MOLECULAR TOXICOLOGY END-POINTS IN RODENT INHALATION STUDIES

Although histopathology will continue to be essential for assessing th e results of rodent inhalation studies, molecular toxicology endpoints are of increasing importance, as these techniques often complement an d extend histopathological examinations. One of the primary uses of mo lecular toxicology is determining the delivered dose of the inhaled ma terial to macromolecules in target tissues. During inhalation studies this is most often done by measuring DNA adducts in the respiratory tr act. DNA adducts may be measured specifically (e.g. using monoclonal a ntibodies or mass spectrometry) or non-specifically (e.g. by using the P-32-post-la-beling assay). Another major use of molecular toxicology techniques is the assessment of cellular and molecular changes in tar get tissues which may precede or be more sensitive than histopathologi c alterations. For example, rates of cellular DNA synthesis occurring in target tissues may be quantified at any time during the study by ad ministering the animals either radiolabelled thymidine or the non-radi olabelled thymidine analog bromodeoxyuridine (BrdU). Pulmonary changes may be assessed in bronchoalveolar lavage fluid using either cellular (e.g. macrophage number, granulocyte number) or biochemical (e.g. alk aline phosphatase, lactate dehydrogenase) techniques. The potential of the inhaled material to produce genetic alterations may be evaluated by examining the chromosomes of pulmonary alveolar macrophages for cyt ogenetic changes. To illustrate the use of these endpoints, an experim ent was conducted to determine the molecular toxicology of aged and di luted sidestream smoke (a surrogate for environmental tobacco smoke) i n rodent inhalation studies. The endpoints measured were DNA adducts i n target and nontarget tissue, chromosome aberrations in pulmonary alv eolar macrophages, and DNA synthesis in the epithelial lining of the n asal turbinates. In conclusion, quantitative molecular toxicology tech niques are expected to assume an increasingly prominent role in the co nduct and interpretation of rodent inhalation studies.