Introduction: What Is Biotransformation?
- Biotransformation refers to the enzymatic and non-enzymatic chemical modification of drugs within the body.
- It is a central component of pharmacokinetics, the broader framework of Absorption, Distribution, Metabolism, and Elimination (ADME).
- While often equated simply with "drug metabolism," biotransformation has consequences well beyond mere drug inactivation: it determines bioavailability, duration of action, formation of active or toxic metabolites, and is a major source of interindividual variability in drug response.
For the emergency physician, understanding biotransformation is essential for:
- Predicting altered drug behaviour in liver disease, renal failure, and extremes of age
- Understanding why prodrugs may fail or toxic metabolites accumulate in overdose
- Recognising drug interactions that cause unexpected toxicity or treatment failure
- Rationalising dose adjustments in the critically ill
Sites of Biotransformation
The liver is the primary and dominant site of drug biotransformation. However, extrahepatic metabolism occurs and is clinically significant:
| Site | Significance |
|---|---|
| Liver | Primary site; rich in CYP450 enzymes; first-pass metabolism |
| Intestinal epithelium | Contributes to first-pass effect for oral drugs; CYP3A4 present |
| Kidney | Significant for selected drugs; glucuronide hydrolysis |
| Blood/plasma | Esterases (e.g. succinylcholine, remifentanil, atracurium) |
| Brain | Emerging significance for CNS-active drugs |
| Lung | Minor role; relevant for inhaled agents |
- First-pass metabolism occurs when an orally absorbed drug passes through the intestinal wall and portal circulation to the liver before reaching systemic circulation.
- This can dramatically reduce bioavailability.
- For example, a drug with 70% first-pass extraction may require much higher oral doses than IV doses to achieve equivalent plasma concentrations.
Consequences of Biotransformation
Biotransformation does not always mean inactivation. The consequences are diverse and clinically important:
| Consequence | Mechanism | ED Example |
|---|---|---|
| Drug inactivation | Active drug → inactive metabolite | Fentanyl → norfentanyl (inactive) |
| Active metabolite formation | Active drug → pharmacologically active metabolite | Diazepam → desmethyldiazepam (active, long-acting) |
| Prodrug activation | Inactive prodrug → active drug | Codeine → morphine (via CYP2D6) |
| Toxic metabolite formation | Drug → reactive/toxic product | Paracetamol → NAPQI (hepatotoxic) |
- The concept of bioactivation is particularly important in toxicology.
- Metabolism can convert an apparently benign substance into a reactive intermediate capable of causing organ damage, as seen in paracetamol overdose where NAPQI production overwhelms glutathione stores, leading to hepatocellular necrosis.
Phase I Metabolism
- Phase I reactions chemically modify the drug structure, typically introducing or unmasking a functional group (-OH, -NH₂, -SH, -COOH).
- The net result is usually a more polar, water-soluble compound that may be pharmacologically active, inactive, or toxic.
Types of Phase I Reactions
| Reaction Type | Description | Example |
|---|---|---|
| Oxidation | Most common; adds oxygen or removes hydrogen | CYP450-mediated hydroxylation of most lipophilic drugs |
| Reduction | Adds hydrogen/electrons; less common | Ketone → alcohol (e.g. haloperidol → reduced haloperidol) |
| Hydrolysis | Cleavage by water (esterases, amidases) | Succinylcholine → succinylmonocholine by plasma cholinesterase; remifentanil by tissue esterases |
| Dehydrogenation | Removal of hydrogen | Ethanol → acetaldehyde by alcohol dehydrogenase |
The Cytochrome P450 (CYP) Enzyme System
- The CYP enzyme system is the most important enzymatic system for Phase I metabolism of therapeutic drugs.
- CYP enzymes are haem-containing monooxygenases located primarily in the endoplasmic reticulum of hepatocytes.
The overall reaction can be summarised as:
$$\text{Drug} + O_2 + NADPH + H^+ \xrightarrow{\text{CYP450}} \text{Drug-OH} + H_2O + NADP^+$$
The CYP isoforms most relevant to emergency and anaesthetic drugs are:
| CYP Isoform | Key Substrates | Notes |
|---|---|---|
| CYP3A4/5 | Midazolam, fentanyl, alfentanil, lidocaine, bupivacaine, rocuronium | Most abundant hepatic CYP; also intestinal |
| CYP2D6 | Codeine, oxycodone, metoprolol, tramadol | Highly polymorphic; poor vs. ultra-rapid metabolisers |
| CYP2E1 | Ethanol, paracetamol, volatile anaesthetics, isoniazid | Induced by ethanol; generates reactive oxygen species |
| CYP2B6 | Ketamine, propofol (minor), methadone | Polymorphic |
| CYP1A2 | Theophylline, caffeine, clozapine | Induced by smoking |
Esterases
Esterases are a separate and critically important class of Phase I enzymes:
- Plasma cholinesterase (pseudocholinesterase / butyrylcholinesterase): Hydrolyses succinylcholine, mivacurium, and ester local anaesthetics (procaine, chloroprocaine, cocaine). Genetic deficiency leads to prolonged neuromuscular blockade with succinylcholine.
- Tissue esterases: Non-specific esterases in blood and tissues metabolise remifentanil and atracurium/cisatracurium (the latter also undergoes Hofmann elimination, a spontaneous, non-enzymatic pH- and temperature-dependent process).
Phase II Metabolism
Phase II reactions conjugate the drug or its Phase I metabolite with an endogenous molecule, producing a larger, more polar, and typically water-soluble compound that is readily excreted in urine or bile.
| Phase II Reaction | Conjugate Added | Example Drug |
|---|---|---|
| Glucuronidation (UGT enzymes) | Glucuronic acid | Morphine → morphine-6-glucuronide (active); paracetamol; propofol |
| Sulfation (SULT enzymes) | Sulfate | Paracetamol; dopamine; steroids |
| Acetylation (NAT enzymes) | Acetyl group | Isoniazid, hydralazine, procainamide |
| Glutathione conjugation (GST enzymes) | Glutathione | NAPQI (paracetamol's toxic metabolite); reactive intermediates |
| Methylation | Methyl group | Catecholamines (COMT), histamine |
- Key point: Phase II metabolites are generally pharmacologically inactive and readily excreted.
- However, notable exceptions exist, morphine-6-glucuronide (M6G) is a potent opioid agonist that accumulates in renal failure (as glucuronides depend on renal excretion), causing prolonged or recurrent respiratory depression.
Phase II reactions do not always require prior Phase I transformation, some drugs (e.g. paracetamol at therapeutic doses, propofol) undergo direct conjugation.
Factors Causing Interindividual Variability in Biotransformation
Variability in drug metabolism can result in greater than 100-fold differences in drug exposure between individuals, this is one of the most clinically significant sources of variable drug response.
Pharmacogenetics
Genetic polymorphisms in CYP enzymes are a major cause of variability. The classification of metaboliser phenotypes is particularly important for CYP2D6:
| Phenotype | Genotype | Clinical Consequence |
|---|---|---|
| Poor metaboliser (PM) | Two non-functional alleles | Reduced activation of prodrugs (e.g. codeine → minimal morphine); accumulation of parent drug |
| Intermediate metaboliser (IM) | One non-functional allele | Intermediate response |
| Extensive metaboliser (EM) | Normal | Expected response |
| Ultra-rapid metaboliser (UM) | Gene duplication | Rapid prodrug activation → toxicity; e.g. codeine → morphine toxicity with neonatal death reported |
For CYP2C9 (warfarin, phenytoin), polymorphisms alter dose requirements significantly, relevant when managing anticoagulant-related bleeding in the ED.
For acetylation (NAT2), slow acetylators metabolise isoniazid and procainamide more slowly, with higher plasma levels and increased risk of toxicity (peripheral neuropathy, lupus-like syndrome).
Age
Biotransformation activity is age-dependent:
| Age Group | Effect on Metabolism |
|---|---|
| Neonates/infants | Reduced CYP activity at birth; increases rapidly to peak at ~2 years |
| Children (2-12 years) | Often higher weight-adjusted metabolic rates than adults |
| Elderly | Reduced hepatic mass, blood flow, and CYP activity; phase I reactions more affected than phase II |
Other Modifiers
| Factor | Effect |
|---|---|
| Liver disease | Reduced Phase I and Phase II capacity; also reduced first-pass effect → increased bioavailability of high-extraction drugs |
| Renal disease | Indirect impairment of hepatic biotransformation; accumulation of phase II metabolites (e.g. M6G) |
| Smoking | Induces CYP1A2; reduces effect of theophylline and some antipsychotics |
| Diet | Grapefruit juice inhibits CYP3A4 → increased plasma levels of many drugs |
| Alcohol | Acute inhibition; chronic use induces CYP2E1 |
| Drug interactions | Inhibitors/inducers alter metabolism of co-administered drugs |
Enzyme Induction and Inhibition
Enzyme Induction
Inducers increase the expression or activity of CYP enzymes, increasing the rate of metabolism of substrates and reducing their effect.
- Onset: days to weeks (requires new protein synthesis)
- Clinically important inducers: rifampicin (potent CYP3A4 inducer), phenytoin, carbamazepine, phenobarbitone, St John's Wort
Enzyme Inhibition
Inhibitors reduce CYP activity, increasing plasma concentrations of substrates, potentially causing toxicity.
- Competitive inhibition: Reversible; occurs immediately
- Mechanism-based (irreversible) inhibition: Inhibitor is metabolised to a reactive intermediate that permanently inactivates the enzyme (e.g. erythromycin, clarithromycin, some azole antifungals on CYP3A4)
| Inhibitor | CYP Inhibited | Clinical Risk |
|---|---|---|
| Fluconazole | CYP2C9, CYP3A4 | ↑ Warfarin effect → bleeding |
| Amiodarone | CYP2D6, CYP2C9 | ↑ Digoxin, ↑ warfarin levels |
| Macrolide antibiotics | CYP3A4 | ↑ Midazolam, statin toxicity |
| Cimetidine | Multiple CYPs | Historical concern; broad inhibitor |
Hepatic Clearance Concepts
Hepatic clearance depends on three factors: hepatic blood flow ($Q_H$), the fraction of drug unbound in plasma ($f_u$), and intrinsic clearance ($CL_{int}$):
$$CL_H = Q_H \cdot \frac{f_u \cdot CL_{int}}{Q_H + f_u \cdot CL_{int}}$$
This relationship defines two important drug categories:
High Extraction Ratio Drugs (ER > 0.7)
- Clearance is primarily flow-limited, proportional to hepatic blood flow
- First-pass effect is extensive; oral bioavailability is low
- Examples: lidocaine, morphine, propranolol, fentanyl
- In shock states (↓ hepatic blood flow), clearance of these drugs falls significantly, clinical implication: reduce doses of flow-limited drugs in haemodynamic compromise
Low Extraction Ratio Drugs (ER < 0.3)
- Clearance is capacity-limited, depends on enzyme activity and protein binding
- Less affected by changes in blood flow; more sensitive to changes in protein binding and enzyme activity
- Examples: warfarin, phenytoin, diazepam
- In hepatic disease or with enzyme inhibitors, these drugs accumulate
Emergency Medicine Relevance
Overdose and Toxicology
- Paracetamol overdose is the archetype of biotransformation-driven toxicity.
- At therapeutic doses, paracetamol is safely conjugated by glucuronidation and sulfation.
- After overdose, these pathways saturate and a greater proportion undergoes CYP2E1-mediated oxidation to NAPQI (N-acetyl-p-benzoquinone imine).
- NAPQI is detoxified by glutathione conjugation, but glutathione stores are exhausted in overdose, leaving free NAPQI to cause covalent binding to hepatocyte proteins and hepatocellular necrosis.
- N-acetylcysteine (NAC) acts as a glutathione precursor/substitute, its efficacy diminishes with time, hence the urgency of early administration.
- CYP2E1 induction by chronic alcohol use and fasting (depletes glutathione) increases susceptibility to paracetamol hepatotoxicity.
- Codeine toxicity in ultra-rapid metabolisers: Codeine is a prodrug requiring CYP2D6-mediated O-demethylation to morphine.
- Ultra-rapid metabolisers (common in certain North African and Middle Eastern populations) generate morphine at unusually high rates, risking respiratory depression, particularly dangerous in neonates breastfed by ultra-rapid metaboliser mothers.
- Succinylcholine apnoea: Plasma cholinesterase (pseudocholinesterase) deficiency, inherited or acquired (liver disease, organophosphate poisoning, pregnancy), prevents normal hydrolysis of succinylcholine, resulting in prolonged neuromuscular blockade.
- In the ED, this presents as the patient who cannot be extubated after RSI.
- Management is supportive ventilation and monitoring.
- Dibucaine number can quantify the degree of enzyme dysfunction.
Altered Metabolism in the Critically Ill
- Hepatic failure: Reduced first-pass metabolism increases bioavailability of high-extraction drugs (e.g. morphine, midazolam); reduced Phase I and II capacity prolongs drug effects
- Renal failure: M6G and other glucuronide conjugates accumulate, causing prolonged opioid effect, particularly important with morphine. Prefer fentanyl (no active renal metabolites) or hydromorphone cautiously
- Shock states: Reduced hepatic blood flow decreases clearance of flow-limited drugs; lidocaine infusions (e.g. for arrhythmia) may accumulate, causing CNS toxicity
- Hypothermia: Hofmann elimination of atracurium is temperature-dependent, at lower temperatures, this degrades more slowly
Drug Interactions in the ED
- Patients receiving amiodarone have inhibited CYP2D6 and CYP2C9, warfarin effect is potentiated, increasing bleeding risk
- Rifampicin as empirical meningitis cover can dramatically reduce levels of co-administered CYP3A4 substrates including midazolam
- Recognising that prodrug failure (e.g. clopidogrel in CYP2C19 poor metabolisers) may explain lack of antiplatelet response in ACS patients
Age-Related Dosing
- Neonates and the elderly represent the extremes of biotransformation capacity.
- Neonatal CYP activity is limited, prolonged drug effects and unexpected toxicity can occur at standard weight-based doses.
- Elderly patients have reduced hepatic mass and blood flow, Phase I reactions are more impaired than Phase II, favouring drugs with purely conjugative metabolism (e.g. lorazepam over diazepam in the elderly, as lorazepam undergoes direct glucuronidation without prior oxidation).