Pharmacokinetics

Animal studies suggest that the minimum therapeutic concentration of pralidoxime in plasma is 4 µg/mL; this level is reached in about 16 minutes after a single injection of 600 mg pralidoxime chloride. In one study of healthy adult volunteers and patients self-poisoned with organophosphate compounds, a single intramuscular injection of 1000 mg of pralidoxime chloride resulted in mean peak plasma levels of 7.5 ± 1.7 µg/mL and 9.9 ± 2.4 µg/mL, respectively. Time to reach the mean peak plasma levels in both groups was similar, 34 minutes in healthy adults and 33 minutes in poisoned patients. Mean half-life was about 3 hours in both groups.

Some evidence suggests that a loading dose followed by continuous intravenous infusion of pralidoxime chloride may maintain therapeutic levels longer than short intermittent infusion therapy. In a cross-over study of seven healthy adults (18 – 50 years) a short intravenous infusion dose of 16 mg/kg over 30 minutes was compared to an intravenous loading dose of 4 mg/kg over 15 minutes, followed by 3.2 mg/kg/hr for 3.75 hours (for a total dose of 16 mg/kg). Results showed that the mean time over which plasma levels were maintained above 4 µg/mL was prolonged in the volunteers who received a loading dose followed by continuous infusion as compared to those who received short infusion therapy (257.5 ± 50.5 min vs. 118.0 ± 52.1 min). Use of continuous intravenous infusion in adult patients with organophosphate poisoning has been described in several case reports, with and without loading doses. Infusion rates ranged from 400 – 600 mg/hr. In one case the blood levels were 11.6 – 13.7 µg /mL when given 400 mg/hr over 5 days (measured at 5, 10 and 18 hours). In another case following an initial loading dose of 1000 mg, blood levels were 11.79 µg/mL when given 500 mg/hr and 17.26 µg/mL when given 600 mg/hr. In the latter case the pralidoxime elimination half-life was 4 hours. In two other cases blood levels were not measured.

Pralidoxime chloride is distributed throughout the extracellular water; its apparent volume of distribution at steady state has been reported to range from 0.60 to 2.7 L/kg. Pralidoxime chloride is not bound to plasma protein.

Pralidoxime chloride is relatively short acting and repeated doses may be needed, unless continuous intravenous infusion is selected. Simulations suggest that after a dose of 1000 mg given intravenously, concentrations fall below 4 µg/mL in about 1.5 hours. The short duration of action of pralidoxime chloride and the necessity for repeated doses should be considered especially where there is any evidence of continuing absorption of the poison. The apparent half-life of pralidoxime chloride is 74 to 77 minutes. The drug is rapidly excreted in the urine by renal tubular secretion, partly unchanged, and partly as a metabolite produced by the liver. After intramuscular administration of 1000 mg of pralidoxime chloride, the renal clearance has been reported to be 7.2 ± 2.9 mL/min/kg in healthy volunteers and 3.6 ± 1.5 mL/min/kg in organophosphate-poisoned patients.

In one study of 11 organophosphate-poisoned pediatric patients (age, 0.8 to 18 years), an intravenous loading dose of 15-50 mg/kg (mean 29 mg/kg) of pralidoxime chloride followed by a continuous infusion of 10-16 mg/kg/hr (mean 14 mg/kg/hr) over 12 to 43 hours (mean 27 ± 8 hours) resulted in an average steady state plasma concentration of 22.2 mg/L (6.9 to 47.4 mg/L) and an average body clearance of 0.88 L/kg/hr (0.28 to 2.20 L/kg/hr). After the continuous infusion was discontinued, determinations of the apparent volume of distribution and half-life ranged from 1.7 to 13.8 L/kg and from 2.4 to 5.3 hours, respectively.

General

PROTOPAM Chloride has been well tolerated in most cases, but it must be remembered that the desperate condition of the organophosphate-poisoned patient will generally mask such minor signs and symptoms as have been noted in normal subjects.

Intravenous administration of PROTOPAM Chloride should be carried out slowly and, preferably, by continuous or intermittent infusion, since temporary worsening of cholinergic manifestations (i.e. tachycardia, cardiac arrest, laryngospasm, and muscle rigidity or paralysis) may occur if PROTOPAM Chloride is infused too rapidly. The intermittent infusion rate should not exceed 200 mg/minute. If intravenous administration is not feasible, intramuscular or subcutaneous injection should be used (see DOSAGE AND ADMINISTRATION).

PROTOPAM Chloride should be used with great caution in treating organophosphate overdosage in cases of myasthenia gravis since it may precipitate a myasthenic crisis.

Because pralidoxime is excreted in the urine, a decrease in renal function will result in increased blood levels of the drug. Thus, the dosage of PROTOPAM Chloride should be reduced in the presence of renal insufficiency.

Laboratory Tests

Treatment of organophosphate poisoning should be instituted without waiting for the results of laboratory tests. Red blood cell, plasma cholinesterase, and urinary paranitrophenol measurements (in the case of parathion exposure) may be helpful in confirming the diagnosis and following the course of the illness, although such tests may be normal in the face of clinically significant organophosphate poisoning. A reduction in red blood cell cholinesterase concentration to below 50% of normal has been seen only with organophosphate ester poisoning.

Drug Interactions

When atropine and pralidoxime chloride are used together, the signs of atropinization (flushing, mydriasis, tachycardia, dryness of the mouth and nose) may occur earlier than might be expected when atropine is used alone. This is especially true if the total dose of atropine has been large and the administration of pralidoxime chloride has been delayed.

The following precautions should be kept in mind in the treatment of anticholinesterase poisoning, although they do not bear directly on the use of pralidoxime chloride: since barbiturates are potentiated by the anticholinesterases, they should be used cautiously in the treatment of convulsions; morphine, theophylline, aminophylline, reserpine, and phenothiazine-type tranquilizers should be avoided in patients with organophosphate poisoning. Prolonged paralysis has been reported in patients when succinylcholine is given with drugs having anticholinesterase activity; therefore, it should be used with caution.

Carcinogenesis, Mutagenesis, Impairment of Fertility

Because pralidoxime chloride is indicated for short-term emergency use only, no investigations of its potential for carcinogenesis, mutagenesis, or impairment of fertility have been conducted by the manufacturer, or reported in the literature.

Pregnancy

Nursing Mothers

It is not known whether this drug is excreted in human milk. Because many drugs are excreted in human milk, caution should be exercised when pralidoxime chloride is administered to a nursing woman.

Pediatric Use

There are no adequate and well-controlled clinical trials that establish the effectiveness of pralidoxime chloride in pediatric patients. Efficacy has been extrapolated from the adult population and is supported by nonclinical studies, pharmacokinetic studies in adults and experience in the pediatric population (see DOSAGE AND ADMINISTRATION). As in adults, laryngospasm, cardiac arrest, tachycardia, and muscle rigidity or paralysis have been reported following rapid intravenous injection. Muscle fasciculations, apnea, and convulsions have also been reported.

Geriatric Use

Clinical studies of PROTOPAM Chloride did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.

Manifestations of Overdosage

Observed in normal subjects only: dizziness, blurred vision, diplopia, headache, impaired accommodation, nausea, slight tachycardia. In therapy, it has been difficult to differentiate side effects due to the drug from those due to the effects of the poison.

Organophosphate Poisoning

Treatment should include general supportive care, atropinization, and decontamination, in addition to the use of PROTOPAM Chloride. Treatment is most effective if initiated immediately after poisoning. Administration of PROTOPAM Chloride should be carried out slowly and, preferably, by infusion. If intravenous administration is not feasible, intramuscular or subcutaneous injection should be used. Generally, little is accomplished if PROTOPAM Chloride is given more than 36 hours after termination of exposure to the poison. When the poison has been ingested, it is particularly important to take into account the likelihood of continuing absorption from the lower bowel since this constitutes new exposure and fatal relapses have been reported after initial improvement. In such cases, additional doses of PROTOPAM Chloride may be needed every three to eight hours. In effect, the patient should be “titrated” with PROTOPAM Chloride as long as signs of poisoning recur. As in all cases of organophosphate poisoning, care should be taken to keep the patient under observation for at least 48 to 72 hours.

If dermal exposure has occurred, clothing should be removed and the hair and skin washed thoroughly with sodium bicarbonate or alcohol as soon as possible.

Supportive care, including airway management, respiratory and cardiovascular support, correction of metabolic abnormalities, and seizure control, may be necessary in cases of severe organophosphate poisoning.

Atropine should be given as soon as possible after hypoxemia is improved. Atropine should not be given in the presence of significant hypoxia due to the risk of atropine-induced ventricular fibrillation. In adults, atropine may be given intravenously in doses of 2 to 4 mg. This should be repeated at 5- to 10-minute intervals until full atropinization (secretions are inhibited) or signs of atropine toxicity appear (delirium, hyperthermia, muscle twitching).

Some degree of atropinization should be maintained for at least 48 hours, and until any depressed blood cholinesterase activity is reversed.

Use of morphine, theophylline, aminophylline, reserpine, and phenothiazine-type tranquilizers should be avoided in patients with organophosphate poisoning (see PRECAUTIONS, Drug Interactions). Prolonged paralysis has been reported in patients when succinylcholine is given with drugs having anticholinesterase activity; therefore, it should be used with caution.

After the effects of atropine become apparent, PROTOPAM Chloride may be administered.

Symptoms Of Nerve Agent And Insecticide Poisoning

PROTOPAM Chloride dosing is based, in part, on the severity of symptoms of nerve agent intoxication. These symptoms include the following:

MILD symptoms:

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Blurred vision and sore eyes

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Teary eyes*

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Runny nose*

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Increased salivation such as sudden drooling*

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Chest tightness or difficulty breathing

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Tremors throughout the body or muscular twitching

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Nausea and vomiting

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Involuntary respiratory secretions

SEVERE symptoms:

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Strange or confused behavior

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Severe difficulty breathing or respiratory secretions

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Severe muscular twitching and general weakness**

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Involuntary urination and defecation*

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Convulsions

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Unconsciousness

ADULT DOSING

PEDIATRIC DOSING (FOR PATIENTS 16 YEARS AND UNDER)

Anticholinesterase Overdosage

As an antagonist to such anticholinesterases as neostigmine, pyridostigmine, and ambenonium, which are used in the treatment of myasthenia gravis, PROTOPAM Chloride may be given in a dosage of 1000 to 2000 mg intravenously followed by increments of 250 mg every five minutes.

Preparation for Administration

PROTOPAM Chloride is supplied as 1000 mg single-dose vials for injection.

For INTRAVENOUS infusion: Reconstitute a single PROTOPAM Chloride 1000 mg vial by adding 20 mL of Sterile Water for Injection, USP, which results in a 50 mg/mL concentration.

The solution should further be diluted with Normal Saline for Injection, USP to achieve a concentration of 10 to 20 mg/mL (e.g. 1000 mg in 100 mL or 2000 mg in 100 mL).

For fluid restricted patients or for rapid administration (over at least 5 min), a maximum concentration of 50 mg/mL may be used.

For INTRAMUSCULAR injection: Reconstitute a single PROTOPAM Chloride 1000 mg vial by adding 3.3 mL of Sterile Water for Injection, USP for an approximate concentration of 300 mg/mL.

Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution and container permit.

Discard unused solution after a dose has been withdrawn.

Storage

Store at 20°-25°C (68°-77°F), excursions permitted to 15°-30°C (59°-86°F) [see USP Controlled Room Temperature].