The effect and mode of action of zinc pyrithione on cell growth. II. In vivo studies.

The effect of topical application of the anti-dandruff agent zinc pyrithione (ZnPTO) on epidermal DNA synthesis in normal and hexadecane-stimulated rat skin was investigated. Autoradiography was used to determine the percentage of epidermal cells labelled with [3H]thymidine (labelling index). ZnPTO at 1% in shampoo base caused a slight increase in the labelling index in normal skin, similar to the effect of the shampoo base alone. No effect of 1% ZnPTO as an aqueous dispersion was observed. ZnPTO at 1% in shampoo base did not reduce the large increase in labelling index produced by hexadecane, nor did shampoo base alone or 1% ZnPTO in water. The shampoo base with or without 1% ZnPTO had only a very slight effect on the histopathology of normal and hexadecane-treated skin, and 1% ZnPTO in water had no effect. It is concluded that the in vitro potential of ZnPTO to cause growth inhibition is not achieved in vivo, presumably because of low percutaneous absorption. This evidence does not support a cytostatic mode of action in clearing dandruff.