Lecture 12B - Topical Administration (skin) Flashcards
What is the skin?
a complex multilayered membrane
What is the stratum corneum?
the outermost layer of the skin, composed of 15-20 layers of flattened, keratinised cells
What is the principle barrier to drug delivery?
the stratum corneum
To be suitable for topical drug deliver, what should an antimicrobial drug be?
have a low MWt (<500Da)
be moderately lipophilic (log P between 1 and 4)
be effective at a low dose (<10mg/day)
What is passive diffusion modelled by?
Fick’s first law of diffusion
J= -D ΔC/ Δx
What do the components of Fick’s law stand for?
J = flux D = diffusion coefficient C = concentration of diffusing drug x = distance
What should a topical formulation be?
stable, but allowing the drug to be released following application
What should the vehicle of topical formulation allow?
some solubility of the drug, but should not retain the drug by being a very good solvent
What are the types of topical formulation?
liquid formulations
semi solid formulations
solid formulations
What are liquid formulations?
single phase solutions and lotions
Advantages of liquid formulations?
rapid short term input of permeant into the skin
Disadvantages of liquid formulations?
poor residence time on the skin which means low drug delivery
What happens to liquid formulations after application on the skin?
evaporation of the solvent (alcoholic vehicles)
cool, soothing effect on the skin
Examples of liquid formulations?
erythromycin solution, clindamycin lotion
treatment of acne
Semi solid formulation examples?
ointments, gels, creams
Advantages of semi solid formulations?
good residence time on the skin
good acceptance from the patients
What are semi solid formulations?
drug in semi solid base (either hydrophobic e.g. liquid paraffin, glycerides, waxes or hydrophilic e.g. PEG)
What does the base determine?
the release of the drug from the formulation
What are ointments?
hydrophobic, fatty preparations
Advantages of ointments?
highly occlusive
increase of transdermal flux
prolonged drug delivery
Disadvantage of ointments?
messy to use and difficult to wash
What are ointments used on?
dry lesions
Examples of ointments?
mupirocin, fusidic acid (impetigo caused by staph infections)
What are gels?
preparations formed from a liquid phase that have been thickened with other components
What can be used to thicken gels?
PEG, carbomer, vegetable gums such as gum tragacanth, guar gum
What might the liquid phase of gels be?
aqueous, alcoholic, solvent may evaporate
Drug release of gels?
equivalent to that of a solution (unless binding of the drug to the polymer, or high viscous gel)
Examples of a gel?
metronidazole gel for the treatment of rosacea
What are creams made of?
2 phased emulsion
water in oil (w/o) or oil in water (o/w)
What is the most common semi solid topical formulation?
creams
Advantages of creams?
w/o emulsions less greasy than ointments
easier to apply
can be washed off the skin surface
Disadvantages of creams?
less occlusive than ointments
less beneficial in dry skin conditions
Examples of creams?
mupirocin, neomycin, fusidic acid
Examples of solid formulatios?
spray powders
What are spray powders?
hydrocarbon propellant to deposit powders on the skin surface
incorporate a volatile solvent (ethanol) that dissolves some drug prior to evaporation
Examples of spray powders?
miconazole nitrate (fungal skin infections such as athlete’s foot
Advantages of spray powders?
can be given directly to the site of infection and easy to repeat treatment
