Use of the local lymph node assay for the estimation of relative contact allergenic potency

The effective toxicological evaluation of skin sensitization demands that potential contact allergens are identified and that the likely risks of sensitization among exposed populations assessed. By definition, chemicals which possess the toxicological property of skin sensitization potentially are capable of causing allergic contact dermatitis (ACD) in humans. However, this hazard is not an all-or-none phenomenon; clear dose-response relationships can be discerned and thresholds identified for both the induction of sensitization and the elicitation of contact dermatitis. Commonly, these parameters are grouped under the heading of potency, determination of which is vital for risk assessment. In the present investigation, the local lymph node assay (LLNA) has been employed to determine the relative potency of a range of 20 chemicals. The parameter used is the estimated concentration required to produce a 3-fold increase in draining lymph-node cell proliferative activity, the EC3 value. These measurements have been compared with an assessment of the human sensitizing potency of the 20 selected chemicals, each being assigned to 1 of 5 classes based on their human sensitizing potency. The EC3 value, derived from LLNA work carried out in acetone/olive oil vehicle, correlated well with the human classification, with the strongest sensitizers having low EC3 values (<0.1%), weaker sensitizers having EC3 values generally in the 1-10% range, and non-sensitizing chemicals having EC3 values in excess of 100%. In conclusion, the derivation of the EC3 for a chemical provides an objective and quantitative estimate of potency that is of considerable utility for skin sensitization risk assessment.