, the famous German physician, said “… the cell is really the ultimate morphological element in which there is any manifestation of life …” in articulating his vision of the cellular basis of disease. The implementation of the microscope as a diagnostic tool and the development of a taxonomy of disease organized around cellular dysfunction contributed greatly to the advances in medicine that took place during the first half of the 20th century. The determination of the double-helix structure of DNA in 1953 by Watson and Crick heralded the replacement of the cellular basis of disease by the modern era of molecular pathogenesis. The tools now at our disposal are phenomenal, rapidly evolving, and supported by remarkable computational power.
We are now in a post-Virchow era, and defining an adverse effect purely by its apical manifestations is no longer enough. The toxicity testing of tomorrow will use new molecular techniques to generate enormous amounts of data describing fundamental subcellular responses to toxicants. A TFACS framework, yet to be designed, will provide a path forward to mining this data, generating a deep understanding of toxicity and adversity at the molecular level.
The NRC report provides some guidance for creation of a TFACS framework. Roughly speaking, one can imagine that the TFACS categories might correspond to the initial sequential steps of toxicity testing as proposed in the NRC report: chemical characterization, assessment of toxicity pathway responses and targeted testing, and dose-response and extrapolation modeling. The testing itself will identify response patterns consistent with vulnerabilities in each of these categories (latent failures) that when sufficiently aligned predispose to an adverse effect (active failure). Analysis of the response patterns of numerous toxicants within the TFACS framework will, over time, develop and refine a Taxonomy of Adverse Effects. This conceptual approach is illustrated in .
FIG. 1. The Swiss cheese model of adverse effects. Chemical exposure may result in vulnerabilities, or latent failures, in TFACS categories, represented by the pieces of cheese. Examples of failures for each of these categories are listed in red. When the response (more ...)
Looking into the future, one imagines that the features that predispose to adversity within the TFACS categories () are similar to those already associated with an adverse effect. At the level of the first TFACS category, chemical characterization, response patterns will be identified by evaluating quantitative structure-activity relationships, physical and chemical properties, environmental concentrations, and possible metabolites and toxic properties of the test chemicals. Vulnerabilities predisposing to an adverse effect will likely include a biologically reactive chemical or metabolic product of a chemical, predicted molecular interactions with critical cellular macromolecules, and chemicals that have a high probability of reaching and persisting in the environment.
FIG. 2. The TFACS framework. Within the three TFACS categories illustrated in this example, chemical characterization, toxicity pathways and dose–response, and extrapolation modeling, there are various features that will be used to identify latent failures (more ...)
Assessment of toxicity pathway responses, the next category within the TFACS framework, will provide another level of predictive power for active failures. Chemical exposure may result in activation of adaptive response pathways (e.g., heat shock protein response pathways) and/or pathways that produce adverse consequences (e.g., proapoptotic pathways). Activation of adaptive pathways, while protective, may be a red flag for eventual adverse outcomes should the pathway perturbation remain for a sufficient period of time or intensity. In the context of toxicity pathway alterations, adaptive responses to toxicant exposure will likely be characterized by reversible changes, a limited scope of toxicity pathway–induced alterations, and gradual changes in dose-response. On the other hand, latent failures predisposing to an adverse effect will likely be characterized by irreversible changes, a spreading of toxicity pathway responses, and abrupt dose-response transitions. As we develop this new framework, we will likely identify selected pathways of concern that may be more indicative of an active failure than other pathways. Therefore, the specific toxicity pathways affected and the biological relevance of the pathway alteration are important considerations when characterizing a response as adaptive versus adverse. In a previous forward-thinking report from the NRC entitled Scientific Frontiers in Developmental Toxicology and Risk Assessment
(Committee on Developmental Toxicology, Board on Environmental Studies and Toxicology, National Research Council, 2000
), signaling pathways important during development were characterized and the importance of understanding how molecular perturbations in these pathways can result in adverse outcomes was stressed. This report provides an excellent example of how adverse effects will be classified in the future based on a more complete understanding of toxicity pathways.
Within the third TFACS category, dose-response and extrapolation modeling, there are additional predisposing manifestations of adversity. The characterization and interpretation of the dose-dependent changes in protective and adverse toxicity pathways are at the core of the new toxicity testing paradigm. At some low dose, a pathway may begin to be disrupted by a toxicant exposure, but the pathway will continue to function due to a homeostatic response (an “adaptive” behavior). At a higher dose, the adaptive response is overwhelmed, and an adverse effect takes place. Dose-response modeling is critical to identifying adverse effects, especially since there can be dose-dependent transitions in the principal mechanism of toxicity (Slikker et al., 2004
). As discussed earlier, an abrupt dose-response transition and a transition in mechanism of toxicity at different doses will likely be indicative of an adverse effect. Physiologically based pharmacokinetic models will be needed to determine if the doses that cause toxicity pathway alterations in vitro
are comparable to the human blood/tissue concentrations that would result from environmental exposure levels. A response will be classified as adverse if the dose required to elicit the effect is environmentally relevant.
The development of the Taxonomy of Adverse Effects can only be accomplished through national and international collaboration among laboratories. This will require the establishment of standardized data collection and integration methods. Publicly available databases must be the norm, facilitating collaboration and comparison of data among laboratories and maximizing the utility of the resource. The large amounts of data that will be generated and stored in these databases must be analyzed to identify adverse and adaptive effects of chemical exposure. Therefore, development of standardized bioinformatic techniques for data analysis will also be necessary. These are just a few of the steps that will be essential to the creation of a robust and comprehensive Taxonomy of Adverse Effects.
A fully fleshed out Taxonomy of Adverse Effects is the holy grail of the new toxicity testing paradigm. Implementing a systems-oriented approach to toxicity testing and developing a TFACS framework of data analysis will inevitably lead to a Taxonomy of Adverse Effects as the output of the next 10–20 years of work (or however long it takes). We all seek a mode of action–based molecular understanding of how the initiating events arising from the interactions of a toxicant with a living system produce adverse effects. One advantage of this new approach is a deeper and coherent appreciation of the contributing components that ultimately manifest as an adverse effect. For commercial aviation, the systematic use of such a coherent approach to accident investigation has markedly decreased the frequency of active failures in the past quarter century. The goal for the future of toxicity testing is exactly the same—to build a testing system that is very robust in its identification and understanding of the predisposing manifestations of adversity.