Biotransformation in Liver Disease

Biotransformation Basics - Liver's Detox Dance

• Definition: Liver's process of chemically altering substances (drugs, toxins) for elimination.
• Location: Primarily liver (smooth ER, cytosol).
• Goal: Convert lipophilic compounds $\rightarrow$ hydrophilic metabolites for easier excretion (urine, bile).
• Phases:
  • Phase I (Functionalization): Introduces/exposes functional groups.
    • Reactions: Oxidation, reduction, hydrolysis.
    • Key Enzymes: Cytochrome P450 (CYP450) family.
  • Phase II (Conjugation): Covalent addition of endogenous polar molecules.
    • Reactions: Glucuronidation, sulfation, acetylation, glutathione conjugation.
    • Result: ↑ Water solubility, ↑ excretion. 📌 Makes It Water Soluble (MIWS for Phase II).

⭐ Cytochrome P450 (CYP450) enzymes are major sites of drug interactions.

Phase I Impairment - Sick Liver's Slowdown

• Cytochrome P450 (CYP450) enzyme system, crucial for Phase I (oxidation, reduction, hydrolysis), is significantly compromised.
• Liver disease (e.g., cirrhosis, severe hepatitis) leads to:
  • ↓ CYP450 enzyme content & activity.
  • ↓ Hepatic blood flow, reducing drug delivery to hepatocytes.
• Consequences for drug metabolism:
  • ↓ Clearance of drugs reliant on Phase I pathways.
  • ↑ Drug plasma half-life ($t_{1/2}$).
  • ↑ Bioavailability of high hepatic extraction ratio drugs (e.g., propranolol, morphine, verapamil).
  • ↑ Risk of adverse drug reactions & dose-dependent toxicity.

⭐ In patients with cirrhosis, the clearance of drugs like diazepam and theophylline (primarily undergoing Phase I metabolism) can be reduced by up to 50-70%, necessitating careful dose adjustments to avoid toxicity_._

Phase II Impairment - Conjugation Conundrum

• Phase II (conjugation) generally more resilient than Phase I in liver disease.
• Significant impairment can occur, especially in severe cases (e.g., cirrhosis).
• Glucuronidation:
  • Often most affected Phase II pathway.
  • ↓ clearance of drugs like morphine, lorazepam.
  • Leads to ↑ drug $t_{1/2}$ & ↑ toxicity.
• Sulfation:
  • Relatively preserved until advanced disease stages.
• Acetylation:
  • Capacity reduced, particularly in slow acetylators (e.g., isoniazid).
• Clinical Impact:
  • Dose adjustments crucial for drugs primarily eliminated by conjugation.
  • Increased risk of drug accumulation and adverse effects.

⭐ Morphine glucuronidation is markedly reduced in cirrhosis, necessitating careful dose titration to avoid prolonged narcosis and respiratory depression_

Clinical Adjustments - Dosing in Distress

• Liver disease: Impairs drug biotransformation (Phase I & II) & excretion.
• Consequences: ↑ bioavailability, ↓ clearance, ↑ drug half-life ($t_{1/2}$), ↑ toxicity risk.
• Assessment: Child-Pugh & MELD scores guide severity.
• Dosing Strategies:
  • ↓ Dose or ↑ dosing interval.
  • Prefer drugs with renal excretion or preserved Phase II metabolism.
  • Avoid hepatotoxic drugs.
  • Therapeutic Drug Monitoring (TDM) if feasible.
• Key Drug Considerations:
  • Opioids (e.g., morphine): ↓ dose significantly.
  • Benzodiazepines: Prefer 📌 L.O.T. (Lorazepam, Oxazepam, Temazepam).
  • Paracetamol: Limit to <2 g/day in severe cirrhosis.

⭐ "L.O.T." drugs (Lorazepam, Oxazepam, Temazepam) are safer in liver disease due to relatively preserved Phase II glucuronidation, unlike often impaired Phase I (CYP450) reactions.

High‑Yield Points - ⚡ Biggest Takeaways

• Reduced Phase I (CYP450) reactions impair metabolism, especially for high first-pass drugs.
• Phase II conjugation (e.g., glucuronidation) is often better preserved than Phase I.
• Oral drug bioavailability ↑ due to ↓ first-pass effect and portosystemic shunting.
• Hypoalbuminemia causes ↓ protein binding, leading to ↑ free drug fraction and toxicity.
• Impaired biliary excretion affects drugs eliminated via bile, prolonging their effects.
• Drug half-life is typically prolonged, requiring dose adjustments and careful monitoring.
• Risk of hepatic encephalopathy ↑ with sedatives or ammonia-producing drugs.