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Questions and Answers
A decrease in drug concentration leads to an increase in the amount of drug absorbed per surface area per time.
False
The larger the surface area of application, the less drug is absorbed.
False
A drug with good solubility in water but poor solubility in lipids can penetrate the skin easily.
False
A shorter contact time with the skin results in greater absorption.
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Premature infants have the same skin absorption rate as young adults.
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Vasoconstricting agents increase the absorption of other drugs.
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Desquamation, or the shedding of the skin layer, decreases the permeability of the skin.
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Inflammation and psoriasis decrease skin absorption.
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A higher partition coefficient of a drug between the vehicle and skin indicates that the drug is more attracted to the vehicle.
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An ideal molecular weight for a drug is 1000 Dalton or more.
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Increasing the contact time of a drug with the skin reduces its absorption.
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The ionized form of a drug penetrates the skin more effectively than the unionized form.
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A higher pKa of a drug indicates that it is more lipophilic.
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Occlusive films reduce skin hydration and enhance drug absorption.
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A drug with a low partition coefficient will have a longer residence time in the skin.
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Increasing the surface area of the skin available for drug absorption will decrease the drug's absorption rate.
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The surface area of hair follicles and sebaceous glands is relatively large compared to the total skin surface.
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Hydrophilic molecules can penetrate the skin easily due to their high solubility in water.
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The absorption capacity of hair follicles and sebaceous glands is very high.
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Regional skin sites with a thicker horny layer have a higher permeation rate for drugs.
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The partition coefficient of a drug affects its ability to penetrate the skin.
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A drug with a high pKa value will have a faster penetration rate through the skin.
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Contact time and residence time have no effect on drug penetration through the skin.
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The solubility of a drug in oil is directly related to its ability to penetrate the skin.
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Study Notes
Biological Factors
- Thinner horny layer increases ease of absorption
- Skin behind ear is thinner and highly perfusable, making it ideal for absorption
- Skin conditions such as inflammation, psoriasis, cuts, abrasions, and dermatitis increase skin absorption
- Premature infants may be born with no stratum corneum, resulting in high drug absorption
- No dramatic difference in skin absorption between young and elderly individuals
Blood Flow
- Vasoconstricting agents, such as topical steroids, can reduce their own absorption or other drugs
- Blood flow affects skin absorption, with increased blood flow increasing absorption
Skin Metabolism
- Prodrugs with good skin absorption can be designed to be metabolized in the skin, entering the systemic circulation as active drugs
- Example: Prednisone
Physico-chemical Factors
- Drug concentration: amount absorbed per surface area per time increases with increase in drug concentration
- Surface area of application: larger surface area results in more drug absorption
- Drug solubility: drug solubility in both water and lipid is required for absorption
- Contact time with skin: longer contact time results in greater absorption
- Partition coefficient (vehicle vs. skin): drug should be attracted to the skin more than the vehicle
- pH and dissociation constant (pKa): unionized form of the drug penetrates better
- Molecular weight, size, and shape: ideal Mwt is 500 Dalton or less
- Skin hydration: occlusive film seals the area, retaining any sweating and limiting absorption
Other Factors
- Hydrophilic molecules may penetrate the skin through 'pores' or openings of hair follicles and sebaceous glands
- Relative surface area of these openings is barely 1% of the total skin surface, and their absorption capacity is very limited
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Description
This quiz covers the biological factors and blood flow factors that influence skin absorption, including skin conditions and individual differences. Test your understanding of these important factors!
