Blood levels of oxime and symptoms in humans after single and multiple oral doses of 2-pyridine aldoxime methochloride

Chemical warfare. Nerve agent poisoning

The threat of civilian and military casualties from nerve agent exposure has become a greater concern over the past decade. After rapidly assessing that a nerve agent attack has occurred, emphasis must be placed on decontamination and protection of both rescuers and medical personnel from exposure. The medical system can become rapidly overwhelmed and strong emotional reactions can confuse the clinical picture. Initially, care should first be focused on supportive care, with emphasis toward aggressive airway maintenance and decontamination. Atropine should be titrated, with the goal of therapy being drying of secretions and the resolution of bronchoconstriction and bradycardia. Early administration of pralidoxime chloride maximizes antidotal efficacy. Benzodiazepines, in addition to atropine, should be administered if seizures develop. Early, aggressive medical therapy is the key to prevention of the morbidity and mortality associated with nerve agent poisoning.

The acetylcholinesterase oxime reactivator HI-6 in man: pharmacokinetics and tolerability in combination with atropine

In a double-blind, placebo-controlled, single-dose ascending pharmacokinetics and tolerance study, we evaluated the bispyridinium oxime HI-6 dichloride monohydrate (62.5, 125, 250, and 500 mg), administered intramuscularly with atropine sulphate, 2 mg, in 24 healthy male volunteers. The plasma HI-6 peak concentration (Cmax) and area under the concentration-time curve (AUC) demonstrated linear pharmacokinetics with low intradose variability, suggestive of uniformity of effect among subjects. HI-6 (500 mg) attained plasma drug concentrations that appeared adequate for practical use as an antidote. The mean +/- SD time to maximum plasma HI-6 concentration (tmax = 0.69 +/- 0.21 h, n = 16), and absorption half-life (t/2a = 0.17 +/- 0.05 h) indicated rapid onset of effect. The volume of distribution (Vd = 0.25 +/- 0.04 L kg-1 TBW) approximated the extracellular fluid volume. A high total body clearance (CL = 252 +/- 52 mL min-1) and short apparent elimination half-life (t/2e = 1.15 +/- 0.19 h) were expected for this polar quaternary ammonium drug. The renal clearance CLr = 137 +/- 33 mL min-1), which approximated the expected glomerular filtration rate, and 24 h urinary excretion of unchanged drug (55 +/- 10%) indicated substantial non-renal elimination. Blood pressure, heart rate, respiratory rate, electrocardiographic parameters, mental acuity, and vision were not altered. Adverse events and changes in serum, urine, and semen laboratory tests were mild. The pharmacokinetics, safety, and apparent efficacy of HI-6 suggest it may be a superior oxime antidote against nerve agent poisoning.

Plasma concentrations of pralidoxime methylsulphate in organophosphorus poisoned patients

Using pharmacokinetic data from healthy human volunteers in a bicompartmental pharmacokinetic model, a repeated dose scheme for pralidoxime methylsulphate (Contrathion) was developed producing plasma levels remaining above the assumed "therapeutic concentration" of 4 mg.l-1. Using the same data, it was found that a concentration of 4 mg.l-1 could also be obtained by a loading dose of 4.42 mg.kg-1 followed by a maintenance dose of 2.14 mg.kg-1.h-1. In order to study the pharmacokinetic behaviour of pralidoxime in poisoned patients, this continuous infusion scheme was then applied in nine cases of organophosphorus poisoning (agents: ethyl parathion, ethyl and methyl parathion, dimethoate and bromophos), and the pralidoxime plasma levels were determined. The mean plasma levels obtained in the various patients varied between 2.12 and 9 mg.l-1. Pharmacokinetic data were calculated, giving a total body clearance of 0.57 +/- 0.27 l.kg-1.h-1 (mean +/- SD), an elimination half-life of 3.44 +/- 0.90 h, and a volume of distribution of 2.77 +/- 1.45 l.kg-1.

Use of pralidoxime-Pd(II) complex for the spectrophotometric determination of the cholinesterase reactivator (PAM-2Cl) in the urine

A simple and rapid method for the spectrophotometric determination of pralidoxime chloride in urine, based on complex formation with palladium(II) without preliminary mineralization of the sample, is described. The proposed method is shown to be reproducible and in good agreement with a reference method, which involved spectrophotometric determination of corresponding oximate ions in ammonium hydroxide solution at 336 nm. Our results indicate that the proposed method is reliable, rapid and of sufficient sensitivity for pralidoxime chloride analysis in urine.

Oral kinetics and bioavailability of the cholinesterase reactivator HI-6 after administration of 2 different formulations of tablets to dogs

A one-compartment open model with first-order absorption was used for comparing new oral formulations of the potent acetylcholinesterase reactivating oxime HI-6. Although mean peak plasma levels did not differ between retard and conventional tablets (21.38 and 20.74 mumol/l), the time for reaching peak levels was significantly longer (5.5 h) with retard than with conventional tablets (2.86 h). Among other pharmacokinetic estimates only absorption half-lives and areas under the concentration-time curve (AUC) were significantly different (P less than 0.05). The AUC with retard tablets was 8.07% and that of conventional tablets 5.42% of intravenous AUC, indicating low bioavailability of oral HI-6 formulations. Potential therapeutic use of HI-6 requires, therefore, further investigations in order to improve its gastrointestinal absorption.

Therapeutic dosing of pralidoxime chloride

Pralidoxime chloride is a useful agent in the treatment of organophosphate poisoning. Poisindex, a widely used poisoning treatment resource, recommends dosing pralidoxime chloride as an intermittent iv infusion every 8-12 hours, whereas other authors have used continuous iv infusion with good results. Available animal data suggest that a serum concentration of 4 micrograms/ml may be a minimal level to protect against the toxic effects of organophosphates. Pharmacokinetic simulations, based on parameters obtained from healthy nonpoisoned subjects, show that pralidoxime levels fall rapidly to less than 4 micrograms/ml within 1.5-2 hours after a 1-g iv bolus. Continuous iv infusion (0.5 g/h) maintains pralidoxime levels greater than 4 micrograms/ml throughout the length of infusion. We conclude that continuous iv infusion of pralidoxime chloride may be the preferred method of administration in patients with acute organophosphate poisoning. Clinical trials will be necessary to document the effectiveness of this regimen.

Pharmacokinetics of pralidoxime chloride in the rat

The pharmacokinetics of pralidoxime chloride (2-PAM) was studied in rats. Different groups of rats were given an intramuscular injection of 2-PAM at one of three doses (20, 40, or 80 mg/kg). This range of doses is used commonly in studies concerned with the efficacy of 2-PAM against poisoning by potent organophosphorus inhibitors of cholinesterase enzyme. Individual, sequential blood samples were collected during the course of the experiment. From these blood samples the plasma concentrations of 2-PAM were determined over time for each animal. Next the relationship of plasma concentration to time was expressed in terms of a standard pharmacokinetic model. Estimates of various pharmacokinetic parameters were calculated using an open, one-compartment model: volume of distribution (Vd), maximal plasma concentration (Cmax), elimination rate constant (k10), absorption rate constant (k01), area under the curve (AUC) and clearance (CL). Of the pharmacokinetic estimates, only Cmax and AUC were found to be statistically significant (p less than 0.0001) when compared across all the doses; these pharmacokinetic estimates were highly correlated with doses with r = 0.998 and r = 0.997, respectively. However, when AUC and Cmax were normalized by dividing through by dose, no significant differences were found in the transformed data. The results of this study in rat indicate that the pharmacokinetics of 2-PAM is linearly related to dose in a range employed in therapeutic studies of 2-PAM.

Treatment of acute organophosphate poisoning: evidence of a direct effect on central nervous system by 2-PAM (pyridine-2-aldoxime methyl chloride)

Management of acute organophosphate poisoning in man includes rapid treatment with atropine and oximes. Oximes are thought to be unable to enter the central nervous system. We describe a case of parathion poisoning in a 3-1/2 year-old child and the effect of treatment with oxime 2-PAM (34 mg/kg) on EEG activity and clinical symptoms. The prompt improvement of cortical electrical activity documented by EEG could not be explained by any improvement of circulatory or respiratory function and has to be considered a direct effect of oximes on the central nervous system.

Effect of N-methylpyridinium-2-aldoxime methane sulphonate (P2S) on rat intestinal, (Na-K)ATPase and adenyl cyclase activities

The effect of N-methylpyridinium-2-aldoxime methane sulphonate (P2S), a drug recommended for prophylactic and therapeutic purposes in organophosphate poisoning, on intestinal (Na-K) ATPase and adenyl cyclase activities, was tested in rats. Intestinal (Na-K) ATPase activity was determined 5 h after intragastric administration of either 0.15 M NaCl or P2S 200 mg/kg body weight. P2S decreased significantly jejunal and colonic (Na-K)ATPase activity, 17.1 +/- 4.8 (S.E.) and 13.5 +/- 3.0, as compared to that in saline-treated rats, 41.5 +/- 3.0 (S.E.) and 25.4 +/- 1.2 mumol Pi/mg protein per h, respectively. Pretreatment with methyl prednisolone did not prevent the decrease in enzyme activity induced by P2S. Mucosal PGE2 and cAMP contents, adenyl cyclase and phosphodiesterase activities, were similar in P2S and saline-treated rats. It is thus suggested that P2S-induced inhibition of intestinal (Na-K)ATPase activity might be among the mechanisms contributing to looseness of the stool frequently observed following P2S administration.

Kinetics of absorption and elimination of pralidoxime chloride in dogs

The kinetics of the absorption and elimination of pralidoxime chloride were investigated in the dog. Similar apparent elimination rate constants were obtained after intravenous, intramuscular, and oral administration. Although oral absorption occurred slowly, intramuscular absorption proceeded rapidly. With in situ techniques, it was found that no absorption occurred from the isolated stomach and duodenum but that absorption did take place from the jejunum and ileum.