10 Questions
What is the term used to describe an abnormal drug response due to a genetic defect?
Idiosyncrasy
What is the primary route of drug excretion?
Kidney
What is the process by which the free form of a drug is filtered from the blood into the urine?
Glomerular filtration
Which of the following drugs is actively transported into the lumen of the proximal convoluted tubules?
Penicillin
What is the effect of alkalinization of urine on the excretion of acidic drugs?
Increases excretion
Which of the following is an example of a drug that undergoes tubular reabsorption?
Lipophilic form of a drug
What is the effect of probenecid on the excretion of penicillin?
Decreases excretion
What is the term used to describe the process of excretion of drugs and their metabolites?
Excretion
Which of the following is an active transport system for the excretion of drugs?
Tubular secretion
What is the effect of increasing the pH of urine on the excretion of basic drugs?
Increases excretion
Study Notes
Phase I Metabolism
- Results in inactivation, activation, maintaining activation, or toxification of drugs
- Examples of inactivation: Adrenaline and Noradrenaline → Vanil Mandilic Acid (VMA)
- Examples of activation: Imipramine (Inactive) → Desipramine (Active)
- Examples of maintaining activation: Phenacetin (Active) → Paracetamol (Active)
- Examples of toxification: Methyl alcohol → Formaldehyde → Blindness
Phase II Reactions
- Also known as "Synthetic" or "Conjugation" reactions
- Involve the conjugation of the drug or a metabolite with an endogenous polar compound
- Examples of polar compounds: glucuronic acid, sulphate, glycine, acetate, glutathione, or methyl group
- Mostly result in drug inactivation, with some exceptions
- Examples of exceptions: morphine (active) → morphine 6-glucuronide (active metabolite), minoxidil (inactive) → minoxidil sulphate (active)
Volume of Distribution (Vd)
- The volume of fluid in the body that contains the drug at the same concentration as the blood
- Drugs with high Vd occupy multiple compartments or are concentrated in tissues
- Drugs with low Vd are retained in the vascular compartment due to high molecular weight or high binding to plasma proteins
- Formula to determine the loading dose: A (mg) = Vd X c (mg/ml)
Dialysis
- Can be done only with small Vd, which means most of the drug is present in the circulation
- Example: Aspirin
Metabolism (Biotransformation)
- Chemical reactions that occur mainly in the liver
- Aim to convert lipophilic (lipid soluble) drugs into water-soluble (hydrophilic, ionized, or polar) metabolites
- Sites of biotransformation reactions: liver, plasma, lung, kidney, skin, and GIT
Excretion
- Routes of drug excretion: kidney (renal excretion), and other routes
- Renal excretion: glomerular filtration, tubular reabsorption, and tubular secretion
- Glomerular filtration: a passive process depending on the glomerular filtration rate
- Tubular reabsorption: a passive process, where the unionized (lipophilic) form of the drug undergoes reabsorption
- Tubular secretion: an active process, where some drugs are actively transported into the lumen of the proximal convoluted tubules (PCT) of nephrons
- Examples of drugs that undergo tubular secretion: penicillin, thiazides, loop diuretics, and probenecid
- pH of urine affects the rate of urinary excretion of drugs; Alkalinization of urine increases urinary excretion of acidic drugs as aspirin
Test your knowledge on the different types of metabolism reactions, including inactivation, activation, maintain activation, and toxification. Learn how drugs are transformed into active or inactive metabolites and understand the consequences of these reactions.
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