Normal human keratinocyte assay to predict the human irritancy response

The potential to cause human skin irritation responses is part of the required information for the registration of new chemicals and for chemicals to conform to the new EU Chemical regulations under Registration, Evaluation, and Authorisation of Chemicals (REACH). The Fund for Replacement of Animals in Medical Experiments (FRAME) has developed an in vitro assay for potential human irritant responses via the skin, employing human passage 1-3 primary keratinocytes. The assay is based on two assessments: a direct assay of keratinocytes and an assay of the effect on macrophages of the mediators released by the keratinocytes. Keratinocyte activity is measured by the resazurin assay. Mediator activity is measured by assaying the release of nitric oxide (NO) following macrophage stimulation. It is also possible to measure release of interleukin-1alpha (IL-1alpha). Additional mediators are also released, but the precise range and composition of all these mediators are not clear at present. From time of exposure, this assay can be completed in 3-7 d; the exact timing is dependent on the rate and degree of recovery.