Comparative and Veterinary Pharmacology

a

b

c

Fig. 2Mode of action of spot-on anti-parasitic drugs (e.g. fipronil or imidacloprid). (a) Oil-based
spot-on administered to skin behind the neck distributes over the skin gradually for 30 days. (b)
Drugs are sequestered by sebaceous glands, which release drugs together with sebum onto skin
over the therapeutic period at a level above that required to kill parasites. (c) Drugs act non-
competitively and with high potency to inhibit flea glutamate-gated chloride channels and with
lower potency to inhibit gamma amino butryric acid (GABA)-gated ones (Narahashi et al. 2007 ).
Re-drawn by George Retseck from his original diagram with permission from the publishers of
Scientific American (Fischetti 2001 )

Drug Delivery Systems in Domestic Animal Species 89