Drug Metabolism (DMPK)

Drug Metabolism: DMPK

Drug metabolism (also called biotransformation) is the biochemical modification of drugs by the body, primarily in the liver. The goal of metabolism is usually to convert lipophilic drugs into more polar (hydrophilic) metabolites that can be excreted renally. Metabolism is a core component of DMPK — Drug Metabolism and Pharmacokinetics — a fundamental discipline in drug development and clinical pharmacology.

Phase 1 Reactions: Functionalisation

Phase 1 reactions introduce or unmask a polar functional group (–OH, –NH₂, –COOH, –SH) on the drug molecule. They typically increase hydrophilicity modestly and often (but not always) reduce pharmacological activity.

Oxidation — the dominant Phase 1 reaction

The cytochrome P450 (CYP) superfamily is responsible for ~75% of all oxidative drug metabolism. CYPs are haem-containing monooxygenases located primarily in hepatocyte smooth ER (microsomes). The overall reaction:

Drug + O₂ + NADPH + H⁺ → Drug-OH + H₂O + NADP⁺

Key CYP isoforms and their major substrates in NZ clinical practice:

| CYP Isoform | Major substrates | Key inducers | Key inhibitors |

|-------------|-----------------|-------------|----------------|

| CYP3A4 (most abundant; ~30% hepatic CYP) | Midazolam, simvastatin, cyclosporin, HIV protease inhibitors, amlodipine, testosterone | Rifampicin, carbamazepine, St John's Wort | Ketoconazole, erythromycin, grapefruit juice (furanocoumarins) |

| CYP2D6 (highly polymorphic) | Codeine, tramadol, metoprolol, haloperidol, amitriptyline, tamoxifen | Rifampicin (weak) | Fluoxetine, paroxetine, bupropion |

| CYP2C9 | Warfarin (S-form), phenytoin, ibuprofen, losartan | Rifampicin | Fluconazole, amiodarone, trimethoprim |

| CYP2C19 | Omeprazole, clopidogrel (activation), diazepam, sertraline | Rifampicin | Omeprazole (auto-inhibits), fluoxetine, fluvoxamine |

| CYP1A2 | Theophylline, clozapine, caffeine, paracetamol (minor) | Cigarette smoke, omeprazole | Ciprofloxacin, fluvoxamine |

Non-CYP oxidation: MAO-A/B (monoamine oxidase) oxidatively deaminates monoamine neurotransmitters (dopamine, serotonin, noradrenaline); inhibited by MAOIs. Flavin-containing monooxygenases (FMO) oxidise nitrogen and sulfur atoms.

Reduction: Less common; aldehyde reductases convert aldehydes to alcohols; nitroreductases reduce aromatic nitro groups (–NO₂ → –NH₂, as in metronidazole activation); azoreductases.

Hydrolysis: Esterases and amidases cleave ester and amide bonds. Examples: aspirin (acetylsalicylate) → salicylic acid + acetate (plasma esterases); procaine → PABA + diethylaminoethanol; suxamethonium → succinic acid + choline (plasma cholinesterase/butyrylcholinesterase). Important clinical point: suxamethonium apnoea occurs in patients with genetic pseudocholinesterase deficiency (BCHE variants).

Phase 2 Reactions: Conjugation

Phase 2 reactions conjugate a polar endogenous molecule to the Phase 1 metabolite (or directly to the parent drug if it already has a polar group), producing a highly water-soluble product almost always pharmacologically inactive and readily renally excreted.

Glucuronidation — most important Phase 2 reaction

Enzyme: UDP-glucuronosyltransferases (UGTs) in hepatic ER.

Reaction: Drug-OH (or –COOH, –NH₂, –SH) + UDP-glucuronate → Drug-glucuronide (highly polar, large, anionic, renally excreted).

Examples: morphine → morphine-3-glucuronide (M3G, inactive) and morphine-6-glucuronide (M6G, active analgesic — accumulates in renal failure, prolonging effect); paracetamol (minor glucuronide pathway); bilirubin (conjugation to bilirubin glucuronide for biliary excretion); oral contraceptive oestrogens (enterohepatic recirculation: gut bacteria deconjugate glucuronides → free oestrogen reabsorbed).

Sulfation

Enzyme: Sulfotransferases (SULTs) in cytosol.

Reaction: Drug-OH + PAPS (3'-phosphoadenosine-5'-phosphosulfate) → Drug-sulfate.

Examples: paracetamol sulfate (major pathway at therapeutic doses); oestrogens; dopamine-sulfate. Sulfation has limited capacity — saturable at high doses; explains why paracetamol overdose overwhelms sulfation and glucuronidation, shunting to CYP2E1-mediated NAPQI formation.

Acetylation

Enzyme: N-acetyltransferases (NAT1, NAT2) using acetyl-CoA.

Reaction: Drug-NH₂ + Acetyl-CoA → Drug-NHCOCH₃.

Examples: isoniazid, hydralazine, procainamide, sulfamethoxazole, dapsone.

Genetic polymorphism in NAT2: ~50% of NZ Europeans are slow acetylators; rapid acetylators are more common in East Asian populations. Clinical significance:

• Slow acetylators on isoniazid: ↑isoniazid plasma levels → peripheral neuropathy risk (prevented with pyridoxine); ↑lupus risk with hydralazine.
• Slow acetylators on procainamide: ↑lupus risk.

Glutathione conjugation: GSH transfers to electrophilic metabolites, catalysed by glutathione-S-transferases (GSTs). Example: NAPQI (reactive paracetamol metabolite) is detoxified by GSH conjugation → mercapturic acid. In overdose, GSH is depleted; N-acetylcysteine replenishes GSH.

Methylation: COMT (catechol-O-methyltransferase) methylates catecholamines and drugs with catechol groups (levodopa, methyldopa). TPMT (thiopurine S-methyltransferase) inactivates mercaptopurine and azathioprine — TPMT deficiency (genetic) leads to toxic drug accumulation and severe myelosuppression.

First-Pass Effect

The first-pass effect (first-pass metabolism) occurs when drug absorbed from the GI tract enters the portal circulation and passes through the liver before reaching systemic circulation. A highly extracted drug undergoes substantial metabolism, dramatically reducing bioavailability. Examples:

• Glyceryl trinitrate (GTN): oral bioavailability ~1% due to near-complete hepatic extraction; administered sublingually or transdermally.
• Morphine: ~25-35% oral bioavailability; significantly higher IV dose equivalent required.
• Propranolol: ~25% oral bioavailability (variable); high first-pass extraction.
• Lignocaine: not given orally for therapeutic use; extensive first-pass (>95% extraction).

Gut wall metabolism also contributes: CYP3A4 in enterocytes metabolises many drugs (e.g., cyclosporin, midazolam) before they even reach the portal vein. P-glycoprotein (P-gp) efflux in the gut epithelium returns absorbed drug back into the intestinal lumen, further reducing bioavailability.

Drug Interactions Mediated by Drug Metabolism

CYP inhibition (most common, clinically significant):

Inhibition raises plasma concentrations of CYP substrates → toxicity risk.

• Warfarin + fluconazole (CYP2C9 inhibition → ↑S-warfarin → bleeding risk)
• Statins + clarithromycin or ketoconazole (CYP3A4 inhibition → ↑statin → myopathy/rhabdomyolysis)
• Codeine + fluoxetine (CYP2D6 inhibition → ↓codeine→morphine conversion → reduced analgesia)

CYP induction (slower onset, 1-2 weeks):

Induction lowers plasma concentrations of CYP substrates → reduced efficacy.

• Rifampicin + warfarin (CYP2C9/3A4 induction → ↓warfarin → thrombosis risk)
• Carbamazepine + oral contraceptive pill (CYP3A4 induction → ↓oestrogen → contraceptive failure)
• St John's Wort (hypericum) + ciclosporin (CYP3A4 induction → transplant rejection)
• Smoking + clozapine (CYP1A2 induction → ↓clozapine → relapse)

Prodrugs

Prodrugs are pharmacologically inactive compounds converted to active drug by metabolic processes. Strategies: mask polar groups to improve oral absorption; improve stability; reduce toxicity.

Examples:

• Codeine (oral prodrug): CYP2D6 O-demethylation → morphine (active analgesic). CYP2D6 ultra-rapid metabolisers → excess morphine production → toxicity. Poor metabolisers → negligible analgesia.
• Clopidogrel: CYP2C19 conversion to active thiol metabolite that irreversibly blocks ADP P2Y12 receptors. CYP2C19 poor metabolisers (PPI co-prescription) → reduced antiplatelet effect → increased MACE risk.
• Enalapril (ester prodrug of enalaprilat): hydrolysis in intestinal wall/liver → enalaprilat (ACE inhibitor); overcomes poor oral bioavailability of enalaprilat.
• Valaciclovir: valine ester of aciclovir → hydrolysis → aciclovir (3-5× improved oral bioavailability).
• Levodopa: dopamine prodrug; DOPA decarboxylase in CNS converts L-DOPA → dopamine; given with peripheral decarboxylase inhibitor (carbidopa) to reduce peripheral conversion and side effects.

Pharmacogenomics in Drug Metabolism

Genetic polymorphisms in metabolising enzymes create inter-individual variability:

| Gene | Phenotype | Clinical consequence |

|------|-----------|---------------------|

| CYP2D6 | UM (ultra-rapid metaboliser) | Codeine → morphine toxicity; tamoxifen → reduced efficacy |

| CYP2D6 | PM (poor metaboliser, ~7% Europeans) | Codeine analgesia failure; ↑metoprolol exposure (bradycardia) |

| CYP2C19 | PM (~2-4% Europeans, ~20% East Asians) | Clopidogrel failure; ↑omeprazole exposure |

| NAT2 | Slow acetylator (~50% Europeans) | ↑isoniazid toxicity; hydralazine lupus |

| TPMT | Deficient (~0.3% population) | Azathioprine/6-MP toxicity; myelosuppression |

| UGT1A1*28 | Reduced activity | ↑irinotecan toxicity (SN-38 accumulation → diarrhoea) |

These polymorphisms are the foundation of pharmacogenomics and are increasingly tested clinically (e.g., pre-emptive CYP2D6/CYP2C19 genotyping before antidepressant or antiplatelet therapy).