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CLINICAL PHARMACOLOGY 2003 (PART 18)

This account is confined to therapy directed primarily at the skin. • Pharmacokinetics of the skin • Topical preparations:Vehicles for presenting drugs to the skin; Emollients, barrier preparations and dusting powders;Topical analgesics; Antipruritics; Adrenocortical steroids; Sunscreens • Cutaneous adverse drug reactions • Individual disorders: Psoriasis.Acne, Urticaria, Skin infections It is easy to do more harm than good with potent drugs, and this is particularly true in skin diseases. Many skin lesions are caused by systemic or topical use of drugs, often taking the form of immediate or delayed hypersensitivity. • rate of diffusion of drug from the vehicle to. | SECTION 3 Drugs and the skin SYNOPSIS This account is confined to therapy directed primarily at the skin. Pharmacokinetics of the skin Topical preparations Vehicles for presenting drugs to the skin Emollients barrier preparations and dusting powders Topical analgesics Antipruritics Adrenocortical steroids Sunscreens Cutaneous adverse drug reactions Individual disorders Psoriasis.Acne Urticaria Skin infections It is easy to do more harm than good with potent drugs and this is particularly true in skin diseases. Many skin lesions are caused by systemic or topical use of drugs often taking the form of immediate or delayed hypersensitivity. Pharmacokinetics The stratum corneum superficial keratin layer is both the principal barrier to penetration of drugs into the skin and a reservoir for drugs a corticosteroid may be detectable even 4 weeks after a single application. Drugs are presented in vehicles e.g. cream ointment and their entry into the skin is determined by the rate of diffusion of drug from the vehicle to the surface of the skin this depends on the type of vehicle see below partitioning of the drug between the vehicle and the stratum corneum a physicochemical feature of the individual drug and degree of hydration of the stratum corneum hydration reduces resistance to diffusion of drug . Vehicles bases1 are designed to vary in the extent to which they increase the hydration of the stratum corneum e.g. oil-in-water creams promote hydration see below . Some vehicles also contain substances intended to enhance penetration e.g. squalane p. 306 . Absorption through normal skin varies with site from the sole of the foot and the palm of the hand it is relatively low it increases progressively on the forearm the scalp the face until on the scrotum and vulva absorption is very high. Where the skin is damaged by inflammation bum or exfoliation absorption is further increased. If an occlusive dressing impermeable plastic membrane is used absorption increases by as much .