An `Adverse Outcome Pathway`(AOP) describes the sequential progression of events evolving in an organism from the first contact of a toxicant at the molecular level, via a subset of following key effects or biological responses to a final adverse outcome at the individual or population level (OECD 2013). Although AOPs can be outlined as a linear cascade of consecutive events, where one common molecular initiating effect is the prerequisite for all subsequent steps, the `adverse outcome` may vary significantly. In this respect AOPs take into account that different molecular initiating events can cause the same adverse outcome as well as that many different `mode-of actions` (MoA) share common key molecular initiating events. Even though the adverse outcome observed in vivo is the result of a sequential cascade of biological events, each step in this pathway may itself be influenced by other pathways ongoing and/or dominating within the biological system of interest. Thus it is the intrinsic chain of causally linked biological events that determines the AOP and in cases where species specific toxicodynamics can influence various possible MoAs different phenotypic outcomes may be triggered. Species related differences associated with specific biological functions expressed by particular cell types, such as the inhibition or generation of reactive oxygen species in ECETOC TR No. 122 47
various organs and/or tissues, may therefore be responsible that the same substance can cause different pathological outcomes based on a common initial molecular event.
The development of an AOP depends largely on the identification of the `molecular initiating event`, the relevant intermediate events and the final adverse outcome. Based on above outlined concept it becomes clear that each AOP will have only one `molecular initiating event` and one `adverse outcome` as apical endpoint. With regard to “lung overload” effects following chronic inhalation to PSP of low acute toxicity, obvious species specific differences exist in the `adverse outcome`. Rats are considered to be particularly sensitive towards PSP induced lung toxicity compared to other species. Although an accumulation of particles in the deep lung is a common finding in all investigated species, significant differences in the phenotypic `adverse outcome` between rats and all other mammalian species exist. Lung tumours have been reported exclusively in rats, but not in mice, hamsters, non-human primates or humans. It is well established that lung `overload` contributes to the observed (species independent) pathogenesis of non-neoplastic lung responses, with the significant impairment of pulmonary particle clearance as `initial event` relevant for AOP considerations. Since the induction of persistent neutrophilic pulmonary inflammation, apoptosis, generation of ROS/RNS and increased cell proliferation are also recurring descriptors of events occurring during “overload” triggered lung pathology, they may be regarded “intermediate events” in the sense of an AOP. However, with regard to the final biological effect tumours have to be considered the `adverse outcome` only in rats whereas non-neoplastic changes, e.g. fibrosis, seem to be the `adverse outcome` in other species. Key events in the context of an AOP, which are relevant for the diversion of the so far common mechanistic sequence of effects leading to different outcomes may be the consequence of differently equipped cell systems, e.g. in the diversity of toxification/detoxification systems, like anti-oxidants impacting the degree of resulting `oxidative stress`, apoptosis as well as DNA repair capacity. A simplified depiction of an AOP linking a common initial event with different phenotypical outcomes is shown in Figure 5.