Pharmacology : Drug Distribution
- Created by: Sophie
- Created on: 16-01-13 15:19
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- Drug Distribution
- Facts
- Once absorbed from site of admin. into plasma, the drug is distributed throughout the body:
- 1. Bulk flow in systemic circulation (~1 min)
- 2. Drugs move into tissues + body fluids by; diffusion and carrier-mediated transport
- Once absorbed from site of admin. into plasma, the drug is distributed throughout the body:
- Binding of drugs to plasma proteins
- At therapeutic concentrations in plasma, many drugs exist mainly in bound form
- The unbound drug is the pharmacologically active drug
- The most important plasma protein is albumin
- The amount of drug that is bound to protein depends on:
- 1. The conc. of free drug
- 2. The affinity of the binding sites
- 3. The concentrationof protein
- Drug interactions
- Binding sites on plasma proteins may bind many diff. types of drug
- Competition occurs between drugs
- If 2 drugs compete in this way (A+B), admin of drug B can reduce the protein binding + hence increase plasma conc. of drug A
- If 2 drugs compete in this way (A+B), admin of drug B can reduce the protein binding + hence increase plasma conc. of drug A
- Competition occurs between drugs
- If a high % is bound then there's potential to be affected by drug interaction
- If a high % is occupied then there's potential to cause drug interaction
- Binding sites on plasma proteins may bind many diff. types of drug
- In tissues
- Drug targets are mosst often in tissues
- Movement of drug from plasma into tissues involves same principles as drug absorption
- Speed of drug movement depends on:
- 1. Bl. flow
- 2. Ability to cross tissue barriers
- 3. pH partition effects
- 4. Partition into body fat
- Speed of drug movement depends on:
- Distribution of drug is not limited to target tissues
- Drug may be higher in liver/kidney than target tissue
- Volume of Distribution
- The apparent volume of distribution (Vd): the volume of fluid required to contain the total amount, Q, of drug at the same conc as present in plasma, Cp
- Vd = Q / Cp
- Size of Vd suggests where the drug goes in the body
- Varies from drug to drug
- Tends to match a body fluid compartment
- Total Body Water (TBF): ~0.5 l/kg
- eg. paracetamol
- Extracellular fluid (ECF): ~0.2 l/kg
- eg. tubocuranine
- Plasma compartment: ~ 0.05 l/kg
- eg. aspririn
- Vd > TBF indicates drug accumulates in specific compartments
- The apparent volume of distribution (Vd): the volume of fluid required to contain the total amount, Q, of drug at the same conc as present in plasma, Cp
- Facts
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