Isopropyl Myristate and Cocoa Butter are not Appropriate Positive Controls for Comedogenicity Assay in Asian Subjects

Background: Comedogenicity is an important consideration in development of topical products, such as cosmetics. An animal test using rabbit ear has long been served as a model for prediction of comedogenicity of topical products, but correlation to the human skin remained controversial. Isopropyl myristate and cocoa butter which are used as positive controls in animal test, also used in human clinical assay but report for the comedogenicity of those two positive controls in human, especially in Asian skin is limited. Objective: To assess the comedogenicity of isopropyl myristate and cocoa butter in Asian skin using a modified human model established by Mill and Kligman. Methods: We selected eight Asian subjects with prominent follicular orifices and the prone to acne type skin on the upper back. Two substances, isopropyl myristate and cocoa butter, were applied on the upper back skin three times a week for four weeks. Petrolatum and non-treated skin served as negative controls. Microcomedones were estimated by cyanoacrylate follicular biopsy at baseline and after four weeks of application. The number of follicles and microcomedones were determined by quantitative image analysis. Results: Microcomedone activities of isopropyl myristate and cocoa butter have no significant difference to negative controls in Asian skin. Conclusion: Comedogenic activity of isopropyl myristate and cocoa butter is too weak that those are not appropriate positive controls for clinical human assay in Asian skin. Further studies are necessary to determine appropriate positive controls in the human model for all ethnic populations.