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Questions and Answers
Explain how the first-pass effect impacts the bioavailability of orally administered drugs.
Explain how the first-pass effect impacts the bioavailability of orally administered drugs.
The first-pass effect reduces the concentration of a drug significantly before it enters systemic circulation, thereby decreasing its bioavailability.
What are two methods to prolong the action of drugs in the body?
What are two methods to prolong the action of drugs in the body?
Prolonging absorption from the site of administration and increasing plasma protein binding are two methods to prolong drug action.
Define clearance in the context of drug metabolism.
Define clearance in the context of drug metabolism.
Clearance is the theoretical volume of plasma from which a drug is completely removed per unit time.
What is the significance of a drug's half-life, and how is it calculated?
What is the significance of a drug's half-life, and how is it calculated?
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Name three categories of drugs contraindicated for lactating mothers and explain why.
Name three categories of drugs contraindicated for lactating mothers and explain why.
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What is biotransformation in the context of drug metabolism?
What is biotransformation in the context of drug metabolism?
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What are Phase-I reactions in biotransformation?
What are Phase-I reactions in biotransformation?
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Can you provide an example of a drug undergoing hydroxylation as a Phase-I metabolic reaction?
Can you provide an example of a drug undergoing hydroxylation as a Phase-I metabolic reaction?
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What role do Phase-II reactions play in drug metabolism?
What role do Phase-II reactions play in drug metabolism?
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How does enzyme induction affect the efficacy of oral contraceptives?
How does enzyme induction affect the efficacy of oral contraceptives?
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Describe the phenomenon of autoinduction with an example.
Describe the phenomenon of autoinduction with an example.
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What is the consequence of enzyme induction on the metabolism of endogenous substrates?
What is the consequence of enzyme induction on the metabolism of endogenous substrates?
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What is the therapeutic use of phenobarbitone in the context of jaundice?
What is the therapeutic use of phenobarbitone in the context of jaundice?
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Study Notes
Biotransformation
- Biotransformation or metabolism is the chemical alteration of a drug in the body.
- It involves two main phases: Phase I (Nonsynthetic reactions) and Phase II (Synthetic or conjugation reactions).
Phase I Reactions
- Involve converting the parent drug into more polar metabolites.
- Examples: Oxidation (Barbiturates), Reduction (Chloramphenicol), Hydrolysis (Procaine).
Phase II Reactions
- Involve coupling between the drug or its metabolites and endogenous substrates (glucuronic acid, sulfuric acid, acetic acid, amino acid).
- Key conjugation reactions:
- Glucuronide conjugation (e.g. chloramphenicol).
- Sulfate conjugation (e.g. Steroids).
- Glycine conjugation (e.g. Aspirin).
- Acetylation (e.g. Sulfonamides).
- Methylation (e.g. Histamine).
Enzyme Induction
- Enzyme induction refers to the increased synthesis of enzymes that metabolize drugs.
Consequences of Microsomal Enzyme Induction
- Decreased intensity or duration of action of drugs inactivated by metabolism (e.g., contraceptive failure).
- Increased intensity of action of drugs activated by metabolism (e.g., paracetamol toxicity).
- Tolerance: Drug induces its own metabolism (autoinduction) (e.g., carbamazepine, rifampin).
- Increased metabolism of endogenous substrates (steroids, bilirubin).
- Intermittent use of inducers can interfere with the dose adjustment of other drugs (e.g., oral anticoagulants (Warfarin)).
Possible uses of enzyme induction
- Congenital nonhaemolytic jaundice: Phenobarbitone hastens clearance of jaundice (due to deficient glucuronidation of bilirubin).
- Cushing's syndrome: Phenytoin can reduce manifestations by enhancing adrenal steroid degradation.
- Chronic poisonings: Faster metabolism of accumulated poisonous substances.
- Liver diseases.
Drugs that inhibit drug-metabolizing enzymes
- Allopurinol
- Omeprazole
- Erythromycin
- Ketoconazole
- Ciprofloxacin
- Isoniazid
First-pass Effect
- The first-pass effect is the significant reduction of drug concentration before reaching systemic circulation due to metabolism in the liver.
- Drugs affected by the first-pass effect:
- Phenobarbitone
- Aspirin
- Isoprenaline
- Propranolol
- Lidocaine
- Hydrocortisone
- Salbutamol
Excretion
- Excretion is the passage of a drug out of the body.
Methods to Prolong Drug Action
- Prolonging absorption from the administration site
- Increasing plasma protein binding
- Retarding metabolism rate
- Retarding renal excretion
Drugs Contraindicated in Lactating Mothers
- Sulfonamides, Tetracycline, Chloramphenicol, Isoniazid
- Morphine, Sedative, hypnotic drugs (Diazepam)
- Phenothiazines, Lithium
- Antithyroid drugs, radioactive iodine
- Senna and related purgatives, metronidazole
- Anticancer drugs, corticosteroids
Clearance
- Clearance is the theoretical volume of plasma from which a drug is completely removed per unit time.
- Calculation: CL = Rate of elimination / Concentration of drug (in blood or plasma)
Half-life (t½)
- The time it takes for the plasma concentration or the amount of drug in the body to be reduced by 50%.
Steady State
- Steady state is achieved when the amount of drug administered equals the amount eliminated.
- This typically occurs after about 5 times the half-life (t½).
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Description
This quiz tests your knowledge on biotransformation, covering essential concepts like Phase I and Phase II reactions, and enzyme induction. Understand how drugs are metabolized in the body, including various conjugation reactions and their implications. Challenge yourself to reinforce your understanding of this critical topic in pharmacology.
