New Options for Eradicating Resistant Head Lice

Marcia L. Buck, Pharm.D., FCCP, FPPAG


Pediatr Pharm. 2012;18(6) 

In This Article


Spinosyns are natural fermentation products of a soil actinomycete bacterium, Saccharopolyspora spinosa. While many spinosyns have been identified, the most common are spinosyn A and spinosyn D. Spinosad, a 5:1 ratio of the two, interferes with nicotinic acetylcholine receptors in insects, causing neuronal excitation. Paralysis then results from neuromuscular fatigue. It has both pediculocidal and ovicidal activity, killing nits as well as adult lice. Spinosad is available as a 0.9% topical suspension (Natroba®, ParaPRO LLC) in a vehicle containing the following inactive ingredients: water, isopropyl alcohol, benzyl alcohol, hexaylene glycol, propylene glycol, cetearyl alcohol, steralkonium chloride, ceteareth-20, hydroxyethylcellulose, butylated hydroxytoluene, and FD&C yellow #6.[4,8,9]

The manufacturer conducted two phase III multicenter, randomized, investigator-blind, comparison studies in adults and children 6 months of age and older.[10] In both studies, patients were randomized to receive either spinosad 0.9% without nit-combing or permethrin 1% with nit-combing. Each patient could be treated up to two times over the 21-day observation period. The primary endpoint was the percentage of subjects who were free of lice 14 days after their last treatment.

In the first study of 180 patients, spinosad was effective in 84.6% of patients, compared to 44.9% of the patients treated with permethrin. Similar results were found in the second trial, with 86.7% of the 83 spinosad patients responding compared to only 42.9% of the 84 patients treated with permethrin. In both studies, the results were statistically significant (p < 0.001). The percentage of patients requiring only one treatment was also higher in the spinosad group.[10]

Before applying spinosad, the 4-ounce bottle should be thoroughly shaken. The suspension should be applied to dry hair, including complete coverage of the scalp. The suspension should be left on for 10 minutes and then washed off with warm water. Use of a nit-comb is not essential, but it is recommended in patients being treated with spinosad to remove dead lice and nits. If lice are seen 7 or more days after the initial treatment, a second treatment should be applied.[4]

Adverse effect information was also gathered in the two comparison trials. The most frequently reported adverse effects were application site erythema (in 3% of patients), ocular erythema (2%), and application site irritation (1%). These percentages were similar or lower than those reported in the permethrin group. Other less common adverse effects included application site dryness or exfoliation, alopecia, and dry skin (all < 1%). The safety and efficacy of spinosad has not been studied in children under 4 years of age.[4,10]

In a study of 14 children between 4 and 15 years of age, plasma spinosad concentrations were measured following application of a spinosad 1.8% suspension for 10 minutes. None of the plasma samples contained a spinosad concentration above the lower limit of detection (3 ng/mL), indicating minimal systemic exposure. Although spinosad 0.9% topical suspension contains only a small amount of benzyl alcohol, like benzyl alcohol 5% lotion, it is not recommended for use in infants less than 6 months of age.[4]


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