JULY 2007 VOLUME 20, NUMBER 7 © Copyright 2007 by the American Chemical Society
Editorial The Future of ToxicologysWrap Up Wither toxicology? We have enjoyed a series of informative, occasionally provocative, commentaries on this subject from individuals with different backgrounds and opinions. The Future of Toxicology series was a highly selective, noncomprehensive exercise designed to uncover relevant issues and to elicit some public discussion. In retrospect, the first objective worked but the second did not. The perspectives were the most heavily downloaded articles of each issue, not surprisingly, since they were both very interesting and freely available on the web. However, they were not extensively discussed, at least not on the CRT website designed for that purpose. This was partly the result of an access glitch for the first few commentaries, but this was eventually corrected. Perhaps readers were not moved by the opinions expressed, maybe they did not agree but were shy about saying it, or perhaps they agreed and felt no need to comment. You can probably imagine which interpretation the journal’s Editor prefers, but we really do not know. Regardless, many interesting ideas were expressed throughout the series. We have decided to catalog a few of them here, through the filter of our own experience. We will focus on general concepts rather than specific examples. Environmental, industrial, and pharmaceutical exposures represent the historical playing field for most research in toxicology. The underlying goal was, and remains, to protect populations from hazardous agents. However, what was once a very descriptive area has become increasingly mechanistic in its endeavors. Like greed, characterized by the character Gordon Gekko in the movie Wall Street, mechanism “clarifies, cuts through, and captures the essence”. One benefit of uncovering mechanism is that it helps group-related agents, thereby enabling predictive toxicology. Another benefit is that mechanism provides a conceptual basis for developing short-term or in vitro assays with which to test compounds. The combination of mechanistically defined structural alerts and facile assays for toxicity screening allows one to identify potentially dangerous compounds relatively quickly and inexpensively. This reduces the routine practice of toxicology to a service that can be conducted in contracting labs. Consequently, there seems to be
a paucity of compelling problems to work on that would generate a broad mandate for large-scale investment in toxicology. Perhaps society should define global warming as a toxicant! This paradoxsthat classical toxicology is dying from its own successesswas raised by several contributors. However, this is a matter of perspective. We would argue that many scientists are practicing toxicology without a license. They might think of themselves as medicinal chemists, cell biologists, neuroscientists, etc.; yet, by seeking to understand how bacteria can be killed, how cancer cells die in response to drugs, or how amyotrophic lateral sclerosis occurs, they are in essence toxicologists. They may be focused on a drug or a disease, but the questions that they are asking and the techniques that they are applying are the same that are highlighted on a monthly basis in CRT. So, it seems that toxicology needs to reach out to complementary fields and exchange information that will be mutually beneficial. This implies that toxicology needs to think of itself in the context of disease as opposed to individual agents. This will open many new opportunities for research in exciting areas where public interest and support are strong.
What Is A Toxicant? Significant overlap exists between the chemistries and the toxicities induced by exogenous and endogenous agents. In general terms, one may consider, for example, that “an aldehyde is an aldehyde” in terms of cellular reactivity and ability to elicit a toxic effect. Yet, biology teaches us that small alterations in structure can have a significant impact on activity. Exploring these differences can teach us about the mechanism of the toxic response and the receptor(s) that mediate it. So, there is good reason to identify the targets of individual reactive compounds and to determine the resulting biological effects. It is anticipated that common mechanisms will continue to surface from such investigations, regardless of whether the toxic compound is endogenous or exogenous in origin. Thus, broadly defined environmental exposures are relevant to understanding the etiology of, for example, cardiovascular or neurodegenerative diseases and vice versa.
10.1021/tx7001564 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/16/2007
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A related issue is the subtle but telling one of nomenclature. Professional toxicologists distinguish between toxicants and toxins. The former refer to small-molecule toxic agents that are environmental, industrial, or pharmaceutical in origin; the latter refer to naturally occurring biological toxic agents, for example, snake venom toxins, alga bloom natural products, etc. The differential nomenclature is intended to draw a box around toxicants to keep out natural products and is analogous to the abandonment of natural products as drugs by the pharmaceutical industry. This artificial distinction between types of toxic agents precludes options for mechanistic insights into the action of toxicants that one might gather from the natural world. Many natural toxins (conotoxin and ciguatoxin) are extraordinarily potent cytotoxic agents that engage critical signaling pathways very efficiently. Is there a chance that environmental or industrial chemicals engage the same pathways but less efficiently? A recent example is provided by the AB5 subtilase cytotoxin, a member of the AB5 family of bacterial toxins such as Shiga, cholera, and pertussis toxin. The AB5 cytotoxin selectively proteolyzes the endoplasmic reticulum chaperone, Grp78, at a single site, which results in its selective inactivation and cell death [(2006) Nature 443, 548-552]. Grp78 is also a target for modification by reactive electrophiles so the study of these toxicants can benefit from understanding natural toxins.
Computational Toxicology What is the role of computation in the future of toxicology? There have been amazing advances in modeling that are very helpful in understanding the consequences of protein or nucleic acid modification, pathway modulation, etc. For example, one can model ternary complexes of DNA template primers, DNA polymerases, and incoming nucleoside triphosphates to predict or explain the mutagenic outcome of replication. Utilizing molecular dynamics calculations, one can develop temporal trajectories to visualize the kinetic process of nucleotide insertion, translocation, and extension. Modeling such as this will have an increasing impact on understanding structure and function not only because increases in the power of computing will enable more difficult challenges to be tackled but also because there are important biological problems that are not easily amenable to wet chemical approaches. Subtle changes in the structure of proteins caused by modification or mutation are sometimes not detectable by X-ray crystallography because the relevant conformation is not the most stable in the crystal or because the resolution of the final structure does not provide fine molecular details to be visualized. Molecular dynamics calculations can provide insights into conformational changes that are not available from classical structural approaches. A major focus for the future of computational toxicology will be integration and analysis of large data sets. The current state of toxicity databases is something of a mess. There are a number of databases, each with differing content, architecture, and searchability, that makes the task of integration extremely difficult. There are well-annotated and curated databases in the private sector, but the pay-to-play necessity of the commercial world makes them inaccessible to many investigators and not part of the academic training environment. The EPA’s Distributed Structure-Searchable Toxicity (DSSTox) database is an example of a recent effort to provide public access to toxicity data in a common format. DSSTox efforts include the annotation of chemical structures, standardization of toxicity data, and open public access to toxicity databases. Database standardization must be a high priority because information in large data sets will become increasingly available from the toxicological, transcriptional, and proteomic analyses that are being routinely
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performed in laboratories worldwide. Actually, there is not a lot of new toxicological data being put in the public domain from academic laboratories because of growing restrictions on the use of animals in laboratory investigations. These types of data are generated in industry, but most of the data are not publicly available. The only major public testing program is the National Toxicology Program at the National Institute of Environmental Health Sciences. However, much of the actual toxicology testing is performed at high doses and its relevance to lower level exposure is uncertain. Extrapolating such data sets from acute, high-level doses to chronic, low-level exposure represents a major computational and experimental problem. Furthermore, at the risk of overstating the point, we must reassert that access to the data is not user friendly. A consequence of the restricted and challenging access to toxicology data is that the venerable field of quantitative structure activity analysis is stuck in the mud. This is not the result of poor computational analysis; in fact, advances in computing and theory are generating many new algorithms for compound analysis. Rather, it reflects the fact that publicly available toxicity data on new compounds are limited, and most of the data sets that are available are clustered in “historically interesting” regions of chemical space. Thus, we are faced with a “BIG S-little a” scenario when it comes to structure-activity relationships. The EPA’s recently established ToxCast program to perform high-throughput analyses of cellular and wholeanimal toxicity on a chosen set of compounds represents the first major commitment to enrich the public domain with biological activity data in years and should trigger a flurry of activity and may rejuvenate structure-activity analysis. The choice of compounds will be critical to expanding our horizons of chemical toxicology.
Low-Dose Extrapolation As stated above, most of the studies used for risk assessment have been conducted at high doses, mainly to ensure an observable response. However, human exposure to toxic compounds is predominantly low dose. There are serious and legitimate debates about how best to extrapolate results from high-dose testing to low-dose exposures. Superimposed on the issue of dose is the issue of species difference. Testing is done in rodents to assess risk for humans. A critical challenge will be to develop a paradigm for using the limited toxicological data available for a given compound to support human risk assessment. Determining the disposition of compounds at doses comparable to actual exposure is an important first step in this process. This includes the identification and quantification of complete metabolic pathways for a given agent. Current analytical methods are capable of meeting this challenge, with mass spectrometry able to detect ultratrace amounts of metabolites, NMR able to provide complete one- and two-dimensional spectra on micrograms of material, and accelerator mass spectrometry able to detect remnants of compounds at attomole levels. This should enable profiling of the complete metabolic disposition of a given compound at levels at which humans are exposed. In fact, these studies can often be performed IN humans.
Training A critical issue that was not directly addressed in an individual perspective is the challenge of attracting and training the next generation of chemical toxicologists. The flow of talented young people into science has diminished over the past decade, a trend that is probably more acute in the field of toxicology. The perception that toxicology is a flailing or antiquated field may have contributed to this attrition. The toxicology community
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must present itself in a more enticing light to potential trainees. Recruiting physician-scientists into the study of toxicology will help bridge the gap from bench to clinic and is needed to translate our field from one of understanding mechanisms to one of prevention and treatment. Chemical toxicology should be particularly attractive to undergraduate and graduate students in chemistry because it is replete with interesting chemical transformations, classical examples of structure-activity, complex biochemical and biological mechanisms, challenging analytical problems, and the opportunity to improve human health. Toxicity is not just the consequence of high-dose exposure but is an important research field for all of biomedical science, as selective toxicity to tumor cells, fungi, and bacteria is key to developing antitumor agents and antibiotics. CRT has been publishing the very best of chemical toxicology for 20 years, and inspection of current issues reveals that it is still at the frontier. CRT is now available in greater than 3300 libraries worldwide and must play a greater role in highlighting the
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opportunities that exist for young investigators in chemical toxicology. The Editors, in close collaboration with the American Chemical Society, have taken up this challenge and over the next few months we will be incorporating many new features to increase not only the journal’s impact on individual scientists but on their careers as well. We look forward to working with all of you to build the future of toxicology. Aaron T. Jacobs and Lawrence J. Marnett* A.B. Hancock Jr. Memorial Laboratory for Cancer Research Departments of Biochemistry, Chemistry, and Pharmacology Vanderbilt Institute of Chemical Biology Center in Molecular Toxicology Vanderbilt-Ingram Cancer Center Vanderbilt UniVersity School of Medicine NashVille, Tennessee 37232 TX7001564
