1.1 Acetaminophen Assays Interpretation and Methodology – Acute Ingestion

The acute ingestion of acetaminophen in quantities of 150 mg/kg or greater may result in hepatic toxicity. However, the reported history of the quantity of a drug ingested as an overdose is often inaccurate and is not a reliable guide to therapy of the overdose. Therefore, plasma or serum acetaminophen concentrations, determined as early as possible, but no sooner than four hours following an acute overdose, are essential in assessing the potential risk of hepatotoxicity. If an assay for acetaminophen cannot be obtained, it is necessary to assume that the overdose is potentially toxic.

Interpretation of Acetaminophen Assays

1. When results of the plasma acetaminophen assay are available, refer to the nomogram in Figure 1 to determine if plasma concentration is in the potentially toxic range. Values above the line connecting 200 mcg/mL at 4 hours with 50 mcg/mL at 12 hours (probable line) are associated with a probability of hepatic toxicity if an antidote is not administered.
2. If the predetoxification plasma level is above the line connecting 150 mcg/mL at 4 hours with 37.5 mcg/mL at 12 hours (possible line), continue with maintenance doses of acetylcysteine. It is better to err on the safe side and thus this line, defining possible toxicity, is plotted 25% below the line defining probable toxicity.
3. If the predetoxification plasma level is below the line connecting 150 mcg/mL at 4 hours with 37.5 mcg/mL at 12 hours (possible line), there is minimal risk of hepatic toxicity, and Acetylcysteine treatment may be discontinued.

Estimating Potential for Hepatotoxicity: The following depiction of the Rumack- Matthew nomogram has been developed to estimate the probability that plasma levels in relation to intervals post-ingestion will result in hepatotoxicity.

The Rumack-Matthew nomogram may underestimate the risk for hepatotoxicity in some patients with risk factors such as chronic alcoholism, malnutrition, or CYP2E1 enzyme inducing drugs (e.g., isoniazid).

Figure 1. Rumack-Matthew Nomogram:

Figure 1. Michael J Hodgman, Alexander R Garrard, A Review of Acetaminophen Poisoning. Crit Care Clin. 28 (2012) 499-516. Stephen J. Wolf, Kennon Heard, et.al, Clinical Policy: Critical Issues in the Management of Patients Presenting to the Emergency Department with Acetaminophen Overdose. Ann Emerg Med. 2007:50:292-313.

1.2 Acetaminophen Assays Interpretation and Methodology – Repeated Supratherapeutic Ingestion

Repeated Supratherapeutic Ingestion (RSI) is defined as ingestion of acetaminophen at doses higher than those recommended for extended periods of time. The nomogram does not apply to patients with RSI. Treatment is based on the acetaminophen and elevated AST/ALT levels indicative of potential toxicity due to acetaminophen. For specific treatment information regarding the clinical management of repeated supratherapeutic acetaminophen overdose, please contact your regional poison center at 1-800-222-1222, or alternatively, a special health professional assistance line for acetaminophen overdose at 1-800-525-6115.

Figure 2. Acetylcysteine Injection Treatment Flow Chart

1Acetaminophen levels drawn less than 4 hours post-ingestion may be misleading.

2With an extended-release preparation, an acetaminophen level drawn less than 8 hours post-ingestion may be misleading. Draw a second level at 4 to 6 hours after the initial level. If either falls above the toxicity line, acetylcysteine treatment should be initiated.

3Acetylcysteine may be withheld until acetaminophen assay results are available as long as initiation of treatment is not delayed beyond 8 hours post-ingestion. If more than 8 hours post-ingestion, start acetylcysteine treatment immediately.

2.1 Administration Instructions (Three-Bag Method: Loading, Second and Third Dose)

Dosing for Patients who weigh 5 kg to 20 kg ( Table 1):

Loading Dose: 150 mg/kg diluted in 3 mL/kg of diluent* administered over 1 hr

Second Dose: 50 mg/kg diluted in 7 mL/kg of diluent* administered over 4 hrs

Third Dose: 100 mg/kg diluted in 14 mL/kg of diluent* administered over 16 hrs

See also Section 2.2 Volume Adjustment: Patients less than 40 kg and Requiring Fluid Restriction

Dosing for patients who weigh 21 kg to 40 kg ( Table 2):

Loading Dose: 150 mg/kg diluted in 100 mL of diluent* administered over 1 hr

Second Dose: 50 mg/kg diluted in 250 mL of diluent* administered over 4 hrs

Third Dose: : 100 mg/kg diluted in 500 mL of diluent* administered over 16 hrs

See also Section 2.2 Volume Adjustment: Patients less than 40 kg and Requiring Fluid Restriction.

Dosing for patients who weigh 41 kg to 100 kg ( Table 3):

Loading Dose: 150 mg/kg diluted in 200 mL of diluent* administered over 1 hr

Second Dose: 50 mg/kg diluted in 500 mL of diluent* administered over 4 hrs

Third Dose: 100 mg/kg diluted in 1,000 mL of diluent administered over 16 hrs

Patients Weighing More Than 100 kg

No specific studies have been conducted to evaluate the use of or necessity of dosing adjustments in patients weighing over 100 kg. Limited information is available regarding the dosing requirements of patients that weigh more than 100 kg. The dose of Acetylcysteine Injection recommended in these patients should be a loading dose of 15,000 mg infused over a period of one hour followed by a first maintenance dose of 5,000 mg over 4 hours and a second maintenance dose of 10,000 mg over 16 hours ( Table 3).

Continued Therapy beyond 21 Hours

While there is no clinical trial data to support infusions beyond 21 hours there is literature that supports continued infusion of acetylcysteine in some rare instances. In cases of suspected massive overdose, or with concomitant ingestion of other substances, or in patients with preexisting liver disease, the absorption and/or the half-life of acetaminophen may be prolonged, in such cases consideration should be given to the need for continued infusion of N-acetylcysteine beyond 21 hours. Acetaminophen levels and ALT/AST & INR should be checked before the end of the 21-hour infusion. If acetaminophen levels are still detectable, or in cases in which the ALT/AST are still increasing or the INR remains elevated, the infusion should be continued, and the treating physician should contact a US regional poison center at 1-800-222-1222, or alternatively, a “special health professional assistance line for acetaminophen overdose” at 1-800-525-6115 for assistance with dosing recommendations.

2.2 Volume Adjustment: Patients less than 40 kg and Requiring Fluid Restriction

The total volume administered should be adjusted for patients less than 40 kg and for those requiring fluid restriction. To avoid fluid overload, the volume of diluent should be reduced as clinically needed. If the volume of the infusion is not adjusted, fluid overload can occur, potentially resulting in hyponatremia, seizure and death. [ see Dosage and Administration (2)]

As Acetylcysteine Injection is hyperosmolar (2600 mOsmol/L), caution is advised when the diluent volume is decreased as the hyperosmolarity of the solution is increased. See Table 4 below for examples.

Single dose vial, preservative-free, discard unused portion. If vial was previously opened, do not use for intravenous administration.

Stability studies indicate that the diluted solution is stable for 24 hours at controlled room temperature.

Note: The color of Acetylcysteine Injection may turn from essentially colorless to a slight pink or purple once the stopper is punctured. The color change does not affect the quality of the product.

2.3 Renal Impairment

No data are available to determine if a dose adjustment in patients with moderate or severe renal impairment is required.

2.4 Hepatic Impairment

Although there was a threefold increase in acetylcysteine plasma concentrations in patients with hepatic cirrhosis, no data are available to determine if a dose adjustment in these patients is required. The published medical literature does not indicate that the dose of acetylcysteine in patients with hepatic impairment should be reduced.

5.1 Anaphylactoid Reactions

Serious anaphylactoid reactions, including death in a patient with asthma, have been reported in patients administered acetylcysteine intravenously.
Acute flushing and erythema of the skin may occur in patients receiving acetylcysteine intravenously. These reactions usually occur 30 to 60 minutes after initiating the infusion and often resolve spontaneously despite continued infusion of acetylcysteine. Anaphylactoid reactions (defined as the occurrence of an acute hypersensitivity reaction during acetylcysteine administration including rash, hypotension, wheezing, and/or shortness of breath) have been observed in patients receiving intravenous acetylcysteine for acetaminophen overdose and occurred soon after initiation of the infusion [see  Adverse Reactions ( 6.1)].  If a reaction to acetylcysteine involves more than simply flushing and erythema of the skin, it should be treated as an anaphylactoid reaction. This usually entails administering antihistaminic drugs and in severe cases may require administration of epinephrine. In addition, the acetylcysteine infusion may be interrupted until treatment of the anaphylactoid symptoms has been initiated and then carefully restarted. If the anaphylactoid reaction returns upon reinitiation of treatment or increases in severity, intravenous acetylcysteine should be discontinued and alternative patient management should be considered.

5.2 Monitoring Patients with Asthma

Acetylcysteine Injection should be used with caution in patients with asthma, or where there is a history of bronchospasm.

5.3 Volume Adjustment: Patients less than 40 kg and Requiring Fluid Restriction

The total volume administered should be adjusted for patients less than 40 kg and for those requiring fluid restriction. To avoid fluid overload, the volume of diluent should be reduced as needed [see Dosage and Administration ( 2)]. If volume is not adjusted fluid overload can occur, potentially resulting in hyponatremia, seizure and death.
 
For specific treatment information regarding the clinical management of acetaminophen overdose, please contact your regional poison center at 1-800-222-1222, or alternatively, a special health professional assistance line for acetaminophen overdose at 1-800-525-6115.

6.1 Clinical Studies Experience

Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in practice.

In the literature the most frequently reported adverse reactions attributed to intravenous acetylcysteine administration were rash, urticaria and pruritus. The frequency of adverse reactions has been reported to be between 0.2% and 20.8%, and they most commonly occur during the initial loading dose of acetylcysteine.

  Loading Dose/Infusion Rate Study

The incidence of drug-related adverse reactions occurring within the first 2 hours following acetylcysteine administration reported in a randomized study in patients with acetaminophen poisoning is presented in Table 5 by preferred term. In this study patients were randomized to a 15-minute or a 60-minute loading dose regimen.

Within the first 2 hours following intravenous acetylcysteine administration, 17% developed an anaphylactoid reaction (18% in the 15-minute treatment group; 14% in the 60-minute treatment group) in this randomized, open-label, multi-center clinical study conducted in Australia to compare the rates of anaphylactoid reactions between two rates of infusion for the intravenous acetylcysteine loading dose  [see Warnings (Section 5) and Clinical  Studies - Loading Dose/Infusion Rate Study (Section 14) ]. 

Postmarketing Safety Study
A large multi-center study was performed in Canada where data were collected from patients who were treated with intravenous acetylcysteine for acetaminophen overdose between 1980 and 2005. This study evaluated 4709 adult cases and 1905 pediatric cases. The incidence of anaphylactoid reactions in adult (overall incidence 7.9%) and pediatric (overall incidence 9.5%) patients is presented in Tables 6 and 7.

8.1 Pregnancy

Pregnancy Category B

There are no adequate and well-controlled studies of Acetylcysteine Injection in pregnant women. However, limited case reports of pregnant women exposed to acetylcysteine during various trimesters did not report any adverse maternal, fetal or neonatal outcomes.

There are published reports on four pregnant women with acetaminophen toxicity, who were treated with oral or intravenous acetylcysteine at the time of delivery. Acetylcysteine crossed the placenta and was measurable following delivery in serum and cord blood of three viable infants and in cardiac blood of a fourth infant at autopsy (22 weeks gestational age who died 3 hours after birth). No adverse sequelae developed in the three viable infants. All mothers recovered and none of the infants had evidence of acetaminophen poisoning.

Reproduction studies were performed in rats at oral doses up to 2,000 mg/kg/day (1.1 times the recommended total human intravenous dose of 300 mg/kg based on body surface area comparison) and in rabbits at oral doses up to 1,000 mg/kg/day (1.1 times the recommended total human intravenous dose of 300 mg/kg based on body surface area comparison). No effects on fertility or harm to the fetus due to acetylcysteine were observed.

8.3 Nursing Mothers

It is not known whether Acetylcysteine Injection is present in human milk. Because many drugs are excreted in human milk, caution should be exercised when acetylcysteine is administered to a nursing woman. Based on the pharmacokinetics of acetylcysteine, it should be nearly completely cleared 30 hours after administration. Nursing women may consider resuming nursing 30 hours after administration.

8.4 Pediatric Use

No adverse effects were noted during intravenous infusion with acetylcysteine at a mean rate of 4.2 mg/kg/h for 24 hours to 10 preterm newborns ranging in gestational age from 25 to 31 weeks and in weight from 500 to 1380 grams in one study or in 6 newborns ranging in gestational age from 26 to 30 weeks and in weight from 520 to 1335 grams infused with acetylcysteine at 0.1 to 1.3 mg/kg/h for 6 days. Elimination of acetylcysteine was slower in these infants than in adults; mean elimination half-life was 11 hours. There are no adequate and well-controlled studies in pediatric patients.

8.5 Geriatric Use

The clinical studies do not provide a sufficient number of geriatric subjects to determine whether the elderly respond differently.

12.1 Mechanism of Action

Acetaminophen Overdose:

Acetaminophen is absorbed from the upper gastrointestinal tract with peak plasma levels occurring between 30 and 60 minutes after therapeutic doses and usually within 4 hours following an overdose. It is extensively metabolized in the liver to form principally the sulfate and glucoronide conjugates which are excreted in the urine. A small fraction of an ingested dose is metabolized in the liver by isozyme CYP2E1 of the cytochrome P-450 mixed function oxidase enzyme system to form a reactive, potentially toxic, intermediate metabolite. The toxic metabolite preferentially conjugates with hepatic glutathione to form nontoxic cysteine and mercapturic acid derivatives, which are then excreted by the kidney. Recommended therapeutic doses of acetaminophen are not believed to saturate the glucuronide and sulfate conjugation pathways and therefore are not expected to result in the formation of sufficient reactive metabolite to deplete glutathione stores. However, following ingestion of a large overdose, the glucuronide and sulfate conjugation pathways are saturated resulting in a larger fraction of the drug being metabolized via the cytochrome P-450 pathway and therefore, the amount of acetaminophen metabolized to the reactive intermediate increases. The increased formation of the reactive metabolite may deplete the hepatic stores of glutathione with subsequent binding of the metabolite to protein molecules within the hepatocyte resulting in cellular necrosis.

Acetylcysteine Intravenous Treatment:

Acetylcysteine has been shown to reduce the extent of liver injury following acetaminophen overdose. It is most effective when given early, with benefit seen principally in patients treated within 8-10 hours of the overdose. Acetylcysteine likely protects the liver by maintaining or restoring the glutathione levels, or by acting as an alternate substrate for conjugation with, and thus detoxification of, the reactive metabolite.

12.3 Pharmacokinetics

Distribution:

The steady-state volume of distribution (Vd ss) and the protein binding for acetylcysteine were reported to be 0.47 liter/kg and 83%, respectively.

Metabolism:

Acetylcysteine may form cysteine, disulfides and conjugates in vivo (N, N'-diacetylcysteine, N-acetylcysteine-cysteine, N-acetylcysteine- glutathione, N-acetylcysteine-protein, etc). Based on published data, it was reported that after an oral dose of 35S-acetylcysteine, about 22% of total radioactivity was excreted in urine after 24 hours. No metabolites were identified.

Elimination:

After a single intravenous dose of acetylcysteine, the plasma concentration of total acetylcysteine declined in a poly-exponential decay manner with a mean terminal half-life (T 1/2) of 5.6 hours. The mean clearance (CL) for acetylcysteine was reported to be 0.11 liter/hr/kg and renal CL constituted about 30% of total CL.

Special Populations:

Gender: Adequate information is not available to assess if there are differences in pharmacokinetics (PK) between males and females.

Pediatric: The mean elimination T 1/2 of acetylcysteine is longer in newborns (11 hours) than in adults (5.6 hours). Pharmacokinetic information is not available in other age groups.

Pregnant Women: In four pregnant women with acetaminophen toxicity, oral or I.V. acetylcysteine was administered at the time of delivery. Acetylcysteine was detected in the cord blood of 3 viable infants and in cardiac blood of a fourth infant sampled at autopsy [see Pregnancy (8.1)].

Hepatic Impairment: In subjects with severe liver damage, i.e., cirrhosis due to alcohol (with Child-Pugh score of 7 to 13), or primary and/or secondary biliary cirrhosis (with Child-Pugh score of 5 to 7), mean T 1/2 increased by 80% while mean CL decreased by 30% compared to the control group.

Renal Impairment: Pharmacokinetic information is not available in patients with renal impairment.

Geriatric Patients: Adequate information on acetylcysteine PK in geriatric patients is not available.

13.1 Carcinogenesis, Mutagenesis, Impairment of Fertility

Long-term studies in animals have not been performed to evaluate the carcinogenic potential of acetylcysteine.

Acetylcysteine was not genotoxic in the Ames test or the in vivo mouse micronucleus test. It was, however, positive in the in vitro mouse lymphoma cell (L5178Y/TK+/-) forward mutation test.

Treatment of male rats with acetylcysteine at an oral dose of 250 mg/kg/day for 15 weeks (0.1 times the recommended human dose of 300 mg/kg based on body surface comparison) did not affect the fertility or general reproductive performance.