Reduction of Acute Inhalation Toxicity Testing in Rats: The Contact Angle of Organic Pigments Predicts Their Suffocation Potential

Determination of acute inhalation toxicity is requested for hazard assessment of substances. In the regulatory-required test, otherwise toxicologically inert solid (dust) aerosols have to be tested up to a very high concentration limit of 5 mg/L of air. By testing a series of organic pigments at this concentration, we found that some pigments were well tolerated (hence, resulting in an LC50 >5 mg/L). For other pigments, we identified obstruction of the airways with subsequent suffocation as the cause of death at this concentration. In these cases, Organization for Economic Cooperation and Development TG 436 requires that additional animals are tested at 1 mg/L air. However, the mortality of animals at the high concentration could have been avoided if the suffocation by obstruction of the airways was predictable. Hence, we investigated the correlation of several physicochemical characteristics with the observed mortality. Test substances with the highest contact angle, a measure of hydrophobicity, produced mortality at high concentrations, whereas the more hydrophilic compounds did not. Therefore, the contact angle of test substances may serve as a predictive parameter for suffocation potential. We propose conducting this characterization before in vivo testing to reduce the number and suffering of animals until further in vitro and in silico approaches are developed to completely replace animal testing.