Paracetamol Overdose and NAC Treatment

Paracetamol Overdose Pathways

  • The diagram illustrates potential reactions of paracetamol in an overdose situation.
  • Paracetamol can undergo different pathways, some toxic and some not.
  • If one pathway is overloaded due to excessive paracetamol, the reaction shifts to another pathway.

Four Ways NAC Works

  • NAC (N-acetylcysteine) has four ways of helping in paracetamol overdose:
    1. Replenishes glutathione (GSH).
    2. Binds directly to NAPQI (N-acetyl-p-benzoquinone imine), the toxic intermediate.
    3. Enhances sulfation.
    4. Acts as an antioxidant.

NAC Enhances Sulfation

  • NAC can increase the sulfation of paracetamol, even before it turns into NAPQI.
  • This leads to more acetaminophen sulfate and less NAPQI production.

Glutathione's Role

  • Glutathione (GSH) conjugates with NAPQI, making it nontoxic.
  • If there's too much NAPQI, the GSH pathway can be overloaded.

NAPQI: The Toxic Intermediate

  • NAPQI is the primary toxic intermediate that binds to proteins in the liver and kidneys, causing damage.
  • The goal is to either stop the production of NAPQI or conjugate it with GSH to neutralize it.

Timing of NAC Administration

  • NAC should be administered as early as possible after a paracetamol overdose.
  • It takes approximately 8-24 hours to completely deplete glutathione (GSH) stores.
  • Early administration can prevent liver damage by ensuring sufficient GSH to conjugate with NAPQI.
  • If NAC is given too late, the patient may already be in the stage of acute toxicity and require more aggressive treatments.

Initial Steps in Overdose Treatment

  • Gastric lavage (stomach pump) is used to wash out any unabsorbed paracetamol from the stomach.
  • Activated charcoal is administered to absorb paracetamol in the gastrointestinal tract, preventing further absorption into the body.
  • Clay can also bind to organic molecules and may be used.

Standard Antidote Treatment

  • The standard antidote for paracetamol overdose is intravenous (IV) infusion of N-acetylcysteine (NAC).

NAC Mechanisms of Action (Detailed)

  1. Precursor to Glutathione (GSH):

    • NAC is a precursor to glutathione, the rate-limiting amino acid required for GSH synthesis.
    • NAC→Cysteine→GlutathioneNAC \rightarrow Cysteine \rightarrow Glutathione
      • NAC replenishes glutathione levels by providing cysteine, a building block for glutathione synthesis.
    • Glutathione is crucial for detoxifying NAPQI.
    • This process takes time.

  2. Direct Binding to NAPQI:

    • Both glutathione and NAC act as nucleophiles and can bind directly to NAPQI, which is an electrophile.
    • The sulfhydryl group (SH) in both molecules is nucleophilic and reacts with NAPQI.
    • At physiological pH (7.4), about one-third of the sulfhydryl groups are fully negatively charged, making them highly reactive.
    • NAPQI+NAC→Non−Toxic ConjugateNAPQI + NAC \rightarrow Non-Toxic \ Conjugate or NAPQI+GSH→Non−Toxic ConjugateNAPQI + GSH \rightarrow Non-Toxic \ Conjugate

  3. Enhancing Sulfation:

    • NAC helps fortify sulfur, which is used by PAPS (3'-phosphoadenosine-5'-phosphosulfate) as a cofactor.
    • This is useful because the body's supply of PAPS is limited.
    • By increasing sulfation, more paracetamol is converted to acetaminophen sulfate, a non-toxic metabolite, thus reducing NAPQI formation.

  4. Quenching Free Radicals (Antioxidant Activity):

    • NAC quenches free radicals by donating an electron, stabilizing the free radical.
    • Free radicals are molecules missing an electron, making them highly reactive.
    • These free radicals steal electrons from lipids, proteins, and DNA, causing damage (free radical cascade).
    • NAC has a spare pair of electrons in its sulfhydryl group.
    • R∙+NAC→RH+NAC∙(stable)R^{\bullet} + NAC \rightarrow RH + NAC^{\bullet} (stable)

Timing Criticality

  • The timing of NAC administration is critical because glutathione (GSH) must be available to react with NAPQI.
  • Liver damage occurs after GSH is depleted.
  • If the overdose occurred a long time ago, paracetamol may have already converted to NAPQI, depleting GSH.
  • In such cases, NAC is still administered even if the exact time of overdose is unknown.

Adverse Effects of Antidotes

  • Antidotes can have adverse effects, so they should be administered sensibly.
  • Doctors will measure paracetamol levels in the blood to determine the appropriate dose of NAC.

Effects of Other Medications and Substances

  1. Phenobarbital:

    • Phenobarbital is an inducer of certain cytochrome P450 enzymes, particularly those involved in NAPQI formation.
    • It increases the toxicity of paracetamol by inducing the formation of more NAPQI.

  2. Ethanol (Alcohol):

    • Ethanol induces CYP2E1, which also enhances the pathway leading to NAPQI formation.
    • Acute and coadministration of ethanol with paracetamol can compete for the same pathway, potentially lowering initial NAPQI formation.
    • Chronic alcohol consumption will induce CYP2E1 increasing NAPQI

  3. Vitamin E (Alpha-Tocopherol):

    • Vitamin E is a lipophilic antioxidant that helps neutralize reactive oxygen species.
    • Important antioxidant protector of cell membranes.
    • Vitamin E sits in cell membranes and quenches lipid peroxy radicals.
    • Vitamin E would help decrease paracetamol toxicity.

Monitoring Liver Damage

  • To monitor liver damage, doctors can measure:
    1. Liver Transaminases (ALT, AST):
      • These enzymes are released into the bloodstream when liver cells are damaged.
      • Elevated levels indicate liver damage.
    2. Bilirubin Levels:
      • Bilirubin is a waste product from the breakdown of heme (hemoglobin).
      • The liver clears bilirubin, but in liver damage, bilirubin levels increase, leading to jaundice.
      • Bilirubin comes from hemoglobin. The heme group that holds the iron in place breaks open, becomes a greenish color, and then is oxidized further through a yellow color.
      • Liver clearing bilirubin is important.
    3. INR (International Normalized Ratio):
      • Measures blood clotting time.
      • The liver makes clotting factors, so damage leads to decreased clotting factor production.
      • Increased INR indicates slower blood clotting due to impaired liver function.
      • The liver makes albumin, the protein that thickens the blood. With liver damage, less albumin is produced.

Normal Dose of Paracetamol vs. Overdose

  • A standard dose of paracetamol (two 500mg tablets, not more than eight tablets in 24 hours) does not typically cause liver damage.
  • At normal doses, 90% of paracetamol is metabolized through sulfation and glucuronidation (phase II pathways), which are non-toxic.
  • Only 10% is converted to NAPQI, and the glutathione stores are sufficient to mop up this small amount.
  • Overdoses exceed this limit, leading to NAPQI accumulation and glutathione depletion.

Drugs that May Cause Cancer

  • Alkylating agents are strongly genotoxic drugs used to treat aggressive cancers.
  • They cause genetic damage, which can initiate cancer development.

Drugs that May Cause Birth Deformities

  • Thalidomide is a drug known to cause birth deformities.

Product Information Document

  • Product information documents contain information about toxicology, adverse drug reactions, dose-limiting ADRs (adverse drug reactions), target organ toxicity, and susceptible populations.