Permethrin

Permethrin belongs to a group of medicines called pyrethroids which are anti-parasitic agents. Permethrin is rapidly absorbed across the insect cuticle. The principal physiological action is the induction of electrochemical abnormalities across the membranes of excitable cells, leading to sensory hyperexcitability, inco-ordination and prostration. When presented in aqueous base, the ovicidal activity of permethrin is increased by the addition of an alcohol.

It is assumed that the mode of action against arachnids (mites) is similar.

Paediatric population

Newborns and infants

The safety and efficacy of permethrin in newborns and infants under 2 months of age have not been established since no data are available from prospective trials or larger case series. A limited number of case reports in the treatment of children under 2 months of age presenting with scabies do not suggest specific safety concerns for the use of topical permethrin in this age group, but no definite conclusion can be drawn.

Investigations with the 5% cream in humans revealed an average percutaneous absorption rate of 0.47 ± 0.3% in healthy subjects and of 0.52 ± 0.3% in patients.

Pharmacokinetic properties were studied in adult subjects only (6 healthy volunteers and 6 patients with scabies).

Absorbed permethrin is rapidly broken down by esterases as well as hydrolases. After oral administration, peak plasma concentrations are reached in approximately 4 hours. The isomeric mixture is then excreted in the urine in the form of glucuronides, sulfates etc as cis-trans CI2CA [(3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropane-1-carboxylic acid)] and after oxidation to 3 PBA (3-phenoxybenzoic acid). After oral application, up to 6% is excreted unchanged in the faeces whilst on dermal application, unchanged permethrin is virtually undetectable.

The application of creme rinse to the hair of the volunteers for the recommended application time resulted in extremely low or undetectable level of permethrin metabolites in plasma and urine samples. In vitro studies have shown that permethrin levels on hair were not affected by chlorine in concentrations used in swimming pools.

From acute and chronic toxicity studies there is no evidence indicating the occurrence of previously unknown adverse effects in humans. Furthermore there is no evidence on relevant genotoxic or carcinogenic potential.

In studies on the reproductive toxicity in mice, rats and rabbits after repeated oral administration of permethrin effects were observed only for doses largely exceeding the exposure expected for the topical use of the 5% cream. Following the intended use of this active substance a serious harmful effect on aquatic organisms (daphnia and fish) and terrestric organisms (plants) is expected after passage of the sewage treatment plant.