Pharmacology : Drug Absorption
- Created by: Sophie
- Created on: 16-01-13 14:53
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- Drug Absorption
- Facts
- The movement of a drug from the site of admin, into plasma
- Drugs must cross one or more biological barriers eg cell membranes
- 4 main ways they cross cell membrane:
- Diffusion through lipid; Diffusion through aquaporins; Solute carrier, membrane transporter; Pinocytosis
- Diffusion through lipids + carrier-mediated transport are important
- Aquaporins are too small to allow drug molecules through
- 4 main ways they cross cell membrane:
- Drug molecules move around the body in 2 ways:
- Bulk flow
- Diffusion
- Drug Diffusion
- Drugs diffuse through biological membranes along a conc. gradient
- 1. Membranes are predominantly lipids
- 2. Lipids are non-polar solvents
- 3. Non-polar solvents dissolve non-polar substances
- 4. Non-polar substances cross membranes easily
- 3. Non-polar solvents dissolve non-polar substances
- 2. Lipids are non-polar solvents
- 1. Membranes are predominantly lipids
- Close correlation between lipid solubility and the permeability of the cell membrane to diff. substances
- Drugs diffuse through biological membranes along a conc. gradient
- Lipid Solubility of Drug
- Degree of lipid solubility varies with chemical composition of drug
- Solubility measured by oil-water partition coefficient
- pH and ionisation
- May drugs are weak acids or bases + so exist in their ionised/ unionised form
- Ionised molecules have very low lipid solubility so cannot cross membrane
- Unionised, uncharged molecules; their lipid solubility depends on chemical nature
- Mostly lipid soluble
- Degree of ionisation can be measured using Henderson- Hasselbach equation
- For a weak base:
- BH+ <--> B + H+
- pKa = pH + log10 (BH+/B)
- BH+ <--> B + H+
- For a weak base:
- May drugs are weak acids or bases + so exist in their ionised/ unionised form
- Effects of drug ionisation
- Concept applies to movement of drugs into other cellular compartmens
- Ionisation affects not only rate at which drugs permeate membranes, but the steady-state distribution of drug molecules between aq compartments
- Ion trapping
- 1. Unionised drug moves across biological membrane
- 2. Enters environment of diff pH
- 3. Drug becomes ionised
- 4. Ionised drug can't cross biological membrane so is trapped
- 3. Drug becomes ionised
- 2. Enters environment of diff pH
- 1. Unionised drug moves across biological membrane
- Ion trapping
- By Carrier-Mediated transport
- Many cells have specialised transmembrane transport systems
- eg. for sugars, amino acids, metal ions
- Carried out by transmembrane proteins called transporters
- Many cells have specialised transmembrane transport systems
- Facts
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