Renal tubular secretion of pralidoxime in man

Pharmacology, Pharmacokinetics, and Tissue Disposition of Zwitterionic Hydroxyiminoacetamido Alkylamines as Reactivating Antidotes for Organophosphate Exposure

In the development of antidotal therapy for treatment of organophosphate exposure from pesticides used in agriculture and nerve agents insidiously employed in terrorism, the alkylpyridinium aldoximes have received primary attention since their early development by I. B. Wilson in the 1950s. Yet these agents, by virtue of their quaternary structure, are limited in rates of crossing the blood-brain barrier, and they require administration parenterally to achieve full distribution in the body. Oximes lacking cationic charges or presenting a tertiary amine have been considered as alternatives. Herein, we examine the pharmacokinetic properties of a lead ionizable, zwitterionic hydroxyiminoacetamido alkylamine in mice to develop a framework for studying these agents in vivo and generate sufficient data for their consideration as appropriate antidotes for humans. Consequently, in vitro and in vivo efficacies of immediate structural congeners were explored as leads or backups for animal studies. We compared oral and parenteral dosing, and we developed an intramuscular loading and oral maintenance dosing scheme in mice. Steady-state plasma and brain levels of the antidote were achieved with sequential administrations out to 10 hours, with brain levels exceeding plasma levels shortly after administration. Moreover, the zwitterionic oxime showed substantial protection after gavage, whereas the classic methylpyridinium aldoxime (2-pyridinealdoxime methiodide) was without evident protection. Although further studies in other animal species are necessary, ionizing zwitterionic aldoximes present viable alternatives to existing antidotes for prophylaxis and treatment of large numbers of individuals in terrorist-led events with nerve agent organophosphates, such as sarin, and in organophosphate pesticide exposure.

A physiologically based pharmacokinetic model for the oxime TMB-4: simulation of rodent and human data

Multiple oximes have been synthesized and evaluated for use as countermeasures against chemical warfare nerve agents. The current U.S. military and civilian oxime countermeasure, 2-[(hydroxyimino)methyl]-1-methylpyridin-1-ium chloride (2-PAM), is under consideration for replacement with a more effective acetylcholinesterase reactivator, 1,1'-methylenebis{4-hydroxyiminomethyl}pyridinium dimethanesulfonate (MMB-4). Kinetic data in the scientific literature for MMB-4 are limited; therefore, a physiologically based pharmacokinetic (PBPK) model was developed for a structurally related oxime, 1,1'-trimethylenebis{4-hydroximinomethyl}pyridinium dibromide. Based on a previous model structure for the organophosphate diisopropylfluorophosphate, the model includes key sites of acetylcholinesterase inhibition (brain and diaphragm), as well as fat, kidney, liver, rapidly perfused tissues and slowly perfused tissues. All tissue compartments are diffusion limited. Model parameters were collected from the literature, predicted using quantitative structure-property relationships or, when necessary, fit to available pharmacokinetic data from the literature. The model was parameterized using rat plasma, tissue and urine time course data from intramuscular administration, as well as human blood and urine data from intravenous and intramuscular administration; sensitivity analyses were performed. The PBPK model successfully simulates rat and human data sets and has been evaluated by predicting intravenous mouse and intramuscular human data not used in the development of the model. Monte Carlo analyses were performed to quantify human population kinetic variability in the human evaluation data set. The model identifies potential pharmacokinetic differences between rodents and humans, indicated by differences in model parameters between species. The PBPK model can be used to optimize the dosing regimen to improve oxime therapeutic efficacy in a human population.

Pharmacokinetics and dosage regimen of 2-pyridine aldoxime in Bubalus bubalis intoxicated with fenitrothion

In the present study, the pharmacokinetics of 2-pyridine aldoxime (2-PAM, 30 mg/kg, i.v.) alone and in conjunction with atropine (0.3 mg/kg; 1/4 i.v., 3/4 i.m.) was investigated in 10 Bubalus bubalis intoxicated with a single oral lethal dose of fenitrothion (435 mg/kg). Based on the kinetic parameters, an appropriate dosage regimen of 2-PAM in B. bubalis was calculated. There was no significant difference between plasma levels and pharmacokinetic parameters of 2-PAM in the two groups of animals, given 2-PAM alone and in conjunction with atropine. The peak plasma concentration of 2-PAM at 1 min was in the range of 189.5-196.6 microg/mL which declined to 9.22-9.98 microg/mL at 4 h. The values of elimination half-life, Vd(area) and total body clearance were 2.41-2.67 h, 0.77-0.95 L/kg and 227.5-245.7 mL/kg/h, respectively. The binding capacity of 2-PAM to plasma proteins of fenitrothion-intoxicated buffalo calves and dissociation rate constant of protein drug complex were 0.015 x 10(-6) mol/g and 2.367 x 10(-6) mol, respectively. Approximately 63% of 2-PAM was bound with plasma proteins. In the treatment of organophosphate insecticide (OPI) toxicity in B. bubalis, an appropriate i.v. dosage regimen of 2-PAM in conjunction with atropine would be 18 mg/kg followed by 15 mg/kg at 4 h interval.

Intramuscular kinetics and dosage regimens for pralidoxime in buffalo calves (Bubalus bubalis)

The plasma levels, disposition kinetics and a dosage regimen for pralidoxime (2-PAM) were investigated in male buffalo calves following single intramuscular administration (15 or 30 mg/kg). The effects of 2-PAM on various blood enzymes were also determined. The absorption half-life, elimination half-life, apparent volume of distribution and total body clearance of 2-PAM were 1.08 +/- 0.19 h, 3.14-3.19 h, 0.83-1.01 L/kg and 184.9-252.1 ml/(kg h), respectively. At doses of 15 and 30 mg/kg body weight, a plasma concentration > or = 4 microg/ml was maintained for up to 4 and 6 h, respectively. Pralidoxime significantly lowered the serum level of transferases, phosphatases and lactate dehydrogenase but did not influence the acetylcholinesterase and carboxylesterase enzymes. The most appropriate dosage regimen for 2-PAM in the treatment of organophosphate toxicity in buffaloes would be 25 mg/kg followed by 22 mg/kg at 8 h intervals.

Trimedoxime and HI-6: kinetic comparison after intravenous administration to mice

The intravenous pharmacokinetics of the oximes HI-6 (pyridinium-1-(((4-carbamoil-pyridinio)metoxy)methyl)2 -(hydroxyiminomethyl)dichloride monohydrate), (132.54 mu mol/kg) and trimedoxime (1,1'-(1,3'-propanedyl)bis((4-hydroxyimino) methyl)-pyridinium dibromide), (55.98 mu mol/kg) in mice was investigated. The concentrations of oximes in plasma determined by high pressure liquid chromatography (HPLC) corresponded to a two-compartment pharmacokinetic open model. The oximes were rapidly eliminated from mice plasma, with half-times of 57.93 min. for HI-6 and 108.08 min. for trimedoxime. Although the oximes passed from circulation into the tissues at approximately the same rate, their transport back to the central compartment was two-times slower in the case of trimedoxime: t(1/2k21) was 77.9 min. for trimedoxime and 41.7 min. for HI-6. The total body clearance (CI(tot)) of HI-6 was about 25% higher than that of trimedoxime. The central compartment volume of HI-6 distribution (V(1)) was greater, whereas the volume of distribution of the peripheral compartment (V(2)) was lower for about 35% with respect to the corresponding parameters of trimedoxime. The calculated pharmacokinetic parameters for the oxime HI-6 and trimedoxime show that trimedoxime is eliminated more slowly in mice, and penetrates better into the peripheral compartment where it remains longer.

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.

Pharmacokinetics of pralidoxime chloride. A comparative study in healthy volunteers and in organophosphorus poisoning

The pharmacokinetics of a cholinesterase reactivating oxime, 2-PAM, was comparatively studied in healthy volunteers and in persons suffering from poisoning by organophosphorus insecticides. The elevation of plasma concentrations of the oxime and prolongation of its "effective half-life" were noticed in poisoned individuals. These changes were attributed to the disturbed general hemodynamics and reduced renal blood flow in organophosphorus intoxication rather than to the direct renal toxicity of anticholinesterases.

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.

Effect of thiamine hydrochloride on the blood level of 2-formyl 1-methyl pyridinium oxime chloride (2-PAM.C1) in rats

The biological half-life of 2-PAM.C1 was found to increase in female rats pretreated with thiamine hydrochloride (10 mg/kg i.m.). No such effect was observed in the male rats.