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What is the primary factor that influences the distribution rate of drugs?
Which of the following drugs would most likely have a homogeneous distribution pattern throughout the body?
Which tissue types are characterized by a low perfusion rate?
Which drug is primarily bound to albumin in the blood?
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What happens to the total concentration of a drug in well-perfused organs?
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Which of the following patterns describes the distribution of most drugs?
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Basic drugs are typically bound to which plasma proteins?
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Which of these drugs is not likely to concentrate in a specific tissue?
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Which property of plasma protein-drug binding indicates that one plasma protein can bind only a limited number of drug molecules?
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What does the term 'non-selective' imply in plasma protein-drug binding?
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Which fraction of the drug is responsible for therapeutic effects?
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What can cause a delayed therapeutic effect of a drug in tissues?
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Which characteristic of a drug enhances its passage through the blood-brain barrier?
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What factor does NOT influence the passage of drugs to the fetus through the placenta?
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What is a potential consequence of drug redistribution in the body?
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Which scenario best describes the concept of ion trapping in the fetus?
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Study Notes
Distribution
- Distribution refers to the movement of drug molecules from the bloodstream into various body compartments and tissues.
- The process involves transport across the capillary membrane.
- Drugs are distributed within the body based on their properties and the characteristics of the tissues they reach.
Drug Distribution Patterns
- Plasma-limited distribution: High molecular weight drugs like dextran remain primarily in the plasma.
- Homogeneous distribution: Small, non-ionized molecules like alcohol distribute evenly throughout body fluids.
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Tissue-specific accumulation: Some drugs concentrate in certain tissues:
- Iodine in the thyroid
- Chloroquine in the liver
- Tetracyclines in bones and teeth
- Highly lipophilic drugs in fat tissue
- Non-homogeneous distribution: The majority of drugs distribute unevenly based on their ability to pass through cell membranes and their affinity for different tissues.
Factors Affecting Drug Distribution
- Diffusion Rate: A higher diffusion rate generally leads to a faster distribution of the drug.
- Tissue Affinity: Some drugs have a strong affinity for specific tissues, leading to their accumulation in those areas.
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Blood Flow (Perfusion Rate):
- Well-perfused organs like the kidneys, liver, brain, and heart receive a high blood flow, resulting in faster drug distribution.
- Organs with low perfusion rates, such as skin, resting skeletal muscle, and bone, have a slower drug distribution.
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Plasma Protein Binding:
- Albumin is the primary protein that binds drugs in the bloodstream.
- Acidic drugs (e.g., salicylates, vitamin C, sulfonamides, barbiturates, penicillin, tetracyclines, warfarin, probenesid) bind to albumin.
- Basic drugs (e.g., streptomycin, chloramphenicol, digitoxin, coumarin) bind to alpha-1 and alpha-2 acid glycoproteins, globulins, and lipoproteins.
Properties of Plasma Protein-Drug Binding
- Saturable: Each protein molecule can bind a limited number of drug molecules.
- Non-selective: Different drugs with varying chemical structures and pharmacological effects can bind to the same protein sites.
- Reversible: The bonds between drugs and proteins are weak, allowing for reversible binding and release.
Impact of Plasma Protein Binding
- Unbound Fraction: Only the portion of the drug that is not bound to proteins (unbound fraction) is free to circulate in the plasma and pass through capillary membranes.
- Bound Fraction: The portion of the drug bound to proteins serves as a reservoir, delaying its elimination and prolonging its effect.
Storage and Redistribution
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Storage: Drugs can accumulate in tissues, acting as a drug reservoir. This may:
- Prolong the drug effect.
- Delay the onset of therapeutic action.
- Reduce the overall amount of drug available for activity.
- Redistribution: Some drugs, particularly highly lipophilic ones (like general anesthetics), can redistribute within the body, moving from well-perfused organs (e.g., CNS) to less perfused tissues (e.g., muscles) and eventually to adipose tissue. This can lead to a decline in drug levels at the target site and termination its effect.
Crossing the Blood-Brain Barrier
- The blood-brain barrier restricts the passage of substances into the CNS.
- Only non-ionized, highly lipophilic, small molecules can readily cross the BBB and exert their effects.
- Inflammation can sometimes make the BBB more permeable, allowing certain antibiotics (e.g., penicillin) to pass through.
Passage of Drugs to the Fetus
- The placenta doesn't act as a complete barrier to drug passage.
- Factors influencing simple passive diffusion affect drug transfer to the fetus:
- Placental blood flow
- Molecular size
- Drug solubility in lipids
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Fetal pH (ion trapping):
- The fetal plasma pH (7.0-7.2) is lower than maternal plasma pH (7.4).
- This difference can trap weak basic drugs in the fetal plasma, potentially leading to higher fetal concentrations compared to maternal levels.
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Description
This quiz explores the concept of drug distribution within the body, focusing on how various factors affect the movement of drug molecules from the bloodstream into tissues. It covers different distribution patterns such as plasma-limited distribution, homogeneous distribution, and tissue-specific accumulation. Test your understanding of these crucial pharmacological principles.
