC and characterized by analytical HPLC and mass spectrometry. The antiproliferative activity of the synthesized compounds was studied against different human tumour cell lines: lung cancer A549, liver cancer HepG2 and normal human fibroblasts GM5657T, in comparison with the activity of cisplatin as a reference drug. The bioconjugate 21 containing the Co complex 4 and the combined phenylalanine and polyarginine cell penetrating sequence P3 shows better activity against the liver cancer line HepG2 than the parent Co(III) complex 4.

A trans-dichloridoplatinum(II) complex of a monodentate nitrogen mustard: Synthesis, stability and cytotoxicity studies

A trans-dichloridoplatinum(II) complex, trans-[Pt^IICl₂(L)(DMSO)] (1) of a monodentate nitrogen mustard, bis(2-chloroethyl)amine (L), was synthesized by the reaction of cis-[Pt^IICl₂(DMSO)₂] &L.HCl in presence of Et₃N. 1 was characterised by NMR, FT-IR and elemental analysis. L is unstable in aqueous solution while 1 displayed moderate stability. In aqueous buffer solution of pD 7.4, 1 starts to loose L slowly upon dissolution and even after 48 h there is still intact/aquated complex present in solution. 1 interacts with the model nucleobase 9-ethyl guanine. The ligand L was non-toxic against MCF-7, A549, HepG2 & MIA PaCa-2 up to 200 μM. In contrast, the Pt(II) complex 1 showed an excellent IC₅₀ (ca. 600 nM) against MIA PaCa-2 and also displayed good IC₅₀ value (3-7 μM) against the other cancer cell lines probed. The in vitro cytotoxicity of 1 is better than cisplatin against each of the treated cancer cell lines and it is not affected by hypoxia as per the in vitro studies. Complex 1 displays higher cellular accumulation than cisplatin and arrests the cell cycle in both S & G2/M phase inducing apoptotic cell death. The G2/M phase arrest is dominant at higher concentrations. The depolarisation of mitochondria by 1 combined with activation of caspase-7 indicates apoptotic cell death. Complex 1 induces low hemolysis of human blood signifying excellent blood compatibility.

Hypoxia-targeted drug delivery

Hypoxia is a state of low oxygen tension found in numerous solid tumours. It is typically associated with abnormal vasculature, which results in a reduced supply of oxygen and nutrients, as well as impaired delivery of drugs. The hypoxic nature of tumours often leads to the development of localized heterogeneous environments characterized by variable oxygen concentrations, relatively low pH, and increased levels of reactive oxygen species (ROS). The hypoxic heterogeneity promotes tumour invasiveness, metastasis, angiogenesis, and an increase in multidrug-resistant proteins. These factors decrease the therapeutic efficacy of anticancer drugs and can provide a barrier to advancing drug leads beyond the early stages of preclinical development. This review highlights various hypoxia-targeted and activated design strategies for the formulation of drugs or prodrugs and their mechanism of action for tumour diagnosis and treatment.

Modulation of the reactivity of nitrogen mustards by metal complexation: approaches to modify their therapeutic properties

Metal complexation of nitrogen mustards shows promise with an ability to control the mustards’ reactivity, perform selective hypoxia activation, overcome resistance, and control GSH deactivation.

Transition metal compounds as cancer radiosensitizers

Combining metallo-drugs with ionising radiation for synergistic cancer cell killing: chemical design principles, mechanisms of action and emerging applications.

Anticancer activity of a chelating nitrogen mustard bearing tetrachloridoplatinum(iv) complex: better stability yet equipotent to the Pt(ii) analogue

Two Pt(iv) complexes cis,cis,trans-[Pt(IV)(L1)Cl4] (1a) & cis,cis,trans-[Pt(IV)(L2)Cl4] (2a) containing the nitrogen mustard moieties -N(CH2CH2Cl)2 & -NHCH2CH2Cl, were prepared in a single step from the Pt(ii) complexes containing -N(CH2CH2OH)2 (1) & -NHCH2CH2OH (2) moieties respectively using only thionyl chloride. The characterization of both the Pt(iv) complexes was performed by NMR, IR, UV and elemental analysis. Complex 1a was also characterized by single crystal X-ray diffraction. 1a crystallized in the I2/a space group. 1a exhibited much higher solution stability than 2a in kinetic studies by (1)H NMR. 1a shows a prodrug like activity as it converts to its Pt(ii) congener, [Pt(II)(L1)Cl2] (3) after 2 days in buffered solution. The binding experiment of 1a with model nucleobase 9-ethylguanine (9-EtG), showed that 1a converts to 3 and forms mono-adducts with 9-EtG. In the presence of reduced glutathione (GSH), the formation of 3 from 1a is quicker and upon the formation of 3 it binds almost instantaneously to GSH to form cis-[PtCl(L1)SG] (3c). Complex 3c transformed within a day to give a free aziridinium ion of L1 (3b) by dissociation. The in vitro cytotoxicity of the complexes and the clinical anticancer drug cisplatin show that 1a is potent against MCF-7, A549, HepG2 and MIA PaCa-2. The potency is highest against MIA PaCa-2 exhibiting an IC50 value of 4.4 ± 0.5 μM. The in vitro cytotoxicity data also showed that between the two complexes only 1a is active against MCF-7, A549 and MIA PaCa-2 in normoxia and hypoxia, both in the presence and absence of added GSH. Even in the presence of excess GSH in hypoxia, 1a exhibits significant cytotoxicity against MIA PaCa-2 and MCF-7 with IC50 values of 4.5 ± 0.3 and 11.2 ± 1.8 μM respectively. Platinum accumulation studies by ICP-MS display greater internalization of 1a, than 2a, 3 and cisplatin inside MCF-7 cells. 1a arrests cell cycle at the G2/M phase in MCF-7, exhibits capability to inhibit metastasis, induces apoptotic cell death and displays blood compatibility with human blood.

Advances in cobalt complexes as anticancer agents

The evolution of resistance to traditional platinum-based anticancer drugs has compelled researchers to investigate the cytostatic properties of alternative transition metal-based compounds. The anticancer potential of cobalt complexes has been extensively studied over the last three decades, and much time has been devoted to understanding their mechanisms of action. This perspective catalogues the development of antiproliferative cobalt complexes, and provides an in depth analysis of their mode of action. Early studies on simple cobalt coordination complexes, Schiff base complexes, and cobalt-carbonyl clusters will be documented. The physiologically relevant redox properties of cobalt will be highlighted and the role this plays in the preparation of hypoxia selective prodrugs and imaging agents will be discussed. The use of cobalt-containing cobalamin as a cancer specific delivery agent for cytotoxins will also be described. The work summarised in this perspective shows that the biochemical and biophysical properties of cobalt-containing compounds can be fine-tuned to produce new generations of anticancer agents with clinically relevant efficacies.

Anticancer activity of a cis-dichloridoplatinum(ii) complex of a chelating nitrogen mustard: insight into unusual guanine binding mode and low deactivation by glutathione

A platinum(ii) complex (2) of a chelating nitrogen mustard shows potency against MIA PaCa2.2displays anti-angiogenic potential and displays excellent cytotoxicity profile even in presence of GSH in hypoxia.

Cobalt complexes with tripodal ligands: implications for the design of drug chaperones

Extensive research is currently being conducted into metal complexes that can selectively deliver cytotoxins to hypoxic regions in tumours. The development of pharmacologically suitable agents requires an understanding of appropriate ligand-metal systems for chaperoning cytotoxins. In this study, cobalt complexes with tripodal tren (tris-(2-aminoethyl)amine) and tpa (tris-(2-pyridylmethyl)amine) ligands were prepared with ancillary hydroxamic acid, β-diketone and catechol ligands and several parameters, including: pK(a), reduction potential and cytotoxicity were investigated. Fluorescence studies demonstrated that only tpa complexes with β-diketones showed any reduction by ascorbate in situ and similarly, cellular cytotoxicity results demonstrated that ligation to cobalt masked the cytotoxicity of the ancillary groups in all complexes except the tpa diketone derivative [Co(naac)tpa](ClO(4))(2) (naac = 1-methyl-3-(2-naphthyl)propane-1,3-dione). Additionally, it was shown that the hydroxamic acid complexes could be isolated in both the hydroxamate and hydroximate form and the pK(a) values (5.3-8.5) reveal that the reversible protonation/deprotonation of the complexes occurs at physiologically relevant pHs. These results have clear implications for the future design of prodrugs using cobalt moieties as chaperones, providing a basis for the design of cobalt complexes that are both more readily reduced and more readily taken up by cells in hypoxic and acidic environments.

Redox activation of metal-based prodrugs as a strategy for drug delivery

This review provides an overview of metal-based anticancer drugs and drug candidates. In particular, we focus on metal complexes that can be activated in the reducing environment of cancer cells, thus serving as prodrugs. There are many reports of Pt and Ru complexes as redox-activatable drug candidates, but other d-block elements with variable oxidation states have a similar potential to serve as prodrugs in this manner. In this context are compounds based on Fe, Co, or Cu chemistry, which are also covered. A trend in the field of medicinal inorganic chemistry has been toward molecularly targeted, metal-based drugs obtained by functionalizing complexes with biologically active ligands. Another recent activity is the use of nanomaterials for drug delivery, exploiting passive targeting of tumors with nano-sized constructs made from Au, Fe, carbon, or organic polymers. Although complexes of all of the above mentioned metals will be described, this review focuses primarily on Pt compounds, including constructs containing nanomaterials.

Synthesis, characterization and biological activities of mononuclear Co(III) complexes as potential bioreductively activated prodrugs

Aiming to investigate the use of tridentate ligands to develop new bireductively activated prodrugs, two N(2)O-donor ligands (HL1: [(2-hydroxybenzyl)(2-(imidazol-2-yl)ethyl)]amine; and HL2: [(2-hydroxybenzyl)(2-(pyridil-2-yl)ethyl]amine) were used to synthesize new Co(III) complexes, 1 and 2. Both complexes were characterized by X-ray crystallography, mass spectrometry, electrochemistry, IR, UV-visible and (1)H NMR spectroscopies. Electrochemical data in methanol revealed that the Co(III)-->Co(II) reduction of 1 (-0.84V vs. normal hydrogen electrode - NHE) is more positive than 2 (-1.13V vs. NHE), while it was expected to be more negative due to better sigma-donor ability of imidazole ring in HL1, compared to pyridine in HL2. Considering that reduction processes on Co(III) center may involve the lowest unoccupied molecular orbital (LUMO), it might play an important role on the electronic properties of the complexes, and could explain the observed redox potentials. Then, geometry optimizations of 1 and 2 were performed using the density functional theory (DFT), and different group participation in their LUMO is demonstrated. Using Saccharomyces cerevisiae cells as eukaryotic model, it is shown that in situ generated reduced species, 1(red) and 2(red), have high capacity to inhibit cellular growth, with IC50 (0.50mM for both complexes) lower than cisplatin IC50 (0.6mM) at the same time of exposure. Regarding to their ability to promote S. cerevisiae cells death, after 24 h, cells became susceptible only when exposed to 1(red) and 2(red): (i) at concentrations higher than 0.5mM in a non-dose dependence, and (ii) in anaerobic metabolism. These data reveal the potential of 1 and 2 as bioreductively activated prodrugs, since their oxidized forms do not present expressive activities when compared to their reduced forms.

Synthetic routes to mixed-ligand cobalt(III) dithiocarbamato complexes containing imidazole, amine and pyridine donors and the X-ray crystal structure of a cobalt(III) bis(dithiocarbamato) histamine complex

The binuclear cobalt complex [Co(2)(Me(2)dtc)(5)](+) reacts with a range of nitrogen donor ligands L' or L'' to form an equimolar mixture of Co(Me(2)dtc)(3) and the mixed-ligand complexes [Co(Me(2)dtc)(2)(L')(2)](+) or [Co(Me(2)dtc)(2)(L'')](+), where (L')(2) is two monodentate ligands and (L'') is one bidentate ligand. The complexes prepared by this route contain the monodentate ligands L'=1-methyl-imidazole, 1-methyl-5-nitro-imidazole and benzimidazole, all of which coordinate to cobalt through an imidazole nitrogen atom. Symmetrical bidentate ligand complexes contain the bisimidazole L''=2,2'-bis(4,5-dimethylimidazole), the diamine L''=1,2-diaminobenzene and the pyridine donors L''=2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine and 1,10-phenanthroline. Two examples of complexes with unsymmetrical bidentate imidazole-amine donors were prepared in which L''=4-(2-aminoethyl)imidazole (histamine) and 2-aminomethylbenzimidazole. All new complexes were fully characterised, and the X-ray crystal structure of the histamine complex [Co(Me(2)dtc)(2)(hist)]ClO(4) is also reported.

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.

Radiolytic and cellular reduction of a novel hypoxia-activated cobalt(III) prodrug of a chloromethylbenzindoline DNA minor groove alkylator

Metabolic reduction can be used to activate prodrugs in hypoxic regions of tumours, but reduction by ionising radiation is also theoretically attractive. Previously, we showed that a cobalt(III) complex containing 8-hydroxyquinoline (8-HQ) and cyclen ligands releases 8-HQ efficiently on irradiation in hypoxic solutions [Ahn G-O, Ware DC, Denny WA, Wilson WR. Optimization of the auxiliary ligand shell of cobalt(III)(8-hydroxyquinoline) complexes as model hypoxia-selective radiation-activated prodrugs. Radiat Res 2004;162:315-25]. Here we investigate an analogous Co(III) complex containing the potent DNA minor groove alkylator azachloromethylbenzindoline (azaCBI, 1) to determine whether it releases 1 on radiolytic and/or enzymatic reduction under hypoxia. Monitoring by HPLC, the azaCBI ligand in the Co(III)(cyclen)(azaCBI) complex (2) slowly hydrolysed in aqueous solution, in contrast to the free ligand 1 which readily converted to its reactive cyclopropyl form. Irradiation of 2 (30-50 microM) in hypoxic solutions released 1 with yields of 0.57 micromol/J in formate buffer and 0.13 micromol/J in human plasma. Using bioassay methods, cytotoxic activation by irradiation of 2 at 1 microM in hypoxic plasma was readily detectable at clinically relevant doses (> or = 1 Gy), with a estimated yield of 1 of 0.075 micromol/J. Release of 1 from 2 was also observed in hypoxic HT29 cultures without radiation, with subsequent conversion of 1 to its O-glucuronide. Surprisingly, overexpression of human cytochrome P450 reductase in A549 cells did not increase the rate of metabolic reduction of 2, suggesting that other reductases and/or non-enzymatic reductants are responsible. Thus the cobalt(III) complex 2 is a promising prodrug capable of being activated to release a very potent cytotoxin when reduced by either ionising radiation or cells under hypoxic conditions.