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Rx2013
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The trans-dermal route is of particular interest for drugs that
- have a short half life
- undergo extensive 1st pass metabolism
- require frequent dosing
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Advantages of TDD systmes
- no first pass
- sustained therapeutic drug levels
- self administration
- non invasive
- allows removal of drug
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limitations of TDD
- poor diffusion of large molecules
- skin irritation
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Most important barrier to drug absorption via TDD
- stratum corneum
- keratinized cells
- protein rich
- intercellular lipid layers
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Transport = Flux=(mg/cm2/sec)=P x A x (Cd-Cr)
Permeability Coefficient = P = [D x K(cm/sec)]/h
- A= surface area of patch
- D= diffusivity of drug in membrane (skin)
- K= partition coefficient (patch/skin)
- C= concentration in patch or skin
- h= thickness of membrane (skin)
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TDD drug candidate
- <20 mg/day
- half life less than 10 hours
- molecular weight < 500 daltons
- melting point < 200 celcius
- skin permeability
- partition coeficient between -1 and 4
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Provides protection from external factors during application period
backing
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moderates rate of drug release
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maintains contact with pt skin and incorporates drug and excipients
adhesive
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protects patch during storage; removed prior to application
liner
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Matrix patch
skin controls rate of diffusion
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reservoir patch
- drug in reservoir with rate controlling membrane
- used for drugs that are quickly absorbed
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drug in adhesive multi layer
- membrane in patch controls rate
- use for drugs that are quickly absorbed
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drug in adhesive single layer patch
- skin controls rate of difusion
- drug dispersed in adhesive
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Drug in adhesive patches
- simple construction
- complex formulation
- skin controlled delivery
- skin conformability
- can be a smaller size
- low dose dumping potential
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Reservoir patches
- complex construction
- simple formulation
- membrane controlled delivery
- poor skin conformability & multiple layers
- cannot be a smaller size
- dose dumping potential
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