To what extent do in vitro tests correctly predict the in vivo residence of nail lacquers on the nail plate?

Pharmaceutical nail lacquers are used to topically treat nail fungal infections. The film's residence on the nail is an important factor, and in the laboratory, rapid in vitro adhesion and water resistance tests are often used to indicate their likely in vivo residence. However, the predictivity of such in vitro tests is unknown. The aim of this work was thus to determine whether, and to what extent, such in vitro tests can correctly predict the in vivo fate of nail lacquers. The in vivo residence of four commercially available nail lacquers (three pharmaceutical and one cosmetic) was determined in 16 volunteers. In vitro, the films' resistance to water, and their adhesion to a model nail plate was measured, and in vitro-in vivo correlations were explored. It was found that the in vitro films' resistance to water correctly predicted the in vivo residence of lacquers, while the adhesion tests did not.