Principles of dose-setting in toxicology studies: the importance of kinetics for ensuring human safety.

Regulatory toxicology seeks to ensure that exposures to chemicals encountered in the environment, in the workplace, or in products pose no significant hazards and produce no harm to humans or other organisms, i.e., that chemicals are used safely. The most practical and direct means of ensuring that hazards and harms are avoided is to identify the doses and conditions under which chemical toxicity does not occur so that chemical concentrations and exposures can be appropriately limited. Modern advancements in pharmacology and toxicology have revealed that the rates and mechanisms by which organisms absorb, distribute, metabolize and eliminate chemicals-i.e., the field of kinetics-often determine the doses and conditions under which hazard, and harm, are absent, i.e., the safe dose range. Since kinetics, like chemical hazard and toxicity, are extensive properties that depend on the amount of the chemical encountered, it is possible to identify the maximum dose under which organisms can efficiently metabolize and eliminate the chemicals to which they are exposed, a dose that has been referred to as the kinetic maximum dose, or KMD. This review explains the rationale that compels regulatory toxicology to embrace the advancements made possible by kinetics, why understanding the kinetic relationship between the blood level produced and the administered dose of a chemical is essential for identifying the safe dose range, and why dose-setting in regulatory toxicology studies should be informed by estimates of the KMD rather than rely on the flawed concept of maximum-tolerated toxic dose, or MTD.

Evaluation of the Inherent Toxicity Concept in Environmental Toxicology and Risk Assessment.

Intrinsic/inherent chemical properties are characteristic, irrespective of the number of molecules present. However, toxicity is an extensive/extrinsic biochemical property that depends on the number of molecules. Paracelsus, often considered the father of toxicology, noted that all things are poisonous. Because dose magnitude (i.e., number of molecules) determines the occurrence of poisonous effects, toxicity cannot be an intrinsic/inherent biochemical property. Thus, toxicology's task is to determine case-specific risks resulting in adverse effects produced by the interaction of toxic doses/exposures, toxic mechanisms, and case-specific influencing factors. Experimental testing results are known to vary within and between chemicals, test organisms, and experimental conditions and repetitions; however, hazard-based approaches treat toxicity as a fixed and constant property. A logical alternative is the standard-risk, case-specific risk model. In this approach, testing data are defined as standard risks where the nature, magnitude, and toxicity effect is standardized to the organism, chemical, and test conditions. Interpolation/extrapolation of standard risks to site-specific conditions (i.e., case-specific risks) is challenging, requiring understanding of the influences of the complex interactions within and between differing species, conditions, and toxicity-modifying factors. Therefore, Paracelsus's paradigm is perhaps better abbreviated as "dose-causality-response", because a key interpretive requirement is establishing toxicity causality by separating mode/mechanism of toxic action from modifying factor influences in overall toxicity responses. Unfortunately, the current knowledge base is inadequate. Moving to a standard-risk-specific-risk paradigm would highlight the importance of improving the toxicity causality knowledge base. Thereby, a rationale would be provided for enhancing the design and interpretation of toxicity testing that is necessary for achieving advances in routine translation of standard-risk to specific-risk estimates-the raison d'être of regulatory risk decision making. Environ Toxicol Chem 2020;39:2351-2360. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

The regulatory challenge of chemicals in the environment: Toxicity testing, risk assessment, and decision-making models.

Environmental assessment for chemicals relies on models of fate, exposure, toxicity, risk, and impacts. Together, these models should provide scientific support for regulatory risk management decision-making, assuming that progress through the data-information-knowledge-wisdom (DIKW) hierarchy is both appropriate and sufficient. Improving existing regulatory processes necessitates continuing enhancement of interpretation and evaluation of key data for use in decision-making schemes, including ecotoxicity testing data, physical-chemical properties, and environmental fate processes. Yet, as environmental objectives also increase in scope and sophistication to encompass a safe chemical economy, testing, risk assessment, and decision-making are subject to additional complexity due to the ongoing interaction between science and policy models. Problems associated with existing design and implementation choices in science and policy have both limited needed development beyond chemo-centric environmental risk assessment modeling and constrained needed improvements in environmental decision-making. Without a thorough understanding of either the scientific foundations or the disparate evaluation processes for validation, quality, and relevance, this results in complex technical and philosophical problems that increase costs and decrease productivity. Both over- and under-management of chemicals are consequences of failure to validate key model assumptions, unjustified standardized views on data selection, and inordinate reification (i.e., abstract concepts are wrongly treated as facts).

The human relevant potency threshold: reducing uncertainty by human calibration of cumulative risk assessments.

The 2008 National Research Council report "Phthalates and Cumulative Risk Assessment: Tasks Ahead," rejected the underlying premises of TEQ-like approaches - e.g., chemicals are true congeners; are metabolized and detoxified similarly; produce the same biological effects by the same mode of action; exhibit parallel dose response curves - instead asserting that cumulative risk assessment should apply dose addition (DA) to all chemicals that produce "common adverse outcomes" (CAOS). Published mixtures data and a human health risk assessment for phthalates and anti-androgens were evaluated to determine how firmly the DA-CAOS concept is supported and with what level of statistical certainty the results may be extrapolated to lower doses in humans. Underlying assumptions of the DA-CAOS concept were tested for accuracy and consistency against data for two human pharmaceuticals and its logical predictions were compared to human clinical and epidemiological experience. Those analyses revealed that DA-CAOS is scientifically untenable. Therefore, an alternative approach was developed - the Human-Relevant Potency-Threshold (HRPT) - that appears to fit the data better and avoids the contradictions inherent in the DA-CAOS concept. The proposed approach recommends application of independent action for phthalates and other chemicals with potential anti-androgenic properties at current human exposure levels.

Review of the toxicity of chemical mixtures containing at least one organochlorine.

An analysis of current research on mixture toxicity was conducted by critically reviewing published journal articles. The scope was limited to complex mixtures (more than two components) where at least one component was a chlorinated organic chemical. Although the basics of dose-response are widely accepted for mixtures, a number of technical issues, including dose metrics and the unquantified influence of toxicity modifying factors, confound data interpretation and restrict the ability to establish reliable determinations of the presence, nature, and extent of additivity. Lack of knowledge about dose level influences and species-specific variations contribute further interpretational limitations. Within this context, available data indicates that most tested mixtures are near or below simple dose/concentration additivity. Exceptions (both positive and negative) tend to occur when tested mixtures have only a few components or where sensitive whole organism or sub-organismal changes are used as the response metric. Available information does not routinely identify the presence of chlorine as a marker either of a particular type of toxicity or consistently greater potency. The most profound difficulty is the problem of clearly defining when and why similarity and dissimilarity of toxic action is expected for a particular mixture. This impediment largely results from the lack of a generally accepted, technical classification for mode/mechanism of toxic action coupled with the lack of a generally accepted classification scheme for mode/mechanism of toxicity interactions.

Review of the toxicity of chemical mixtures: Theory, policy, and regulatory practice.

An analysis of current mixture theory, policy, and practice was conducted by examining standard reference texts, regulatory guidance documents, and journal articles. Although this literature contains useful theoretical concepts, clear definitions of most terminology, and well developed protocols for study design and statistical analysis, no general theoretical basis for the mechanisms and interactions of mixture toxicity could be discerned. There is also a poor understanding of the relationship between exposure-based and internal received dose metrics. This confounds data interpretation and limits reliable determinations of the nature and extent of additivity. The absence of any generally accepted classification scheme for either modes/mechanisms of toxic action or of mechanisms of toxicity interactions is problematic as it produces a cycle in which research and policy are interdependent and mutually limiting. Current regulatory guidance depends heavily on determination of toxicological similarity concluded from the presence of a few prominent constituents, assumed from a common toxicological effect, or presumed from an alleged similar toxic mode/mechanism. Additivity, or the lack of it, is largely based on extrapolation of existing knowledge for single chemicals in this context. Thus, regulatory risk assessment protocols lack authoritative theoretical underpinnings, creating substantial uncertainty. Development of comprehensive classification schemes for modes/mechanisms of toxic action and mechanisms of interaction is needed to ensure a sound theoretical foundation for mixture-related regulatory activity and provide a firm basis for iterative hypothesis development and experimental testing.

Influence of soil half-life on risk assessment of carcinogens.

Risk estimates for contaminants in soil are currently calculated assuming that concentrations remain unchanged over time. In reality, biological and physicochemical processes can substantially diminish contaminant concentrations in soil. For exposure periods typically evaluated in USEPA risk assessments, failure to consider the decline in contaminant levels from environmental transport and degradation can result in a significant overestimation of the average daily dose of toxicant. This overestimation may be up to 2- to 3-fold for compounds with long half-lives (15-20 years) in soil and as much as 40-fold for compounds with short half-lives (0.5 years). Overestimation of dosages affects estimation of cancer risks because of the assumption that the probability of cancer increases directly with the cumulative dose of carcinogen. Thus, assuming static contaminant concentrations in soil adds unacknowledged conservatism to cancer risk estimates and target concentration limits. Furthermore, as significant time may elapse before future-use scenarios could possibly occur, soil half-life can affect the estimation of noncarcinogenic health hazards as well. Therefore, an increase in target concentration limits for some compounds could be allowed and corresponding remediation costs reduced by considering how soil half-life changes the dosage calculation. Specific examples of the influence of soil degradation rates on estimates of cancer risk are presented and the degree of added conservatism imparted to risk assessments through assumption of static site contaminant levels is discussed. Considering the potential importance of this parameter for risk assessment and risk management decisions, soil degradation of contaminants under site-specific conditions should be performed whenever possible and incorporated into the risk assessment exercise. When the soil degradation rate cannot be measured or reliably predicted, an estimate of the degree of conservatism should be made to provide risk managers with an appreciation of the degree of uncertainty in the calculation of risk.

Drug sensitivity of heat-resistant mouse B16 melanoma variants.

Induction of transient thermotolerance by heat or other cytotoxic stressors has been reported to confer a moderate degree of drug resistance to tumor cells in vitro. In this study, a genetically stable, heat-resistant mouse B16 melanoma variant (W-H75) was tested for its sensitivity to various cytotoxic and antiproliferative agents. The heat-resistant W-H75 cells displayed a moderate two- to threefold resistance to doxorubicin, VP-16, VM-26, colchicine, cis-dichlorodiammineplatinum(II), HgCl2, and CdCl2. Marginal resistance to 4'(9-acridinylamino)methanesulfon-m-anisidide vinblastine, 1,3-bis(2-chloroethyl)-1-nitro-sourea, and NaAsO2 was observed, while no difference in sensitivity to the anticancer drugs, actinomycin D and camptothecin, was observed. Although W-H75 cells were generally more resistant than the parental cells to most of the agents that were tested, they were collaterally sensitive to the antimetabolites methotrexate and 6-mercaptopurine. Resistance of the W-H75 cells to epipodophyllotoxins and anthracyclines was not due to differences in steady-state drug accumulation. For the epipodophyllotoxin VP-16, resistance may be related to a relative decrease in the number of drug-induced DNA strand breaks in W-H75 cells. However, no difference in DNA strand breakage was observed between W-H75 and parental cells which were treated with doxorubicin, suggesting that resistance to this drug occurred by a different mechanism. The possible involvement of glutathione and glutathione S-transferase in resistance was also investigated. The glutathione content in W-H75 cells was 35% higher than that in the parental line. However, glutathione S-transferase activity appeared to be identical in both cell lines. Two other heat-resistant B16 melanoma variants, B-H103 and R-H92, were also tested for sensitivity to doxorubicin and VP-16. In contrast to the W-H75 cells, these two heat-resistant variants were hypersensitive to doxorubicin. The B-H103 cells were also hypersensitive to VP-16. This study suggests that selection for cellular resistance to heat may result in cells that have an altered sensitivity to drugs.