Acetaminophen Overdose Case File

Acetaminophen Overdose Case File

Eugene C.Toy, MD, William E. Seifert, Jr., PHD, Henry W. Strobel, PHD, Konrad P. Harms, MD

❖ CASE 40

A 20-year-old female was brought to the emergency department after being found on the dormitory room floor nauseated, vomiting, and complaining of abdominal pain. Her friends were concerned when she did not show up for a biochemistry final at the local university. The patient had been under a lot of stress with finals, a recent breakup with a boyfriend, and trying to find a job. In the dormitory room, one of her friends noticed an empty bottle of Tylenol (acetaminophen) near the bed with numerous pills lying on the ground near their friend. On arrival to the emergency department, the patient was found to be in moderate distress and vomiting. The patient was quickly assessed, and laboratory work was obtained. Patient had a hypokalemia noted on electrolytes and elevated liver enzymes. Her white blood cell count was normal. Her urine drug screen was negative, and her acetaminophen blood level was above 200 μg/mL. The emergency department physician prescribes oral N-acetylcysteine to help prevent toxicity from the acetaminophen.

◆ What is the pathophysiology of the liver toxicity?

◆ What is the biochemical mechanism whereby the N-acetylcysteine helps in this condition?

ANSWERS TO CASE 40: ACETAMINOPHEN OVERDOSE

Summary: A 20-year-old college student under increasing stress was found in moderate distress with nausea, vomiting, and abdominal pain with empty bottle of Tylenol (acetaminophen) at her bedside. Oral N-acetylcysteine is prescribed.

◆ Pathophysiology: Acetaminophen is metabolized via the cytochrome P450 enzymes into a deleterious product N-acetyl benzoquinoneimine, an unstable intermediate, which causes arylated derivatives of protein, lipid, ribonucleic acid (RNA), and deoxyribonucleic acid (DNA), causing destruction of these compounds. Because the liver has high levels of cytochrome P450 enzymes, it is the major organ affected by acetaminophen overdose.

◆ Biochemical mechanism of N-acetylcysteine: As glutathione is used to conjugate the acetaminophen toxic metabolite, the antidote Nacetylcysteine helps to facilitate glutathione synthesis by increasing the concentrations of one of the reactants of the first synthetic step.

CLINICAL CORRELATION

The patient described has all the initial signs of a deliberate overdose of acetaminophen. Normally acetaminophen is cleared by conjugation with either glucuronic acid or sulfate followed by excretion. Metabolism also takes place, producing an active intermediate capable of binding tissue macromolecules. These conjugative and metabolic pathways involve a number of enzymes that may themselves be compromised to such an extent that the threshold for the concentration that constitutes an overdose is substantially lowered. More typically, overdose concentrations are the result of deliberate ingestion, as in this clinical case, or accidental ingestion, often involving either a child who finds a bottle of acetaminophen and consumes its contents or a disoriented elderly person who loses track of how many tablets have been consumed. Usually, the acetaminophen serum level is drawn and plotted on a nomogram to determine the possibility of hepatic damage. Hepatocyte necrosis with clinical manifestations of nausea and vomiting, diarrhea, abdominal pain, and shock may ensue. Few survivors of an overdose have long-term hepatic disease. The initial therapy is gastric lavage, activated charcoal, supportive care, and administration of N-acetylcysteine.

APPROACH TO GLUTHATHIONE AND ACETAMINOPHEN

Objectives

1. Know about the role of glutathione in protection of acetaminophen overdose.

2. Understand that acetaminophen overdose can lead to liver toxicity.

3. Know about the effect of glutathione.

4. Be aware of the mechanism of action of N-acetylcysteine in treating acetaminophen toxicity.

Definitions

Phase I drug metabolism: Oxidative metabolism of drugs usually mediated by cytochrome P450 leading to hydroxylation or epoxidation of substrate compounds.

Phase II drug metabolism: Conjugative metabolism of oxidized drugs usually involving hydration of epoxides by epoxide hydrase producing phenolic derivatives, conjugation by uridine diphosphate (UDP)-glucuronyl transferase to produce glucuronide adducts or S-alkylated adducts, or sulfation by sulfotransferase producing sulfated derivatives.

Drug toxicity: Aberrant reaction to a therapeutic agent often depending on individual variations in either the quantity or activity level of specific drug metabolizing enzymes toward the drugs or on individual genetic polymorphisms of drug metabolism enzymes giving higher or lower activities or product profile produced by the genetic variant versus the normally expressed enzyme.

DISCUSSION

The major pathway of removal of acetaminophen is by formation of a glucuronide conjugate. The reactions required for formation of acetaminophen glucuronide are shown in Figure 40-1 and depend on the generation of activated glucuronic acid. The first phase is the formation of activated glucuronic acid from glucose. Glucose is phosphorylated to glucose 6-phosphate by hexokinase in an adenosine triphosphate (ATP)-requiring reaction, which constitutes the first step of glycolysis, the basal pathway for cellular energy generation. Phosphoglucomutase, which plays a critical role in glycogen formation, converts glucose 6-phosphate to glucose 1-phosphate. Glucose 1-phosphate is activated to uridine diphosphate (UDP)-glucose by UDP-glucose pyrophosphorylase using UTP and producing pyrophosphate as an additional product. Pyrophosphate is rapidly hydrolyzed to 2 mol of phosphate by pyrophosphatase; this pulls the reaction toward the formation of UDP-glucose. This reaction is also on the pathway to the formation of glycogen. The last step in this activation phase

Figure 40-1. Formation of UDP-glucuronate and acetaminophen glucuronide.

is the oxidation of the sixth carbon of glucose in UDP-glucose to the acid level forming UDP-glucuronic acid. This reaction is catalyzed by UDP-glucose dehydrogenase, which also produces 2 mol of nicotinamide adenine dinucleotide (NADH) as an additional product. UDP-glucuronyl transferase, in the next phase of the reaction, catalyzes the transfer of the glucuronide group to the hydroxyl group of acetaminophen forming acetaminophen glucuronide, which is eliminated without toxic effects to the organism.

Alternatively, acetaminophen can be conjugated with organic sulfate for elimination. This alternative pathway shown in Figure 40-2 also consists of

Figure 40-2. Formation of 5′-phosphoadenosine 3′-phosphosulfate (PAPS) and sulfated acetaminophen.

two phases. The first phase consists of preparing activated sulfate for transfer as 5′-phosphoadenosine 3′-phosphosulfate (PAPS), and the second consists of transfer of the sulfate moiety to acetaminophen. In the first step of the first phase, ATP-sulfurylase catalyzes the formation of pyrophosphate and adenosine-5′-phosphosulfate (APS) from ATP and sulfate. In the second step, APS-kinase catalyzes the formation of PAPS and adenosine diphosphate (ADP) from ATP and APS. (The structure for PAPS is shown in Figure 40-2.) In the second phase, phenolsulfotransferase, one of a group of sulfotransferases, transfers the sulfate group from PAPS to acetaminophen to yield adenosine-3′,5′-bisphosphate and acetaminophen sulfate, which is eliminated. In the event that this pathway and the glucuronide forming pathway are both overwhelmed by overdose, more acetaminophen is metabolized by the cytochrome P450 pathway.

Oxidative metabolism of acetaminophen by the cytochrome P450 system, shown in Figure 40-3, is catalyzed by various cytochromes P450 such as CYP2E1 and others. The deleterious product is N-acetyl benzoquinoneimine, an unstable intermediate shown in Figure 40-3, which can

Figure 40-3. Metabolism of acetaminophen.

react with cellular macromolecules damaging them and the integrity of the cells wherein the metabolic alteration occurs. The tissue with the highest concentration of cytochromes P450 is the liver, and it is there (along with kidney) that acetaminophen causes the most damage. In the liver, N-acetyl benzoquinoneimine can form arylated derivatives of protein, lipid, RNA, and DNA, causing destruction of these compounds as well as any larger structure with which they are associated, for example, cellular and subcellular membranes, leading to hepatocyte lysis and loss of cellular contents, such as enzymes, to the circulation. However, N-acetyl benzoquinoneimine may also disrupt Ca²⁺ balance, leading to dramatically increased intracellular Ca²⁺ concentrations, that is, 20 μM versus 0.1 μM, which is the normal concentration. Ca²⁺ is a potent and therefore well-regulated signal. Thus dramatic increases in Ca²⁺ concentration would have deleterious effects on the balance of many cellular processes, especially energy generation. The low serum calcium and elevated liver enzyme serum levels seen in the patient reflect hepatocyte lysis.

What mechanisms protect against this metabolite-induced destruction of cellular integrity? The primary defense against radical metabolite intermediatemediated damage is the glutathione (GSH) system. Glutathione is a tripeptide with an active sulfhydryl group that plays a role in protection of cellular macromolecules from attack by radicals such as organic hydroperoxides or active metabolic intermediates such as N-acetyl benzoquinoneimine. The formation of glutathione is summarized in Figure 40-4. γ-Glutamylcysteine synthetase catalyzes the formation of the dipeptide γ-glutamylcysteine from the amino acids glutamate and cysteine using energy provided by the hydrolysis of ATP to ADP and phosphate. Glutathione synthetase catalyzes the addition of glycine to γ-glutamylcysteine to form the tripeptide glutathione, again using 1 mol of ATP.

The role of the glutathione in detoxifying N-acetyl benzoquinoneimine is shown in Figure 40-3. The adduct acetaminophen glutathionate is no longer toxic to cells and can be excreted without further damage. Any process depleting glutathione levels would compromise the ability of the cell to protect itself against N-acetyl benzoquinoneimine. This includes deficiencies of any of the enzymes involved in the synthesis of glutathione or that keep glutathione in its reduced state as well as any other radical generating process that consumes glutathione. In the case of acetaminophen overdose, in addition to giving activated charcoal to absorb excess stomach and intestinal acetaminophen or its conjugates, a strategy to replenish glutathione concentration is important. The most frequently administered compound is N-acetylcysteine. Easily obtainable and readily soluble, it serves to facilitate glutathione synthesis by increasing the concentrations of one of the reactants of the first synthetic step. The committed dipeptide product being made, glutathione is synthesized to replenish depleted supplies. N-acetylcysteine seems most effective when given less than 10 hours after ingestion of acetaminophen but is recommended within the first 35 hours after ingestion.

Figure 40-4. Glutathione formation.

COMPREHENSION QUESTIONS

[40.1] A patient admitted to the emergency room with nausea and vomiting showed low serum potassium, elevated blood enzymes, and acetaminophen blood level above 200 μg/mL. The patient was diagnosed with acetaminophen overdose. Acetaminophen is a widely used analgesic. What is the most probable explanation of how a high dose of acetaminophen might have led to a toxic condition?

A. Acetaminophen itself is toxic.

B. Acetaminophen is metabolized to a potential toxic product.

C. Acetaminophen is metabolized to a potential toxic product that is fully conjugated.

D. Acetaminophen is metabolized to a potential toxic product that is partially conjugated.

E. None of the above.

[40.2] In acetaminophen toxicity, which of the following compounds is a potential toxic intermediate?

A. Acetaminophen glucuronide

B. Acetaminophen sulfate

C. N-Acetyl benzoquinoneimine

D. Acetaminophen glutathionate

E. N-Acetyl-p-aminophenol

[40.3] Glutathione is a critical tripeptide involved in conjugation reactions and in reactions that protect cells from reactive oxygen species. Which of the following components compose glutathione?

A. Glutamic acid, alanine, methionine

B. Glutamine, alanine, cysteine

C. Glutamate, glycine, cysteine

D. Alanine, glycine, cysteine

E. Methionine, glycine, cysteine

Answers

[40.1] D. Acetaminophen is not itself toxic, but its intermediate metabolite N-acetyl benzoquinoneimine can be toxic unless it is adequately conjugated. In the case presented, a high dose of acetaminophen overwhelmed the conjugative processes, allowing the toxic intermediate to interact with body components and thus causing the nausea, vomiting, and elevated blood enzymes observed.

[40.2] C. N-acetyl benzoquinoneimine is toxic, whereas acetaminophen glucuronide, acetaminophen sulfate, and acetaminophen glutathionate are nontoxic acetaminophen conjugates. N-acetyl-p-aminophenol is another name for acetaminophen.

[40.3] C. Glutathione (γ-glutamylcysteinylglycine) is a tripeptide of glutamic acid, cysteine, and glycine in which the amino terminal glutamate residue is in a peptide linkage through its side-chain carboxyl group to the cysteine residue.

BIOCHEMISTRY PEARLS

❖ The major pathway of removal of acetaminophen is by formation of a glucuronide conjugate.

❖ Acetaminophen is oxidized by the cytochrome P450 system, yielding the deleterious product N-acetyl benzoquinoneimine, an unstable intermediate that can react with cellular macromolecules and thus damage them.

❖ The liver has a high concentration of cytochrome P450 and is particularly susceptible to acetaminophen toxicity.

❖ N-acetylcysteine is the antidote to acetaminophen toxicity and facilitates glutathione synthesis by increasing the concentrations of one of the reactants of the first synthetic step.

References

Goodman AG, Gilman LS, eds. The Pharmological Basis of Therapeutics, 10th ed. New York: Mc-graw-Hill, 2001.

HomeCase FilesAcetaminophen Overdose Case File