A tier approach for evaluating the respiratory allergenicity of low molecular weight chemicals

A multi-level approach for evaluating low molecular weight chemicals as respiratory sensitizers is proposed. The approach involves four levels of testing that utilize both in vitro and in vivo methods. Tier 1 evaluates structure-activity information to determine if the chemical can covalently modify carrier molecules. It also includes a literature search to determine if the compound belongs to a family of chemicals that has been reported to induce hypersensitivity. Tier 2 tests the chemical's potential to haptenate carrier molecules (i.e., protein) under in vitro conditions. Positive results in Tiers 1 and 2 lead to testing in a guinea pig injection model to assess chemical immunogenicity (Tier 3). A positive result at this level leads to testing in a guinea pig inhalation model to address questions about relevant routes of chemical exposure and allergenicity (Tier 4). Tier 4 results are used in determining safe chemical exposure levels. We have evaluated three chemicals using this scheme: phthalic anhydride, reactive black b dye, and toluene diisocyanate. All three have reactive groups and haptenate protein in vitro. They induce a humoral immune response when injected into guinea pigs at equimolar concentrations, and they sensitize animals via inhalation exposure. The severity of the response (antibody titer and respiratory reactivity) can be used to rank-order the chemicals in terms of allergenic "potency." Our data indicate that this approach can detect chemical allergens and can be used to characterize them as moderate or strong respiratory sensitizers.