Formation of ferrihaemoglobin with aminophenols in the human for the treatment of cyanide poisoning

Past, present and future of cyanide antagonism research: From the early remedies to the current therapies

This paper reviews milestones in antidotal therapies for cyanide (CN) spanning early remedies, current antidotal systems and research towards next generation therapies. CN has been a part of plant defense mechanisms for millions of years. It became industrially important in the nineteenth century with the advent of CN assisted gold mining and the use of CN as a pest control agent. The biochemical basis of CN poisoning was actively studied and key mechanisms were understood as early as 1929. These fundamental studies led to a variety of antidotes, including indirect CN binders that generate methemoglobin, direct CN binders such as hydroxocobalamin, and sulfur donors that convert CN to the less toxic thiocyanate. Research on blood gases at the end of the twentieth century shed new light on the role of nitric oxide (NO) in the body. The discovery of NO’s ability to compete with CN for enzymatic binding sites provided a previously missed explanation for the rapid efficacy of NO generating antidotes such as the nitrites. Presently used CN therapies include: methemoglobin/NO generators (e.g., sodium nitrite, amyl nitrite, and dimethyl aminophenol), sulfur donors (e.g., sodium thiosulfate and glutathione), and direct binding agents [(e.g., hydroxocobalamin and dicobalt salt of ethylenediaminetetraacetic acid (dicobalt edetate)]. A strong effort is being made to explore novel antidotal systems and to formulate them for rapid administration at the point of intoxication in mass casualty scenarios. New antidotes, formulations, and delivery systems are enhancing bioavailability and efficacy and hold promise for a new generation of improved CN countermeasures.

Which cyanide antidote?

Cyanide has several antidotes, with differing mechanisms of action and diverse toxicological, clinical, and risk-benefit profiles. The international medical community lacks consensus about the antidote or antidotes with the best risk-benefit ratio. Critical assessment of cyanide antidotes is needed to aid in therapeutic and administrative decisions that will improve care for victims of cyanide poisoning (particularly poisoning from enclosed-space fire-smoke inhalation), and enhance readiness for cyanide toxic terrorism and other mass-casualty incidents. This paper reviews preclinical and clinical data on available cyanide antidotes and considers the profiles of these antidotes relative to properties of a hypothetical ideal cyanide antidote. Each of the antidotes shows evidence of efficacy in animal studies and clinical experience. The data available to date do not suggest obvious differences in efficacy among antidotes, with the exception of a slower onset of action of sodium thiosulfate (administered alone) than of the other antidotes. The potential for serious toxicity limits or prevents the use of the Cyanide Antidote Kit, dicobalt edetate, and 4-dimethylaminophenol in prehospital empiric treatment of suspected cyanide poisoning. Hydroxocobalamin differs from these antidotes in that it has not been associated with clinically significant toxicity in antidotal doses. Hydroxocobalamin is an antidote that seems to have many of the characteristics of the ideal cyanide antidote: rapid onset of action, neutralizes cyanide without interfering with cellular oxygen use, tolerability and safety profiles conducive to prehospital use, safe for use with smoke-inhalation victims, not harmful when administered to non-poisoned patients, easy to administer.

The reaction of cell-free oxyhemoglobin with nitrite under physiologically relevant conditions: Implications for nitrite-based therapies

Nitric oxide (NO*) participates in the regulation of a wide array of biological processes and its deficit contributes to the severity of many diseases. Recently, a role of NO deficiency that occurs as a result of intravascular hemolysis and increases in levels of cell-free hemoglobin in the pathway of chronic anemic pathologies has been suggested. Experimental evidence for deoxyhemoglobin-catalyzed reduction of nitrite to NO* leads to the possibility of nitrite infusion-based therapies to correct NO* deficits. However, the presence of plasma hemoglobin also raises the possibility of deleterious free radical-mediated oxidative damage from the reaction between nitrite and oxyhemoglobin in the vasculature. We show that the conditions required for the reaction between nitrite and oxyhemoglobin to exhibit free radical-mediated autocatalytic kinetics are highly unlikely to occur in the plasma compartment, even during extensive hemolysis and with pharmacological nitrite doses. Although the presence of haptoglobin enhances the rate of the reaction between nitrite and oxyhemoglobin, common plasma antioxidants-ascorbate and urate, as well as catalase-prevent autocatalysis. Our findings suggest that pharmacological doses of nitrite are unlikely to cause free radical or ferrylhemoglobin formation in plasma originating from the reaction of nitrite with cell-free oxyhemoglobin in vivo.

Cyanides

Cyanide is a likely weapon for terrorists due to its notoriety, lethality, and availability. Poisoning results in central nervous system and cardiovascular dysfunction due to inhibition of oxidative phosphorylation. Laboratory findings of anion gap metabolic acidosis and hyperlactemia aid in confirming the diagnosis. Treatment for significant poisonings includes aggressive supportive care and administration of antidotes such as sodium nitrite, sodium thiosulfate, and hydroxocobalamin. Survivors of significant poisonings can have long-term neurologic dysfunction.

Medical management of incidents with chemical warfare agents

Successful management of incidents with chemical warfare agents strongly depends on the speed of medical help and the ability of helpers to react properly. Though the general principles of clinical toxicology, such as decontamination, stabilization, patient evaluation and symptomatic treatment are similar for many toxicants, chemical warfare agents deserve special attention because of their very high inhalative and cutaneous toxicity, rapid onset of the disease and multiple organ failures. This article describes the medical management of mass casualties with blister agents, nerve agents and blood agents from the viewpoint of a clinical toxicologist. Characteristic diagnostic signs, decontamination procedures and therapeutic schemes for these agents are described. Treatment options are discussed. The importance of planning (e.g. antidote availability) and preparedness is emphasized.

Hyperoxia in lethal methemoglobinemia: effects on oxygen transport, tissue oxygenation, and survival in pigs

BACKGROUND

Treatment of severe methemoglobinemia includes the avoidance of methemoglobin-inducing drugs, the application of methylene blue, and the administration of supplementary oxygen. However, the efficacy of the latter on oxygen transport, tissue oxygenation, and survival in the treatment of extreme methemoglobinemia is ambiguous. The objective was to assess whether using hyperoxic ventilation as the sole therapeutic intervention (i.e., ventilation with pure oxygen, Fio2 1.0) improves the short-term (6-hr) survival rate during otherwise lethal methemoglobinemia.

DESIGN

Prospective, randomized, controlled study.

SETTING

Experimental animal laboratory of a university hospital.

SUBJECTS

Fourteen anesthetized, mechanically ventilated pigs.

INTERVENTIONS

After induction of profound methemoglobinemia (60 +/- 2%) by the injection of 15 mg/kg 4-dimethylaminophenol, artificial ventilation either was continued with room air (G 0.21, n = 7) or was changed over to hyperoxic ventilation (G 1.0, n = 7). A constant level of methemoglobinemia was maintained by continuous infusion of 4-dimethylaminophenol throughout a 6-hr follow-up period.

MEASUREMENTS AND MAIN RESULTS

All animals died within the 6-hr follow-up period, but survival time was prolonged in animals ventilated with pure oxygen (G 0.21, 105 +/- 30 mins; G 1.0, 210 +/- 64 mins, p < .05). No differences were encountered between G 0.21 and G 1.0 with respect to the investigated variables of macrohemodynamics, oxygen transport, and tissue oxygenation.

CONCLUSIONS

Hyperoxic ventilation has negligible effects on oxygen transport and tissue oxygenation during lethal methemoglobinemia; nevertheless, survival was increased without severe adverse reactions provoked by hyperoxic ventilation.

Oscillatory oxido-reductive reaction of intracellular hemoglobin in human erythrocyte incubated with o-aminophenol

When human erythrocytes were incubated with o-aminophenol at pH 7.0 at 37 degrees C for 46 hours, intracellular oxyhemoglobin was completely oxidized to methemoglobin during the initial 6 hours, and methemoglobin formed was then reduced to oxyhemoglobin during the following 20 hours. This was demonstrated by the changes in absorption spectra of intracellular hemoglobin. Such oscillatory behavior of intracellular hemoglobin during reaction with o-aminophenol was explained by the fact that o-aminophenol has the ability to both oxidize oxyhemoglobin and reduce methemoglobin. In order to study the mechanism of oxido-reductive reactions of hemoglobin with aromatic reductants including o-aminophenol, the oxidation of ferrous hemoglobin and reduction of methemoglobin with various aromatic reductants such as o-aminophenol, 2-amino-4-methyl-phenol, 2-amino-5-methylphenol, and homogentisic acid were investigated under various conditions. It was found that oxyhemoglobin was oxidized by these aromatic compounds, and the oxidation rate was accelerated in the presence of inositol hexaphosphate, but was not affected in the presence of catalase and superoxide dismutase, except for the case with homogentisic acid. The oxidation of ferrous hemoglobin by these compounds did not proceed under anaerobic conditions. Methemoglobin was reduced by these aromatic compounds, and the reduction rate was much accelerated in the presence of inositol hexaphosphate, but was not affected in the presence of catalase and superoxide dismutase, except for the case with homogentisic acid. The reduction of methemoglobin by these compounds proceeded under anaerobic conditions, suggesting that ferric heme of hemoglobin reacts directly with aromatic reductants. On the basis of these results, the mechanism of oxido-reductive reaction of ferrous and ferric hemoglobin with aromatic reductants was proposed.

Failure of intravenous N-acetylcysteine to reduce methemoglobin produced by sodium nitrite in human volunteers: A randomized controlled trial

STUDY OBJECTIVE

To determine whether intravenous N -acetylcysteine (NAC) produces a clinically significant decline in sodium nitrite-induced methemoglobinemia in human volunteers.

METHODS

We conducted a randomized, control crossover trial with each subject serving as his own control. Methemoglobinemia was induced with intravenous sodium nitrite (4 mg/kg) administered over 10 minutes starting at time 0. At time 30 minutes, subjects were randomly assigned to treatment with intravenous NAC for 100 minutes (150 mg/kg over 1 hour followed by 14 mg/kg per hour for 40 minutes) or administration of an equal volume of 5% dextrose in water. Each subject received the alternative treatment after an interval of at least 1 week. Blood methemoglobin concentrations were measured by multiwavelength co-oximetry at time 0, 15, 30, 50, 70, 90, 110, and 130 minutes. Area under the methemoglobin concentration-time curve (AUC) between 30 and 130 minutes was compared between groups using a 2-tailed, paired t test.

RESULTS

There were no statistically significant differences in the control and treatment groups with respect to baseline hemoglobin or methemoglobin concentrations, as well as nitrite-induced methemoglobin concentrations at the initiation of treatment (0.85+/-0.06 g/dL, 0.88+/-0.04 g/dL; mean+/-SEM; P =.31). Mean AUC for the control group (77.1+/-5.7 g x min/dL) was significantly lower than the mean AUC for the treatment group (84.5+/-4.7 g x min/dL); P =.01).

CONCLUSION

Intravenous NAC failed to enhance methemoglobin reduction in this model.

Comparison of hematologic consequences and efficacy of p-aminophenones in mice

Controlled methemoglobin (MHb) formation is one strategy employed to counter cyanide (CN) toxicity. Currently available MHb formers present certain drawbacks and limitations. The purpose of this study was to characterize, in mice, the hematologic effects of the MHb-forming compound p-aminopropiophenone (PAPP), and two structurally-related p-aminophenones, p-aminoheptanoylphenone (PAHP) and p-aminooctanoylphenone (PAOP). Although these three p-aminophenones have been shown previously to be efficacious as pretreatments against CN, a more complete understanding of their hematologic effects is lacking. In addition, because the active form of PAPP has been shown to be its N-hydroxy metabolite, the N-hydroxy metabolites of PAPP, PAHP and PAOP were also tested. Using a hemoximeter, blood samples obtained -2 to +180 min relative to intramuscular (i.m.) or intraperitoneal (i.p.) drug injections were evaluated. Sodium nitrite (NaNO(2)) and the appropriate solvents served as the positive and negative controls, respectively. Dose-, time-, route-, and compound-related effects were observed. MHb and sulfhemoglobin levels increased, whereas levels of those parameters related to oxygen-carrying capacity of the blood, such as, oxygen saturation and oxyhemoglobin decreased. In general, the effects of PAHP and PAOP were longer lasting than those of PAPP and NaNO(2). Furthermore, PAPP and NaNO(2) were equally effective with either route of administration. Conversely, PAHP and PAOP showed larger effects when administered i.p. versus i.m. The animals treated with N-hydroxy metabolites of the p-aminophenones also showed similar changes in the hematological parameters measured. N-hydroxy PAPP was shown to be the most rapidly acting MHb-forming compound examined in this series. It could achieve therapeutic concentrations of MHb within 2 min and thus may be considered as a treatment for CN intoxication. Although additional work is needed, these data provide information that will be useful for the successful development of improved anti-CN MHb formers.

Anonlethal, Anesthetized Canine Model for Efficacy Evaluation of Anticyanide Therapy

Efficacy evaluations of anticyanide therapeutic compounds historically have used lethality in unanesthetized animals as the toxic endpoint, and a nonlethal, repeated-testing, anesthetized canine model has been reported. Time to respiratory arrest (TRA) induced by a continuous, slow intravenous (IV) infusion of sodium cyanide (Na CN) was shown to be a consistent, well-defined endpoint in anesthetized canines. Thirty seconds after respiratory arrest (RA), an IV bolus of a methemoglobin-forming compound reversed the respiratory effects. This study was designed to determine the variability in TRA, survivability of anesthetized dogs repeatedly infused with Na CN and treated with hydroxylamine, replicability of the procedure within an animal and between animals, and development of trends in data. Four animals were anesthetized, intubated, catheterized, and instrumented to record respiratory rate and heart rate. Immediately following baseline data collection, a 4-mgl m L. Na CN solution w as infused at a rate of 2 m Llmin. Following a 10-s period without functional respiration, infusion was stopped and this moment was designated as RA. Hydroxylamine therapy was administered 30 s after RA. There were 4 replicate experiments per animal with a 1-week washout period between replicates. Interanimal variability in TRA was significant (p = <.04). The average variability in TRA within an animal was less than 10%. Baseline heart and respiratory rates were not altered significantly. Data confirm, by replicate testing, the utility of this model.

Evaluation of methemoglobin as an autologous intravascular MRI contrast agent

Methemoglobin (MetHb) was evaluated as an intravascular paramagnetic contrast agent. Methemoglobin formation was induced by 4-dimethylaminophenol (4-DMAP), causing a reduction in blood T2* in vitro. The 4-DMAP generated metHb with a time constant of 62 s. A 4-DMAP bolus did not decrease measurably the signal intensity in the in vivo rabbit kidney in the first pass. At steady state, a MetHb concentration of 24.8 +/- 2.3% resulted in a signal decrease of 9.2 +/- 2.6% in the kidney. Methemoglobin is an effective vascular T2* relaxation agent, but the formation of MetHb by 4-DMAP is too slow for first-pass imaging. A more effective conversion agent resulting in a bolus of at least 25% MetHb within 5 s would result in a detectable first-pass signal and a viable contrast technique.

Cyanide toxicity in mice pretreated with diethylamine nitric oxide complex

1. Since the literature suggested a portion of the overall toxicity of cyanide (CN) may be affected by nitric oxide, we investigated a long acting NO releasing complex (diethylamine/nitric oxide (DEA/NO)) which may exhibit vasodilatory as well as other nitric oxide effects to determine its ability to modify CN toxicity. Sodium nitrite, a vasodilator commonly used to treat cyanide toxicity thought to act by methemoglobin (MHb) formation, can be rapidly transformed to nitric oxide (NO). 2. Mice (n = 10 per dose) were administered one of five doses of sodium cyanide (NaCN) intraperitoneally (4.28, 5.08, 6.03, 7.17 and 8.52 mg kg-1). DEA/NO was given intravenously (20 mg kg-1) 2 min prior to NaCN. As a control, NG-monomethyl-L-arginine (L-NMMA), which inhibits NO synthesis, was administered intravenously (70 mg kg-1) to mice, 3 min prior to DEA/NO. 3. Before CN toxicity studies, we determined whether DEA/NO was producing MHb by collecting tail vein blood from mice and measuring MHb levels. For example, 4 min after DEA/NO administration (5, 10, and 20 mg kg-1), MHb levels were 1.27 +/- 0.28%, 2.60 +/- 0.26% and 6.53 +/- 0.54% respectively. O2 capacity was also decreased in a dose related manner. Carboxyhemoglobin or percent O2 saturation, on the other hand, was not significantly inhibited. The LD50 increased from 5.75 +/- 0.026 (CN alone) to 7.66 +/- 0.021 mg kg-1 (CN+DEA/NO) resulting in a protective ratio of 1.73. 4. Results suggest the following: (1) L-NMMA, which inhibits the synthesis of endogenous NO, appears to exacerbate the DEA/NO (or exogenous NO) response; (2) DEA/NO appears to reduce the toxicity of CN which suggests that a portion of CN toxicity may be affected by a NO component; and (3) low DEA/NO doses may act via a direct effect while higher doses (40 mg kg-1) may allow for formation of a concentration of MHb which can bind CN to form cyanomethemoglobin and reduce the toxicity of CN.

Therapeutic efficacy of sodium nitrite and 4-dimethylaminophenol or hydroxylamine co-administration against cyanide poisoning in rats

1. The therapeutic efficacy of combined treatment of sodium nitrite (NaNO2) and 4-dimethylaminophenol (DMAP) or hydroxylamine (H2NOH) was investigated in potassium cyanide (KCN) intoxication in male rats. 2. Therapy with NaNO2 (0.27 mmol kg-1) + DMAP (0.09 mmol kg-1) or NaNO2 + H2NOH (0.09 mmol kg-1, produced a protection index (ratio of LD50 of KCN in rats receiving therapy to an LD50 of KCN in rats given only 0.9% saline) of 2.5 and 2.0 respectively. 3. Both the regimens exhibited a beneficial effect in terms of improving the survival time and postural defects in rats exposed to 2 LD50 KCN. 4. NaNO2 + DMAP showed a significant protective effect in the disposition of the plasma cyanide level at different time intervals. 5. The NaNO2 + DMAP regimen was superior to NaNO2 + N2NOH in terms of reactivating the inhibited brain cytochrome oxidase enzyme. 6. The addition of sodium thiosulphate (Na2S2O3) in both the regimens increased the degree of protection. 7. The results suggest that combined therapy with NaNO2 + DMAP could significantly reduce the toxic effects of cyanide, compared with NaNO2+H2NOH treatment.

Cyanide and methemoglobin kinetics in smoke inhalation victims treated with the cyanide antidote kit

STUDY OBJECTIVE

To evaluate serial cyanide, methemoglobin, and carbon monoxide levels in smoke inhalation patients.

SETTING

Regional poison center and regional toxicology treatment center.

PARTICIPANTS

Seven critically ill smoke inhalation patients referred to the regional poison center.

INTERVENTIONS

Peak level and half-life were determined by obtaining serial carboxyhemoglobin, cyanide, and methemoglobin levels.

RESULTS

The mean observed half-life of cyanide was 3.0 +/- 0.6 hours. Methemoglobinemia was evaluated in four patients after sodium nitrite administration. The peak measured methemoglobin levels (mean, 10.5% +/- 2%; range, 7.9% to 13.4%) did not occur until a mean of 50 minutes (range, 35 to 70 minutes) following administration of sodium nitrite. The total oxygen-carrying capacity reduced by the combination of carboxyhemoglobin and methemoglobin was never more than 21% (range, 10% to 21%) in this series.

CONCLUSION

The administration of sodium nitrite to smoke inhalation patients in the presence of concomitant carbon monoxide poisoning may be relatively safe.

The antidotal action of sodium nitrite and sodium thiosulfate against cyanide poisoning

The combination of sodium thiosulfate and sodium nitrite has been used in the United States since the 1930s as the primary antidote for cyanide intoxication. Although this combination was shown to exhibit much greater efficacy than either ingredient alone, the two compounds could not be used prophylactically because each exhibits a number of side effects. This review discusses the pharmacodynamics, pharmacokinetics, and toxicology of the individual agents, and their combination.

Nitrates, nitrites and N-nitrosocompounds: a review of the occurrence in food and diet and the toxicological implications

Data on occurrence of nitrate, nitrite and N-nitrosocompounds in food and drinking water, and on total dietary intakes are reviewed. Metabolic, toxicological and epidemiological studies are surveyed and the implications with respect to safety evaluation are addressed. It is concluded that, on the basis of recent long-term animal studies and of clinical experience in man, the current Acceptable Daily Intake (ADI) allocated to nitrate by the Joint FAO/WHO Expert Committee on Food Additives of 0-5 mg/kg body weight/day (expressed as sodium nitrate) might be increased to 0-25 mg/kg body weight/day. Based on similar criteria, the ADI for nitrite would be 0-0.1 mg/kg body weight/day (expressed as sodium nitrite). In view of the known carcinogenicity of N-nitrosocompounds, exposure to these compounds in food should be minimized by appropriate technological means, such as lowering the nitrite concentration in preserved foods to the minimum required to ensure microbiological safety and use of inhibitors of nitrosation like alpha-tocopherol or ascorbic acid. Further work is needed to define the minimal levels of nitrite in foods needed to inhibit outgrowth of Clostridium botulinum and toxin production.

Activation and detoxication of aminophenols. II. Synthesis and structural elucidation of various thiol addition products of 1,4-benzoquinoneimine and N-acetyl-1,4-benzoquinoneimine

1. Nine thioethers of 4-aminophenol with beta-hydroxyethylmercaptan, ranging from mono- to tetra-substituted thioadducts, were prepared from synthetic 1,4-benzoquinoneimine and characterized by 1H-n.m.r. and u.v. spectroscopy. For each compound, extinction coefficients and pKa values of the amino group were determined. 2. Five thioethers of 4-aminophenol with glutathione (GSH) were prepared and characterized by 1H-n.m.r. and u.v. spectroscopy with their respective extinction coefficients and pKa values. Two further thioadducts were tentatively assigned by their u.v. spectroscopic properties. 3. Reaction products of 1,4-[U-14C]benzoquinoneimine and GSH were studied, indicating formation of 4-amino-2-(glutathione-S-yl)phenol, 4-amino-2,3,6-tris(glutathione-S-yl)phenol as the main products. Formation of glutathione disulphide (GSSG) was not detected. In contrast, N-acetyl-1,4-[U-14C]benzoquinoneimine was partly reduced by GSH and formed only the 2-substituted thioadduct. 4. Investigation of the product orientation in the reductive addition of GSH to 2-(glutathione-S-yl)-1,4-benzoquinoneimine and 3-(glutathione-S-yl)-1,4-benzoquinoneimine, respectively, showed that the 3-substituted derivative formed mainly the 3,5-di-substituted thioadduct, whereas the 2-substituted compound formed mainly the 2,3,6-tri-substituted thioadduct. 5. Formation of thioadducts which autoxidize markedly faster than the parent aminophenol indicates that thioether formation is not an obligatory detoxication process.

Depletion of mitochondrial coenzyme A and glutathione by 4-dimethylaminophenol and formation of mixed thioethers

4-Dimethylaminophenol (DMAP), an antidote in cyanide poisoning, has been shown to produce kidney lesions in rats, to damage isolated rat kidney tubules and to impair mitochondrial functions as already described for 4-aminophenol. Since DMAP upon oxidation forms bis- and tris-substituted thioethers with GSH, it was anticipated that mitochondrial toxicity of DMAP might result from CoA depletion. In a model reaction DMAP was oxidized by oxyhemoglobin in the presence of CoA and GSH resulting in formation of tris-(CoA-S-yl)-DMAP, tris-(GSH-S-yl)-DMAP and two mixed thioethers, namely, (CoA-S-yl)-bis-(GSH-S-yl)-DMAP and (GSH-S-yl)-bis-(CoA-S-yl)-DMAP. The compounds were isolated by HPLC and identified spectroscopically, by amino acid analysis and Raney-Nickel desulfuration. Rat liver mitochondria (5 mg protein/ml) incubated under state IV conditions with 20 and 50 microM DMAP were depleted of GSH and total coenzyme A with formation of GSSG and the above-mentioned thioethers which were quantified by isotope dilution techniques using [14C]-labelled DMAP and the isolated, inactive thioethers. The results confirm earlier suggestions that part of the cytotoxicity of DMAP may result from depletion of vital mitochondrial thiols, particularly CoA. Since 4-aminophenol reacts analogously, similar cytotoxic effects can be expected from compounds which on (aut)oxidation form quinoid systems capable of 1.4-addition reactions with nucleophilic thiols.

Ferrihaemoglobin formation by amyl nitrite and sodium nitrite in different species in vivo and in vitro

The ferrihaemoglobin (HbFe3+) formation by amyl nitrite (AN) or sodium nitrite (NaNO2) was studied in different species including man, in vivo and in vitro. In in vivo studies AN was administered intravenously (i.v.), intramuscularly (i.m.), by inhalation, or orally. NaNO2 was injected i.v.. AN i.v. produced HbFe3+ much more rapidly than NaNO2 in dogs, cats, rabbits, and rats. In dogs, i.m. injection of AN was followed by a very slow linear increase in the HbFe3+ content. Inhalation of AN did not lead to HbFe3+ formation in dogs unless it was rebreathed in a closed (bag) or not completely open (gas mask) system. HbFe3+ was produced by oral AN in dogs, the effect being enhanced by addition of DMSO. Inhalation of AN by human volunteers in a gas mask and from ampoules crushed close to the nose did not induce haemoglobin oxidation to a practically significant extent, but it was associated with headache, tiredness, dizziness, and a fall in blood pressure. In in vitro studies, in contrast to NaNO2, AN produced HbFe3+ instantaneously in erythrocytes of various species and in purified human haemoglobin. AN 1 mol yielded 2 mol Fe3+. Only 20% of the oxygen released during the oxidation of haemoglobin by AN or NaNO2 was recovered. In 0.2 M phosphate buffer, pH 7.4, 0.01 mol O2/mol AN was consumed. CO2 was released in the presence of AN, but not of NaNO2, from blood, plasma, and 0.02 M NaHCO3 solution. The ratio (lactate)/(pyruvate) decreased when HbFe3+ was formed by AN or NaNO2.

The effect of oxygen on in vitro studies on methemoglobin production in man and dog blood using 4-dimethylaminophenol

Methemoglobin production induced by the addition of 4-dimethylaminophenol to human or beagle blood in vitro is inhibited at high oxyhemoglobin levels. The effect is similar in the two species and probably results from conformational change in hemoglobin consequent on oxygen binding.

Effect on blood and plasma cyanide levels and on methaemoglobin levels of cyanide administered with and without previous protection using PAPP

Hydrogen cyanide was administered intravenously at doses of 0.67 or 1.34 mg kg-1 to beagle bitches after protection with oral p-aminopropiophenone (0.5 mg kg-1). Hydrogen cyanide was also administered to unprotected bitches at the lower level (0.67 mg kg-1) only. PAPP protection caused sequestration of cyanide inside the red cells. In the case of the lower dose of cyanide this resulted in a lower plasma cyanide in protected than unprotected bitches. In the case of the higher dose it resulted in survival, despite 1.34 mg kg-1 being a known lethal dose. It is concluded that prior administration of PAPP ameliorated the effects of cyanide poisoning.

Species differences in methaemoglobin production after addition of 4-dimethylaminophenol, a cyanide antidote, to blood in vitro: a comparative study

Methaemoglobin production after addition of DMAP to blood of various species, has been studied in vitro. The study was undertaken both with blood, as taken, and after equilibration with atmospheric oxygen. Considerable interspecies variation in methaemoglobin production was found. When the initial rate of methaemoglobin formation was considered only marmoset and human blood showed any marked degree of inhibition by equilibration with atmospheric oxygen.

Therapeutic problems in cyanide poisoning

In three patients with severe acute cyanide poisoning, a cyanosis was observed instead of the bright pink skin coloration often mentioned as a sign in textbooks. Treatment of cardiopulmonary insufficiency is as essential as antidotal therapy and the use of sodium nitrite and 4-DMAP is not without risk as, in practice, the methemoglobin-level induced is difficult to control.

Interspecies differences in rhodanese (thiosulfate sulfurtransferase, EC 2.8.1.1) activity in liver, kidney and plasma

Rhodanese levels have been measured in liver, kidney and plasma from a number of species. Liver activity was low in marmosets, pigeons and beagle bitches. Levels were high in rats and somewhat lower in hamsters and guinea pigs while levels in two strains of rabbits were intermediate between guinea pigs and marmosets. The relationship between hepatic and plasma rhodanese and cyanide sensitivity is discussed.

Kinetics of methaemoglobin production. (1). Kinetics of methaemoglobinaemia induced by the cyanide antidotes, p-aminopropiophenone, p-hydroxyaminopropiophenone or p-dimethylaminophenol after intravenous administration

Methaemoglobin profiles have been studied by using ISIS, a simulation package, and NONLIN, a non-linear least-squares analysis regression program. A simple kinetic model which satisfactorily describes methaemoglobin profiles after p-aminopropiophenone (PAPP) administration and 4-dimethylaminophenol (DMAP) administration has been developed. The two compounds differed mainly in their effective rates of elimination. The model less satisfactorily described methaemoglobin profiles after p-hydroxyaminopropiophenone (PHAPP) administration.

Comparison of nitrite treatment and stroma-free methemoglobin solution as antidotes for cyanide poisoning in a rat model

The standard nitrite/thiosulfate regimen for cyanide poisoning was tested in our rat model. By modifying the treatment regimen and the nitrite solution an effective antidote against an LD90 of cyanide could be produced. However, this treatment was effective against two times the LD90 only when administered ten minutes prior to cyanide injection. These results are in marked contrast to our results with stroma-free methemoglobin solutions (SFMS) which showed SFMS to be a highly effective antidote against four times the LD90 when administered 30 seconds after an intravenous injection of cyanide. SFMS proved to be an effective antidote for two times the LD90 when administered up to sixty seconds after the cessation of respiration.

Stroma-free methemoglobin solution: an effective antidote for acute cyanide poisoning

Several aspects of stroma-free methemoglobin solution (SFMS) as a cyanide antidote were investigated using a rat model. Stroma-free methemoglobin solution was more than 90% effective against multiples of the LD90 of cyanide up to and including four times the LD90 and approximately 50% effective against multiples up to and including eight times the LD90. Highly concentrated solutions of SFMS (33 g/dl) did not differ significantly from less concentrated solutions of SFMS (16 g/dl) when compared on the basis of efficacy. Administration of large doses of SFMS alone resulted in no apparent morbidity or mortality. It could be that SFMS is a safe and effective alternative antidote for the treatment of cyanide poisoning.

4-Dimethylaminophenol and dicobalt edetate (Kelocyanor) in the treatment of experimental cyanide poisoning

The efficacy of dicobalt edetate (Kelocyanor) was compared against that of 4-dimethylaminophenol (DMAP) in experimental cyanide poisoning. DMAP gave better survival. The efficacy has been related to the toxicity of the two compounds.

Plasma free cyanide and blood total cyanide: a rapid completely automated microdistillation assay

Techniques are presented which provide direct measurement of both free cyanide (CN-) in plasma and total CN- in whole blood. Loss of total CN- from blood is prevented by conversion to cyanmethemoglobin. Both free and total CN- are assayed by a completely automated method providing readout 17 minutes after sampling. No prior isolation technique is required and sensitivity is adjustable to cover a broad range of CN- concentrations from 1 to 4000 uM. Precision of blood CN- values from 2 to 2500 uM is within +/- 2.3%. No interference results from thiocyanate or thiosulfate at a concentration of approximately 1 mM.

2,4-Difluoroaniline and 4-fluoroaniline exposure: monitoring by methaemoglobin and urine analyses

Two possible methods for monitoring exposure to 2,4-difluoroaniline and 4-fluoroaniline have been investigated: measurement of methaemoglobin content in blood and measurement of urinary metabolites. Experiments using rats dosed by the oral route as a model system show that measurement of methaemoglobin content provides a very rapid and simple monitoring method, but is not very sensitive. Measurement of the ortho-hydroxy metabolites of the two compounds, as their benzoxazole derivatives, provides a much more sensitive, but complicated technique. Details of both methods are presented.

The acute intravenous and oral toxicity in mice, rats and guinea-pigs of 4-dimethylaminophenol (DMAP) and its effects on haematological variables

The single dose oral toxicity of 4- dimethylaminophenol (DMAP) was examined in mice, rats and guinea-pigs and the intravenous toxicity in mice. The oral LD50 in female mice was 946 mg kg-1, in male rats, 780 mg kg-1, in female rats 689 mg kg-1 and in female guinea-pigs 1032 mg kg-1. The intravenous LD50 in female mice was 70 mg kg-1. Dosing with DMAP at the maximum tolerated dose produced Heinz body haemolytic anaemia in rats and Heinz bodies unaccompanied by anaemia in mice. Rats showed biochemical and histological evidence of renal tubular changes.

Radical formation during autoxidation of 4-dimethylaminophenol and some properties of the reaction products

4-Dimethylaminophenol (DMAP), after intravenous injection, rapidly forms ferrihaemoglobin and has been successfully used in the treatment of cyanide poisoning. Since DMAP produces many equivalents of ferrihaemoglobin, it was of interest to obtain further insight into this catalytic process. DMAP autoxidizes readily at pH regions above neutrality, a process which is markedly accelerated by oxyhaemoglobin. The resulting red-coloured product was identified as the 4-(N,N-dimethylamino) phenoxyl radical by EPR spectroscopy. The same radical was also produced by pulse radiolysis and oxidation with ferricyanide. The 4-(N,N-dimethylamino)phenoxyl radical is quite unstable and decays in a pseudo-first order reaction (k = 0.4 sec-1 at pH 8.5, 22 degrees) with the formation of p-benzoquinone and dimethylamine. This observed decay rate is identical with the rate of hydrolysis of N,N-dimethylquinonimine. When a solution containing the phenoxyl radical was extracted with ether, half the stoichiometric amount of DMAP was recovered. Hence it is apparent that the phenoxyl radical decays by disproportionation yielding DMAP and N,N-dimethylquinonimine. The latter product then quickly hydrolyses. The equilibrium of this disproportionation reaction is far towards the radical side, and the pseudo-first order hydrolysis controls the radical decay rate. p-Benzoquinone rapidly reacts with DMAP (k2 = 2 X 10(4) M-1 sec-1) with the formation of the 4-(N,N-dimethylamino)phenoxyl and the semiquinone radicals. This reaction explains the autocatalytic phenoxyl radical formation during autoxidation of DMAP. DMAP is not oxidized by H2O2 or O-.2 but the 4-(N,N-dimethylamino)phenoxyl radical is very rapidly reduced by O-.2 (k2 = 2 X 10(8) M-1 sec-1). In addition, the phenoxyl radical is quickly reduced by NAD(P)H or GSH with the formation of NAD(P)+ or GSSG. Since DMAP is also able to reduce two equivalents of ferrihaemoglobin (provided that the ferrohaemoglobin produced is trapped by carbon monoxide), electrophilic addition reactions of the phenoxyl radical seem unimportant in contrast to N,N-dimethylquinonimine. Hence, during the catalytic ferrihaemoglobin formation, DMAP is oxidized by oxygen which is activated by haemoglobin, and the phenoxyl radical oxidizes ferrohaemoglobin. This catalytic process is terminated by covalent binding of N,N-dimethylquinonimine to SH groups of haemoglobin (and GSH in red cells).

Differences in the reactions of isomeric ortho- and para-aminophenols with hemoglobin

The metabolites of phenacetin, 2-hydroxyphenetidine and 4-nitrosophenetol, rapidly produced ferrihemoglobin both in vivo (dogs) and in vitro. At low concns, 2-hydroxyphenetidine was superior to 4-nitrosophenetol in ferrihemoglobin formation. The kinetics of ferrihemoglobin formation by 2-hydroxyphenetidine in solutions of purified human hemoglobin was biphasic and exhibited an unusual dose response. Similar to p-aminophenols, 2-hydroxyphenetidine was oxidized by oxyhemoglobin, and the oxidation product(s) were reduced by ferrohemoglobin with the formation of ferrihemoglobin. In addition, these oxidation products condensed to 2-amino-7-ethoxy-3H-phenoxazine-3-one (u.v., i.r., 1H-NMR and mass spectroscopy). This metabolite produced ferrihemoglobin by itself and was responsible for the slow phase of ferrihemoglobin formation observed with 2-hydroxyphenetidine. This condensation reaction, which was also observed with 2-aminophenol, prevented thioether formation of the transient o-quinonimines with the cysteine residues of hemoglobin and reduced glutathione as observed with 4-aminophenol and 4-dimethylaminophenol. Phenoxazone formation, which depends on the square of the o-quinonimine concn, was negligible at micromolar concns. At similar concns addition reactions to thiols prevailed also with 2-hydroxyphenetidine and 2-aminophenol. Other electrophilic reactions, e.g. with primary amino groups of amino acids, were insignificant. These dose-dependent differences in the reactions of isomeric aminophenols may explain the low nephrotoxicity of those o-aminophenols capable of forming phenoxazones when given in a single dose. This self-detoxication of some o-quinonimines, however, should not function during long-term exposure to repetitive low doses of such o-aminophenols.

The red cell as a sensitive target for activated toxic arylamines

During biotransformation of arylamines, activated phase I metabolites, like aminophenols and hydroxylamines, occasionally escape the liver and exert allergic, toxic or carcinogenic effects in sensitive target organs. The first organ in contact with these proximate toxic compounds is the blood where oxyhemoglobin activates proximate to ultimate toxic derivatives. Thereby hydroxylamines and oxyhemoglobin are co-oxidized to nitrosoarenes and ferrihemoglobin. Because of an enzymic cycle, severe methemoglobinemia can occur even with small, catalytic amounts of hydroxylamines. Reactive oxygen intermediates, if not eliminated enzymically, may be responsible for hemolysis, Heinz body formation, and green pigments. In addition, nitrosoarenes bind covalently to hemoglobin and membranes and deplete glutathione by formation of glutathione-sulfinamides. Aminophenols, on the other hand, have to be activated first by oxyhemoglobin to phenoxyl radicals and quinonimines, which are reduced back with simultaneous ferrihemoglobin formation. Hence, aminophenols catalytically transfer electrons from iron to oxygen. This catalytic cycle is terminated by side reactions: p-quinonimines form adducts with glutathione and hemoglobin. Thereby the physiological functions of hemoglobin can be greatly altered as shown for 4-dimethylaminophenol. o-Quinonimines either condense to the respective phenoxazones, or if condensation is hindered, they form adducts, mainly with thiols. The different pathways for o-aminophenols are concentration-dependent, with adduct formation being favoured at low concentrations. Thus, methemoglobin formation poorly correlates with the implications of reactive electrophilic intermediates.

Stroma-free methemoglobin solution as an antidote for cyanide poisoning: a preliminary study

Effective treatment of cyanide poisoning requires rapid diagnosis, good supportive treatment and the use of a specific antidote. The currently available antidotes offer demonstrated efficacy along with significant potential adverse side effects. We have investigated an alternate approach to antidote therapy for cyanide poisoning by using Stroma-Free Methemoglobin Solution ( SFMS ). Rats injected with an LD100 intravenous dose of cyanide were treated with SFMS equal to 1.5% of their total body hemoglobin. There was a highly significant increase in the survival rate of the treated group compared to saline controls. The potential advantages of SFMS over current antidotes include an immediate onset of action, rapid elimination of cyanide from the body and a mode of action that doesn't compromise any of the patients' oxygen carrying capacity. SFMS shows promise as a significant adjunct in the treatment of cyanide poisoning.