🧪 General Principles🟢 BasicDrug Metabolism

Enzyme Induction and Inhibition

How one drug can speed up or slow down the liver enzymes that break down another drug, changing its level in the body and causing interactions.

Key points

Many drugs are broken down by a family of liver enzymes called cytochrome P450, and some drugs change how active these enzymes are.
An enzyme inducer increases the amount of these enzymes over days to weeks, so drugs broken down by them are cleared faster and their effect falls.
An enzyme inhibitor blocks the enzymes quickly, so a drug that relies on them for its clearance builds up and its effect, and its toxicity, rises.
This is a common cause of drug interactions: an inducer can make another drug fail, while an inhibitor can push another drug towards toxic levels.
Because induction needs new enzyme to be made, it comes on and wears off slowly, whereas inhibition can happen with the first few amounts given.
Grapefruit juice inhibits the enzyme CYP3A4 in the gut wall, and it can raise the blood level of several affected drugs.

Memory trick

Inducers speed up and lower levels (slow, needs new enzyme); inhibitors block and raise levels (fast, from the start); grapefruit inhibits gut enzymes.

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🧪 General Principles🟢 BasicDrug Metabolism

Enzyme Induction and Inhibition

How one drug can speed up or slow down the liver enzymes that break down another drug, changing its level in the body and causing interactions.

Key points

Many drugs are broken down by a family of liver enzymes called cytochrome P450, and some drugs change how active these enzymes are.
An enzyme inducer increases the amount of these enzymes over days to weeks, so drugs broken down by them are cleared faster and their effect falls.
An enzyme inhibitor blocks the enzymes quickly, so a drug that relies on them for its clearance builds up and its effect, and its toxicity, rises.
This is a common cause of drug interactions: an inducer can make another drug fail, while an inhibitor can push another drug towards toxic levels.
Because induction needs new enzyme to be made, it comes on and wears off slowly, whereas inhibition can happen with the first few amounts given.
Grapefruit juice inhibits the enzyme CYP3A4 in the gut wall, and it can raise the blood level of several affected drugs.

Memory trick

Inducers speed up and lower levels (slow, needs new enzyme); inhibitors block and raise levels (fast, from the start); grapefruit inhibits gut enzymes.

All of Pharmacology More General Principles