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

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.

In Vitro Characterization of a Threonine-Ligated Molybdenyl-Sulfide Cluster as a Putative Cyanide Poisoning Antidote; Intracellular Distribution, Effects on Organic Osmolyte Homeostasis, and Induction of Cell Death

Binuclear molybdenum sulfur complexes are effective for the catalytic conversion of cyanide into thiocyanate. The complexes themselves exhibit low toxicity and high aqueous solubility, which render them suitable as antidotes for cyanide poisoning. The binuclear molybdenum sulfur complex [(thr)Mo2O2(μ-S)2(S2)]- (thr - threonine) was subjected to biological studies to evaluate its cellular accumulation and mechanism of action. The cellular uptake and intracellular distribution in human alveolar (A549) cells, quantified by inductively coupled plasma mass spectrometry (ICP-MS) and cell fractionation methods, revealed the presence of the compound in cytosol, nucleus, and mitochondria. The complex exhibited limited binding to DNA, and using the expression of specific protein markers for cell fate indicated no effect on the expression of stress-sensitive channel components involved in cell volume regulation, weak inhibition of cell proliferation, no increase in apoptosis, and even a reduction in autophagy. The complex is anionic, and the sodium complex had higher solubility compared to the potassium. As the molybdenum complex possibly enters the mitochondria, it is considered as a promising remedy to limit mitochondrial cyanide poisoning following, e.g., smoke inhalation injuries.

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.