The local lymph node assay in practice: a current regulatory perspective

Following the formal acceptance of the local lymph node assay (LLNA) as an Organization for Economic Cooperation and Development (OECD) guideline in April 2002, the UK Health and Safety Executive (HSE) informed notifiers that this was now the method of choice for the assessment of skin sensitization potential under the EU notification scheme for new industrial chemicals (NONS). This paper summarizes the experience of the HSE for the 2-year period immediately following the issuing of this statement, during which 48 LLNA study reports were assessed for notification purposes. The issues discussed here include adherence to the OECD guideline, interpretation of results, and classification outcomes. Generally, notifying laboratories followed the OECD guideline successfully, with regard to the sex/ strain/numbers of mice used, the precise process used for measurement of cell proliferation, and the use of recommended vehicles and positive controls. Initially, use of the individual animal approach (measuring the cell proliferation in each animal rather than for a pooled dose group) highlighted problems caused by technical inexperience, but these were overcome by practice. Toxicity or irritation were found to be minor factors in dose selection; more important was the choice of vehicle to correctly maximize the test substance concentration, while maintaining appropriate application properties. Contrary to concerns that the LLNA would prove to be less sensitive or more sensitive than the traditionally used Guinea Pig Maximization Test (GPMT), the proportion of new substances classified as skin sensitizers was within the range observed in previous years. Although the sample size is relatively small, the experience of the HSE indicates that the LLNA is satisfactory for routine regulatory use.