Paper I
2013 September (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Microsomal enzyme induction

Q33 marksShort Notes

Answer

Microsomal enzyme induction is the increase in synthesis (and hence activity) of hepatic microsomal (CYP450) drug-metabolizing enzymes caused by certain drugs/chemicals, resulting in increased metabolism (usually inactivation) of the inducing drug itself and/or other co-administered drugs.

Enzyme inducers

  • Rifampicin, Phenobarbitone, Phenytoin, Carbamazepine, Griseofulvin, chronic alcohol use

Consequences

  • Reduced efficacy of co-administered drugs due to increased metabolism — e.g., oral contraceptives (contraceptive failure), warfarin (reduced anticoagulant effect), corticosteroids
  • Autoinduction — some drugs induce their own metabolism, reducing their own efficacy over time (e.g., carbamazepine)
  • Increased toxicity if the drug has a toxic metabolite (e.g., increased hepatotoxic metabolite formation with isoniazid + rifampicin)
  • Can be used clinically — phenobarbitone induces glucuronyl transferase, used in neonatal hyperbilirubinaemia

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