Immunotoxicology: role in the safety assessment of drugs.

The immunotoxic effects of drugs are divided into immunosuppression, immunostimulation, hypersensitivity and autoimmunity. The major adverse consequences of immunosuppression are infectious complications and virus-induced malignancies. Flu-like reactions, more frequent autoimmune diseases and hypersensitivity reactions to unrelated allergens, and inhibition of drug-metabolising enzymes are the adverse effects related to immunostimulation. Hypersensitivity reactions are the most frequent immunotoxic effects of drugs. They include immune-mediated ('allergic') and non immune-mediated ('pseudoallergic') reactions. Drug-induced autoimmune reactions, either systemic or organ-specific, are seemingly rare. A review of drug-induced immunotoxic effects demonstrates that immunotoxicity is a significant cause of morbidity and even mortality. As immunotoxicologists have long focused on immunosuppression, the nonclinical immunotoxicity safety assessment of unexpected immunosuppression is based on a number of relatively well standardised and validated animal models and assays. However, there is no general consensus regarding the minimal requirement for this assessment. Many different assays can be used to extend the assessment case by case. Few animal models and assays have been validated for use in the nonclinical safety assessment of unexpected immunostimulation. The situation is worse regarding the prediction of hypersensitivity and autoimmune reactions. Our limited understanding of the molecular and cellular mechanisms of immunotoxicity accounts, at least partly, for this situation. Recent guidelines for the immunotoxicity safety assessment of drugs, even though conflicting on several points, will serve as an impetus not only to refine current animal models and assays, but also to search for better alternatives. The new data generated will have to be interpreted and extended to animal species other than just rodents. Likewise, animal results will have to be compared with findings in humans. The search for immunological endpoints that can be used in several animal species and in humans will therefore become essential. Specific endpoints and clinical criteria that can be included in clinical trials to further investigate the potential for immunotoxicity of new drugs will have to be defined. Because immunotoxicity plays a key role in drug-induced adverse effects, the role of immunotoxicology in drug safety assessment is indisputable and the systematic nonclinical as well as clinical immunotoxicity assessment of every new drug is deemed essential.