Week 23 - topical and transdermal

Question 1

Why apply drugs to the skin?

Answer 1

1. Skin surface treatment
2. Targeting of dermal layers (topical drug delivery)
3. Systemic delivery (transdermal drug delivery)

Question 2

Structure of the skin

Answer 2

-The largest organ in the human body
-Approximately 15 % of human body weight with and a
surface area of around 1.8 m2
Layers:
-Stratum corneum
-Viable epidermis
-Dermis
-Subcutaneous fat

Question 3

How do drugs permeate the skin?

Answer 3

-The most obvious route would appear to be via the sweat
ducts and hair follicles / sebaceous glands – but this is not the case
->Permeation occurs directly across the stratum corneum,
which is also the main barrier to drug delivery via this route

Question 4

Permeation: drug factors

Answer 4

-The drug needs to have an intermediate partition
coefficient, i.e. not too polar, not too non-polar
-If a weak acid or a weak base then should be in the
unionised form
-Low molecular weight (<500 Daltons)

Question 5

Permeation: formulation factors

Answer 5

Drug concentration in formulation – maximum “flux”
exhibited when vehicle is saturated
Occlusivity (seals wound to prevent water loss) promotes drug penetration:
-Leads to skin hydration
-Ointments versus creams / gels
-o/w versus w/o creams
-Release from the vehicle – drugs (solutes) held firmly in the
formulation exhibit low fluxes
-Presence of cosolvents and/or penetration enhancers

Question 6

Permeation: biological factors

Answer 6

->Skin structure varies to some extent around the body and
this can affect permeation
Genitals > head & neck > trunk > arm > leg
-Age – dermis not fully mature until 3 to 6 months’ old
-Race – stratum corneum water content varies
-Skin condition (injury, disease)
-Natural variation between individuals