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Ca(II) ions are critical for the proper function of neurons by contributing to synaptic signaling and regulating neuronal plasticity. Dysregulation of Ca(II) is associated with a number of pathologies that cause neurodegeneration; therefore the ability to monitor Ca(II) intracellularly is an important target for molecular imaging. Contrast-enhanced MR imaging is a promising modality for imaging changes in Ca(II) concentrations. However, the majority of Ca(II) responsive MR agents are limited to the extracellular space or hindered by poor cellular uptake. Here, we describe a new class of multimodal, bioresponsive Ca(II) magnetic resonance agents that are coupled to the NIR probe IR-783. This new design is based on previous generations of our Ca(II) MR agents but overcomes two significant challenges: (1) the presence of the NIR probe dramatically increases cellular uptake of the agent and (2) provides histological validation of the MR signal using NIR fluorescence imaging. IR-783 targets organic anion transporter polypeptides, and we demonstrate that the agents are not toxic in HT-22 or U-87 MG cells up to 20 μM. The cellular uptake of complex 1 was measured to be greater than 16 femtomoles per cell (where ∼1 femtomole/cell is detectable in acquired MR images). Complex 1 is simultaneously detectable by both MR and NIR fluorescence imaging in vitro and is activated (turned on) by intracellular Ca(II) at concentrations between 1 and 10 μM.

Coordination complexes have emerged as prominent modulators of amyloid aggregation via their interaction with the N-terminal histidine residues of amyloid-β (Aβ). Herein, we report the synthesis and characterization of a novel cobalt(III) Schiff base complex with methylamine axial ligands, and we present both computational and experimental data demonstrating the reduction of β-sheet formation by this complex. The computations include molecular dynamics simulations of both monomeric and pentameric Aβ, which demonstrate decreased formation of β-sheet structures, destabilization of preformed β-sheets, and suppression of aggregation. These results are consistent with a dose dependence in experimental bulk aggregation data using thioflavin T fluorescence, and overall this study demonstrates useful drug activity of the cobalt complex.

The use of metals in medicine has grown impressively in recent years as a result of greatly advanced understanding of biologically active metal complexes and metal-containing proteins. One landmark in this area was the introduction of cisplatin and related derivatives as anticancer drugs. As the body of literature continues to expand, it is necessary to inspect sub-classes of this group with more acute detail. This chapter will review preclinical research of cobalt complexes coordinated by Schiff base ligands. Cobalt-Schiff base complexes have a wide variety of potential therapeutic functions, including as antimicrobials, anticancer agents, and inhibitors of protein aggregation. While providing a broad introduction to this class of agents, this chapter will pay particular attention to agents for which mechanisms of actions have been studied. Appropriate methods to assess activity of these complexes will be reviewed, and promising preclinical complexes in each of the following therapeutic areas will be highlighted: antimicrobial, antiviral, cancer therapy, and Alzheimer's disease.

The transition from a non-invasive to an invasive phenotype is an essential step in tumor metastasis. The Snail family of transcription factors (TFs) is known to play a significant role in this transition. These TFs are zinc fingers that bind to the CAGGTG Ebox consensus sequence. Co(III) -Ebox is a cobalt(III) complex attached to an Ebox oligonucleotide that confers specificity towards Snail TFs. Co(III) -Ebox has been shown to inhibit Snail-mediated embryonic neural crest development in Xenopus laevis, but its efficacy in inhibiting Snail-induced cancer cell invasiveness has not been explored. Here, we describe the efficacy of Co(III) -Ebox in inhibiting the invasive aspects of heregulin-β1(HRG)-treated breast cancer cells. Co(III) -Ebox was found to inhibit the capacity of Snail to repress target genes after HRG induction. Snail inhibition by Co(III) -Ebox reduced the invasive propensity of cells in 2D and 3D, thereby demonstrating promise in inhibiting metastasis., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Cobalt(III) Schiff base complexes ([Co(acacen)(L)2](+), where L = NH3) inhibit histidine-containing proteins through dissociative exchange of the labile axial ligands (L). This work investigates axial ligand exchange dynamics of [Co(acacen)(L)2](+) complexes toward the development of protein inhibitors that are activated by external triggers such as light irradiation. We sought to investigate ligand exchange dynamics to design a Co(III) complex that is substitutionally inert under normal physiological conditions for selective activation. Fluorescent imidazoles (C3Im) were prepared as axial ligands in [Co(acacen)(L)2](+) to produce complexes (CoC3Im) that could report on ligand exchange and, thus, complex stability. These fluorescent imidazole reporters guided the design of a new dinuclear Co(III) Schiff base complex containing bridging diimidazole ligands, which exhibits enhanced stability to ligand exchange with competing imidazoles and to hydrolysis within a biologically relevant pH range. These studies inform the design of biocompatible Co(III) Schiff base complexes that can be selectively activated for protein inhibition with spatial and temporal specificity.

Long-term cell tracking using MR imaging necessitates the development of contrast agents that both label and are retained by cells. One promising strategy for long-term cell labeling is the development of lipophilic Gd(III)-based contrast agents that anchor into the cell membrane. We have previously reported the efficacy of monomeric and multimeric lipophilic agents and showed that the monomeric agents have improved labeling and contrast enhancement of cell populations. Here, we report on the synthesis, characterization, and in vitro testing of a series of monomeric lipophilic contrast agents with varied alkyl chain compositions. We show that these agents disperse in water, localize to the cell membrane, and label HeLa and MCF7 cells effectively. Additionally, these agents have up to tenfold improved retention in cells compared to clinically available ProHance(®).

Transcription factors are key regulators in both normal and pathological cell processes. Affecting the activity of these proteins is a promising strategy for understanding gene regulation and developing effective therapeutics. Co(III) Schiff base complexes ([Co(acacen)(L)2](+) where L=labile axial ligands) have been shown to be potent inhibitors of a number of zinc metalloproteins including Cys2His2 zinc finger transcription factors. Inhibition by [Co(acacen)(L)2](+) of the target protein is believed to occur through a dissociative exchange of the labile axial ligands for histidine (His) residues essential for function. Here, we report a series of spectroscopic investigations with model peptides of zinc fingers that elucidate the interaction between [Co(acacen)(L)2](+) complexes and zinc finger transcription factors. Observed changes in NMR chemical shifts and 2D (1)H-(1)H NOESY NMR spectra demonstrate the preference of [Co(acacen)(L)2](+) complexes to coordinate His residues over other amino acids. The conformation of [Co(acacen)(L)2](+) upon His coordination was characterized by (1)H NMR spectroscopy, near-UV CD, and electronic absorption. These studies reveal that the resulting His-coordinated [Co(acacen)(L)2](+) complex possesses an octahedral structure. The effects of [Co(acacen)(L)2](+) complexes on the zinc-finger structure were assessed by the degree of hydrogen bonding (probed by 2D NMR spectroscopy) and secondary-structure profiles measured by far-UV CD. These structural studies demonstrate the ability of [Co(acacen)(L)2](+) complexes to disrupt the ββα structure of zinc fingers, resulting in primarily random-coil conformations. A mechanism is described wherein [Co(acacen)(L)2](+) complexes inhibit zinc finger transcription factor activity through selectively coordinating His residues in the zinc finger by dissociative ligand exchange and disrupting the ββα structural motif required for gene regulation., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

This paper describes the activation of a biologically inert Co(III) Schiff base [Co(III)-SB] complex to its protein inhibitor form by photoinduced electron transfer (PET) from a colloidal PbS quantum dot (QD, radii of 1.5-1.7 nm) to the cobalt center, with a charge separation time constant of 125 ns. Reduction of the Co(III)-SB complex initiates release of the native axial ligands, promoting replacement with the histidine mimic 4-methylimidazole. The rate of ligand displacement increases by a factor of approximately 8 upon exposure of the PbS QD/Co(III)-SB mixture to light with an energy greater than the energy of the first excitonic state of the QDs, from which PET occurs. These results suggest an approach for the preparation of inorganic therapeutic agents that can be specifically coupled to a biologically active site by cooperative redox binding ligation.

Inorganic complexes are versatile platforms for the development of potent and selective pharmaceutical agents. Cobalt possesses a diverse array of properties that can be manipulated to yield promising drug candidates. Investigations into the mechanism of cobalt therapeutic agents can provide valuable insight into the physicochemical properties that can be harnessed for drug development. This review presents examples of bioactive cobalt complexes with special attention to their mechanisms of action. Specifically, cobalt complexes that elicit biological effects through protein inhibition, modification of drug activity, and bioreductive activation are discussed. Insights gained from these examples reveal features of cobalt that can be rationally tuned to produce therapeutics with high specificity and improved efficacy for the biomolecule or pathway of interest., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Cobalt(III) Schiff base complexes, such as [Co(acacen)L(2)](+), inhibit the function of Zn(II)-dependent proteins through dissociative exchange of the axial ligands with key histidine residues of the target protein. Consequently the efficacy of these compounds depends strongly on the lability of the axial ligands. A series of [Co(acacen)L(2)](+) complexes with various axial ligands was investigated using DFT to determine the kinetics and thermodynamics of ligand exchange and hydrolysis. Results showed excellent agreement with experimental data, indicating that axial ligand lability is determined by several factors: pK(a) of the axial ligand, the kinetic barrier to ligand dissociation, and the relative thermodynamic stability of the complexes before and after exchange. Hammett plots were constructed to determine if the kinetics and thermodynamics of exchange can be modulated by the addition of an electron-withdrawing group (EWG) to either the axial ligand itself or to the equatorial acacen ligand. Results predict that addition of an EWG to the axial ligand will shift the kinetics and thermodynamics so as to promote axial ligand exchange, while addition of an EWG to acacen will decrease axial ligand lability. These investigations will aid in the design of the next generation of [Co(acacen)L(2)](2+), allowing researchers to develop new, more effective inhibitors.

The kinetic and thermodynamic ligand exchange dynamics are important considerations in the rational design of metal-based therapeutics and therefore, require detailed investigation. Co(III) Schiff base complex derivatives of bis(acetylacetone)ethylenediimine [acacen] have been found to be potent enzyme and transcription factor inhibitors. These complexes undergo solution exchange of labile axial ligands. Upon dissociation, Co(III) irreversibly interacts with specific histidine residues of a protein, and consequently alters structure and causes inhibition. To guide the rational design of next generation agents, understanding the mechanism and dynamics of the ligand exchange process is essential. To investigate the lability, pH stability, and axial ligand exchange of these complexes in the absence of proteins, the pD- and temperature-dependent axial ligand substitution dynamics of a series of N-heterocyclic [Co(acacen)(X)(2)](+) complexes [where X = 2-methylimidazole (2MeIm), 4-methylimidazole (4MeIm), ammine (NH(3)), N-methylimidazole (NMeIm), and pyridine (Py)] were characterized by NMR spectroscopy. The pD stability was shown to be closely related to the nature of the axial ligand with the following trend toward aquation: 2MeIm > NH(3) ≫ 4MeIm > Py > Im > NMeIm. Reaction of each [Co(III)(acacen)(X)(2)](+) derivative with 4MeIm showed formation of a mixed ligand Co(III) intermediate via a dissociative ligand exchange mechanism. The stability of the mixed ligand adduct was directly correlated to the pD-dependent stability of the starting Co(III) Schiff base with respect to [Co(acacen)(4MeIm)(2)](+). Crystal structure analysis of the [Co(acacen)(X)(2)](+) derivatives confirmed the trends in stability observed by NMR spectroscopy. Bond distances between the Co(III) and the axial nitrogen atoms were longest in the 2MeIm derivative as a result of distortion in the planar tetradentate ligand, and this was directly correlated to axial ligand lability and propensity toward exchange.

Cobalt(III) Schiff base complexes have been used as potent inhibitors of protein function through the coordination to histidine residues essential for activity. The kinetics and thermodynamics of the binding mechanism of Co(acacen)(NH(3))(2)Cl [Co(acacen); where H(2)acacen is bis(acetylacetone)ethylenediimine] enzyme inhibition has been examined through the inactivation of matrix metalloproteinase 2 (MMP-2) protease activity. Co(acacen) is an irreversible inhibitor that exhibits time- and concentration-dependent inactivation of MMP-2. Co(acacen) inhibition of MMP-2 is temperature-dependent, with the inactivation increasing with temperature. Examination of the formation of the transition state for the MMP-2/Co(acacen) complex was determined to have a positive entropy component indicative of greater disorder in the MMP-2/Co(acacen) complex than in the reactants. With further insight into the mechanism of Co(acacen) complexes, Co(III) Schiff base complex protein inactivators can be designed to include features regulating activity and protein specificity. This approach is widely applicable to protein targets that have been identified to have clinical significance, including matrix metalloproteinases. The mechanistic information elucidated here further emphasizes the versatility and utility of Co(III) Schiff base complexes as customizable protein inhibitors.

Magnetic resonance imaging (MRI) has long been used clinically and experimentally as a diagnostic tool to obtain three-dimensional, high-resolution images of deep tissues. These images are enhanced by the administration of contrast agents such as paramagnetic Gd(III) complexes. Herein, we describe the preparation of a series of multimodal imaging agents in which paramagnetic Gd(III) complexes are conjugated to a fluorescent tetrapyrrole, namely, a porphyrazine (pz). Zinc metalated pzs conjugated to one, four, or eight paramagnetic Gd(III) complexes are reported. Among these conjugates, Zn-Pz-8Gd(III) exhibits an ionic relaxivity four times that of the monomeric Gd(III) agent, presumably because of increased molecular weight and a molecular relaxivity that is approximately thirty times larger, while retaining the intense electronic absorption and emission of the unmodified pz. Unlike current clinical MR agents, Zn-Pz-1Gd(III) is taken up by cells. This probe demonstrates intracellular fluorescence by confocal microscopy and provides significant contrast enhancement in MR images, as well as marked phototoxicity in assays of cellular viability. These results suggest that pz agents possess a new potential for use in cancer imaging by both MRI and near-infrared (NIR) fluorescence, while acting as a platform for photodynamic therapy.