Stability of 5% Permethrin Cream Used for Scabies Treatment

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To the Editors: Human scabies is caused by the human mite (Sarcoptes scabiei var. hominis). In addition to the discomfort caused by the intensely itchy lesions, infestations often become secondarily infected.1 Scabies is transmitted by person-to-person body contact, and infants and children are particularly at a greater risk for infection.2 The American Academy of Pediatrics considers topical permethrin 5% the treatment of choice for scabies infestation because of its safety and efficacy profile.1,2 One treatment with permethrin 5% cream is usually effective in eradicating scabies.3 Some experts recommend retreatment one week later, while others recommend retreatment only if live mites are observed. A thin layer of the cream should be applied to all areas of the body including the face and scalp in infants and young children. The cream should be washed off after 8–14 hours.4 Under those conditions permethrin is exposed to normal body temperature of 37°C. This cream is usually kept in kitchen or bathroom cabinets exposed to wide temperature and humidity fluctuations. There are few studies published concerning permethrin stability. We assessed the stability of permethrin in a 5% cream formulation for the topical treatment of scabies and to provide information about the stability of the drug. Permethrin (cis:trans ratio of 25:75) (Supelco Inc; Bellefonte, PA) and drug formulation (permethrin 5% cream) were provided by UniPharma S.A. (El Papiol, Barcelona, Spain). We checked the stability of permethrin 5% cream stored at 40 ± 2°C and 75 ± 5% of relative humidity for up to 6 months. In these conditions 3 samples were periodically obtained from each batch (n = 3) of drug formulation tested. Sample treatment was carried out and concentration measurements were determined by using a newly developed and validated high-performance liquid chromatography method. There was no apparent degradation (less than 5% changes in assay from its initial value) of either isomer under the conditions of the study and for at least the 6-month period. In conclusion, permethrin appeared to be stable in a 5% cream formulation used for scabies treatment at 40°C ± 2°C and 75 ± 5% of relative humidity during at least 6 months. ACKNOWLEDGEMENTS This study received partial financial support from Unipharma S.A. and the Bosch i Gimpera Foundation of the University of Barcelona. Pilar Modamio, PhD Cecilia F. Lastra, PhD Joan Sebarroja, PharmD Eduardo L. Mariño, PhD Clinical Pharmacy and Pharmacotherapy Unit Department of Pharmacy and Pharmaceutical Technology Faculty of Pharmacy University of Barcelona Barcelona, Spain