Figure 1: Metabolism of Benzo (a) pyrene

To establish the validity of a test system, it is critical to demonstrate target tissue exposure. This is especially true in cases where there is an adverse outcome in the in vitro mammalian cell bases assay (ICH S2(R1), 2012). This evidence of target tissue exposure can be demonstrated either by showing cytotoxicity to the target tissue or by directly measuring a drug or related toxic metabolite in the target tissue.

In vivo , assessment of cytotoxicity can be conducted by histopathological evaluation of the target tissue or by analyzing changes in the blood biochemistry values.

The direct measurement of drug-related substances can be performed in blood, plasma, or target tissues. Autoradiographic techniques can be used to assess tissue exposure to these substances. ((ICH S2(R1), 2012; Kirkland et al., 2019). Demonstration of target tissue exposure is critical in validating a NAM that can be used as a follow-up to anin vitro assay. Therefore, the goal of this study was to verify that in CEGA tested chemicals can reach the fetal chicken liver (target organ) following administration into the air sac of the fertilized egg at sufficient levels to produce genotoxic effect(s), and to form of sufficient amounts of reactive metabolite(s) from a parent compound upon reaching the target tissue.