Toxogonin: oral administration to man

Internal standard high-performance liquid chromatography method for the determination of obidoxime in urine of organophosphate-poisoned patients

Obidoxime is an antidote approved for reactivation of inhibited acetylcholinesterase in organophosphate poisoning. HPLC methods were described for its determination in blood or aqueous solutions but not for the determination in urine. Since data for renal obidoxime excretion ranged from 2.2 to 84% of administered dose in healthy volunteers depending on the route of administration and little is known about pharmacokinetics of obidoxime in severely intoxicated patients we developed an internal standard (HI 6) reversed-phase HPLC method for determining obidoxime in urine. The mobile phase consisted of methanol, the counter ion 1-heptane sulfonic acid and tetrabutylammonium phosphate, the stationary phase involved a 5 microm reversed-phase column (125x4 mm). Obidoxime was detected spectrophotometrically at 288 nm. The limit of quantification (LOQ) was 1 microM, the limit of detection (LOD) 0.5 microM. Linear calibration curves were obtained in a concentration range from 1 to 1000 microM. Intra- and inter-day precision C.V.s were below 4%. Accuracy was 95.9% in the LOQ range. Using this method, we were able to quantify obidoxime in urine of an organophosphate poisoned patient. Based on this data we calculated that 58% of the administered dose was excreted in the urine.

Cholinesterase status, pharmacokinetics and laboratory findings during obidoxime therapy in organophosphate poisoned patients

1 The effectiveness of oxime therapy in organophosphate poisoning is still a matter of debate. It appears, however, that the often cited ineffectiveness of oximes may be due to inappropriate dosing. By virtue of in vitro findings and theoretical considerations we concluded in the preceding paper that oximes should preferably be administered by continuous infusion following an initial bolus dose for as long as reactivation of inhibited acetylcholinesterase (AChE) can be expected. This conclusion has called for a clinical trial to evaluate such oxime therapy on the basis of objective parameters. 2 Before transfer to the intensive care unit (ICU), 5 patients received primary care by an emergency physician. In the ICU, atropine sulphate was administered i.v. upon demand according to the endpoints: no bronchorrhoea, dry mucous membranes, no axillary sweating, heart rate of about 100/min. Obidoxime (Toxogonin) was given as an i.v. bolus (250 mg) followed by continuous infusion of 750 mg/24 h. 3 Intoxication and therapy were monitored by determining erythrocyte AChE (eryAChE) activity, reactivatability of the patient's eryAChE ex vivo, plasma cholinesterase activity, the presence of AChE inhibiting compounds, as well as the concentrations of obidoxime and atropine in plasma. 4 Obidoxime was effective in life-threatening parathion poisoning, in particular when the dose absorbed was comparably low. In mega-dose poisoning, net reactivation was not achieved until several days after ingestion, when the concentration of active poison in plasma had declined. Reactivatability in vivo lasted for a longer period than expected from in vitro experiments. 5 Obidoxime was quite ineffective in oxydemetonmethyl poisoning, when the time elapsed between ingestion and oxime therapy was longer than 1 day. When obidoxime was administered shortly after ingestion (1 h) reactivation was nearly complete. 6 Obidoxime levels of 10-20 microM were achieved by our regimen, and atropine could rapidly be reduced to approx. 20 microM, as attained by continuous infusion of 1 mg atropine sulphate/h. Maintenance of the desired plasma levels was not critical even when renal function deteriorated. 7 Signs of transiently impaired liver function were observed in patients who showed transient multiorgan failure. In the present stage of knowledge, we feel it advisable to keep the plasma concentration of obidoxime at 10-20 microM, although the full reactivating potential of obidoxime will not then be exploited. Still, the reactivation rate, with an apparent half-time of some 3 min, is twice that estimated for a tenfold higher pralidoxime concentration.

HLö 7 dimethanesulfonate, a potent bispyridinium-dioxime against anticholinesterases

HLö 7 dimethanesulfonate (1-[[[4-(aminocarbonyl)pyridinio]methoxy]methyl]-2,4-bis [(hydroxyimino)methyl]pyridinium dimethanesulfonate) is a broad-spectrum reactivator against highly toxic organophosphorus compounds. The compound was synthesized by a new route with the carcinogenic bis(chloromethyl)ether being substituted by the non-mutagenic bis(methylsulfonoxymethyl)ether. The very soluble dimethanesulfonate of obidoxime was also prepared by this way. HLö 7 dimethanesulfonate is the first water-soluble salt of HLö 7 that should be suitable for the wet/dry autoinjector technology, because aqueous solutions of HLö 7 are not very stable (calculated shelf-life 0.2 years when stored at 8 degrees C, 1 M solution, pH 2.5). The crystalline preparation contains 96% of the syn/syn-isomer, less than 2% of the syn/anti-isomer and some minor identified by-products. HLö 7 was very efficient in reactivating acetylcholinesterase (AChE) blocked by organophosphates as long as ageing did not prevent dephosphylation. HLö 7 was superior to HI 6 (1-[[[4-(aminocarbonyl)pyridinio]methoxy]methyl]-2- [(hydroxyimino)methyl]pyridinium dichloride) in reactivating soman and sarin-inhibited AChE from erythrocytes, and literature data indicate that HLö 7 exceeds HI 6 by far in reactivating tabun-inhibited AChE. In atropine-protected, soman-poisoned mice HLö 7 was three times more potent than HI 6 (protective ratio 5 versus 2.5), and in sarin-poisoned mice HLö 7 was 10 times more potent than HI 6 (protective ratio 8 for both oximes). In atropine-protected guinea-pigs HLö 7 was less effective than HI 6 (protective ratio: 2.3 versus 5.2 for soman; 5.2 versus 6.8 for sarin; 4.3 versus 3.8 for tabun). The mean survival time of anaesthetized guinea-pigs exposed to 5 LD50 soman (6.3 min) was increased by atropine (27 min) and atropine + HLö (57 min). HLö 7 alone did not prolong the survival. The most impressive effect of HLö 7 was on respiration: 3 min after i.v. injection of HLö 7 and atropine, the depressed respiration increased rapidly to 60% of control and remained at that level during the observation period (60 min). With atropine alone, respiration recovered only slowly. Behavioural and physiologic parameters were determined in atropine-protected mice exposed to a sublethal soman dose. The running performance was significantly improved by HLö 7. Even central symptoms, e.g. hypothermia and convulsions, were decreased markedly by HLö 7 (evaluation 60 min after poisoning). The pharmacokinetic data for HLö 7 in male beagle dogs are similar to those of HI 6.(ABSTRACT TRUNCATED AT 400 WORDS)

A review of organophosphate poisoning

Many organophosphate compounds are pesticides widely used for the control of insect vectors. They are not ideal agents because they lack target vector selectivity, and have caused severe toxicity and even death in humans and domestic animals. Their toxicity has been recognised since the 1930s, when they were also developed for use as chemical warfare agents. The mechanism of action of organophosphates has been determined in some depth; the understanding of the toxic effects resulting from the inhibition of cholinesterase activity, causing accumulation of acetylcholine at nerve endings has played a major part in providing a rationale for specific antidote treatment using atropine and oximes. However, the most suitable oxime for reactivation of cholinesterases has still not been established with certainty, although pralidoxime is widely recommended. Chronic toxicity, particularly the neuropathic effects, merits further study because it contributes substantially to the long term morbidity in cases of severe acute, or chronic, exposure. Prevention of potentially toxic organophosphate exposure, particularly amongst employees in industries manufacturing or using the compounds and in the most susceptible groups of the population, such as the young and the elderly, should be sought wherever possible. Government authorities should be encouraged to control organophosphate product licensing, manufacture, storage, import, methods of use and delivery, food contamination and disposal.

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.

Q-T prolongation and polymorphous ("torsade de pointes") ventricular arrhythmias associated with organophosphorus insecticide poisoning

It is not generally appreciated in the Western world that organophosphorus poisoning may be associated with a serious and often fatal cardiac complication: Q-T interval prolongation with malignant ventricular arrhythmias of the "torsade de pointes" type. This insidious complication may lead to delayed, sudden death after the patients appears to be well on the way to recovery from the other, more dramatic respiratory and neurologic symptoms. In this study 15 patients with organophosphorus poisoning are described. Q-T prolongation was observed in 14 and malignant tachyarrhythmias in 6. In view of the dismal prognosis of these patients, ventricular pacing, previously used with success in other conditions associated with this syndrome, was tried in four patients and successfully shortened the Q-T interval and eliminated the arrhythmias. Isoproterenol did the same in a fifth patient. Awareness of this lethal, but preventable complication of organophosphorus poisoning is called for. Careful electrocardiographic monitoring is necessary until the Q-T interval returns to normal. Electrical pacing appears to be the treatment of choice for the tachyarrhythmias.

Administration of obidoxime tablets to man. Plasma levels and side reactions

Twenty-four male volunteers were given obidoxime tablets in quantities ranging from 1.84-3.58 g in a single dose, or 7.36 g divided into 4 equal doses. With the lowest dose, average peak plasma level of the drug was 1.9 mug/ml and after the highest single dose it was 5.6 mug/ml, both attained 1.5 h after administration. In the multiple-dosed individuals, plasma levels of the oxime increased gradually following each additional dose, reaching a peak of 3.5 mug/ml after the last dose. Thirteen individuals complained of one or more of the following side effects: pallor, nausea, pyrosis, headache, generalized weakness, sore throat, and paresthesia of the face muscles. Activities of blood cholinesterase, glutamic oxalacetic transaminase, glutamic pyruvic transaminase, as well as hematocrit values, heart rate, and blood pressure were not affected. It is postulated that due to the undesirable side effects, the general use of obidoxime tablets should not be recommended. However, prophylactic oral treatment with obidoxime could be considered for persons at high risk of organophosphate poisoning or when parenteral administration might not be feasible.