Adversity: A Pathologist’s Perspective
Discovery Sciences, Janssen Research & Development, Spring House, PA, USA
Worldwide regulatory agencies require data from toxicologic studies in animals in order to govern the introduction of drugs, consumer products, and food additives to the human population, as well as chemical exposure in the environment. Toxic effects in these animal studies are generally expected to occur in a dose-dependent manner in relation to their incidence and severity, allowing the determination of dose levels where substantial effects occur or where these effects do not occur. Toxic effects generally manifest as changes in cell or tissue morphology and/or function. Gross, microscopic and clinical pathological evaluation are central to toxicological assessment in these studies, and often help in deciding which doses produce an adverse or a nonadverse effect.
Before determining ‘adversity’, a pathologist must address various questions and make decisions at various levels (Figure 7.1). At an individual animal level, a finding in a tissue or organ has to be judged as noteworthy and must be differentiated from artefacts caused by post mortem change or tissue processing. The pathologist has to give the finding an appropriate and a globally accepted and harmonised term. At the study level, the pathological findings must be judged in relation to the test article. To differentiate a test article effect from a chance finding, consideration should be given to the dose response, the ranges of natural variation and the biological plausibility (e.g. clinical/nonclinical data for other compounds in same class, mode of action) (ECETOC, 2000; Lewis et al., 2002).
Often the terms ‘treatment’ and ‘test article’ are used interchangeably when describing an effect. It is important to distinguish whether a finding is ‘treatment’- or ‘test article’-related. ‘Treatment’ indicates the effects likely to be attributed to a procedure (e.g. haemorrhages around the eye due to retro-orbital bleeding). In a study designed with appropriate controls, procedure- or vehicle-related changes are expected to occur in all dose groups, but in studies with a small number of animals or no control animals (exploratory studies), these findings may not be distributed evenly in all the groups and can complicate the interpretation.
This chapter provides a pathologist’s perspective on how to determine the adversity of pathological findings in toxicity studies. It will also discuss the definitions of various toxicity terms, such as ‘lowest observable adverse effect level’ (LOAEL), ‘lowest observable effect level’ (LOEL), ‘no observable adverse effect level’ (NOAEL) and ‘no observable effect level’ (NOEL), along with challenges in defining adversity and communication of NOAEL in study reports.
As part of a risk assessment, toxicology studies in animals are conducted in order to identify and characterise the toxic effects of chemicals and drugs (hazard identification). Based on the dose response in a toxicological study, various dose-related indices (LOAEL, NOAEL, NOEL) commonly used in risk assessment can be calculated. In a well-designed study, doses are selected which produce a clear toxic effect, a LOAEL and either a LOEL, a NOAEL or a NOEL.
As defined in various toxicology publications (e.g. Klaassen, 2013; Derelanko and Auletta, 2014):
- LOAEL is the lowest dose at which there are statistically and/or biologically significant increases in the frequency or severity of adverse effects between the exposed population and its appropriate control.
- NOAEL is the highest experimental dose at which there are no statistically and/or biologically significant increases in the frequency or severity of adverse effects between the exposed population and its appropriate control.
- NOEL is the highest experimental dose at which there are no test article-related effects (adverse or nonadverse) observed in the exposed population, when compared with its appropriate control.
It is noteworthy that the NOAEL is a measured or estimated value and may be different from the true no-adverse-effect level, which lies somewhere between the measured NOAEL and the measured LOAEL (Lewis et al., 2002; Filipsson et al., 2003). Once the NOAEL or NOEL is established from animal toxicity studies, regulatory guidelines are harmonised around the calculation of a safe starting dose for human clinical trials or the determination of allowable exposure limits (FDA, 2005; ICH, 2009; EPA, 2012).
It is a well accepted fact amongst toxioclogists and pathologists that there is no consensus on the definition of ‘adversity’ or ‘adverse effect’, or even of ‘NOAEL’. This can be seen from a quick review of the published literature and various regulatory guidelines (Table 7.1). In general, however, all definitions indicate that an adverse effect is a change in biochemical, functional or structural parameters that may impair performance and generally have a harmful effect on the growth, development or life span of a nonclinical toxicology model (Dorato and Engelhardt, 2005).
Table 7.1 Selected definitions of ‘adverse effect’ from the published literature.
|EPA (2007)||‘Adverse effect: A biochemical change, functional impairment, or pathological lesion that affects the performance of the whole organism, or reduces an organism’s ability to respond to an additional environmental challenge.’|
|FDA (2005)||‘…an adverse effect observed in nonclinical toxicology studies used to define a NOAEL for the purpose of dose-setting should be based on an effect that would be unacceptable if produced by the initial dose of a therapeutic in a phase 1 clinical trial conducted in adult healthy volunteers.’|
|IPCS (2004)||‘…change in the morphology, physiology, growth, development, reproduction, or life span of an organism, system, or (sub) population that results in an impairment of functional capacity, an impairment of the capacity to compensate for additional stress, or an increase in susceptibility to other influences.’|
|Sergeant (2002)||‘Adverse effects are changes that are undesirable because they alter valued structural or functional attributes of the entities of interest…The nature and intensity of effects help distinguish adverse changes from normal…variability or those resulting in little or no significant change.’|
|Lewis et al. (2002)||‘…biochemical, morphological or physiological change (in response to a stimulus) that either singly or in combination adversely affects the performance of the whole organism or reduces the organism’s ability to respond to an additional environmental challenge.’|
|Eaton and Gilbert (2008)||‘The spectrum of undesired effects of chemicals is broad. Some effects are deleterious and others are not…[Regarding drugs], some side effects…are never desirable and are deleterious to the well-being of humans. These are referred to as the adverse, deleterious, or toxic effects of the drug.’|
Much controversy exists regarding the relevance of extrapolating toxicity findings from nonclinical studies to a clinical setting. According to some definitions, an adverse finding is an adverse finding regardless of its relevance to humans. The US Food and Drug Administration (FDA) definition states that ‘the use of NOAEL as a benchmark for dose-setting in healthy volunteers should be acceptable to all responsible investigators. As a general rule, an adverse effect observed in nonclinical toxicology studies used to define a NOAEL for the purpose of dose-setting should be based on an effect that would be unacceptable if produced by the initial dose of a therapeutic in a phase 1 clinical trial conducted in adult healthy volunteers’ (FDA, 2005). Note that the FDA does not ask that the pathologist or toxicologist determine whether the adverse effect would occur in humans or comment in any way on the relevance of the finding for humans; it simply asks, if it occurred, would it be acceptable for a healthy volunteer who can derive no therapeutic benefit from the drug?
The difference in approach likely arises from the nature of chemicals and from differences between the agencies responsible for making regulatory decisions. For example, risk assessment performed by the US Environmental Protection Agency (EPA) for the registration and marketing of agrochemicals relies heavily on animal toxicity data, with little or no information on human exposure. Environmental chemicals are not intended to provide therapeutic benefits. In contrast, for a pharmaceutical drug, critical factors in the registration and marketing are human efficacy and safety data. In early drug development, due to lack of human-exposure data, animal-toxicity data generally help in making ‘go or no-go’ efficacy and safety decisions; later in development, toxicity data can still be decisive, especially regarding reproductive/developmental toxicity or carcinogenic potential.
With the growth of advanced technologies and high-throughput approaches in the field of toxicity testing, there is a strong interest in finding alternatives to animal testing (NRC, 2007). At the centre of this strategy is a revamping of testing to focus on the molecular mechanisms of toxicant effects. Using high-throughput assays and human cells in vitro, large amounts of in vitro data can be generated. In order to make biological-interpretation and regulatory decisions based on these data, it is critical to re-evaluate the criteria of adversity that consider the toxicant-induced mode of action at the molecular and cellular levels. A Health and Environmental Sciences Institute (HESI) committee was tasked with discussing approaches to identifying adverse effects in the context of 21st-century toxicity testing. The committee (Keller et al., 2012) recently published what appears to be a practical definition of adversity in this context, adopted from the IPCS risk-assessment terminology (IPCS, 2004), defining an adverse effect as: ‘A change in morphology, physiology, growth, development, reproduction, or life span of a cell or organism, system, or (sub)-population that results in an impairment of functional capacity, an impairment of the capacity to compensate for additional stress, or an increase in susceptibility to other influences.’
Toxicologists and pathologists have not been consistent in applying judgements in determining whether an observed effect in a nonclinical toxicity study is adverse or nonadverse. A nonadverse finding may be given in the following categories: ‘no significant effect on organ function’, ‘no significant effect in overall health’, ‘finding does not result in organ failure’, ‘finding being reversible or nonprogressive’ ‘finding being an adaptive response’ and ‘finding has no counterpart in humans or occurs through a mechanism not relevant to humans’. Thus, the decision over the adverse nature of a test article-related finding in a nonclinical toxicology study is often subject to discussion, challenge and reinterpretation (Dorato and Engelhardt, 2005).