Cyanide Scavenging by a Cobalt Schiff-Base Macrocycle: A Cost-Effective Alternative to Corrinoids

A Potential Antidote for Both Azide and Cyanide Poisonings

There do not appear to be any established therapeutics for treating azide poisoning at this time, and presently available antidotes to cyanide poisoning are far from ideal, being particularly impractical for use if multiple victims present. The cobalt (II/III) complex of the Schiff-base ligand trans-[14]-diene (5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene (CoN4[14]) is shown to act as an effective antidote to both azide and cyanide toxicity in mice. Groups of animals challenged with an LD40 dose of NaCN (100 µmol/kg i.p.) exhibited significantly faster recovery from knockdown and fewer (zero) deaths if given CoN4[14] (50 μmol/kg i.p.) 2 minutes after the toxicant. Groups of animals challenged with an essentially lethal dose of NaCN (1.5 x LD50 = 150 µmol/kg i.p.) all survived if given the CoN4[14] (75 μmol/kg i.p.) 5 minutes before the toxicant dose. These data represent improved antidotal capability over the Food and Drug Administration-approved cobalt-based cyanide antidote hydroxocobalamin. Recovery of animals challenged sublethally with NaN3 (415 μmol/kg i.p.) was assessed employing a modified pole-climbing test. Mice given the CoN4[14] antidote (70 μg/kg i.p.) 5 minutes after the toxicant dose recovered twice as fast as the controls given no antidote. The interactions of cyanide and azide with CoN4[14] in vitro (buffered aqueous solutions) have been further investigated by a combination of spectroscopic approaches. The Co(II) form of the complex is able to bind two CN- anions while only binding a single N3 -, providing a reasonable explanation for the difference between their therapeutic abilities. SIGNIFICANCE STATEMENT: The Schiff-base complex CoN4[14] is shown to be an effective antidote to cyanide in mice, with improved therapeutic capabilities compared to the Food and Drug Administration-approved cobalt-containing hydroxocobalamin. CoN4[14] is also antidotal in mice toward azide poisoning, for which there is seemingly no approved therapy currently available. The activity toward cyanide involves a "redox-switching" mechanism that could be a common, but largely unrecognized, feature of all cobalt-based cyanide antidotes in use and under development.

Identification of Platinum(II) Sulfide Complexes Suitable as Intramuscular Cyanide Countermeasures

The development of rapidly acting cyanide countermeasures using intramuscular injection (IM) represents an unmet medical need to mitigate toxicant exposures in mass casualty settings. Previous work established that cisplatin and other platinum(II) or platinum(IV)-based agents effectively mitigate cyanide toxicity in zebrafish. Cyanide's in vivo reaction with platinum-containing materials was proposed to reduce the risk of acute toxicities. However, cyanide antidote activity depended on a formulation of platinum-chloride salts with dimethyl sulfoxide (DMSO) followed by dilution in phosphate-buffered saline (PBS). A working hypothesis to explain the DMSO requirement is that the formation of platinum-sulfoxide complexes activates the cyanide scavenging properties of platinum. Preparations of isolated NaPtCl₅-DMSO and Na (NH₃)₂PtCl-DMSO complexes in the absence of excess DMSO provided agents with enhanced reactivity toward cyanide in vitro and fully recapitulated in vivo cyanide rescue in zebrafish and mouse models. The enhancement of the cyanide scavenging effects of the DMSO ligand could be attributed to the activation of platinum(IV) and (II) with a sulfur ligand. Unfortunately, the efficacy of DMSO complexes was not robust when administered IM. Alternative Pt(II) materials containing sulfide and amine ligands in bidentate complexes show enhanced reactivity toward cyanide addition. The cyanide addition products yielded tetracyanoplatinate(II), translating to a stoichiometry of 1:4 Pt to each cyanide scavenger. These new agents demonstrate a robust and enhanced potency over the DMSO-containing complexes using IM administration in mouse and rabbit models of cyanide toxicity. Using the zebrafish model with these Pt(II) complexes, no acute cardiotoxicity was detected, and dose levels required to reach lethality exceeded 100 times the effective dose. Data are presented to support a general chemical design approach that can expand a new lead candidate series for developing next-generation cyanide countermeasures.

A Comparison of Potential Azide Antidotes in a Mouse Model

Three cobalt-containing macrocyclic compounds previously shown to antagonize cyanide toxicity have been comparatively evaluated for the amelioration of sublethal azide toxicity in juvenile (7-8 weeks) Swiss-Webster mice. The lowest effective doses were determined for hydroxocobalamin, a cobalt porphyrin, and a cobalt-Schiff base macrocycle by giving the antidotes 5 min prior to the toxicant, 27 mg (415 μmol) /kg sodium azide. Both male and female mice were evaluated for their response to the toxicant as well as the antidotes, and no significant differences were noted once weight differences were taken into account. Two of the three compounds significantly decreased the recovery time of azide-intoxicated mice at 10 min after the administration of sodium azide, as determined by a behavioral test (pole climbing). Additionally, azide was determined to cause a several degree drop (∼3 °C) in measured tail temperature, and warming the mice led to a more rapid recovery. The mice were also shown to recover more rapidly when given sodium nitrite, 24 mg (350 μmol)/kg, 5 min after the toxicant; this treatment also suppressed the azide-induced tail temperature decrease. Electron paramagnetic resonance (EPR) measurements of mouse blood treated with sodium azide demonstrated the presence of nitrosylhemoglobin at levels of 10-20 μM which persisted for ∼300 min. The presence of the methemoglobin azide adduct was also detected by EPR at a maximum level of ∼300 μM, but these signals disappeared around 200 min after the administration of azide. The treatment of mice with ¹⁵N sodium azide proved that the nitrosylhemoglobin was a product of the administered azide by the appearance of a two-line hyperfine (due to the ¹⁵N) in the EPR spectrum of mouse blood.

A Cobalt Schiff-Base Complex as a Putative Therapeutic for Azide Poisoning

There is presently no antidote available to treat azide poisoning. Here, the Schiff-base compound Co(II)-2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]heptadeca-1(17)2,11,13,15-pentaenyl dibromide (Co(II)N₄[11.3.1]) is investigated to determine if it has the capability to antagonize azide toxicity through a decorporation mechanism. The stopped-flow kinetics of azide binding to Co(II)N₄[11.3.1] in the absence of oxygen exhibited three experimentally observable phases: I (fast); II (intermediate); and III (slow). The intermediate phase II accounted for ∼70% of the overall absorbance changes, representing the major process observed, with second-order rate constants of 29 (±4) M^-1 s^-1 at 25 °C and 70 (±10) M^-1 s^-1 at 37 °C. The data demonstrated pH independence of the reaction around neutrality, suggesting the unprotonated azide anion to be the attacking species. The binding of azide to Co(II)N₄[11.3.1] appears to have a complicated mechanism leading to less than ideal antidotal capability; nonetheless, this cobalt complex does protect against azide intoxication. Administration of Co(II)N₄[11.3.1] at 5 min post sodium azide injection (ip) to mice resulted in a substantial decrease of righting-recovery times, 12 (±4) min, compared to controls, 40 (±8) min. In addition, only two out of seven mice "knocked down" when the antidote was administered compared to the controls given toxicant only (100% knockdown).

Reaction Kinetics of Cyanide Binding to a Cobalt Schiff-Base Macrocycle Relevant to Its Mechanism of Antidotal Action

The Co(II/III)-containing macrocycle, cobalt 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17)2,11,13,15-pentaenyl cation, or CoN₄[11.3.1], is a potential cyanide-scavenging agent. The rate of reduction of Co(III)N₄[11.3.1] by ascorbate is reasonably facile under pseudo-first-order conditions; a second-order rate constant of 11.7(±0.4) M^-1 s^-1 was determined at 25 °C and pH 7.4, along with the activation parameters for the reaction (ΔH^⧧ = 53.9(±0.8) kJ mol^-1; ΔS -79(±3) J mol^-1 K^-1). It follows that any cyanide-decorporating capability of the cobalt complex should depend more on the cyanide-binding characteristics of Co(II)N₄[11.3.1] than the oxidized form. The kinetics of the reaction of cyanide with Co(II)N₄[11.3.1] under anaerobic pseudo-first-order conditions is rapid and resulted in a linear dependence on the cyanide concentration, k_HCN = 8 × 10⁴ M^-1 s^-1, with a nonlinear intercept of 420 s^-1 at 10 °C, pH 7.6. The observed reaction rate increases significantly with increasing pH. A rate law is suggested, k_obs = k'[X] + (k_HCN + k_CNK_a/[H⁺])[HCN], where k_CN is estimated to be ∼2 × 10⁶ M^-1 s^-1. Activation parameters for the reaction with HCN (ΔH^⧧ = 10.7(±0.4) kJ mol^-1; ΔS^⧧ = -153(±1) J mol^-1 K^-1) suggest an associative mechanism. In the presence of excess oxygen, i.e., at higher levels than free oxygen in vivo, the reaction rate was too fast to be measured, and the final product was the oxidized complex, Co(III)N₄[11.3.1], where any cyanide ligands had been lost. This is much more rapid than the oxidation of the parent compound by oxygen, for which a second-order rate constant of 0.5(±0.02) M^-1 s^-1 at 25 °C was obtained. The study has gone some way toward enhancing our understanding of the reaction of Co(II)N₄[11.3.1] with cyanide. The fast reaction rate implies a high efficacy of the cyanide-scavenging capability of the complex and further supports the suggestion stemming from our previous work that Co(II)N₄[11.3.1] could prove to be a better and more cost-effective cyanide antidote than the FDA-approved hydroxocobalamin.

Assessing modulators of cytochrome c oxidase activity in Galleria mellonella larvae

Caterpillars of the greater wax moth, Galleria mellonella, are shown to be a useful invertebrate organism for examining mitochondrial toxicants (inhibitors of electron transport) and testing putative antidotes. Administration of sodium azide, sodium cyanide, or sodium (hydro)sulfide by intra-haemocoel injection (through a proleg) results in a dose-dependent paralysed state in the larvae lasting from <1 to ~40 min. The duration of paralysis is easily monitored, because if turned onto their backs, the larvae right themselves onto their prolegs once they are able to move again. The efficacy of putative antidotes to the three toxicants can routinely be assessed by observing shortened periods of paralysis with larvae given toxicant and antidote compared to larvae administered only the same dose of toxicant. The validity of the approach is demonstrated with agents previously shown to be antidotal towards cyanide intoxication in mice; namely, sodium nitrite and CoN₄[11.3.1] (cobalt(II/III) 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(7)2,11,13,15-pentaenyl cation). These same compounds are shown to be antidotal towards all three toxicants in the G. mellonella caterpillars; findings that may prove important in relation to azide and sulfide poisonings, for which there are currently no effective antidotes available. The observation that sodium nitrite ameliorates cyanide toxicity in the larvae is additionally interesting because it unambiguously demonstrates that the antidotal action of nitrites does not require the involvement of methemoglobin, contributing to the resolution of an ongoing controversy.

Rationally designed molecules for resurgence of cyanide mitigated cytochrome c oxidase activity

Cytochrome c oxidase (CcOX) containing binuclear heme a₃-Cu B centre (BNC) mechanises the process of electron transfer in the last phase of cellular respiration. The molecular modelling based structural analysis of CcOX - heme a₃-Cu B complex was performed and the disturbance to this complex under cyanide poisoning conditions was investigated. Taking into consideration the results of molecular docking studies, new chemical entities were developed for clipping cyanide from the enzyme and restoring its normal function. It was found that the molecules obtained by combining syringaldehyde, oxindole and chrysin moieties bearing propyl/butyl spacing groups occupy the BNC region and effectively remove cyanide bound to the enzyme. The binding constant of compound 2 with CN^- was 2.3 × 10⁵ M^-1 and its ED₅₀ for restoring the cyanide bound CcOX activity in 10 min was 16 µM. The compound interacted with CN^- over the pH range 5-10. The comparison of the loss of enzymatic activity in the presence of CN^- and resumption of enzymatic activity by compound 2 mediated removal of CN^- indicated the efficacy of the compound as antidote of cyanide.

Rationally Designed Circularly Arranged Sextuple Molecule with Dimethoxyphenolic Tentacles for Ample Hunting of Cyanide

Herein, we report the design, synthesis, and cyanide-scavenging behavior of circularly arranged sextuple molecule 4. The six syringaldehyde units carrying equal number of dimethoxyphenolic moieties projecting at the periphery make the molecule highly efficient for cleaning up cyanide from the aqueous solution. The stoichiometric data 1:6 showed that six units of cyanide interact with one unit of compound 4. The association constant of the compound for cyanide was 2.5 × 10⁴ M^-1, and its detection limit for cyanide was 10 nM. The compound was also found to remove cyanide bound to cytochrome c oxidase.

Modified vitamin B12 derivatives with a peptide backbone for biomimetic studies and medicinal applications

This short review highlights the author’s group research on modified vitamin B[Formula: see text] derivatives with a peptide backbone as (1) inhibitors of B[Formula: see text]-dependent enzymes and as (2) models of cofactor B[Formula: see text]-protein complexes.

A Comparison of the Cyanide-Scavenging Capabilities of Some Cobalt-Containing Complexes in Mice

Four cobalt-containing macrocyclic compounds previously shown to ameliorate cyanide toxicity have been comparatively evaluated with an acute sublethal toxicity model in conscious (unanesthetized) adult male Swiss-Webster mice. All of the compounds (the cobalt-corrins cobalamin and cobinamide, a cobalt-porphyrin, plus a cobalt-Schiff base macrocycle) given 5 min prior to the toxicant dose significantly decreased the righting-recovery time of cyanide-intoxicated mice, but the doses required for maximal antidotal effect varied. Additionally, all of the compounds tested significantly reduced the righting-recovery time when administered at either 1 or 2 min after cyanide intoxication, but none of the compounds tested significantly reduced the righting-recovery time when delivered 5 min after the toxicant dose. Using the lowest effective dose of each compound determined during the first (prophylactic) set of experiments, neuromuscular recovery following cyanide intoxication in the presence/absence of the cobalt-based antidotes was assessed by RotaRod testing. All the compounds tested accelerated recovery of neuromuscular coordination, and no persistent impairment in any group, including those animals that received toxicant and no antidote, was apparent up to 2 weeks postexposures. The relative effectiveness of the cobalt compounds as cyanide antidotes are discussed and rationalized on the basis of the cyanide-binding stoichiometries and stability constants of the Co(III) cyano adducts, together with consideration of the rate constants for axial ligand substitutions by cyanide in the Co(II) forms.

Relative Propensities of Cytochrome c Oxidase and Cobalt Corrins for Reaction with Cyanide and Oxygen: Implications for Amelioration of Cyanide Toxicity

In aqueous media at neutral pH, the binding of two cyanide molecules per cobinamide can be described by two formation constants, K_f1 = 1.1 (±0.6) × 10⁵ M^-1 and K_f2 = 8.5 (±0.1) × 10⁴ M^-1, or an overall cyanide binding constant of ∼1 × 10¹⁰ M^-2. In comparison, the cyanide binding constants for cobalamin and a fully oxidized form of cytochrome c oxidase, each binding a single cyanide anion, were found to be 7.9 (±0.5) × 10⁴ M^-1 and 1.6 (±0.2) × 10⁷ M^-1, respectively. An examination of the cyanide-binding properties of cobinamide at neutral pH by stopped-flow spectrophotometry revealed two kinetic phases, rapid and slow, with apparent second-order rate constants of 3.2 (±0.5) × 10³ M^-1 s^-1 and 45 (±1) M^-1 s^-1, respectively. Under the same conditions, cobalamin exhibited a single slow cyanide-binding kinetic phase with a second-order rate constant of 35 (±1) M^-1 s^-1. All three of these processes are significantly slower than the rate at which cyanide is bound by complex IV during enzyme turnover (>10⁶ M^-1 s^-1). Overall, it can be understood from these findings why cobinamide is a measurably better cyanide scavenger than cobalamin, but it is unclear how either cobalt corrin can be antidotal toward cyanide intoxication as neither compound, by itself, appears able to out-compete cytochrome c oxidase for available cyanide. Furthermore, it has also been possible to unequivocally show in head-to-head comparison assays that the enzyme does indeed have greater affinity for cyanide than both cobalamin and cobinamide. A plausible resolution of the paradox that both cobalamin and cobinamide clearly are antidotal toward cyanide intoxication, involving the endogenous auxiliary agent nitric oxide, is suggested. Additionally, the catalytic consumption of oxygen by the cobalt corrins is demonstrated and, in the case of cobinamide, the involvement of cytochrome c when present. Particularly in the case of cobinamide, these oxygen-dependent reactions could potentially lead to erroneous assessment of the ability of the cyanide scavenger to restore the activity of cyanide-inhibited cytochrome c oxidase.