Consequences of Microsomal Enzyme Induction

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Chapter: Essential pharmacology : Pharmacokinetics; Metabolism Excretion Of Drugs, Kinetics Of Elimination

Decreased intensity and/or duration of action of drugs that are inactivated by metabolism, e.g. failure of contraception with oral contraceptives.


CONSEQUENCES OF MICROSOMAL ENZYME INDUCTION

 

Decreased intensity and/or duration of action of drugs that are inactivated by metabolism, e.g. failure of contraception with oral contraceptives.

 

Increased intensity of action of drugs that are activated by metabolism. Acute paracetamol toxicity is due to one of its metabolites—toxicity occurs at lower doses in patients receiving enzyme inducers.

 

Tolerance—if the drug induces its own metabolism (autoinduction), e.g. carbamazepine, rifampin.

 

Some endogenous substrates (steroids, bilirubin) are also metabolized faster.

 

Precipitation of acute intermittent porphyria: enzyme induction increases porphyrin synthesis by derepressing δaminolevulenic acid synthetase.

 

Intermittent use of an inducer may interfere with adjustment of dose of another drug prescribed on regular basis, e.g. oral anticoagulants, oral hypoglycaemics, antiepileptics, antihypertensives.

 

Interference with chronic toxicity testing in animals.

 

Drugs whose metabolism is significantly affected by enzyme induction are—phenytoin, warfarin, tolbutamide, imipramine, oral contraceptives, chloramphenicol, doxycycline, theophylline, griseofulvin, phenylbutazone.

 

Possible Uses Of Enzyme Induction

 

Congenital nonhaemolytic jaundice: It is due to deficient glucuronidation of bilirubin; phenobarbitone hastens clearance of jaundice.

 

Cushing’s syndrome: phenytoin may reduce the manifestations by enhancing degradation of adrenal steroids.

 

Chronic poisonings: by faster metabolism of the accumulated poisonous substance.

 

Liver disease.

 

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