Assessment of cellular and molecular metrics for dose selection in an in vivo comet assay: A case study with MDI

The in vivo comet assay can evaluate the genotoxic potential of a chemical in theoretically any tissue that can be processed to a single cell suspension. This flexibility enables evaluation of point-of-contact tissues using a relevant route of test material administration; however, assessing cytotoxicity is essential for the interpretation of comet results. Histopathological evaluation is routinely utilized to assess cytotoxicity, but temporal- and cell-specific considerations may compromise applicability to the comet assay. In the present study, 1,1'-methylenebis(4-isocyanatobenzene) (4,4'-MDI) was administered to rats for 6 h by nose-only inhalation, and the comet assay was conducted to evaluate genotoxicity in the site-of-contact tissue (bronchoalveolar lavage cells) and distal tissues (liver and glandular stomach). Given the reactive nature of MDI, cellular and molecular metrics at the site-of-contact- including inflammation, macrophage activation, apoptosis/necrosis, and oxidative stress- were used to set appropriate exposure concentrations, in addition to the standard systemic measures of toxicity. In the range-finding study, a concentration of 4 mg/m³ was considered the maximum noninflammatory concentration; hence target concentrations of 2, 5, and 11 mg/m³ were selected for the comet study. In the lung lavage, MDI exposure substantially increased total protein and β-glucuronidase, along with cellular apoptosis. Although MDI did not increase the comet assay response (% tail DNA) in any of the tissues examined, the positive control (ethyl methanesulfonate, EMS) significantly increased % tail DNA in all tissues. In total, these data indicate that appropriate cellular and molecular measurements may facilitate dose selection to discern cellular status in the comet assay.